Definition. of the most non-Newtonian fluids that we

Definition.

Figure 2

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      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 Armour

 

 

·       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 created 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 a shear-thinning fluid, caused by the addition
of a relatively 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

A real life example of a rheopectic fluid is cream. If you
stir cream once it won’t have any effect. But if you continually add a force of
stirring it will increase its viscosity and become thicker.

Application

There is ongoing research into 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 it. Body armor and combat vehicle armor are key areas where efforts
are being made to use rheopectic materials. 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

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.

 

Solder pastes used
in electronics manufacturing printing processes are thixotropic.

 

Thread-locking
fluid is a thixotropic adhesive that cures anaerobically.

 

Thixotropy has been
proposed as a scientific explanation of blood liquefaction miracles such as
that of Saint Januarius in Naples.

 

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.

 

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

three models presented below
are mostly applied (Seyssiecq & Ferasse, 2003).

·       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 (Pevere & Guibaud, 2006). 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 Newtonian behaviour

t0 > 0 & n = 1 Þ Bingham plastic behaviour

 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 (Pevere et al., 2006). 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 Newtonian behaviour n > 1 Þ Dilatant behaviour

 

·      
Bingham model

The Bingham model (Eq. 7)
describes the flow curve of a material with a yield stress and a constant
viscosity at stresses above the yield stress (i.e. a pseudo-Newtonian fluid
behaviour; Seyssiecq & Ferasse, 2003). The yield stress (t0) is the shear stress (t) at shear rate
(g) zero and the viscosity (h) is the slope
of the curve at stresses above the yield stress.

t = t0 + h * g (7)

t0 = 0 Þ Newtonian behaviour

 t0 > 1 Þ Bingham plastic behaviour

x

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