1. EE502-PMP Introduction to MEMS
Linear Electrostatic Induction Gate Valve With Variable
Aperture Filter
Matt Kerschbaum
Instructor: Prof. Karl Böhringer
University of Washington
Seattle, WA 98195
Abstract
An examination is made of a microfluidic device which
integrates gate valve and filtering capability into a single
unit. A linear electrostatic induction drive is used to provide
motive force for valve positioning. Filter aperture size is
varied using a mobile mesh superimposed on a fixed mesh
with both meshes having the same opening pattern. Power
consumption is minimized with the inclusion of a “locking”
device which maintains the desired mobile mesh position.
Introduction
Microfluidic systems frequently require flow control
and particle filtering. Filtration is used as a means of
isolating or separating material of different sizes. Early
efforts by Yang [1] and Kittilsland [2] utilized fixed
meshes. Later efforts by Shen [3] simplified fabrication
efforts, but maintained the fixed pore size. A very early
attempt at a variable aperture size can be found in Fleischer
[4], but a literature search seems to indicate that, while
multiple meshes may be present on the same device, a
single adjustable mesh is not common.
Fluid flow control can be accomplished using gate
valves such as [5]. A low-power, “latchable” valve can be
found in [6].
Theory
There exist a wide variety of electro-mechanical actuators,
and each has unique advantages and disadvantages.
However, the electrostatic film actuator has shown to be
capable of high drive forces while being relatively simple to
manufacture. While drives such as [7] [8] can provide high
motive force, the electrostatic induction motor of [9] is
elegant in its simplicity and requires control signals on only
one side of the drive. This eliminates the need for
penetrations of the fluid boundary and greatly simplifies the
physical construction of the device.
The planar electrostatic surface-drive makes use of the long
duration of dielectric relaxation in resistive materials
relative to the electrode control signals. A charge is first
induced on the resistive material and then electrode polarity
is reversed. Because the metal electrodes redistribute their
charge almost instantaneously relative to the dielectric,
both vertical and lateral forces can be created. The
electrostatic drive operates with electrodes in groups of 3
(refer to Figure 1). Two electrodes are used to
electrostatically induce charges in the sliding plate
(images1, 2) and, when polarities are swapped, provide a
repulsive force to “lift” the sliding plate free of the stator
(3). The third terminal polarity is selected to control the
direction of the sliding plate (4). Once charge has been
induced into the sliding plate, subsequent charging cycles
are reduced. Charge relaxation time can be found by (1.1).
For copper, TR=1.5x10-19
s as compared to 51.7 days for an
insulator like quartz. For this reason, the sliding resistor
should have conductivity on the order of between 10-12
and
10-15
mhos/m.
σ
εε R
RT 0
= (1.1)
Figure 1-Surface Drive Control
The device consists of 7 major components (Fig. 2):
1. Main Body/Substrate.
2. Retaining Plate (shown transparent).
3. Fixed Aperture.
4. Movable Aperture.

2. 5. Locking Cantilever Springs.
6. Locking Mechanism/Stator.
7. Stator Electrodes.
Figure 2-Device in partially opened position.
Theory of Device Operation (refer to Fig. 1&2):
1) Initial charge is induced on movable grate (4).
2) Electrode polarities are reversed to create
electrostatic repulsion, which forces (4) upwards
against the cantilever springs (5).
3) Third terminal electrode polarity is selected for the
intended direction (negative for motion towards
the right edge). This shifts the movable grate one
“step” in the desired direction.
4) Once in the desired location, electrodes are set to
ground and spring pressure forces the movable
grate down. The tapered shape of the “teeth” tends
to align the grate.
Figure 3 - Locking Mechanism.
Mechanics of Motion:
A free-body diagram of the mobile grate gives:
xxGratex FmaF == −∑ (1.2)
gmFFmaF GrateSpringyyGratey −−== −∑ 2 (1.3)
The resistance due to viscous fluid has been ignored in
(1.2) due to low velocities of travel as has the contact
friction between the springs and the sliding grate. In (1.3),
the springs are assumed to be identical, and the device is
assumed to be oriented vertically as shown in Fig.2 (hence
the third term).
The spring constant of the cantilevered springs can be
found using [10].
3
3
4l
Ewt
k = (1.4)
To allow motion in the x-direction, the mobile grate must
be raised free of the locking “teeth” and the maximum
force to overcome the springs can be determined by:
Toothyspring khkF 22)max( max =∆= − (1.5)
Due to charge neutrality, the charge induced in the sliding
plate must be the same as that of the inducing electrode
(See Figure 1, pictures 2 and 3). A local charge density, qj,
which was induced by the j-th electrode results in a force:
2
2
4 r
q
F
j
y
πε
= (1.6)
In the case of Fig.1 (picture 2 and 3), F is directed in the –y
and +y directions, respectively. Also in picture 3, a
horizontal force is created due to the opposing charge
distribution on the adjacent electrodes.
Experiments
Various attempts were made to simulate the system
using COMSOL Multi-physics using the Electro-Statics
and Electro-Mechanics modules. Due to processor memory
limitations, a simulation was made on the thin film device
described in [9] and a simplified 2-D model was utilized. A
voltage pattern from Figure 1, Picture 1 (V+, V-, GND)
was used. The computed voltage and electric field patterns
are shown in Fig. 5/6.
Figure 4 - 2-D Thin Film Actuator

3. Figure 5 - 2D Voltage Pattern
Figure 6- Electrode Electric Field
Figure 7-Induced Charge Distribution
Discussion
Due to my unfamiliarity with COMSOL, I spent a lot
of time trying to learn how to use the various features, but
was unable to achieve the results I was after. As an
example, in Figure 7, the polarity of the induced charge is
correct, but the magnitudes are not close to what I would
have expected.
Conclusions
This was a complicated project, and I wish that I would
have had more familiarity with COMSOL to be able to
achieve the results I was hoping for. However, I did learn a
great deal.
Summary
This paper investigated a method for incorporating a
microfluidic valve along with a variable-sized aperture
mesh.
References
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