Microfluidics_Final_Draft_Merge_Final_PDFPDF

Theoretical
Development of a
Microfluidic
Insulation Device
ME6122 Microfluidics
Coursework
April 2016
Jamie Fogarty 10100598
Vivek Raman 15053873
Evan Tierney 12145262
Florent Fege-Bourchanin 15048357

Abstract
Microfluidics is the science and technology of systems that manipulate minute amounts of fluids, in
the range of 10-9
-10-18
litres, using channels with dimensions in the tens to hundreds of micrometres.
These systems are mainly used for analyses purposes, e.g. DNA analysis, biological/chemical agent
detection sensors; however they have been explored and used for other purposes such as micro-
flow devises. These are, for instance, micro heat exchangers & micropumps. The aim of this paper is
to theoretically develop a micro-flow system aimed at significantly improving the insulation of
windows, to minimise the heat loss of businesses and homes in cold countries.
A literature review is conducted, identifying window insulation best practises. A scale analysis is
conducted to determine if heat exchanger systems benefit from minimisation. A micro-flow system
devised by Hatton et. al. (2016) is considered. This system is a thin, transparent, bioinspired,
convective cooling layer for building windows. It contains microvasculature, millimetre scale, fluid-
filled channels, positioned on the inner surface of the window and aimed to convect sun radiation
thermal energy of windows in sunny countries, to reduce the energy input of air conditioning units.
The paper from Hatton et. al. (2016) contained experimental results that were used to obtain a heat
transfer coefficient of such a micro flow system. This figure, for a 10x10cm2
window section and heat
transfer surface area, was calculated to be 200W/m2
K. A thermal resistance network was then
formulated for a double glazed window to obtain the range of temperatures from the inside of the
building to the outside. With this, the best positioning for a micro flow system aimed at insulation
was determined to be the outside of the window. This was determined as the outside surface of the
window could be heated up to the same temperature as the inside to diminish the flow of heat and
provide better insulation. The energy required to increase the temperature of the outer surface of
the window to 20°C (the temperature of the inner room) is 23W for a 10x10cm2
window section, or
2.3kW for a full scale 1x1m2
window. Next the best channel geometry was selected by considering
the channels hydraulic resistance and pumping power. The rectangular geometry was selected,
considering the ease of fabrication in comparison with the other geometries and due to the required
pumping power, Pp=9W for 10x10cm2
or Pp=900W for 1x1m2
. The most effective channel network
structure was determined also. This is determined by intuition, and a zig-zag network was
considered the most effective as the flowing water will cover more surface area of the glass. Next a
choice of pumps and valves were considered. The fabrication of the microfluidic system had a
number of possible ways. Having in mind the component (pumps and valves) to be added on to the
system, a reliable method which would give thermal and mechanical stability was required. Soft
lithography technique which not only provided the requirements, but also cheaper to manufacture
was adopted. The manufacture of reliable microfluidic components is a very difficult task and a lot of
thought was put into the choice of pump and valve. The Centrifugal pump was selected because this
method provides a wider range of flow rates and is more energy efficient than other pumps that
were studied. A number of valves were looked at in this study in order to determine which would be
best suited to our micro-flow system. The solenoid valve is an automatic valve that uses low
amounts of energy and would fit neatly into our system, it is for this reason the solenoid valve was
chosen.
Finally the impact of society was determined to be a reduction in carbon foot print. This reduction is
a result of the improved insulation of the window sections, and less energy required to maintain the
household/business at a comfortable temperature. The savings determined for a 1x1m2
was a
function of the energy input (energy required to heat water to room temperature (RT), and the
pumping power required to achieve the necessary flow rate of water of the window surface) and the
thermal energy lost through the window section. The energy saved was calculated to be 1.75kW.

This is a 29% saving, considering the energy inputs into the micro flow system and the heat losses
through the 1x1m2
window section.

Nomenclature
Symbol Definition Units
𝑸̇ 𝒄𝒐𝒏𝒅 Conduction heat rate W
𝑸̇ 𝒄𝒐𝒏𝒗 Convection heat rate W
𝑷 𝒆 Peclet number dimensionless
𝑷 𝒑 Pumping power W
𝑷 𝒓 Prandtl number dimensionless
𝑸̇ Volumetric flow rate m3
/s
𝑹 𝒂 Rayleigh number dimensionless
𝑹 𝒄𝒐𝒏𝒅 Thermal resistance by conduction K/W
𝑹 𝒄𝒐𝒏𝒗 Thermal resistance by convection K/W
𝑹 𝒆 Reynolds number dimensionless
𝑻∞ Environment temperature K
𝑻 𝑺 Surface temperature K
𝒎̇ Mass flow rate kg/s
A Cross-section area m²
Cp Mass heat capacity J/kgK
Dh Hydraulic diameter m
g Acceleration due to gravity m/s²
h Convective heat transfer coefficient W/m²K
k Thermal conductivity W/mK
L Total length of the channels m
n Number of channels dimensionless
Nu Nusselt number dimensionless
P Perimeter m
Q Heat exchanged W
R Thermal resistance K/W
u Fluid velocity m/s
U Heat transmission coefficient W/m².K
w Air layer width m
x Window dimension m
β Thermal expansion coefficient /K
δ Channel spacing m
ΔP Pressure drop Pa
ΔT Difference of temperature K
μ Dynamic viscosity Pa-s
ν Kinematic viscosity m²/s
ρ density Kg/m^3
τc Thermal convective time constant s
τd Thermal diffusivity time constant s

1. Introduction
In the world of engineering, immense effort is imposed on conserving space and minimising weight
and material, while still achieving the end goal. Through this concept, humanity has witnessed the
most rapid technology development in its history- the miniaturisation of electronic devices.
Microelectronics was the most significant enabling technology of the last century, with integrated
circuits and progress in information processing (Nguyen and Wereley 2002). Microelectronics has
been a platform for work, discovery and invention. Presently poised at the limit of photolithography
technology (having a structure size of 100nm), the progression of microelectronics has slowed down
(Moore, 1980). Lagging behind the miniaturisation of electrical devices, efforts were placed on the
development of miniaturised non-electronics. In the late 70s, silicon technology was extended to
machining mechanical micro devices (otherwise known as microelectromechanical systems (MEMS))
(Peterson, 1982). The development of micro flow sensors, micropumps, and microvalves in the late
80s dominated the early stage of microfluidic. In recent years, the advent of new microtechnologies
have significantly enabled microfluidic protocols (such as the fabrication of microchannels & novel
valves and pumps).
Microfluidics is the science, and technology of systems that process or manipulate small amounts of
fluids (10-9
-10-18
litres), using channels with dimensions of tens to hundreds of micrometres
(Whitesides 2006). In recent years, significant interest in this technology has spurred emergence of
fields known as micro total analysis systems (µTAS) or lab-on-a-chip devices (LOCs). These systems
are predominantly used for analyses purposes and are used in medical, pharmaceutical and defence
applications; for instance, in drug delivery, DNA analysis, and biological/chemical agent detection
sensors on micro systems (Akbari 2011). For the specified applications, and others, microfluidic
systems offer a number of capabilities, such as; minute quantities of samples and reagents, high
resolution and sensitivity, lower costs in comparison to the macroscale, short analysis times, and
small footprints for analytical devices (Tian and Finehout 2008). These capabilities are inherent of
the scale of the microfluidic systems, i.e. their miniaturisation. It should be noted that the scaling
down of the transport phenomena also poses some other fundamental differences against the
macroscopic scale, in relation to the physical effects governing the flow. This aspect, and also the
fact that only in recent times specific technologies have been developed to allow for complex
microfluidic protocols, deems it a new science and technology chips ("Microfluidics and microfluidic
devices: a review - Elveflow" 2016). The scaling effects and microfluidic protocols will be introduced
and discussed in sections 5 and 6 respectively.
Despite analysis purposes, microfluidic has been explored for other applications. Other micro-
systems can be subdivided into three categories; (1) MEMS: Micro-Electro-Mechanical Systems (for
instance, air bag acceleration sensors, HD reader, etc.). (2) MOEMS: Micro-Opto-Electro-mechanical
Systems (for instance, micro endoscope, etc.). (3) MFD: Micro-flow devices (for instance, micro heat
exchangers, micro-pumps, etc.) (Morini 2004). Under consideration for the project at hand, micro
system consideration will be placed on a MFD in the context of micro-heat exchangers. The
theoretical development of a micro-flow device will be conducted. This device will be aimed at
improving the insulation of homes and businesses, reducing heating costs, and in turn carbon foot-
print. The theoretical development will aim to characterise the flow, the heat transfer, and specify
the potential for fabrication methodologies, essentially aiming to establish a basis for prototyping.
This paper will firstly analyse existing methodologies of window insulation to gain an understanding
for the platform being considered.

2. Literature Review
In order to assess the viability of the insulating window described in this coursework, it is useful to
run through the existing window technologies. Whether these technologies are commercially
available, in current development, or only at the idealised stage, they are described in many energy
and building oriented reviews (Selkowitz 1979) and (Clarke et al. 1998, pp. 231-41). Indeed, the
insulation of buildings is a major issue for the construction industry in terms of efficiency, economy
and ecology (Konroyd-Bolden and Liao 2015, pp 245-54). Going from the optimisation of the classical
double glazed window to the most sophisticated glazing systems, this section will describe various
window technologies and will compare their efficiency.
It is evident that a window requires a trade-off between its two main functions:
- Daylight entry
- Insulate the inside building from the outside environment (heat, wind, rain, etc.)
To understand the basis on which each technology is based, it is necessary to understand how the
heat is transmitted through a window.
Figure 1 Schematic of the whole heat transmittance phenomenon through a classical window
(Chow et al. 2010, pp.212-20)
The window is subject to the three modes of heat transmission as shown in Figure 1:
- Thermal conduction through the material, dependent on the thickness and the thermal
conductivity of the material.
- Thermal convection, dependent on the surface of the window and the environmental
conditions.
- Heat radiation essentially coming from the sun, part of which is transmitted to the room, a
second part of it is reflected by the window and a third part is absorbed by the window.
Given that conduction and convection represent 1/3 of the heat transmission against 2/3 for the
radiation, the efficiency of a window to reduce heat loss will depend on which mode, or modes, the
technology will play.

2.1 Passive window technologies
Here the term “passive” is used in the sense that the window will not evolve or adapt according to
the conditions applied. Whatever the external conditions; the window will have the same thermal
resistance.
2.1.1 Traditional windows: single and double glazing
Single glazing is the first solution historically found that let daylight enter the house and protect
from the outside at the same time. It is the simplest solution but also the least efficient. The thermal
resistance of the window is the thermal resistance by conduction of the material, which is essentially
the thermal resistance of the glass (thermal conductivity k of ≃1W/m. K). Thus, if an increase of
thermal resistance of a single glazed window is desired, it is only possible by increasing the width of
the glass. Therefore, good thermal resistance involves a high thickness of the glass and also a heavy
window, which is a disadvantage. The overall transmission coefficient U is estimated at ≃6.0 W/m².K
(Singh et al. 2008, pp 1596-1602) (Warner 1995). This value will be the reference to compare the
other technologies in term of heat losses. The higher the U value, the less the window insulates, and
vice versa.
It was only in the late 70’s that double glazed appeared, becoming the most widespread window
technology in houses to such an extent that in some countries it became a mandatory standard.
Between two panes of glass stand a layer of air, whose thermal conductivity is 40 times less than a
single glaze window sheet, with a value of ≃0.025W/m. K. The average U value for double glazing
windows is 3.0 W/m².K (Singh et al. 2008, pp 1596-1602) (Warner 1995). The thickness of the air
layer w is very important. Indeed, the greater the thickness the higher the thermal resistance,
resulting in a low U value. However, from a certain value of w, the stagnant air is forced to be in
motion due to the difference of temperature between outside and inside: a phenomenon of natural
convection occurs. This critical value of w corresponds to the value from which the Rayleigh number
is higher than 1708.
𝑅𝑎 =
𝑔.𝛽.𝛥𝑇.𝑤3
𝜈²
×
𝐶𝑝.𝜇
𝑘
(1)
Where:
- g is the acceleration due to gravity,
- β is the thermal expansion coefficient,
- 𝛥𝑇 is the difference of temperature between outside and inside,
- 𝜈 is the kinematic viscosity,
- Cp is the specific heat capacity,
- 𝜇 is the dynamic viscosity,
- k is the thermal conductivity.
Thus the objective is to have a minimum Rayleigh number for a maximum width of air layer which
can be improved in two different and compatible ways: add a solid separation between the two glass
panels and/or replace the air with another gas inside the window.
2.1.2 Addition of vertical separations
Equation 1 depicts that from a certain value of w, natural convection occurs. The first approach
against this phenomenon is to reduce w without reducing the total quantity of air between the two
panes of glass. This is achieved by adding a solid vertical separation between the two panes of glass.
Thus, rather than having only one air layer, there are two distinct air layers. These air layers are

separated by a glass pane, turning it into a triple glazed window. Not merely are there two air layers,
but there is the additional thermal conductivity of the middle glass pane which decreases the U
value to 2.0 W/m.K. By increasing the middle pane width, the thermal resistance also increases at
the expense of a heavier window and in turn a higher expense for material. Another approach is to
use a plastic film rather than a middle glass pane to make lighter. This solution is obviously less
efficient with a U value of 2.5 W/m.K compared with the triple glazing and needs film tension to
avoid optical diffraction. Moreover, the plastic film has to be perfectly transparent for a good
transmittance of the visual daylight. This window design has the merit of being lighter and cheaper
than the triple glazing. Obviously, it is possible to add as many films as desired (Warner 1995),
however after a certain threshold the U value doesn’t decrease significantly as the thermal
conduction becomes negligible in comparison to solar radiation.
2.1.3 Substitute gas
The use of air to fill a double glazed window is the first idea which comes to mind, as air is
inexhaustible and present everywhere. According to Equation 1 it is evident that to keep a maximum
value for w with a minimum value for Ra, it is necessary to change β, ν, μ, Cp and k which were until
now the properties of air. Researchers replaced the air layer with another gas whose value of
𝛽.𝐶𝑝.𝜇
𝜈².𝑘
is lower than air, but with a lower value of thermal conductivity k. Several candidates such as Argon
and Krypton gases had the properties needed (Ismail 2008 pp.710-9) (Lolli and Andresen 2016 pp.64-
76). For example while
𝛽.𝐶𝑝.𝜇
𝜈².𝑘
= 9.88. 106
𝑚−3
. 𝐾−1
and k=0.02624 W/m.K for water,
𝛽.𝐶𝑝.𝜇
𝜈².𝑘
= 4.5. 105
𝑚−3
. 𝐾−1
and k=0.00865 W/m.K for Krypton at T=300K giving a U value of 2.7
W/m.K (Selkowitz 1979). It is easy to notice that adding separations is quite more efficient than a
substitute gas, but a combination of both methods is possible giving very low thermal transmittance
at the expense of cost (Ismail 2008 pp.710-9).
It is possible to replace filling gas by vacuum which gives a very good U value of 0.5 W/m.K (Cuce and
Cuce 2016, pp 1345-57). Indeed, vacuum is by definition the absence of matter so the heat cannot
be transmitted by conduction and convection through vacuum. This remaining 1.5 W/m.K is due to
the solar radiations which are not suppressed, given that the light is able to travel through vacuum.
Although a very good technique of insulation, such a window is much more complicated to fabricate
because mechanics solicitations applied to the window are completely different from conventional
glazing.
2.1.3 Coated and tainted glass
So far, all the technologies viewed were based on the reduction of thermal conduction through the
window without decreasing the impact of solar radiation on the room temperature, which is perfect
for winter conditions when sunlight radiation is desired, minus the outside temperature. In order to
reduce the rate of solar radiations during hot days in summer there are two possibilities: the first
one is to increase the capacity of the window to reflect the sunlight and the second one is to
increase the capacity of the glazing to absorb radiations. Coated glass is a window on which a
coating is applied to reflect or to absorb radiations. Heat mirror coatings increase the proportion of
reflected rays thanks to reflecting materials and very good surface conditions. Absorbing coatings
are, most of the time, a tinted film which absorbs a part of the sunlight spectrum. They are made
with low-emissivity materials. With heat from radiation being 50% from visible rays and 50% from
infra-red rays, the absorption of IR rays are targeted in order to let the daylight enter the room
(Chavez-Galan et al. 2007 pp.13-19). Nevertheless, it is difficult to solely absorb IR rays without
absorbing visible spectrum but it remains achievable (Qi et al. 2016 pp.30-5). Thus coated glass is

very efficient in term of heat insulation with a U value of 2.0 W/m.K for a coated double glazing
(Singh et al. 2008, pp 1596-1602) (Warner 1995).
Tainted glass is based on the same principle than coated glass, but it is the glass which is tainted and
not a coating in order to inhibit the sunlight spectrum radiating the room. As it the glass itself which
absorbs radiation and not an additional coating, the glass is hotter and transmits more heat to the
room. This makes tainted glass less efficient than coated glass, but the latter has the disadvantage to
be sensitive to environment and so to be easily damageable. Moreover, whether it is tainted or
coated glass, both suppress a large part of light spectrum which obliges to use artificial light inside
the room.
It is thru mixing rays absorption and reduction of thermal conduction, that the best thermal
insulation results are obtained but also the biggest cost of manufacture and mounting with regard to
passive window technologies. In some environments, it is not necessary to have an ultra-performing
insulating window. Additionally, the requirement is not the same in a cold place, where sunlight heat
is required, and in a hot place where sunlight heat is undesirable from a building thermal efficiency
standpoint. This explains why all the technologies presented are frequently used all around the
world. So far, solutions appear to be fixed, that is to say that in each case the window keeps its
characteristics. It keeps its advantages and also its disadvantages all the time, which is not the case
of “active” window technologies.
2.2 Active window technologies
This second section describes advanced technologies which need energy to highlight their best
insulation characteristics. These are switchable systems which have the ability to adapting their
insulation capacity, depending on the weather conditions.
2.2.1 Smart windows
Smart windows use the ability of chromogenic materials. These materials change their capacity to
transmit solar radiations by colorizing itself. The optical change is done in applying certain conditions
to the material. In controlling the colorizing conditions, this technology becomes very interesting for
environment requireing adaptation and it arouses the curiosity of a lot of researchers (Baetens et al.
2010, pp.87-105). Wittwer et al. (2004, pp.305-14) studied gasochromic windows whose coloration
is activated by H2 exposition to tungsten oxide present in the glazing. Gardiner et al. (2009, pp.301-
6) and Macrelli (1995, pp.123-31) studied Polymer Dispersed Liquid Crystals (PDLC) technology. This
technology, with application of a small voltage to the liquid crystals, changes it’s visual transmittance
according to the frequency of the current. The type of smart window the most widespread is
certainly the electrochromic (EC) one (Rosseinsky and Mortimer 2001, pp.783-93). As PDLC, EC
needs electric energy to reduce its visual transmittance to a value between 6.2 to 68% according to
Piccolo et al. (2009, pp.832-844) whereas it is 90% for clear glass. Tungsten oxyde 𝑊𝑂3is inserted in
outside pane of the double glazing as shown in Figure 2.

Figure 2: Schematic of EC double glazing (Piccolo et al. 2009, pp.832-844)
The advantage is obvious: when the day is warm and sunny, a cool temperature in the room is
preferred and achieved by switching on the voltage, the window behaves like a coated double
glazing (U=2.0W/m.K). When the day is cold but still sunny, the switched off system will let the sun
rays come into the room which will maintain a pleasant temperature, while protecting from the cold
temperature outside. According to Bechinger and Gregg (1998, pp.405-10) and Granqvist (2014,
pp.1-38) although seemingly the ideal solution, it would become very expensive in energy cost if it
was used on glazed buildings. Therefore, Bechinger and Gregg (1998) propose to combine
photovoltaic cells with electrochromic windows in order for the window to produce its own energy it
needs to taint itself. Chromogenic windows are in development and look promising for the future.
2.2.2 Inside flow windows
For approximately 10 years, Chow et al. (2006) dedicate themselves to propose innovative solutions
to reduce energy cost due to conditioned air in warm climate countries. In 2006, Chow et al.
discussed the availability of a window ventilated with air circulating between a traditional window
and an additional coated pane. Among several designs, Chow et. al. (2006) identify the window with
outside ventilation as shown in Figure 3 as the best air flow window proposition.
Figure 3: Schematic of air flow windows (Chow et al. 2006, pp.1910-8)
It is important to consider that the larger the air gap and the higher the air velocity; the less energy
required from air conditioning. The system works as a heat exchanger. It turns out that air is a bad
thermal conductor compared with liquids such as water. Thus in 2011, Chow et al. discussed the
design of a window with a water flow between the two glass panes of a classical double glazing. That

way, rather than heating the inside room, outside heat heats the water between the moment water
enter and the moment water leaves the window. Even with low water velocities in the range of
0.01m/s, Chow affirms that water flow windows are suitable for warm climate issues.
2.2.3 Microfluidic window
In the continuity of Chow’s work, researchers from Harvard (Hatton et al. 2013,pp.429-36) proposed
a microfluidic technology window. Microfluidic is the science of fluids at the micro scale; hence it has
the general advantages to require less power and less consumable than macro scale systems. Their
prototype is based on the manner in which the human body regulates its temperature, by designing
microchannels between a glass and an elastic PDMA panes. The equivalent of a soda can volume of
water circulates due to gravity through the network involving a 9°C cooling of the window. Hatton et
al. (2016) work will be deeper detailed in this coursework because it is on microfluidic window that
our analysis is based.
3. Objectives
The overall aim of this paper is to theoretically develop a microfluidic-system aimed at insulating
windows to minimise heat loss in businesses and homes of cold countries. This will be achieved by:
 Define the benefits of micro-fluidic heat exchanger over its macroscopic counterpart
 Analysing other window insulation methodologies
 Theoretical and experimental calculations to quantify flow, heat transfer, pressure drop and
flow resistance characteristics of the proposed design
 Comparison of flow geometries to determine the most suitable
 Determination of the most suitable network structure for the flow channels
 Analysing suitable technologies, i.e. pumps and valves
 Consideration of materials and fabrication methodologies to ensure the feasibility of
production
 Determine impact on society
o Reduction of carbon foot print and other potential aspects
 Proposition of a spin-out
o Calculations will be made to quantify the insulation potential, and also potential for
cost saving for businesses and home.
o Other spin-outs shall also be considered
4. Scaling Benefits to Flow and Heat Transfer
To reiterate, it was said that the fundamental equations that describe the physics regime of
microscopic and macroscopic regimes are the same. This is indeed valid as long as the continuum
hypothesis holds. The continuum hypothesis, in basic terms, is an idealization that takes the mean
values of many particles instead of considering the discrete molecular nature, in order to factor in
the full range of intermolecular forces and molecular motions. For a liquid, this condition is satisfied
at a scale of 10nm. Under this length scale, discrepancies occur rending the fundamental equations
inaccurate in describing the physical regime of microfluidics. It should be noted that for this project,
the microfluidic system will be considered under the continuum hypothesis constraints.
As previously stated, the miniaturisation of the flow domain poses significant benefits due to the
governing of a different physical regime, in comparison to the macroscopic scale. Despite the
fundamental equations used to characterise the physics of the flow are the same for both
macroscopic and microscopic scale, effects significant at the macroscopic level can be unimportant

at the microscopic levels and vice versa, effects largely neglected at the macroscopic level have to be
factored in at the microscopic level due to their dominance on a smaller scale.
Without necessity of delving into an extensive mathematical description, essential insight into the
physical effects governing the flow may be obtained through a simple scale analysis of the key
dimensions of the flow conduit. Firstly, consider the Reynold's number. This is used to categorized
the flow into two flow regimes; laminar and turbulent. Laminar is characterised by smooth and
constant fluid motions, whereas turbulent flow is characterised by vortices and flow fluctuations
(Kakaç 2010). Physically, the two regimes differ in terms of the relative importance on viscous and
inertial forces. The relative importance of these forces for a given flow condition, or to the extent
the fluid is laminar, is measure by the Reynold’s number.
𝑅𝑒 =
𝜌𝑢𝐷ℎ
𝜇
(2)
The hydraulic diameter is a length scale and is proportional to the Reynold’s number by Re ~ L.
Therefore, on a microfluidic scale L would be significantly small, reducing Reynold’s number into the
laminar regime. At low Re, the viscous effects dominate inertial effects and a completely laminar
flow occurs. In the laminar flow system, fluid streams flow parallel to each other and the velocity at
any location within the fluid stream is invariant with time when boundary conditions are constant
(Lin and Basuray 2011). This implies that convective mass transfer occurs only in the direction of the
fluid flow, and mixing can be achieved only by molecular diffusion (Beebe et. al., 2002). By contrast,
at high Re the opposite is true. The flow is dominated by inertial forces and characterized by a
turbulent flow. In a turbulent flow, the fluid exhibits motion that is random in both space and time,
and there are convective mass transports in all directions (Weigl et. al., 2003). Intuition would depict
that turbulence is essential for effective heat transfer, however on a small scale this may not be the
case.
In a paper written by Tuckerman and Pease (1982) a conclusion was drawn that the heat transfer
coefficient for laminar flow through microchannels may be greater than that for turbulent flow. In
the laminar regime (Re~O(1), common for microfluidic systems), mixing is predominantly achieved
by molecular diffusion and advection. Solely considering diffusion, molecular diffusion is defined as
the process of spreading molecules from a region of higher concentration to one of lower
concentration by Brownian motion, which results in a gradual mixing of material (Capretto et al.
2011). Considering diffusion in one dimension, the time t required for a species to diffuse scales
quadratically with x, the channel width, that is t ~ L2
. Therefore at the micro scale, this diffusion time
would be significantly small.
In more specific terms of heat transfer, it is relevant to discuss the thermal diffusivity. The efficiency
of heat transfer can be characterized by the time it takes to heat up a fluid volume in contact with a
thermal reservoir to a temperature close to the reservoir temperature (the same arguments apply to
the reservoir process, namely cooling) (Hardt and Schönfeld 2007). For heat transfer occurring
purely by conduction, this thermal diffusion time scale is given by;
𝜏 𝑑 =
𝐿2
𝑘
(3)
Where it should be understood that 𝜏 𝑑 only characterises the real heat transfer time scale in an
order-of-magnitude sense. As 𝜏 𝑑 ~ L2
, the minute length scale characterised by micro-flow system
offers a significantly shortened thermal diffusivity time scale.

In specific terms of microfluidic forced convection heat transfer between a channel wall and a fluid,
in a general case it is not possible to derive a simple scaling law, as the flow pattern changes with the
length of the channel. Despite this, in the case for fully developed laminar flow, the time scale for
time scale for forced-convection heat transfer 𝜏 𝑐 displays the same dependence on L as the thermal
diffusivity time scale 𝜏 𝑐~𝐿2
. Thus shrinking the channel diameter has a pronounced effect on the
heat transfer to the fluid and rapidly diminished the temperature difference between the fluid and
solid (Hardt and Schönfeld 2007).
Another significant dimensionless parameter that becomes important in microscale flow and heat
transfer is the Peclet Number which is defined as the product of the Reynolds number and the
Prandtl number (Re . Pr ). This number signifies the ratio of rates of advection to diffusion. Peclet
number enumerates the axial conduction effect in flow. In macro-sized conduits, Pe is generally large
and the effect of axial conduction may be neglected (Yazicioglu and Kakaç 2010). However as the
channel dimensions get smaller, it may become important. The Peclet number scales with L, i.e. Pe ~
L therefore on a small scale the effect of axial conduction becomes quite significant, and is another
factor to aid heat transfer minus turbulence.
Inclusive to the shortened thermal diffusivity and convention time scales, microfluidic systems offer
a large surface area to volume ratio. Surface area is important as it is the area of intimate contact
between the fluid and the channel walls. The surface area to volume ratio varies as 1/D, as the
diameter decreases, surface area to volume ratio increases. The surface area of a channel available
for heat transfer is directly related to the conduction rate, convection rate, and heat transfer
coefficient. Therefore the higher the surface area the higher these coefficients. Noting that the
reduced volume makes for higher efficiency.
The flow in consideration for this project and application is the laminar regime, however due to the
scaling factors previously discussed, despite the smoothness and linearity of the flow, there is
significant potential for effective heat transfer due to the high surface area and other factors
discussed. The next section outlines a previous study conducted, where a microfluidic system was
fabricated to cool widows in countries where sun light increase air conditioning bills significantly.
This section will outline and quantify the potential for heat transfer of these microfluidic systems.
5. Quantification & Justification
Before conducting the theoretical development of a MFD, it is first necessary to gain insight into the
heat transfer mechanism possible and to quantify some essential parameters. Outlining windows
major role in building efficiency Hatton et al. (2013) developed a thin, transparent, bio-inspired,
convective cooling layer for building windows and solar panels that contains microvasculature with
millimetre-scale, fluid-filled channels. Sun-ward windows are subjected to high levels of thermal
radiation, with heat propagating through the window and heating the adjacent room. This heat load
induces heightened air-conditioning costs which reduce building efficient by up to 40% (D&R
International, Ltd., 2011). The microfluidic device devised by Hatton et. al. (2013), aims to reduce
heating by solar absorption through convective cooling. Water is pumped through the microfluidic
device at room temperature or below, with the heated glass transferring heat to the device and
reducing heat transfer to the room and thus the air-conditioning units.
Experiments were conducted with two designs to determine the extent that the microfluidic
network lining a window surface can efficiently control window temperature, note each microfluidic
design tested had a 10x10cm2
window. The channels were fabricated on polydimethylsiloxane
(PDMS) to allow for optimum transparency, and the channels lined one side of the PDMS sheet and

were formed completely when the PDMS layers bonded to the glass window. The two designs are
called Diamond 1 and Diamond 2, having channel cross-sections (width by height) of 1 mm by 0.10
mm and 2 mm by 0.10 mm, respectively.
A pane of glass was heated using an incandescent light source (50 cm from the glass) to an initial
temperature ranging from 35 to 40°C. Water maintained at room temperature (RT, 21 1C) was then
pumped through the microvascular channels at flow rates of 0.20, 2.0 and 10 mL/min. Changes in
surface temperature were visualized using an IR camera as a function of the flow rate.
Taking from the results, it was found that a modest flow rate of 2.0 mL/min of RT water was able to
produce cooling from an average 37°C and 39°C to approximately 30°C for the 1 and 2 mm wide
channels of Diamond 1 and 2, respectively. The experimental results from the authors will be used to
quantify the heat transfer coefficient of such a micro-flow device, in aid to develop a thermal
insulation micro-fluidic system. Figure 4 illustrates a schematic of the microfluidic device, its
positioning and also an image of the microfluidic device with and without water in the channels.
Figure 4: Microfluidic convective cooling system network (Hatton et. al., 2013). (A) Schematic of microfluidic device and it's
positioning. (B) Microfluidic device without water (LHS) and with water (RHS) in the channels.
5.1 Theory and Resistance Network
The next logical path, to ensure the reader understands the heat transfer mechanism, is to define
the theory used to quantify the potential for microfluidic insulation / heat transfer.
In order to acquire the total channel length in the zig-zag design, seen in Figure 1, it was necessary to
manipulate Pythagoras Theorem. The resulting equation is:
𝐿 = [∑ [4√2(𝑥 − 𝛿𝑛)2]
𝑥/𝛿
𝑛=0,1,2… ] − 2√𝑥2 (4)
Where L is the total channel length, x=width (or height, as it’s a square window), δ= channel spacing,
n is the number of channels defined as n=(x/δ)-1, (considering 0 as a integer i.e. 3 channels would
have a set n of {0, 1, 2}). The second factor accounts for the longest channel from diagonal to
diagonal, there may only be two in the network.
The Reynold’s number may be calculated by;
𝑅𝑒 =
𝜌𝑢𝐷ℎ
𝜇
(2)

Where 𝐷ℎ =
4𝐴
𝑃
The Nusselt number may be related to the heat transfer coefficient as follows;
𝑁𝑢 =
ℎ𝑑
𝑘
(5)
Where the Nusselt number is calculated from the following correlation;
𝑁𝑢 = 1.86(𝑅𝑒 𝑃𝑟)
1
3 (
𝐷ℎ
𝐿
)
1
3
(
𝜇 𝑏
𝜇 𝑤
)0.14
(6)
(Bird et al., 2007)
The conduction rate can be calculated from the following relations;
𝑄̇ 𝑐𝑜𝑛𝑑 = 𝑘𝐴
∆𝑇
𝐿
=
∆𝑇
𝑅 𝑐𝑜𝑛𝑑
(7)
Where the conductive thermal resistance is given by;
𝑅 𝑐𝑜𝑛𝑑 =
𝐿
𝑘𝐴
(8)
The convection rate may be defined as;
𝑄̇ 𝑐𝑜𝑛𝑣 = ℎ𝐴 𝑠(𝑇𝑠 − 𝑇∞) =
(𝑇𝑠−𝑇∞)
𝑅 𝑐𝑜𝑛𝑣
(9)
Where the convective thermal resistance is;
𝑅 𝐶𝑜𝑛𝑣 =
1
ℎ𝐴 𝑠
(10)
In general, the thermal resistance may be calculated by;
𝑅 =
∆𝑇
𝑄̇
(11)
Note, Equations 5, 7, 8, 9, 10 & 11 are sourced from Çengel and Ghajar, 2015.
The heat transfer coefficient of the wind can be calculated by the following equations proposed by
(Lokmanhekim (1975));
ℎ 𝑜𝑢𝑡 = {
8.07𝑣0.605
𝑣 > 2𝑚/𝑠
12.27 𝑣 < 2𝑚/𝑠
(12)
The pressure drop through a channel is related to the hydraulic resistance and flow rate by;
∆𝑃 = 𝑅ℎ𝑦𝑑 𝑄 (13)
The dimensional specifications to consider in the conduction of the theoretical calculations are
present in Table 1.
Channel Dimensions Diamond 1 Diamond 2
Width (mm) 1 2
Height (mm) 0.1 0.1
Total Channel Length (m) 2.4 4.8
Window area (m2
) 1x10-4
1x10-4
Table 1: Diamond 1 & 2 specifications

5.2 Justification
Sun may be a prominent thermal efficiency factor is most countries; however other countries suffer
from the opposite effect. Mild countries such as Ireland suffer extensively with cold, wet and cloudy
conditions, with sun minimally shining. Building efficiency, in regards to thermal, is directed towards
insulating heat inside a building. Sunlight is not avoided, and its presence is desired. The aim of the
present study is to devise if a bio-inspired microvascular system such as that developed by Hatton et
al. (2013) could be used in reverse, to insulate heat from leaving the building through the windows.
5.3 Heat Transfer Coefficient
The first protocol is to analyse and quantify some aspects of the experimentation conducted in the
main study. Taking the Diamond 2 design, some calculations were conducted, both theoretically and
from the authors (Hatton et. al., 2013) experimental results, to obtain a heat transfer coefficient for
the microfluidic device.
First of all it was required to calculate the total length of channel present in the Diamond design. The
network was configured in an evenly distant cross or zig-zag-like configuration with channel spacing
δ=5.87, 11.9mm. From this spacing knowledge, the manipulation and summation of Pythagorean
Theorem (eqn. 4) was used to obtain an estimate of channel length.
The total channel length for Diamond 2 was estimated at 2.4m. With knowledge of the fluid
properties, channel geometry and flow rate, using Equation 5 the Reynolds number was estimated
to be 32. Using a Nusselt number correlation presented in Equation 6, the Nusselt number was
obtained to be 0.117. With knowledge of the thermal conductivity of water, the heat transfer
coefficient for the microfluidic system could be backed out using Equation 5. The heat transfer
coefficient was estimated to be 1,483 W/m2
K. This value is quite high, and to verify this figure the
temperatures given in the experimental analysis conducted by Hatton et. al. (2013) can be used to
check.
From the experimental results, Diamond 2 with water at room temperature and a flow rate of
2ml/min cooled the glass by 9°C. With knowledge of the glass conductivity and geometry, the
thermal resistance may be calculated using Equation 8. Using the resistance calculated, the energy
to change the window temperature by 9°C can be calculated using Equation 11. With the energy
known, rearranging equation 9, the heat transfer coefficient can be backed out. The experimental
heat transfer coefficient correlated with the theoretical figure and is around 200 W/m2
K. As this
figure comes from experimental results, this figure is more representative of the heat transfer
possible by the device. The reasoning that the theoretical heat transfer coefficient, of 1,483 W/m2
K,
may not be achievable may be due to the complexity of the channel network. The flow rate of 2
ml/min may not be consistent throughout the network and may loan to the significant inaccuracy.
Also, the Nusselt correlation used is essential an idealisation, making numerous assumptions about
the heat transfer domain. For instance, it calculates for a single straight channel of the specified
length, and does not factor the network structure as an entirety. Effective theoretical solutions may
be obtained through use of CFD.
Despite the fact that the channel geometry, 0.1x2mm, and area for heat transfer being small,
10x10cm, the heat transfer coefficient of the system is substantial. Next, we propose to conduct a
feasibility analysis, to see if the microfluidic system can be applied to thermal insulation practises.

5.4 Insulation
A few influential factors must be considered in the feasibility analysis. These factors mainly consists
position of microfluidic system (inside or outside of window), and channel network. In the main
study the microfluidic system was positioned on the inside of the window.
Taking a double glazed window section, 1x1m2
, with glass thickness of 5mm and an air gap of 12mm
a thermal resistance network is configured. The thermal resistance of the glass and stagnant air is a
function of conductivity and values are known. The thermal resistance of the air side both inside and
outside the window are dependent on heat transfer coefficient of air which is a function of the air
velocity. The heat transfer coefficient for outside can be calculated by obtaining wind speed on the
day thru Equation 12. The heat transfer coefficient inside is seen to be in the range of 0-7, from
experimental values of (Wallentén 2001). Taking the worst case scenario a heat transfer coefficient
of 7 will be used.
5.5 Thermal Resistance Network
The thermal resistance network represents, from left to right; the convective resistance of the
natural convection in the room (1), the conduction throw the glass panel (2), stagnant air (3) and
glass panel (4) respectively, and finally the convective resistance of the microfluidic system (5) and
outdoor convective resistance (6). Note, as the microfluidic device now has consideration for a mild
country, the effect of sunlight radiation will be neglected.
The indoor heat transfer coefficient is assumed to be 7, the outdoor heat transfer coefficient was
calculated using equation 12 using an on-the-day weed speed of 3.61m/s. This was calculated to be
h=17.54 W/m2
K. The outside temperature was taken to be T∞=7°C, while the indoor temperature
was set a room temperature, Tin=20°C.
Number (Tx) Resistance (K/W) Number (Tx) Temperature (°C)
1 0.142 1 17.07
2 5.2e-3 2 16.96
3 0.416 3 8.39
4 5.2e-3 4 8.23
5 5e-3 5 8.18
6 0.057 6 7
Table 2: Thermal Resistance and Temperatures, from the inside to the outside of a double glazed glass window.
T4 defines the surface temperature of the outside of the glass, while T1 defined the surface
temperature of the inside of the glass. The aim of the microfluidic system is to decrease the rate of
heat transfer from inside to outside. The most feasible way to do this is to decrease the temperature
difference between the inside and outside surface of the glass. This can be done by positioning the
microfluidic system on the outer surface of the glass. The water can be at room temperature, with
T1 T4T3
Tin T∞
T5
Rcond
Rconv Rcond Rcond Rconv Rconv
① ② ③ ④ ⑤
Figure 5: Thermal Resistance Network from the inside to the outside of the double
glazed window. The thermal resistance area as follow; (1) indoor convection (2) first
plane of glass (3) air gap (4) second pane of glass (5) microfluidic system convection (6)
outside convection.
⑥
T2

the tank heated inside by natural convection or some other methodology. The aim will be to
increase the outer surface temperature of the glass. Using equation 9, and a value of 200W/m2
K for
the heat transfer coefficient, the energy required to increase the window temperature to 20°C is
23W for a 10x10cm2
window section, or 2.3kW for a full scale 1x1m2
window.
5.6 Channel geometry choice
In order to ensure maximum efficiency in heat transfer, the geometry of the channels is to be
considered. The geometry utilised by the authors, Hatton et. al. (2013), is compared to other
geometrical options firstly under hydraulic resistance factor. This will indicate how much the flow is
restricted by the channel geometry. This is quite an important factor as if there is high hydraulic
resistance present, a higher pumping power will have to be achieved to override the pressure loss
and to maintain an effective heat transfer coefficient.
Shape Rhyd (Pa-s /m3
) ΔP (GPa)
Diamond: 1 2 1 2
Circle 5.59x1015
9.29 x1015
0.093 0.154
Rectangle 3.07x1016
2.97 x1016
0.512 0.495
Triangle 4.74 x1016
7.48 x1016
0.789 1.24
Parabola 6.3 x1016
6.3 x1016
1.05 1.05
Square 6.82 x1017
7.1 x108
11.36 0.71
Table 3: The Hydraulic Resistance and Pressure Drop of Both Diamond 1 and 2 for various channel geometries (for 10x10cm
2
section).
Table 3 presents the resistance of various geometries. The equations used are present in Appendix A
and reference to Bruus, (2008). The hydraulic resistance of Diamond 1 and Diamond 2 were used to
dimension the other shapes used. This was done by manipulating the hydraulic diameters for a given
shape in order to give the same value as the two designs. In this method, the shapes are
representative to one another.
The parabola, rectangular and square geometry can be configured so that the working fluid will be
able to maintain direct contact with the window for effective heat transfer and no thermal contact
influence. The circle poses a low resistance, however this may be harder to fabricate on a microscale
level and also extra material will have to be used in order to bind to the window. The triangle
geometry would introduce fabrication costs that would diminish their appeal. Considering
fabrication limitations and the generated hydraulic resistance, the rectangle and parabola would be
the most feasible options to consider. In the fabrication of microchannel, a perfect shape is rarely
achieved, therefore if seeking a rectangle; a parabola might be the end shape. Nonetheless, a
parabola is deemed a suitable geometry.
Pumping power may be defined as the pressure drop multiplied by the flow rate:
𝑃𝑝 = ∆𝑃. 𝑄̇ (𝑊) (14)
The associated required pumping power to achieve the specified flow rate through the entirety of
the fluid channels is presented in Table 4.

Pp (W)
Diamond: 1 2
Circle 1.55 2.57
Rectangle 8.53 8.26
Triangle 13.16 20.77
Parabola 17.5 17.5
Square 17.33 28.81
Table 4: Pumping power for the channel geometries considered (10x10cm
2
section, representative of Diamond 1 & 2
configurations.
Aiming for the rectangular shape, it is necessary to ensure as accurate fabrication as possible. This is
required as the pumping power increases nearly 2 fold when the regular shapes turns into a
parabola.
5.7 Network Structure
In their researches of auto-insulating windows, researchers from Harvard have decided to settle the
microfluidic channels as a regular grid. Channels cross each other forming right-angles corners on
the whole structure with an angle of 45° in respect with the horizontal axis which make it a very
simple geometry of straight lines. Given that the diagonal of a square is equal to the length of the
side multiplied by √2 (≃ 1.41). Consequently the distance that a particle of the fluid travels through
channels is also multiplied by 1.41 compared with the distance travelled if the channels were
verticals. This involves that the heat exchange surface is all the more high and so that the heat
transfer is more efficient.
Figure 5 Schematic of the channels network as designed by Hatton et Al.
With this structure on a 1x0.5m² window, the fluid which crosses the window from the top to the
bottom thru use of gravity, can take many different ways but the distance travelled will always be
the same that is to say approximately 1.41 times the height of the window. On arrival to a crossroad,
the fluid has only two choices of road which are equal length as shown in Figure 5. Such a structure
ensures good homogeneity of temperature in the whole window.

5.7.1 Network Justification
The reason why such a network has been used is not based on a scientific analysis, but rather on an
intuition coming from a biomedical fact. Researchers wanted to recreate the way the human body
reacts to control its temperature. Therefore, they tried to design a network similar to the blood
vessels of human body. The designed network of crossroads is the closest geometry to the blood
ramifications. It remains a very simple solution to mathematically model, design and to fabric as
well. It is obvious that the constriction or the dilatation of blood vessels have a true incidence on
heat transfers between the outside and inside body, but it is not proved that the blood network
geometry have also one. Indeed, if the blood network is like it is, it is in order to feed muscles and
organs in blood all along the body but nothing more. Hence several other ways to design the
network would have been considered. Being a new field and the first microfluidic insulating window
system, no other designs have been tested, and so the comparison is impossible. However common
sense allows considering different solutions and eliminating others.
5.7.2 Study of other possibilities
Some other possibilities of design may be imagined and discussed such as straight vertical channels,
straight horizontal channels, serpentine channels.
Description Advantages Disadvantages
Straight vertical
channels: The fluid is
driven from the top to
the bottom by straight
vertical channels.
This is a very simple
design which allows a
good circulation of the
fluid achieved by
gravity. The fabric is
also simple with
unidirectional straight
lines. The fluid travels
on the longest
dimension of the
window.
If we compare to the
grid network from
Hatton et. al. (2013),
with this system the
fluid covers 1.41 less
distance and so the
heat transfer is less
efficient.
Straight horizontal
channels: The fluid is
driven from one side to
another by straight
vertical channels.
This is a very simple
design and fabric
system as well.
The fluid requires
pumping to travel and
the distance covered is
the width which is
most of the time half
the length of the
window.

Figure 6: Prefabricated Solenoid
Microvalve
Serpentine channels: The
fluid is driven through a
unique serpentine
channel from the top to
the bottom and going
through all the width of
the window.
Most optimised design
to cover the most of the
window surface
allowing a good heat
transfer.
This system needs a
pumping and the
channels are trickier to
fabric with all the
bends. According to
the throughput, the
fluid may be already
heated when arriving
at half length of the
window. Therefore
heat transfer is less
efficient until the
outlet.
Table 5: Comparison of different channels structure: straight vertical channels, straight horizontal channels and serpentine
channels.
Although the idea to design a network as a regular grid was not based on analysis and calculations, if
we look at the other simple possibilities, the grid seems to be the best solution. Of course it is
possible to combine all the solutions above to find the best way to transfer the heat to the fluid
rather than to the inside of the house. Given that it is a new field, the simplest designs are preferred
and that is why the grid of Hatton et al. (2013) is adopted in our auto-insulating window.
6. Additional Components
6.1 Microvalves
The successful commercialisation of fully integrated microfluidic systems has been delayed due to
the lack of reliable microfluidic components, i.e., micropumps and microvalves. Therefore, even
though there has been a large development with microfluidic components, they are still the most
difficult task. Research on microfluidics, including micropumps, micromixers and world-to-chip
microfluidic interfaces has been thoroughly reviewed, but rarely that on microvalves.
In the case of the insulating window presented in this paper, a valve which would be located at the
fluid inlet is prescribed. The microvalve (because fluid is going through microchannels) would allow
to control the flow rate that is to say the velocity of the fluid which is directly linked with the
Reynolds number and so the heat transfer.
Microvalves found today generally fall into one of two categories: active microvalves, using
mechanical and non-mechanical moving parts, as well as external systems, and passive microvalves,
using only mechanical and non-mechanical moving parts. The following are three microvalves:
6.1.1 Solenoid Microvalve
A solenoid is a coil of wire wound into a tight helix, similar to a
compressed spring. When an electric current is passed through the
solenoid it induces a magnetic field. The magnetic field can be
varied by varying the electric current. If a metallic object is place
within the solenoid then the induced magnetic field can cause the
object to displace. A solenoid microvalve is a simple solenoid with
an actuator inside of it. The actuator is moved by the induced
magnetic field cause by the electric current and is located above the
channel of a microfluidic device. When the electric current flows the
actuator pushes down on the ceiling of the channel to obstruct its

Figure 8: Quake Microvalve
Figure 7: Screw Microvalve
flow. For the solenoid microvalve to be effective it requires an elastomeric lab-on-a-chip, and is
rather bulky. This would be an effective valve for our insulated window as it is compact and could be
integrated into our system easily.
6.1.2 Screw Valve
This valve is a very low tech microvalve that needs little more than
a screw. The screw is used in this microfluidic device very similarly
to the actuator in the solenoid valve. When the screw is twisted it
deflects the membrane of the channel and in turn stops the flow.
In order to prevent damage to the microfluidic device a ball can be
placed beneath the screw. The only power that is required is that
produced by the manual twisting of the screw. This valve has a
small profile and is for easy to use. This would be an effective valve
for our microfluidic system as we are attempting to save energy
and this valve uses virtually none.
6.1.3 Quake valves
In order for quake valves to work they require
additional channels. These channels are often
positioned perpendicular to the target
channel. Both the additional channels and the
targeted channels share a thin, common
membrane. When air is allowed to flow
through the additional channel at the right
pressure, the common membrane is deflected
and obstructs the flow of fluid. This
completely changed the field of microfluidics since its arrival in 2000, allowing such feats as 400
simultaneous PCR reactions.
These microvalves may have changed the field of microfluidics but they certainly have their
drawbacks. Firstly, there is more planning required as you need to incorporate an additional layer of
pneumatic channels as well as route all your channels so that they don’t overlap in positions you
don’t want them to. If there is a small change in the design of your microfluidic device then you
could be required to undergo a massive redesign. While the solenoid valve certainly has a big
footprint over the lab-on-a-chip due to the size of the solenoid, the Quake valves keep the area
around the device cleaner as it only requires the inclusion of an additional layer. For this valve to
work a tank of pressurized air must be kept nearby, this therefore hinders the mobility of the device.
The pneumatic valves can be controlled electronically, allowing a device consisting of multiple
independent valves to become more automatic.

6.1.4 Pneumatic Vavles
The working principle of pneumatic micro valves relies on the deflection of Polydimethylsiloxane
(PDMS) membrane to interrupt flow. Pneumatic pressure is applied to the PDMS via a dedicated
channel termed as “control line” or “control channel”. Another reason for selecting pneumatic micro
valves is that they can be easily integrated with soft lithography processes. However, there seems to
be a crucial issue while using a PDMS based micro valve. It is the geometrical mismatch between the
rectangular shape of the micro channel cross section and the round shaped deformed PDMS
membrane. This mismatch is pre-determined to lead to fluid leakage and affects the correct valve
operation. Considering the system’s leakage rate of 1μL/min , a trade-off would have to be made
with the design. This is increasing the aspect ratio, which would limit the leakage rate consequently.
Figure 9: Top view of the device (Courtesy of Zahra et al. (2015))
The micro valve features a cavity sealed with volatile fluid below a corrugated membrane. Polysilicon
heater grids are above the cavity floor, and the cavity is partially filled with pentane to increase the
thermal efficiency. The cavity is on par with the micro channel arrangement.
Fluid flow control in microfluidic devices devices is essential. To ensure the proper flow control of
our micro channel reinforced glass, a suitable method of control system is required. The micro-
channel system within the window will be empty when on installation and removal, for this reason a
micro-valve is needed to control the flow. The solenoid micro-valve described above is the chosen
valve for this system for a number of reasons. It is an automatic low cost valve that uses very little
energy. This valve is also compact and can easily fit into the wall surrounding the window with only a
switch extruding.

6.2 Micropumps
Although the window design allows the fluid to circulate through the microchannels thanks to
gravity and so without energy, a pumping system is necessary. Indeed, the fluid has to be driven
from the outlet located at the bottom of the window to the inlet located at the top. For this, a
simple macro pumping system is sufficient. However as part of microfluidic study, micropumping
systems are considered. Additionally, use of a pump offers an option of flow rates to change the
achievable heat transfer coefficient.
6.2.1 Electro Osmotic flow (EOF):
Liquid pumping in the microfluidic channels can be achieved by using electro osmotic flow (EOF).
Laminar flow in the microfluidic channel allows liquid-liquid extraction and microfabrication to occur
within the channels. In addition to this, valving and mixing are also required to obtain better flow
control.
This method seems reliable because it does not require moving parts. The EOF is designed by the
electric current flowing in a network of resistor using Kirchoff’s laws, i.e. fluid control established by
designing different channels of different solution resistances and by applying different voltages.
Manipulating the direction and magnitude of EOF is done by modifying the surface of the
microchannel.
Avoidance of a secondary Hydrodynamic flow (HDF) is necessary to obtain better control using EOF
alone. The following steps can be used to achieve this:
 All solution reservoirs are filled to the same liquid level.
 The inlet reservoir is closed to the atmosphere using a valve.
Better EOF control and more reproducible capillary electrophoresis (CE) separation is achieved using
the above-mentioned steps.
EOF is achieved by employing Anionic and cationic exchange beads (5𝜇𝑚 diameter). The reactions
occurrence by employing these beads at different pH-levels is made use of in achieving EOF.
Maintaining the pH-level of liquid is an important task. This is because the Electro osmotic flow is
directly dependant on pH-level and the width of the micro-channels. At low pH, the cationic
exchange beads are protonated, and thus this channel with cationic beads is pumping. But the
anionic exchange beads are neutral at low pH, and thus this channel is non pumping. The reverse
could be achieved by using high pH. In this manner, the flow dependences at low and high pH
compensated each other, which produces an enhanced flow rate at low pH. However, there is some
backflow from the pumping channel to the non-pumping channel. To overcome this backflow, a
smaller section of a 50𝜇𝑚 wide channel filled with smaller sized beads (0.5 𝜇𝑚 diameter) can be
employed.
The addition of another section of different sized channel prevents backflow into the non-pumping
channel. This would also enhance the flow rate at higher pH and results in less pH dependence in the
EOF speed (Li, 2010).
The EOF method of pumping is efficient to some extent. However, as the microchannel reinforced
glass used in the test is big enough to exceed the scope of EOF. Thus as a result, other pumping
methods such as Pressure-driven flow had to be considered.

6.2.2 Centrifugal Pumping:
The Centrifugal pumping uses centrifugal force to drive the fluids through microchannel. A rotating
plastic disc is employed to achieve centrifugal pumping. At first, the fluid is loaded at the centre of
the disc. Various flow rates (5 ml/s to > 0.1 ml/s) can be achieved in our channel at different rotation
speeds (60 to 3000 rpm). This pumping method provides a wider range of flow rates than EOF. It is
sensible to note that the centrifugal flow is insensitive to various physiochemical properties (e.g., pH,
ionic strength) of liquids and works well even in different conditions of the channel (e.g., wall
adsorption, trapped air bubbles). We noted that the method had one limitation that the flow
direction cannot be reversed.
Besides pumping, centripetal acceleration is created. A maximum fluid velocity of up to 12m/s and a
corresponding radial acceleration in excess of 106 g's can been produced within a diamond shaped
micro chamber (55*55 𝜇𝑚). This test is carried out on a device produced with PDMS. This was one of
the reasons for choosing this method of pumping.
7. Fabrication
Microfluidics incorporates a variety of methods for device fabrication. Machining methods such as
drilling, milling, sand blasting etc. can be used. Based on the availability and the level of smoothness
required, laser cutting method can be employed. One of the common methods used in microfluidics
is stereo lithography technique which is more or less the same as 3D printing which builds up the
device layer by layer. Deep reaction ion etching is another notable method. Replication processes
such as moulding and hot embossing can also be applied.
The choice of fabrication method and material is defined by the device structure and constraints.
Based on the proposed design, one of the important factors to be considered is the temperature
range
The microfluidic channel structure has to be thermally and mechanically stable in order to meet the
requirements of the project. Selection of a suitable fabrication technique is essential. A rapid
fabrication technique utilizing two-step soft lithography to realize rigid microfluidic device which is
not only term is considered. The technique enables the production of rigid micro structures including
inlet and outlet ports by utilizing a mould master made from a soft material, conveniently and
inexpensively.
Pressure handling capability of the device is one of the important parameters to be considered. This
is because the project requires external pumping for the purpose to be achieved. A device made
from thermosetting epoxy resin instead of hard material shows high replication accuracy, even with
high aspect ratio micro structure and demonstrates high mechanical stability at higher pressures (0.4
Mpa at micro-level). This technique enables the possibility of fabricating micro structures with high
aspect ratio, high depth and low surface roughness.
Microfluidic devices for heavy metal detection, in which organic solvents are generally used to
extract the metals, have employed hard materials for their chemical resistance. However, our
project deals mainly with water and does not require any special material which has chemical
resistance. In contrast, Polydimethylsiloxane (PDMS) is a typical soft material used for various
microfluidic devices, and can also be used to specific fluid components such as pumps, valves and
regulators because of its elasticity. PDMS is very easily fabricated with low cost, high reproducibility,
quick curing, facile processing, and controllable adhesion by modification of the surface
composition.

The fabrication process involves two steps. First a mould master is made from a rigid material (cast
iron), and then a flexible PDMS device is replicated from the mould master by pouring, curing and
releasing. Because PDMS is elastic, it is easily mouldable and there is no requirement of etching or
embossing. It is to be noted that the fabricated substrate is easy to seal with other substrates by the
adhesive property and deformability of PDMS. Thus, this technique not only turns out to be
inexpensive but also easy to manufacture.
Figure 9: Fabrication procedure (Courtesy of Mogi et al. 2014)
A resin plays an essential role. The main requirement of the resin is the optical transparency. This is
critical because of the application to windows. A material that has the young’s modulus, as close as
that of PMMA was required. The material is also required to have strong adhesion with resin, silicon
substrates and glass in order to be employed in window application. Based on the literature, Stycast
1266, Emerson and Cuming (a thermosetting epoxy resin) proved to be suitable for the technique.
This material seemed to comply with all the requirements of technique. The material is optically
transparent to visible light (400 – 700nm). It was found that it has an absorbance similar to that of
PDMS.
The procedure consists of four steps. As shown in Figure 9, the first step is pre-mold master
fabrication. During this step, PDMS is poured onto the rigid template and cured, and then the pre-
mold master is released. Inlet and outlet ports are created at this stage.

The second step is to fabricate the mold master. A mold master with micro structured is replicated
from the pre-mold master with the positive structure onto a soft material. For smooth removal of
the mold master, CHF3 plasma under suitable conditions is required.
The third step is to replicate a rigid device with microstructures from the mold master. A suitable
resin (as discussed above) (Stycast 1266) is poured onto the soft material mold master and thermally
cured to harden sufficiently. Due to the elasticity of the material, it is possible to remove the
replicated rigid structure without any straining or cracking.
Finally the replicated device is bonded to a resin, glass and silicon substrate, and then tubes are
connected to the inlet and outlet ports. The resin used easily bonds with glass simply by curing at
75℃ for 10 minutes on a hot plate. Then the device ports are connected to silicon tubes with a
hardening resin. (Mogi et al., 2014). The same procedure is followed to develop the entire
crosslinked channels.
8. Impact on society
To quantify the energy lost through a double glazed window, the resistances in Table 3 can be used
in the following equation to depict the flow of heat from hot (inside) to cold (outside)
𝑄 =
∆𝑇
𝑅
(𝑊) (15)
Taking the temperature difference as 13 degrees and summing the resistances from inside to
outside, a 10x10cm2
without the microfluidic device allows 63W of heat energy to flow from inside
to outside, or 6.3kW for a 1x1m2
window section. With the addition of the microfluidic system this
would be reduced to near 0. The energy required to achieve this consists of (1) the pumping power
(2) the energy required to heat the water.
The pumping power associated with both Diamond 1 & 2 for a rectangular cross section and the
associated channel lengths is around 9W or 900W for a 1m2
window section. The energy required to
heat the water can be calculated by:
𝑄 = 𝐶 𝑝 𝑚̇ ∆𝑇 (16)
The mass flow rate can be calculated from an idealised flow rate equation proposed by the authors
Hatton et. al. (2013) and is as follows:
𝑄̇ = 67 𝐿𝑊 (𝑚𝑙/𝑚𝑖𝑛) (17)
Taking a 1x1m2
window section (L=1 & W=1), the flow rate calculated from Equation 17 can then be
converted into a volumetric flow rate and multiplied by the density to achieve a mass flow rate. The
mass flow rate was calculated to be 66.9x10-3
kg/s. With this figure, one can acquire an idea of the
mass of water that will be required to be in the closed circuit, and in turn the amount of water that
will have to be heated. Substituting 66.9 x10-3
kg into Equation 16, and also adding in the required
temperature difference and specific heat capacity of water, 3.65kW of energy is required to heat the
water. Table 6 presents an energy balance and the energy saved per second.

Energy In (W) Energy lost (W)
Pp 900 Heat Loss 6300
Water Heating 3650 - -
Total Input 4550 Total Loses 6300
Net Savings 1750
Table 6: Energy Balance to determine total energy to be saved per 1x1m
2
window section
The MFD has a theoretical potential to save 1.75kW of energy when in use. Considering the indoor
heat transfer coefficient, advantage can be made of the indoor heat to reduce the Wattage input to
heat the water.
The impact on society would be a reduced carbon foot print form the reduction in energy required
to maintain the household/business at a feasible temperature.
9. Spin-off
This microfluidic system can be utilised in households and businesses in cold areas. The system
would reduce the energy required to maintain the living/work space at a comfortable temperature.
Considering the calculations conducted for a household temperature of 20°C and a 1x1m2
double
glazed window section, it was estimated that 6.3kW of energy is lost to the extremities on a day with
an outside temperature of 7°C and a wind speed of 3.61m/s. The microfluidic system has potential
to better insulate the window sections, reducing heat loss to near 0. The addition of energy input to
achieve this goal (4.55kW) is less of that than the lost heat (6.3kW). In the calculations conducted in
Section 5, it was found that for this window section 1.75kW of energy can be saved. This is a 28%
saving, considering the heat losses and energy inputs for this window section alone.
Considering the price per kWh of electricity (From Electric Ireland), 18.26 cent (24 hour rate)
("Electric Ireland Standard Domestic Elec | bonkers.ie" 2016), 32c can be saved per hour per 1x1m2
window section. On a larger scale, for a building unit with 100m2
of window, this would induce a 32€
saving per hour. Considering 40 hours of business per week, summing to an estimate of 2,080 hours
per year, this saving amounts to 66,500€. This is quite a substantial amount and would incline the
benefits of the microfluidic system considered. Note, these figures are representative to the
conditions considered in the theoretical development of the fluidic system. In order to better
quantify the potential of the system, an experimental analysis (using various; flow rates, inside
window surface temperature, outside window surface temperature, and indoor and outdoor heat
transfer coefficients) would have to be conducted. Nonetheless, there is definitely a potential for
saving and reduction in carbon footprint.
Note, as the microfluidic system is aimed at reducing the temperature difference between the inside
surface of the glass and the outside surface of the glass (ΔT=0), the associated thermal resistance of
the window (inclusive of the microfluidic device) is therefore, by Equation 11, 0 K/W. The U-value is
defined as the inverse of the sum of thermal resistances; therefore the theoretical U value is 0
W/m2
K. In the calculations conducted, after positioning of the microfluidic devices, the heat loss due
to the external wind was assumed to be negligible; however this is not the case. In order to obtain a
U-value for the entire system, the most accurate method would be to conduct experimentations, as
theoretical calculations may be misleading.

10. Conclusions
The most significant conclusion drawn from the development of this theoretical insulation micro-
flow device is that there is definitely potential for prototyping, due to the simplicity of the device,
and potential for energy savings and carbon reduction. Other conclusions drawn are as follows:
 MFD is best positioned on the outside of the window.
 Considering hydraulic resistance and fabrication limitations the rectangular and parabola
channel geometry were depicted the most suitable.
 The zig-zag network structure offered the highest surface area for heat transfer, makes
advantage of gravity to add to the pumping power, and as a result is deemed the most
suitable structure.
 The centrifugal pump was deemed the best one for this application owing to its ability to
provide a wider range of flow rates when compared to electro osmotic flow. Moreover, the
centrifugal pumping is insensitive to various physiochemical properties like pH and ionic
strength which comes in handy while using various other fluids in the microfluidic system.
 The solenoid valve was deemed the best valve as it cost efficient, energy efficient and
compact.
 The MFD has a potential to save 1.75kW of energy per 1m2
of double glazed window,
transcribing to a electricity saving of 18.24 cent per hour, as per Electric Ireland 24 hour
rates.
 For an industrial scaled use, of 100m2
of window section, this saving is calculated at 66,500€
per 2,080 hour working year.
 The energy saving directly reduces the amount of carbon footprint.
It was also noted that the savings, both monetary and heat transfer related, are representative of
the conditions used in the theoretical analysis. In order to accurately define these parameters, it
would be necessary to conduct an experimental analysis, in which the input conditions (flow
conditions and thermal conditions) are varied.

11. Appendices
Appendix A
Figure A1 presents a chart of hydraulic resistance for various micro-channel geometries.
Figure A1: Hydraulic resistance equations for various micro-channel geometries (Bruus, 2008)

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