SHEAR THICKENING FLUIDS (STFS); DEFINITION, THE AFFECTING FACTORS, AND THEIR GENERAL APPLICATIONS: A REVIEW

Abstract

Shear thickening fluids STFs are a new kind of materials that consist of nano/micro particles that dispersed in another material such as polymer. In this review paper, the definition of this type of materials and the mechanism of its work are explained. In addition, the main factors that can effect on this behavior and their applications in different area from simple applications to complex applications are shown briefly.

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SHEAR THICKENING FLUIDS (STFS); DEFINITION, THE AFFECTING
FACTORS, AND THEIR GENERAL APPLICATIONS: A REVIEW
Nabeel Hasan Al-Mutairi
Polymers and Petrochemical Engineering Industries Department,
College of Materials Engineering, University of Babylon, Al-Hilla, Iraq
E-mail: nabeelemg90@gmail.com.
ABSTRACT:
Shear thickening fluids STFs are a new
kind of materials that consist of nano/micro
particles that dispersed in another material
such as polymer. In this review paper, the
definition of this type of materials and the
mechanism of its work are explained. In
addition, the main factors that can effect on
this behavior and their applications in
different area from simple applications to
complex applications are shown briefly.
Keywords: Shear thickening fluids; STFs;
Body armor; Dilatant; Non-Newtonian flow.
I. INTRODUCTION TO SHEAR THICKENING
FLUIDS (STFS):
This type of fluid has a non-Newtonian
behavior as its viscosity increases with
increasing shear rate; it is often seen in
concentrated colloidal dispersion. This kind of
materials show a solid like behavior for a split
second when the stress is applied on the material
and return to its liquid behavior after the
removal of stress[1]–[4]. Figure 1 shows the two
main types of fluids: Non-Newtonian (the
viscosity change with changing the shear rate)
such as shear thinning (pseudoplastic) and shear
thickening (dilatent), and Newtonian fluids (the
viscosity constant with shear rate change)[5].
Figure 1: Types of fluids[5].
At low shear rates, shear thinning
behavior appears, and with increasing shear rate,
the viscosity increases to a higher value and
turns into shear thickening. These fluids are
characterized by the fact that removing the shear
stress leads to a decrease in viscosity. This type
of fluid has good mechanical properties because
of the unique rheological properties that make it
distinctive applications, as shown in Figure 2 [6].
Figure 2: The shear thickening fluid behavior [7].

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Figure 2 represents the general scheme of
STF, which consists of three regions: The first
region represents the occurrence of shear
thinning that is evident at shear rates that are
less than the critical shear rate γc, the second
region represents the shear thickening that
occurs between the critical shear γc and the
greatest shear, and the last region is the
occurrence of shear thinning.
II. FACTORS AFFECTING ON SHEAR
THICKENING FLUIDS:
Shear thickening behavior is affected by
many factors related to the dispersed medium,
including volume fraction, particle size, shape
and dispersion, and also depends on the
properties of the continuous medium.
A. The volume fraction of the added particles has
a great effect on the appearance of the shear
thickening behaviour, as by increasing the
volume fraction, the critical shear rate appears in
a closer area, which means an increase in
viscosity, as shown by Figure 3 and 4.
Figure 3: Shear rate/Viscosity curve as a function
of different volume fraction[8].
Figure 4: Critical shear rate onset dependence on
the volume fraction[9].
B. Particles size: particle size has an opposite
effect on shear thickening, as the larger the
particle size, the less shear thickening appears,
as shown in Figure 4 [10].
Figure 4: Particle size effect on the shear
thickening behavior[10].
C. The particles size distribution: The blocks that
consist of different sizes of particles are more
compact in that the small particles fill the spaces
between the large particles. when shear stress is
applied, the small particles act as a lubricant for
the flow of large particles and thus lead to a
decrease in the viscosity of the fluid, as shown by
Figure 5 and 6 [11].
Figure 5: SEM image of large and small particles
in shear thickening fluid[12].
Figure 6: The effect of particle diameter (left)
and particle size distribution (right) on the shear
thickening behavior [13].

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D. The shape of the particles: The particle
shape has a great effect on the shear thickening
behavior. Studies have shown that the rod-
shaped particles are more effective than the plate
because the rod arranges itself in the direction of
flow. Spherical particles produce higher viscosity
because they disperse higher energy, as shown
by figure 7 [11].
Figure 7: The effect of different shape of particles
on the viscosity/shear rate behavior[14]and [4].
E. Attractive forces between particles: The
attraction between particles has an important
effect to obtain the required rheological
properties, as the forces must be stable between
particles. To get shear thickening there should be
no attraction between the particles as shown by
figure 8 [11].
Figure 8: The chemical induction effect on the
shear thickening behavior, (flocculated-no
interaction between the particles/shear
thinning), and (deflocculated- there are an
interaction between the particles/shear
thickening)[14]and [4].
III. APPLICATION OF SHEAR THICKENING FLUIDS:
These liquids have many different
applications from industrial applications to
medical applications, as the use of these liquids
in a specific application depends on the materials
used in terms of the type of particles as well as
the type of polymer. In this section, some of the
important application are listed.
A. Shock absorbers and suspension
systems, which consist of an insulating fluid and
polar particles called ER fluids, are affected by an
electric field. Magnetic particles can be used and
MR fluids are known to be affected by the
magnetic field[15].
Figure 9: Shock absorber system for
automotive[15].
B. Shear thickening fluids are used for the
purpose of protection against sudden shock to
the screens of devices, especially smart devices,
and automotives (paint, and mirrors) where the
surface of the screen is covered with this type of
material[16].
C. Shear thickening fluid is used for the
purpose of smoothing surfaces, especially
complex surfaces, as it is an easy, fast and low
cost method. SiC particles are characterized by
high hardness and strength, which are used for
the cleaning and smoothing of rust and oxidized
objects, as in figure 10 [17].

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Figure 10: Shear thickening fluid and SiC
particles for polishing and cleaning[17].
D. Shear thickening fluids are used in oil
extraction operations during drilling to prevent
an explosion in the well when the drill reaches a
gap containing gases at high pressure[18].
E. The most common and widely used
application is in the field of armor industry to
protect people from stabbing and bullets, where
layers of Kevlar or aramid fibers are used and
covered with Shear thickening fluid, thus
providing protection, ease of movement and low
cost. As the number of layers used is less in the
case of Shear thickening fluid as in figure 11[19].
Personal protection of the body from dangers
and injuries is an important need for people,
especially soldiers and policemen, to protect
them from attacks and attacks that they may
encounter in their daily work [14].
This led to the manufacture of clothing or
covers that are resistant to these conditions to
protect them from any attacks and attacks that
may lead to their death. These protective
clothing are called shields and are defined as any
cover used to protect the body from any danger,
external influence or violent strike. Animal skins,
natural fibres, cotton, silk and iron have been
used as shields throughout history[20], [21].
The shields used today should not only be
stabbing resistant, but rather bulletproof. It is
made of metal, ceramic and solid polymeric
panels with several layers to provide adequate
protection. The problem with these armors is the
weight as well as the restriction of movement, so
the use of nano-liquids known as liquid armor
that adopts the shear thickening behavior has
been resorted to[22], [23].
Figure 11: Kevlar woven fibers and shear
thickening fluid for body armor[24].
IV. CONCLUSION;
This review paper discussed the shear
thickening fluids because it’s a new type of
materials that have properties of different
materials. Its shown that under specific shear
rate the material will behave like shear thinning
and after the shear rate increase and exceeds
critical shear rate the material will show the
solid like behavior shear thickening behavior. In
addition, its shown that there are several factors
effect on the behavior of this material such as
volume fraction, particle size and its distribution,
particle shape, and attraction forces, some
factors make this behavior appear at low shear
rates while other factors make it appear at high
shear rates. In addition, it's found that there are
several applications of these material, the kind of
application depends on the dispersed particle
type.
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