Fluids Mechanics

1. Suspension (chemistry)
From Wikipedia, the free encyclopedia
In chemistry, a suspension is a heterogeneous mixture containing solid particles that are sufficiently
large for sedimentation. Usually they must be larger than 1 micrometer.[1]
The internal phase (solid)
is dispersed throughout the external phase (fluid) through mechanical agitation, with the use of
certain excipients or suspending agents. Unlike colloids, suspensions will eventually settle. An
example of a suspension would be sand in water. The suspended particles are visible under a
microscope and will settle over time if left undisturbed. This distinguishes a suspension from
a colloid, in which the suspended particles are smaller and do not settle.[2]
Colloids and suspensions
are different fromsolutions, in which the dissolved substance (solute) does not exist as a solid, and
solvent and solute are homogeneously mixed.
A suspension of liquid droplets or fine solid particles in a gas is called an aerosol or particulate. In
the atmosphere these consist of fine dust and soot particles, sea
salt, biogenicand volcanogenic sulfates, nitrates, and cloud droplets.
Suspensions are classified on the basis of the dispersed phase and the dispersion medium, where
the former is essentially solid while the latter may either be a solid, a liquid, or a gas.
In modern chemical process industries, high shear mixing technology has been used to create many
novel suspensions.
Suspensions are unstable from the thermodynamic point of view; however, they can be kinetically
stable over a large period of time, which determines their shelf life. This time span needs to be
measured to ensure the best product quality to the final consumer. “Dispersion stability refers to the
ability of a dispersion to resist change in its properties over time.” D.J. McClements.[3]
In chemistry, a suspension is a mixture of two or more substances. In a suspension, very small pieces of solid are spread
through a liquidbut do not dissolve. [1]
If left still, the solid pieces will separate from the liquid and either fall to the bottom or
rise to the top. Sand in water is a suspension. Suspensions may separate quickly or stay suspended for a long time,
depending on what they contain.
If a liquid is suspended in another liquid, it is called an emulsion. Milk is an emulsion. If a liquid is suspended in a gas it is
an aerosol. Mistis an aerosol.

2. Gel
From Wikipedia, the free encyclopedia
For other uses, see Gel (disambiguation).
An upturned vial of hair gel
A gel (coined by 19th-century Scottish chemist Thomas Graham, by clipping from gelatine[1]
) is a solid, jelly-like material that
can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-
linked system, which exhibits no flow when in the steady-state.[2]
A cross-link is a bond that links one polymer chain to
another.By weight, gels are mostly liquid, yet they behave like solids due to a three-dimensional cross-linked network within
the liquid. It is the crosslinking within the fluid that give a gel its structure (hardness) and contribute to the adhesive stick
(tack).[3]
In this way gels are a dispersion of molecules of a liquid within a solid in which the solid is the continuous phase
and the liquid is the discontinuous phase.
IUPAC definition
Gel: Nonfluid colloidal network or polymer network that is expanded throughout its whole
volume by a fluid.[4]
Note 1: A gel has a finite, usually rather small, yield stress.
Note 2: A gel can contain:
(i) a covalent polymer network, e.g., a network formed by crosslinking polymer chains
or by nonlinear polymerization;
(ii) a polymer network formed through the physical aggregation of polymer chains,
caused by hydrogen bonds, crystallization, helix formation, complexation, etc., that

3. results in regions of local order acting as the network junction points. The resulting
swollen network may be termed a “thermoreversible gel” if the regions of local order
are thermally reversible;
(iii) a polymer network formed through glassy junction points, e.g., one based on
block copolymers. If the junction points are thermally reversible glassy domains, the
resulting swollen network may also be termed a thermoreversible gel;
(iv) lamellar structures including mesophases {Ref.[5]
defines lamellar crystal and
mesophase}, e.g., soap gels, phospholipids, and clays;
(v) particulate disordered structures, e.g., a flocculent precipitate usually consisting
of particles with large geometrical anisotropy, such as in V2O5 gels and globular
or fibrillar protein gels.
Note 3: Corrected from ref.,[6]
where the definition is via the property identified in Note 1
(above) rather than of the structural characteristics that describe a gel.[7]
Hydrogel: Gel in which the swelling agent is water.
Note 1: The network component of a hydrogel is usually a polymer network.
Note 2: A hydrogel in which the network component is a colloidal network may be referred
to as an aquagel.
Note 3: Definition quoted from refs.[7][8][9]
Contents
[hide]
1 Composition
o 1.1 Cationic polymers
2 Types
o 2.1 Hydrogels
o 2.2 Organogels
o 2.3 Xerogels
3 Properties
4 Animal produced
5 Applications
6 See also
7 References
8 External links
Composition[edit]
Gels consist of a solid three-dimensional network that spans the volume of a liquid medium and
ensnares it through surface tension effects. This internal network structure may result from

4. physical bonds (physical gels) or chemical bonds (chemical gels), as well as crystallites or other
junctions that remain intact within the extending fluid. Virtually any fluid can be used as an
extender including water (hydrogels), oil, and air (aerogel). Both by weight and volume, gels are
mostly fluid in composition and thus exhibit densities similar to those of their constituent liquids.
Edible jelly is a common example of a hydrogel and has approximately the density of water.
Cationic polymers[edit]
Cationic polymers are positively charged polymers. Their positive charges prevent the formation of
coiled polymers. This allows them to contribute more to viscosity in their stretched state, because
the stretched-out polymer takes up more space. Gel is a colloid solution of dispersion phase as
liquid and dispersion medium as solid.
Types[edit]
Hydrogels[edit]
See also: Superabsorbent polymer and Self-healing hydrogels
Hydrogel is a network of polymer chains that are hydrophilic, sometimes found as a colloidal gel in
which water is the dispersion medium. Hydrogels are highly absorbent (they can contain over
99.9% water[citation needed]
) natural or synthetic polymers. Hydrogels also possess a degree of
flexibility very similar to natural tissue, due to their significant water content. Common uses for
hydrogels include:
currently used as scaffolds in tissue engineering. When used as scaffolds, hydrogels may
contain human cells to repair tissue.
hydrogel-coated wells have been used for cell culture[10]
environmentally sensitive hydrogels which are also known as 'Smart Gels' or 'Intelligent Gels'.
These hydrogels have the ability to sense changes of pH, temperature, or the concentration
of metabolite and release their load as result of such a change.
as sustained-release drug delivery systems.
provide absorption, desloughing and debriding of necrotic and fibrotic tissue.
hydrogels that are responsive to specific molecules, such as glucose or antigens, can be
used as biosensors, as well as in DDS[clarification needed]
.
used in disposable diapers where they absorb urine, or in sanitary napkins
contact lenses (silicone hydrogels, polyacrylamides, polymacon)
EEG and ECG medical electrodes using hydrogels composed of cross-linked polymers
(polyethylene oxide, polyAMPS and polyvinylpyrrolidone)
water gel explosives
rectal drug delivery and diagnosis
encapsulation of quantum dots

5. Other, less common uses include
breast implants
now used in glue.
granules for holding soil moisture in arid areas
dressings for healing of burn or other hard-to-heal wounds. Wound gels are excellent for
helping to create or maintain a moist environment.
reservoirs in topical drug delivery; particularly ionic drugs, delivered by iontophoresis (see ion
exchange resin)
Common ingredients are e.g. polyvinyl alcohol, sodium polyacrylate, acrylate polymers
and copolymers with an abundance of hydrophilic groups.
Natural hydrogel materials are being investigated for tissue engineering; these materials include
agarose, methylcellulose, hyaluronan, and other naturally derived polymers.
Organogels[edit]
See also: Organogels
An organogel is a non-crystalline, non-glassy thermoreversible (thermoplastic) solid material
composed of a liquid organic phase entrapped in a three-dimensionally cross-linked network. The
liquid can be, for example, an organic solvent, mineral oil, or vegetable oil.
The solubility and particle dimensions of the structurant are important characteristics for
the elastic properties and firmness of the organogel. Often, these systems are based on self-
assembly of the structurant molecules.[11][12]
Organogels have potential for use in a number of applications, such as
in pharmaceuticals,[13]
cosmetics, art conservation,[14]
and food.[15]
An example of formation of an
undesired thermoreversible network is the occurrence of wax crystallization in petroleum.[16]
Xerogels[edit]
A xerogel /ˈzɪ ərɵ dʒ ɛ l/ is a solid formed from a gel by drying with unhindered shrinkage.
Xerogels usually retain high porosity (15–50%) and enormous surface area (150–900 m2
/g), along
with very small pore size (1–10 nm). When solvent removal occurs under supercritical conditions,
the network does not shrink and a highly porous, low-density material known as an aerogel is
produced. Heat treatment of a xerogel at elevated temperature produces
viscous sintering (shrinkage of the xerogel due to a small amount of viscous flow) and effectively
transforms the porous gel into a dense glass.
Properties[edit]
Many gels display thixotropy – they become fluid when agitated, but resolidify when resting. In
general, gels are apparently solid, jelly-like materials. By replacing the liquid with gas it is possible
to prepare aerogels, materials with exceptional properties including very low density, high specific
surface areas, and excellent thermal insulation properties.

6. Animal produced[edit]
Some species secrete gels that are effective in parasite control. For example, the long-finned pilot
whale secretes an enzymatic gel that rests on the outer surface of this animal and helps prevent
other organisms from establishing colonies on the surface of these whales' bodies.[17]
Hydrogels existing naturally in the body include mucus, the vitreous humor of the eye, cartilage,
tendons and blood clots. Their viscoelastic nature results in the soft tissue component of the body,
disparate from the mineral-based hard tissue of the skeletal system. Researchers are actively
developing synthetically derived tissue replacement technologies derived from hydrogels, for both
temporary implants (degradable) and permanent implants (non-degradable). A review article on
the subject discusses the use of hydrogels for nucleus pulposus replacement, cartilage
replacement, and synthetic tissue models.[18]
Applications[edit]
Many substances can form gels when a suitable thickener or gelling agent is added to their
formula. This approach is common in manufacture of wide range of products, from foods to paints
and adhesives.
In fiber optics communications, a soft gel resembling "hair gel" in viscosity is used to fill the plastic
tubes containing the fibers. The main purpose of the gel is to prevent water intrusion if the buffer
tube is breached, but the gel also buffers the fibers against mechanical damage when the tube is
bent around corners during installation, or flexed. Additionally, the gel acts as a processing aid
when the cable is being constructed, keeping the fibers central whilst the tube material is extruded
around it.
See also[edit]