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Introduction.

Figure 1

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Fluids are around us everywhere. For
example, fluids, which we exposed to it every day, like the air we breathe and the water we drink. These behave in a
familiar way, taking the shape of their container and flowing easily. These
fluids called the Newtonian fluids –named after the physicist newton-. On the
other hand, there is some fluids do not have the same properties which called
the non-Newtonian fluid. This report defines the non-Newtonian fluids, the
types of them, give some examples and talk about some of the application on
them.

Definition.

Figure 2

Non-Newtonian fluids are fluids that do
not obey the Newton law of viscosity. In these fluids the viscosity varies with shear rate and one single
measurement is not sufficient to know the properties of the fluid. Also there
are other factors affect flow properties like temperature and pressure.

Non-Newtonian fluids change their
viscosity or flow behaviour under stress. If you apply a force to these fluids
(say you hit, shake or jump on them), the sudden action can cause them to get
thicker and act like a solid, or in some cases it results in the opposite
behaviour and they may get runnier than they were before. Remove the stress
(let them sit still or only move them slowly) and they will return to their
earlier state.

Figure 3 oobleck

One of
the most non-Newtonian fluids that we may not know is the mixture of cornflour
and water which called oobleck. This fluid is a runny fluid, but if you apply a
stress on it suddenly acts a solid and its particles will hold on to each other
in a strong way. You can also make from it a solid part in your hand, but when
you stop moving your hand it will return in its liquid phase again. In this
case, oobleck’s viscosity increases with the applied stresses.

Types of
non-Newtonian fluids.

Not
all the non-Newtonian fluids behave in the same way when a stress applied. Some
of them become more fluid and the other become more solid. Also some of the
non-Newtonian fluids’ behavior differs due to the amount of stress applied and
others their behavior differs due to the length of time the stress applied on
it. Non-Newtonian fluids are divided into two main categories: time dependent
which includes shear thinning, shear thickening, Bingham, and Herschel Bulkey.
And time independent includes thixotropic and rheopectic.

·       Shear thickening fluids or dilatant.

In this type
the viscosity will changes dependent on the force applied, it becomes more
harder when the level of applied stress increases ( like oobleck) .

ü

Figure 4 behavior when stress applied

common example of shear
thickening fluids is a mixture of cornstarch and water called oobleck . where people can run over this kind of
solutions and yet, they will sink if they stand still

ü  Quicksand

ü  Silly
putty

ü  printing
inks

ü   vinyl resin pastes

ü   suspensions at high solid content such as wet
beach sand which shows its dilatancy through the fact it stiffens when trodden
on

Applications :

Shock absorption Systems
Automotive Suspension – Magnetic particles suspended
Impact Stress Cushioning – Sport / Athletics
Accident damage and injury mitigation – Transport
Impulse Distribution Systems
Smart Body Armor

·       Shear thinning fluids
or Pseudoplastic.

Unlike the shear
thickening fluids, this type gets runnier when the stress or the force applied
increases.

The graph above shows how both dilatant and pseudoplastic
non-Newtonian fluids behave as a force is applied. The key thing here is that
it doesn’t matter how long the force is applied for, changes in viscosity only
depend on the size of the force.

Examples

include ketchup, motor oil, paints and blood.

When modern paints are applied the shear formed by the
brush or roller will allow them to thin and wet out the surface evenly. Once
applied the paints regain their higher viscosity which avoids drips and runs.

Ketchup is from a shear-thinning fluid, caused by adding small
amount of Xanthan gum – usually 0.5%.

Application

Shear thinning proves useful in many applications, from
lubricating fast-moving engine parts to making an otherwise stiff biocompatible
hydrogel injectable

·       Rheopectic fluids.

It is a type of
fluid that gets more viscous when they are stressed over time. It will not get
more viscous when applying an instantaneous force. It requires sustainable
force to increase the viscosity.

Example

Gypsum
paste ,Cream

for cream. If you turn cream once it won’t have any effect.
But if you continually add a force of turning it will increase its viscosity
and become thicker.

Application

There is research on new ways to make and use rheopectic
materials. There is great interest in possible military uses of this
technology. Moreover, the high end of the sports market has also begun to
respond to this technology. Work is also being done to use these materials in
other kinds of protective equipment, which is seen as potentially useful to
reduce apparent impact stress in athletics, motor sports, transportation
accidents, and all forms of parachuting. In particular, footwear with
rheopectic shock absorption is being pursued as a dual-use technology that can
provide better support to those who must frequently run, leap, climb, or
descend.

·       Thixotropic fluids.

Unlike the rheopectic fluids, thixotropic
fluids get runnier when applied a sustainable stress on it. Also it does not
get runnier when applying an instantaneous force.

This
graph shows how both rheopectic and thixotropic non-Newtonian fluids behave as
a force is applied. The key thing here is that the force has to be sustained –
the longer the force is applied the more the viscosity changes

Figure 6 Behavior when stress is applying

Example

Paint ,Cosmetics ,Asphalt ,Glue

Applications

v Many kinds of
paints and inks—e.g. plastisols used in silkscreen textile printing—exhibit
thixotropic qualities. In many cases it is desirable for the fluid to flow
sufficiently to form a uniform layer, then to resist further flow, thereby
preventing sagging on a vertical surface. Some other inks, such as those used
in CMYK-type process printing, are designed to regain viscosity even faster,
once they are applied, in order to protect the structure of the dots for
accurate color reproduction.

v Solder pastes used
in electronics manufacturing printing processes are thixotropic.

fluid is a thixotropic adhesive that cures anaerobically.

v Thixotropy has been
proposed as a scientific explanation of blood liquefaction miracles .

v Semi-solid casting
processes such as thixomoulding use the thixotropic property of some alloys
(mostly light metals) (bismuth). Within certain temperature ranges, with
appropriate preparation, an alloy can be put into a semi-solid state, which can
be injected with less shrinkage and better overall properties than by normal
injection molding.

v Fumed silica is
commonly used as a rheology agent to make otherwise low-viscous fluids
thixotropic. Examples range from foods to epoxy resin in structural bonding
applications like fillet joints.

·
Rheological mathematical models

There are several rheological
mathematical models applied on rheograms in order to

transform them to information
on fluid rheological behaviour. For non-Newtonian fluids the

models presented below are mostly applied .

·       Herschel Bulkley
model

The Herschel Bulkley model
is applied on fluids with a non linear behaviour and yield stress. It is
considered as a precise model since its equation has three adjustable
parameters, providing data . The Herschel Bulkley model is expressed in
equation 5, where t0 represents the yield stress.

? = t0 + K * g n (5)

The consistency index parameter (K) gives an idea of the viscosity of the fluid. However, to be able
to compare K-values for different fluids they should have similar flow behaviour
index (n). When the flow behaviour index is close to 1 the fluid´s behaviour
tends to pass from a shear thinning to a shear thickening fluid. When n is
above 1, the fluid acts as a shear thickening fluid. According to Seyssiecq and
Ferasse (2003) equation 5 gives fluid behaviour information as follows:

t0 = 0 & n
< 1 Þ Pseudoplastic behaviour  t0 = 0 & n > 1 Þ Dilatant behaviour

Herschel-Bulkley fluids include both shear thinning and shear
thickening materials. The practical examples of such materials are greases,
colloidal suspensions, starch pastes, tooth pastes, paints, and blood flow in
an artery.

·
Ostwald model

The Ostwald model (Eq. 6),
also known as the Power Law model, is applied to shear thinning fluids which do
not present a yield stress . The n-value in equation 6 gives fluid behaviour
information according to:

? = K * g (n-1) (6)

n < 1 Þ Pseudoplastic behaviour n > 1 Þ Dilatant behaviour

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