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Polymer Suspension Based Shear Thickening Fluid Fabric

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Polymer suspension based shear thickening fluid-fabric for protective applications was a new idea. Shear thickening fluid was also known as STF. Generally STF contains liquid medium and solid particles which can be inorganic or organic based. Basically, the viscosity of the Shear Thickening Fluid (STF) will be increased when shear stress increasing. By exploiting this fluid deformation from STF, for the fabric itself, the friction between yarn-yarn and fiber-fiber will be increased drastically when force applied over the fabric composite. Then, the fabric itself will be having higher energy absorption when STF impregnated with the fabric itself compared with un-impregnated fabric composite.

The reason for this selection was motivated by previous works in this area, as well as the beneficial characteristics provided by these two materials. US Patents 5599290 and 5545128 have developed Bone fracture prevention garment and method". In these two works, carried out by the same authors, the patent outlines a design used to resolve the issue of the harmful effects of impacts on the bones of elderly individuals. This design was described as being composed of the following: "the component includes a dilatant material that was relatively stiff near the time of impact and relatively fluid at other times. In a preferred embodiment, the invention provides a hip pad, possessing a thickness small enough to be compatible with wearer acceptability, that conforms to the shape of the body during everyday activities such as walking, sitting, and sleeping, and was thus comfortable to the wearer" (5-14).

In order to model the rheological behavior of these fluids, it has been proposed to use a power law analysis with the relationship between the viscosity and shear rate represented by

η = k ýn-1

Where k is the fluid's consistency and n is the power law exponent specific to the region of high or low viscosity. According to Barnes, the shear-thinning and shear-thickening regions can be accounted for by, "using the sum of two power laws, with one value of n less than unity and one greater". (8)

The rheology studied will be focused on colloidal suspension rather than hard particle suspension. Hard particle suspension like silica particle, was extensively studied by many researched over the world like Wagner and Lee Barnes (5 - 8) .They claimed, the hard particle suspended in the polyethylene glycol (carrier fluid) will exhibit shear thickening behavior by varying the volume fraction of the particle over carrier fluid from, above 40% to 55%. 30% by volume fraction will not exhibit shear thickening behavior but in Ragvahan experiments showed (10), by using 10% volume fraction of fumed silica suspended in the Polypropylene glycol (PPG), shear thickening behavior had occurred but likely referred to particle agglomeration( flocculated gel) rather than "hydrocluster" because fumed silica, naturally have low surface area and larger particle size. In the experiment, the onset transition called critical shear rate seems having two points, the higher and the lower shear strain, due to the original structure of the carrier fluid itself (polypropylene glycol). The formation of vinyl group and straight chain in PPG gave two formations of clusters (10).

Colloidal suspension study was not emphasized and thoroughly studied because of the complexity of the colloidal system itself, which normally involved many factors such as steric and electrostatic stabilize, stability of the suspension and the formation of the hydrocluster via modification of the interaction particle of the colloidal suspension. In chapter 2, theoretical discussion about colloidal suspension which contributed to the new phenomenon of Shear Thickening Fluid (STF) such as structure-relationship of the colloidal suspension in polymer solution and Solid/Liquid transition (SLT) and also liquid/Solid transition (LST) [17]. According to the DVLO theory, stabilization of particle in the suspension with steric and electrostatic (will be have detailed explanation in the chapter 2) charges are important parameters to be investigated. The colloid particles were also known as surface charge particle. It involved the attraction and repulsive force for every single particle in the colloid dispersion or suspension, which contributed to the shear thickening behavior of the STF.

In this paper, colloidal suspension rheology will be main topics, by relating it with the structure-relationship and LST theory. In the chapter 2, more detailed onset transition of STF will be extensively discussed. Conventionally, the onset transition involved Order-Disorder transition (ODT) and hydrocluster.

The main criteria in Shear Thickening Fluid (STF) were to predict critical shear rate which was the critical point when the particles were suspended in the STF. It started to cluster with each other, exhibiting shear thickening behavior upon increase in shear rate. There were many factors in STF, some of them were, volume fraction dependence (between particle and carrier fluid), particle shape and size dependence (porosity) and particle interaction dependence. The particles can be normal charge (hard particle such as fumed silica, clay and any metal oxide particle) or colloidal particle (with charge).

In this paper, fumed silica will be used as a main material for hard particle as well as and colloid dispersion. Fumed silica will be turned into colloidal suspension by surrounding the charging over its surface particle. The main principal of colloidal suspension for this study was based on DVLO theory. Therefore, by studying the electrokinetic for each suspension, relation between this study with measured rheology through the power law index and yield stress value, there are possibilities to relate the structure- relationship in colloidal suspension of STF with onset transition (SLT & LST) evaluated. There were two basic requirements for exhibiting shear thickening behavior from colloidal suspension [raghavan, wagner, barnes], firstly, the volume fraction of the solid in the suspension must be very high and secondly, suspension must be nonflocculated or deflocculated

STF Fabric composite will be impregnated with STF has great potential in bullet proof application especially for soft armor. The vest will be having higher flexibility and less heavy than conventional soft armor. Wagner and his team claimed, STF will increase the friction between yarn-yarn and fiber-fiber by 500% using pull out yarn test. Also, by using NIJ as reference stabbed and puncture resistance test (NIJ 115.00) for STF Fabric composite, it easily passed level 1 protection. For the bullet test, also NIJ as reference, it showed interesting results. Instead of higher penetration from the bullet (9mm) to the conventional fabric composite, for STF-Fabric composite, bullet was deflected away (rebound) and had less significant mark over the top of the clay in tested frame panel. Basically, fabric composite will be placed over the top of the clay. The mark of the clay will be used as reference for the dissipated energy (energy absorption) from the test. A bigger diameter of the mark and the depth for the mark was deeper indicating the fabric composite has a low dissipated energy system. Smaller diameter of the mark and less depth of the indented clay, showed, a higher dissipated energy system for the fabric composite.

1.2 Problem Statement

Wagner found that STF had rheological characteristic of dilatant but studied on the use of hard particle in suspension. Colloidal suspension of fume silica in polymeric aqueous media was performed by other researcher but they studied only ionic strength and critical shear rate effect. In depth study on formation of hydroclusters in colloid need to evaluated to relate structure relationship between shear thickening behavior of polymer suspension with regard to composition (volume fraction), particle porosity, size and shape of particle. Hence, viscosity measurements were evaluated for different polymer systems to determine the effect of these systems on critical shear rate and shear thickening phenomena. Stabbed and puncture resistant fabric composite employing STF with high spike and knife impact performance can be affected by the formulation of STF and adhesion between STF suspension and fabric yarn. Hence, this study is hoped to solve the problems faced by indepth understanding on rheological and performance aspects in the development of STF fabric composite for protective applications.

1.3 Objectives Of Study

The specific objectives of the project include:

  • To determine physical colloid properties of colloidal suspension from hard sphere particles and colloidal dispersion via zeta potential studies.
  • To determine the rheological behaviors of the colloidal suspension from the effects of repulsion system present during steady shear experiments.
  • To fabricate STF fabric composite using various layers of Kevlar 49 and cotton fabric and determine stab and puncture resistance of STF fabric composite according to standard NIJ 115.00 tests.


Bazhenov, S. (1997). Dissipation of Energy by Bulletproof Aramid Fabric." Journal of Materials. Science, 32, 4167-4173.

Cunniff, P. (1992An Analysis of the System Effects in Woven Fabrics Under Ballistic Impact, Textile Research Journal, 62, 495-509

3. Egres Jr., R. (2005). Stab performance of shear thickening fluid (STF)-fabric compositesfor body armor applications. International SAMPE Symposium and Exhibition, 50, 2369-2380.

4. Lee, Y. (2003). The ballistic impact characteristics of Kevlar woven fabrics impregnated with a colloidal shear thickening fluid. Journal of materials science, 38(13), 2825-2833

Maranzano, B.J. and Wagner, N.J., ( 2001) The effects of interparticle interactions and particle size on reversible shear thickening: hard-sphere colloidal dispersions, Journal of Rheology, 45(5), 1205-1222, 2001

Maranzano, B.J., Wagner, N.J., Fritz, G., Glatter, O., (2000) Surface charge of 3-(trimethoxysilyl)propyl methacrylate (TPM) coated Stöber silica colloids by zeta-phase analysis light scattering and small angle neutron scattering, Lagmuir 16, 10556-10558.

Maranzano, B.J., Wagner, N.J. (2001), the effects of particle size on reversible shear thickening of concentrated colloidal dispersions, J. Chem. Phys. 114 10514-10527.

Barnes HA (1989) Shear-thickening (''dilatancy'') in suspensions of nonaggregating solid particles dispersed in Newtonian liquids. J Rheology, 33, 329-366

I.F.Efremov, The Dilatancy of Colloidal Structures and Polymer Solutions Translated from Uspekhi Khimii, 51 285-310 (1982), Russia Chemical Reviews, 51 (2), 1982

SRINIVASA R. RAGHAVAN AND SAAD A. KHAN Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905

G. Fritz, B. J. Maranzano, N. J. Wagner, N. Willenbacher 2002, High frequency rheology of hard sphere colloidal dispersions measured with a torsional resonator, Journal of Non-Newtonian Fluid Mechanics, 102, 2, pp 149-156

J. Schuster, D. Heider, K. Sharp, M. Glowania, 2008, Thermal conductivities of three-dimensionally woven fabric composites Composites Science and Technology, 68, 9, pp 2085-2091

Young Sil Lee Norman J. Wagner Dynamic properties of shear thickening colloidal suspensions Rheol Acta (2003) 42: 199-208

Hoffman RL (1974) Discontinuous and dilatant viscosity behavior in concentrated suspensions. II. Theory and experimental tests. J Colloid Interface Sci 46:491-506

Hoffman RL (1997) Explanations for the cause of shear thickening in concentrated colloidal suspensions. J Rheology 42:111-123

Egres, R.G., Lee, Y.S., Kirkwood, J.E., Kirkwood, K.M., Wetzel, E.D., and Wagner, N.J. 2003. "Novel flexible body armor utilizing shear-thickening fluid (STF) composites." Proceedings of 14th International Conference on Composite Materials. San Diego, CA.July 14 - 18, 2003.

Horst Henning Winter et. Al: Rheology of Polymers near Liquid-Solid Transitions



2.1 Shear Thickening Fluid (STF): Introduction & Review

Shear thickening fluid, basically have two different types of fluid behavior shear thinning and shear thickening. There are two types of shear thickening behavior in the fluid behavior. First, Shear thickening is a non-Newtonian flow behavior (dilatant) observed as an increase in viscosity with increasing shear rate or applied stress also known as time independent material (Barnes, 1989; Maranzano and Wagner, 2001; Lee and Wagner, 2003). Non-Newtonian flow behavior (rheopectics) observed as an increase in viscosity with increasing duration of stress (shear rate constant) known as the time dependent materials which have a memory to deform over period of time ( figure 1).


Concentrated colloidal suspensions consisting of solid/hard spheres particles dispersed in a carrier liquid have been shown to exhibit rheological shear thickening behavior resulting in large, sometimes discontinuous increases in viscosity above a critical shear rate. This transition from a flowing liquid to a solid-like material is due to the formation of shear induced transient aggregates, or "hydroclusters," that dramatically increase the viscosity of the fluid (Barnes, 1989; Maranzano and Wagner, 2001; Lee and Wagner, 2003). For stabilize the STF and avoiding agglomeration behavior, co-solvent is added and it must be done because STF has a very strong molecule interaction between particles and carrier fluid (attraction force) rather than repulsive force when force is applied at specific time (critical shear rate and critical shear stress) [Barnes, 1989; Maranzano and Wagner, 2001; Lee and Wagner, 2003].


The Shear Thickening Fluid (STF) is the combination of the particles suspended in the carrier fluid. Figure 2 showed the STF curve when shear stress applied on the material. The particles used can be made of various materials, such as Silica Dioxide or other oxides, or polymers such as Polystyrene (PS) or Polymethyl methacrylate (PMMA), or other polymers from emulsion polymerization. Generally, particles can be in various shapes like spherical, elliptical, disk-like or clay particles (Barnes, 1989).

The particles can be stabilized in solution or dispersed by charge, Brownian motion, grafted polymers and others. Then, pH value of a suspension also contributed to the stability of suspension via colloidal condition such as steric repulsion and electrostatic system. The effects of pH value, concentration of the surfactant, and ionic strength of the surfactant are major factors for the colloidal suspension. This influence parameter is due to the agglomeration particle size (F. Rey, M.A. Ferreira et al. 1995).These are basic parameters in the rheological suspension and colloidal suspension system. Many studies of shear thickening fluid system (Barnes 1989, Hoffman 1998, Wagner 2004), focus on the colloidal particles only such as nanoparticles colloidal silica and monodisperse silica, which is it is well known as a very stable individual particle interaction in the suspension [Brownian suspension].

In this paper, electrostatic stabilize and steric repulsion system is used to control the interparticles interaction in the rheological suspension. Electrostatic stabilize is less studies due to the complexity in the rheological suspension. Concentration and ionic strength of the surfactant are the main parameter of this paper and the final pH value is critical measured in order to exhibit rheological shear thickening. F. Rey, and M.A. Ferreira in their paper "Effect of concentration, pH, and ionic strength on the viscosity of solutions of a soil fulvic acid" claims, all the parameter in this studies showed the dramatic instant result of the suspension due to the gel point of the suspension or well known as isoelectric point (iep) by adjusting H+ present in the rheological suspension. in addition of steric repulsion system, making a barriers for the individual surface particle is a intention in this paper. The double layers of the barriers (thickness) due to the zeta potential and electrokinetic theories are independent from the shear stress during rheological experiments. Because of the main intention of this paper are to determine the factors involving onset transition of the STF due to the interparticle interaction dependence which are closely related to the colloidal suspension rheology and the effects of the onset transition (rheological behavior) for the stab and puncture resistance test of fabric composite.

Then, the co-solvent that are used, it can be aqueous in nature and non-aqueous which can be chosen to stabilize suspension system The co-solvent should be environmentally stable like ethanol and methanol, so that, they remain integral to the fabric and suspended during service. Another function of co-solvent is to lower the viscosity of the STF, so that impregnation process of fabric composite becomes efficient and easy. By adjusting the viscosity of the STF, areal density of final fabric composite can be alter and also monitored. The particles must get through and suspended in the fiber-fiber and yarn-yarn [lee wagner et al 2003]. The result from the good impregnation process is the friction force between yarn-yarn and fiber-fiber will be increased [lee wagner et al 2003].

But in this paper, by using information from wagner and other researchers, a variation of the drying temperature for the STF and STF- fabric composite will be designed and tested via thermal degradation technique ( pre-degradation) and swelling behavior of the STF-fabric composite after at the different drying temperature. The idea are to avoid pre-degradation region and swelling behavior of the STF in the drying process which is believed will reduced the performance of the fabric composite due to the following factors, degradation of the STF and STF-fabric composite due to the drying temperature and swelling behavior of STF. By combination those factors, believed, the internal friction between fiber-fiber and yarn-yarn will be reduced which is making inefficiency of energy absorbtion or dissipated energy for fabric composite (Wagner 2004).

2.2 Mechanism of Shear Thickening Fluid (STF)

The mechanism of STF normally can be described via hydrocluster theory and order-disorder theory. The similiarity of these theories is a idea of a new macrostructure formation occurred, furthermore, it are closely related to rheological experiments such as temperature and time factors. A few researchers like Barnes (1989), Hoffman (1998) and wagner (2004), claimed colloidal factors such as different types of carrier fluid, particles porosity, and volume fraction of particle suspended. But, these two theories still can't explain in detail about the mechanism of STF. Wagner and his team claimed the formation of hydrocluster must be a deflocculated suspension and major parmaters of STF are volume fraction, interparticle interaction, and particle porosity. A new theoretical transition will be proposed for STF behavior. Liquid-Solid transition (LST) or semi-solid transition involving the relaxation state of the fluid during transition which is normally include the changes of loss and storage modulus during transition and believed new formation of a new macrostructure are formed during rheological experiment. This a new macrostructure formed due to the formation of the attraction force and repulsion force in the STF, which is believed closely related with double layer theories in zeta potential theory. LST theories is based on the transition of the material due to the changing of the complex modulus which are can be translated into the formation of gel (stiffness) and relaxation state of the material during near transiton and at the onset transition.

2.2.1 Hydrocluster

The most related theories to the shear thickening behavior are the hydrocluster theory. Basically, the hydrocluster will be occurred when the balance force from shearing flow in the concentrated suspension and the force rising from particle - particle (interparticles) interaction (Bender and Wagner 1995).There are two conditions, first via colloidal factors (steric and electrostatic repulsion) and particle interactions between particle-particle and particle-carrier fluid (Bender and Wagner 1995). Figure 3 showed the formation of hydrocluster, when the applied shear stress on the STF.

The formations of hydrocluster are still extensively investigated by researcher in the entire world. The main interest of this unique behavior is the onset transition (critical shear rate) from liquid state to solid or gel state. This phenomenon involving the rapid changes of fluid viscosity in a second whether applied it with shear stress, applied electric field (refer to the ER fluid), and applied magnetic field (refer to the MR fluid). There are specific equipments for detection of this phenomenon such as optical rheometer, small angle neutron scattering (SANS), and two beam light laser scattering. But with all this equipments, still the formation of hydrocluster is unknown phenomena.

According to this theory, at the lower stress (below critical stress, maximum volume packing fraction) interparticle interaction either Brownian motion or electrostatic, making the concentrated suspension is easily flow (shear thinning behavior and viscosity of the suspension become lower) due to the particles slippage in the carrier fluid. As the stress is increased, the attraction forces is slightly increases than repulsion force in between particles of the concentrated suspension (particles aggregation become larger and the viscosity of the suspension become higher). When the magnitudes of the shearing force are equilibrium to the interparticle interaction, the particles in the concentrated suspension become "cluster" also known as hydrocluster.


This theory is first suggested by Brady (Bossis and Brady 1989) as a result from stokesian Dynamics simulation and then, supported by optical rheological experiment by wagner (Bender and Wagner 1995). Basically, these phenomenons are closely related with phenomenon of "Resonance" in physic. In physics, resonance is the tendency of a system to oscillate at greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies (or resonance frequencies). At these frequencies, even small periodic driving forces can produce large amplitude oscillations, because the system stores vibration energy [the fundamental of physic 1999].

2.2.2 Order-Disorder Theory

The order-Disorder mechanism is first suggested by Hoffman (Hoffman 1972) which obeserved the monodispersed supension under shear generate different patterns at critical shear rate ( before and after). According to this theory, when the suspension is sheared, particles is ordered in the specific formation ( hexagonal or pentagonal) of packed layers parallel to the plane of shear. After a crtical stress is reached, the instabilities in the formation of packed layers become greater and particles are out of the formation. Then, these particles collide and jammed to each other and produce in the rise of viscosity.


An example of these pattern can be seen in figure 4 Hoffman monitored the monodispersed suspension under shear and showed that figure 4(A) corresponds to the order formation of the packed layers while figure 4(B) is disorder or random formation of the packed layers after critical shear stress is reached.


The illustration of the phenomenon for the order- disorder formation of the packed layers that suggested by Hoffman can be seen in figure 5. those formation is captured by the Hoffman by using a simple shear in figure 4 (A) is same formation in the figure 5 (A). Hoffman predicted that, those formation occur due to the strong surface bonding in the monodisperse suspension which normally reffered to the hydrogen and van der waals bond. In figure 5 (B), the disorder formation occurs when sample shear above critical shear stress. Particle are collide and jammed each other and produce in rise of viscosity due to the agglomeration of the particles.

2.2.3 Introduction Liquid-Solid Transition (LST)

In this paper, a new concept theoretical onset transition of STF will be proposed. The main ideas of this theoretical concept are the macrostructure of the fluid are changes during onset transition from liquid state to the solid or gel state, and liquid- solid transition state which is normally involving the relaxation state (?????). The transition also can be state as semi-solid transition at the critical shear rate. This paper, also will be investigated the formation of the transition by using colloidal particles which are stabilize by using two system, steric and electrostatic repulsion system.

LST involves many such of factors, such as theory of gelation, branching theories, and percolation theories. All the theory in LST are closely related to the formation of the macrostructure, whether effect on the temperature surrounding or over time.

  • Theory of Gelation

The LST of polymers is also technically important since it occurs in nearly all of the common fabrication processes. Examples are injection molding of semi-crystalline polymers (where the surface quality of the finished parts may be affected by gelation shear thickening fluid (STF) and processing of crosslinking polymers. Therefore, the onset transition for STF can be detected by using LST.

There are several theories in gelation are normally used in LST. First is branching theories and second is percolation theory. The onset transition are very important to the STF for comparison with the hydrocluster formation theories and order - disorder theory and proposed new theoretical idea for the rheological behavior due to the factors affecting the performance of STF such as molecular weight dependence, volume fraction dependence and particle - particle interaction dependence. Those all factors which affecting the performance of STF had been discussed in previous sub-chapter.

2.2.4 Description of the Phenomena for Shear Thickening Fluid (STF)

Basically, the phenomenon of STF is investigated by using a lot of parameter in the last two decades. Volume fraction, particle porosity and interparticle interaction dependence is a major parameters for STF. In this paper, molecular weight of carrier fluid is added in the STF's parameter, in order to increase the potential parameter for the STF's phenomenon.

  • Volume Fraction Dependence

Volume fraction factor is the main parameter in the shear thickening fluid [STF]. In general, a solid or hard particle which is suspended in the carrier fluid such as ethylene glycol, polyethylene glycol or other carrier fluid which are aqueous in nature or non-aqueous in nature can exhibit shear thickening behavior at the minimal range of volume fraction in between 30% to 49%. Above 50% of volume fraction, the rheological shear thickening behavior can be measured at lower shear rate but it depending on the complex viscosity of the suspension which is it is related to another parameters like particle size and porosity. Meaning, surface area and aspect ratio of the particle are greater influenced on the final viscosity of the STF.

One parameter that has a huge effect on the critical shear rate is the volume fraction. At low volume fractions (below 0.5), shear thickening is either less dramatic or not significant [Characterization of Shear-Thickening Fluid-Filled Foam system for Use in Energy Absorption Devices, Jose 2004].

Wagner and his team also claims, colloidal silica ( 14nm) which is suspended in the polyethylene glycol, less than or 30% volume fraction of STF will producing less or no significant of thickening behavior either at low shear rate or higher. Raghavan and khan which studying the rheological behavior of fumed silica suspended in low molecular weight polypropylene glycol claim, non-flocculated suspension exhibit shear thickening at 10% [w/w] under steady flow and strain-thickening under oscillatory shear. Strain-thickening refer to the abrupt increase in the complex modulus [complex viscosity].

Fumed silica generally known has higher agglomeration size up to 140 micron. Therefore, volume fraction has less significant affected on the shear thickening behavior. The agglomeration size is the main factor in the Raghavan and Khan studies (figure 2.2.4).

The results of Barnes demonstrate that at volume fractions in the range of 50 %, the shear thickening behavior is expected and predictable. In addition, theoretical analysis of the maximum volume fraction of monodispersed suspensions predicts this value is Ømax=0.605, where this value corresponds to, "the volume fraction for a cubically stacked hexagonal packing" (Boersma et al. 1989).

  • Molecular Weight of the Carrier Fluid Dependence

Molecular weight of the carrier fluid is a new parameter in the STF. Wagner and his team reported that the viscosity of the carrier fluid is important to predict the onset transition from shear thinning to the shear thickening behavior in the STF. A difficulty occurs from getting exacts experimental measurement of the effect of carrier fluids due to the reality that changing the carrier fluid affects the interparticles interaction. In this paper, by monitoring the zeta potential of the carrier fluid, those two effects (molecular weight dependence and Particle-Particle Interaction Dependence) can be separated.

The onset transition of STF will be a main indicator for this parameter neither the suspension are flocculated or deflocculated. The idea of used difference molecular weight is cames from colloidal suspension which prepared by raghvan, in his experiment polypropylene glycol (PPG) as a main carrier fluid. He are experimental the effect of the rheological behavior for fumed silica suspended in PPG at lower concentration (mass fraction), and showed the STF behavior at low yield stress (figure 6). The experiment by raghvan is difference from wagner and barnes, which used colloidal particle from Nissan Chemicals (MP4540) and suspended in PEG 200 at high concentration for exhibit shear thicknening behavior (figure 6).


Therefore, when turnable fumed silica particle into colloidal fumed silica via steric and electrostatic repulsion system, suspended it in the different molecular weight of carrier fluid at various particle loadings, believed, the suspension will exhibited shear thickening behavior at low yield stress.

  • Particle-Particle Interaction Dependence

Interparticles interactions are very important in determining the shear thickening behavior of a suspension. Flocculated suspension will not exhibit shear thickening (Barnes 1989), but instead they will show shear thinning, as shown in figure 7.


Basically, the flow behavior of a suspension is extremely affected by interparticle interaction. These phenomenons also refer to the final condition which refers to the pH value of a suspension. Therefore in table 1, the Floc sizes are monitored due to the effect of pH value, electrolytes, and polymer on a kaolin suspension. Because of flocculation is expected to begin at lower pH values, namely edge to face that kaolin is positively charged and de-flocculation at higher pH values it carries a net negative charge on the surface. (Nongkhran Chaiwong 2008).


The flocculation of kaolin depended on pH, electrolytes and polymers flocculants. Floc size and floc strength increased with increasing of cation valency in the electrolytes and increasing of molecular weight in the polymers (Nongkhran Chaiwong 2008).

The information that gathered from table 1 is important for the colloidal suspension and STF. The shear thickening behavior of a suspension must not have flocculation, from that reason, monitoring a suspension by avoiding flocculation is necessary.

2.3 Colloidal Suspension Rheology

In this paper, the rheological studies of Shear thickening Fluid (STF) are focusing on the colloidal particles. The scopes of studies are focused on the non-Newtonian fluid behavior such as viscosity, power law index, tangent teta, and modulus of the STF.

2.3.1 Viscosity

There are a lots of definition of viscosity, measuring the internal friction of fluid is a simple explanation. The friction becomes apparent when a layer of fluid is made to move in relation to another layer. The greater the friction, the higher amount of force required causing this movement, and it is called "shear".

Isaac Newton defined viscosity by considering the two parallel flat a distance "dx" and are moving in the same direction at different velocities. The assumption by Isaac Newton, required force in order to stabilize or maintain is proportional to the difference in speed through the liquid, or the velocity gradient (dv/dx). To express this, Newton wrote in equation and also can be illustrated in figure 8:


Where viscosity is a constant for a given material and is called its "Viscosity". The velocity gradient is a measure of the change in speed at which the intermediate layers move with respect to each other. It describes the shearing the liquid experiences and is thus called "shear rate". This will be symbolized as "y

" and its unit of measure is called the "reciprocal second" (sec-1).

Brookfield viscometer is used to determine the viscosity's value of the STF. By using special small sample adapter from BROOKFIELD and helped from Brookfield's agent in Malaysia, determination value of the shear rate, shear stress, power law index, tangent teta, and loss & storage modulus of STF become reality. Those theoretical mathematical equations will be used for determination of new theoretical relationship of STF also known as liquid-solid transition (LST).


Viscosity can be determined mathematically by this formula:


. LST researched normally are focused on the transition of the storage modulus to the loss modulus in the crosslinking polymers such as natural and synthetics latex. In this paper, special small adapter (21/13R) is used to determine the value of STF's viscosity. The HB model of viscometer is used because the high concentration of the silica particle suspended in the carrier fluid, resulted the STF's viscosity is high.

  • Cylindrical Spindles

The following equations apply to cylindrical spindles only, on any Brookfield


SHEAR RATE (sec-1) = = 0.93N

SHEAR STRESS (Dynes/cm2) Ï„ =

Viscosity (Poise) η =

Definitions: = angular velocity of spindle (rad/sec)

, N = RPM

RC = radius of container (cm)

Rb = radius of spindle (cm)

X = radius at which shear rate is being calculated (cm)

M = torque input by instrument (dyne - cm)

L = effective length of spindle (cm)


2.3.2 Power Law Index & Tangent Teta

According to the engineering department of BROOKFIELD, there is sort direct relationship between power law index and tangent teta which can be reviewed in the equation 3 and 4. The equation is the mathematical developed by ostwald(??). Basically, power law index indicates the flow behavior such as when "n" is equal to 1, the material is Newtonian fluid, but "n" is less than 1, the material is shear thinning and "n" is more than 1, the material is shear thickening fluid.



The standard calibration for BROOKFIELD viscometer has been developed in this paper by using the Newtonian fluid such as water and silicon oil. The standard calibration procedure is designed to minimize error reading from torque spring during experiment. The idea comes from the basic inertia forces of the unwinding and winding spring during experiment. Therefore, by using Newtonian fluid as calibration fluid, meaning the value of power law index will be exactly 1.

The relationship between power law index and tangent teta for small sample adapter can be reviewed in equation. When the value of"n"is 1, meaning the tangent teta is 45 degrees. Therefore, when the tangent teta is 45 degrees, calibrated procedure for that viscometer is good. The standard calibration procedure can be reviewed in the chapter 3.


Furthermore, the idea of locking tangent teta to 90 degrees in single head rheometer is bad idea because the onset transition is from 0 degrees to 180 degrees as shown in figure 9. Normally, x-axis is represents strain, the applied deformation, and the arrows represent the resultant stress, or torque, vectors.

The stress response for a purely elastic material will be in phase with the strain yielding a tangent teta of 0°. For a purely viscous material, the stress response will be in phase with the strain rate producing a tangent teta of 90°. [Brookfield's engineering department]

The inertia torque contribution from motor and geometry can be 180° in tangent teta.  The instrument measures the raw or total tangent teta.  The tangent teta of the sample needed to calculate the viscoelastic properties is determined by subtracting the inertial contribution from the total tangent teta. As the raw phase signal is directly measured by the rheometer, it is an easily accessible signal that should be reported. The benefit of viewing the raw phase is it allows one to assess the amount of phase correction being applied to the measurement [Brookfield's engineering department].

Furthermore, tangent teta also will be used for determine the onset transition for the STF by relating it with LST theories. Figure 10 showed the relationship between structure property relationships with tangent teta. This onset transition is closely related with complex modulus which will be discussed later in the loss & storage modulus.

The researched for the onset transition between tangent teta and complex modulus is lacked because of the complexity for the determine factors affecting the onset transition. Example, researched on the STF, widely focused on the effects of the particles suspended in the carrier fluid such as, the formation of the hydrocluster due to the different ionic strength of the surfactant[?????], and the effects of the shear thickening behavior of the STF on the fabric composite for the stab and bullet resistance applications [??????].


In this paper, believed, tangent teta is a major factor for the shear thickening behavior, whether in hydrocluster theories or in order - disorder theory. Proposing the Liquid - solid transition is major contributor in this paper. The reason is simple, the ability of the STF changing from liquid state (gel formation) into solid state are closely related to the LST behavior, which had been discussed earlier. That onset transition neither, due to the Brownian motion (particles moved randomly and collide each others) or the effects of the surface potential energy or the combination both factors.

Last but not least, a lot of the rheological behavior especially for STF will be discussed in this paper. For the relationship between tangent teta and complex modulus will be discussed in the next sub-chapter. The new theoretical relationship for viscoelastic properties between tangent teta and complex modulus will be revealed in this paper.

2.3.3 Storage & loss modulus of the elasticity

Normally, assessment for viscoelasticity are done by doing testing such as creep and recovery tests and straight away shows the fluid's behavior is more solid- or liquid-like in its response. Usually, for this test, controlled shear stress and strain (CS), where an oscillatory strain, γ, with an amplitude, γa, and angular velocity,.

But in this paper, by using a BROOKFIELD viscometer as a basic instrument testing equipment for rheological studies of STF, and getting all the information as well as advice from BROOKFIELD's engineering department in Malaysia for theoretical storage and loss modulus. The difficulties of this theoretical is comes from to determine the shear strain of the STF by using normal viscometer. There is a impossible to determine shear strain in normal viscometer because of the missing sensor which is related to the fluid deformation (amplitude) via applied shear stress and [Brookfield's engineering department].

The complex modulus of elasticity, G*, is defined as


and denotes to the total resistance of the fluid to the response strain. The complex modulus of the elasticity can be broken down to its real and imaginary parts as


There is a basic relationship in complex modulus of elasticity. But in this paper, the angular velocity is different because the instrument for the testing is viscometer and not rheometer. By using special small adapter (refer appendix) from BROOKFIELD's, finding the value of the shear rate, and shear stress become realistic.

An angular velocity of the spindles (rad/sec) using BROOKFIELD's viscometer,, is defined as


For oscillatory strain principle, an angular velocity is defined as

, (8)

Because of much different principles in controlled shear rate, which viscometer is using a principle of varying the motor speed to get shear response from the deformation (shear strain) fluid, while, the variation of the frequencies is used by rheometer (an oscillatory).There is basic understanding of viscoselasticity which had been translated into figure 11 below.


The fluid deformation (shear strain) which, it is translated to the tangent teta, have simple relation with elastic and viscous stress. The relation is defined in equation


, (10)

Equation shows the relationship between complex viscosity with viscosity and elasticity. While from figure showed the relation between elastic and viscous stress, which it can be also defined as


The intention is to get the value of the elasticity from the equation. Because of the complex modulus which it can be reviewed in equation, also have relations with elasticity and an angular velocity. The relations can be defined in equation.


Therefore, by relating complex modulus with tangent teta, storage and loss modulus can be determined. Those two equations can be reviewed in equations13 and 14

, (13)


All the equations for the determined storage & loss modulus are guided and advices from the BROOKFIELD's engineering department.

2.3.4 Newtonian fluid

The type of flow behavior which Newton assumed for all fluids is called "Newtonian". A Newtonian fluid is represented graphically in figure 12. Graph A shows that the relationship between shear stress and shear rate is a straight line. Graph B shows that the fluid's viscosity remains constant as the shear rate is varied. Water and thin motor oils are example of Newtonian fluid.


So, at a given temperature the viscosity of a Newtonian fluid remains constant regardless of which viscometer model, spindle or speed is used to measure it.

The behavior of Newtonian liquids in experiments conducted at constant temperature and pressure has the following features:

the only stress generated in simple shear flow is the shear stress S, the two normal stress differences are zero

the shear viscosity doesn't vary with shear rate

the viscosity is constant with respect to the time of shearing and the stress in liquid falls to zero immediately the shearing is stopped

The viscosities measured in different types of deformation are always in simple proportion to one another.

2.3.5 Non-Newtonian fluid

Non-Newtonian fluid can be time -dependent or time-independent fluid. Refer to the table show two major clusters of non-Newtonian fluid, shear thinning and shear thickening behavior.There are several types of non-Newtonian flow behavior, characterized by changes fluid's viscosity to variations in shear rate.



Bingham plastic



Table 2.2 showed the classification of the Non-Newtonian fluids. Non-Newtonian fluids can be classified into two types, time dependent behavior (T.D.B) and time independent behavior (T.I.B).


Classes of Nonlinear Fluids in Non-Newtonian Fluids




Time dependent

(memory materials)



Time independent

(non-memory materials)



With yield stress

Bingham plastic


  • Non-Newtonian fluids - time independent


fluid displayed a decreasing viscosity with an increasing shear rate, some examples include paints and emulsions. This type of behavior is called shear-thinning.


is characterized by an increasing viscosity with an increase in shear rate, some examples include clay slurries, candy compounds, corn starch in water, and sand/water mixtures. Dilatancy is also reffered to as shear-thickening liquids.


Liquid behaves like solid under static conditions. A certain amount of force must be applied to the fluid before any flow is induced. This force is called yield value. Tomato catsup is an example of such fluid. Once the yield value is exceeded and flow begins, plastic fluids may display Newtonian, pseudoplastic or dilatant flow characteristics.

  • Non-Newtonian fluids - time dependent

Some fluids display a change in viscosity with time under conditions of constant shear rate.


fluid undergoes a decrease in viscosity with time, while it is subjected to constant shearing (greases).


fluid's viscosity increases with time as it is sheared at a constant rate.

2.3.6 Special Characteristics (Rheology) of Dispersions, Suspension and Emulsions

Dispersions, suspension and emulsions, which are multiphase materials consisting of one or more solid phases dispersed in a liquid medium, can be affected to rheological by number factors.

In this paper, the particle aggregation or dispersion stability is a main concern during rheological experiment. In chapter 3, a method procedure is developed to minimize all those factors which are affected during experiment. The stability of dispersion system is particularly important when measuring viscosity. If the dispersion system has a tendency to settle, producing a non-homogeneous fluid, the rheological behavior of the system will be change and the inconsistency viscosity value of the dispersion system.

All the possibility is included in the calculation especially for Liquid - Solid Transition (LST), to produce very reliable outcome later. All the processes or procedures for monitoring all those possibilities are used zeta potential theories (DVLO).

2.4 Colloidal Suspension

Colloidal suspension is defined as dispersion of the solid particles in a liquid medium. Examples paints, muds, and slurries. The main idea of using colloidal suspension for STF are to enhance the electrokinetic mobility of silica particle in STF and to reduce swelling behavior of silica particle after impregnated into fabric during drying process. Basically, colloidal suspension is referred to the stability of the suspension, and zeta potential will be used to monitor the suspension stability by referring DVLO theory.

In the colloid technology, flocculation, de-flocculation, and coagulation terms wa very important during designed the suspension or emulsion system. Words flocculation is refer to the particles aggregate (floc) during aggregation process. The floc may or may not sediment or phase separate (Zetasizer Nano series technical note). Normally, those state is refer to the stability of the colloidal system. This flocculation process is reversible process, sometimes also called as de-flocculation. If the aggregate change to new form of formation (particle size increased) and much denser, and this situation called coagulation. Natural rubber latex is a closed example of the material that undergoes coagulation process and this coagulation process is irreversible process. The process of colloidal system which is related with flocculation and coagulation can be seen in figure 18.


In this paper, the colloidal particles will be used in STF. Therefore, zeta potential via pH titration method (acid-base method) will be used for monitoring stable system for colloidal suspension in the STF. Colloid stability is not a main issue in this paper, but the interests are come from the effect of the pH value and the strength of electrolytes on the electrokinetic mobility of the STF. The main idea is to alter the zeta potential property and monitoring the effect of it on the STF's rheology which is focused on the critical shear rate at a constant volume fraction.

2.4.1 Zeta potential

In 1940, Derjaguin, Verway, Landau, Oberbeek developed a theory which dealt with colloidal stability known as DVLO theory. There are lots of techniques to measure zeta potential such as titration, and Electrophoretic light scattering (ELS). In this paper, titration technique is used to determine zeta potential property and isoelectric point (iep) of uncharged and colloidal particles. DVLO theory is used as main guidelines for determine uncharged and colloidal particles effect on the STF's suspension.

2.4.2 DVLO theory

DVLO theory related to the stability of dispersion system by monitoring the electrical double layer repulsive and Van de Waals attractive forces. The idea of the good suspension or emulsion in stability is to create a repulsive barrier forces for preventing two particles approaching one another and adhering together. But if the collide particle's with sufficient energy in overcome repulsive barrier, the attractive force will pull them.

There are lots of factors that affect the stability of the dispersion such as different particles has a different unique stable system that will be designed depends on the following criteria:

Types of particles such as mineral or non-mineral materials, monodisperse or polydispersed and etc.

Types of dispersion or suspension technique that will be used such as high speed and shearing disperser, and using ultrasonic treatment. All the technique basically has a unique effect on the particles such as the effect of charges from ultrasonic treatment on the dispersion system which is related to the effect of electrostatic system on the vibration of particle in dispersion or suspension systems.

Particles concentration in the dispersion or suspension system. Low particles concentration in the system has higher or stable system than high particles concentration.

Water or solvent system. Normally, that type of system involving primary and secondary system for stabilizing the suspension such as NR latex have a main system for stabilizing its own system, protein, and ammonia solution for secondary system.

The attractive and repulsive force must be equal or repulsive force is greater than attractive force, the stable system for dispersion, suspension or emulsion will be achieved.

Controlling those stable systems normally is complicated, involving lots of parameter and factors. Anand Yethiraj studies showed, there are seven factors which are to control and get stable colloidal dispersion system. The main interest of his studies is to determine the phase transition kinetics (thermodynamic) effect on the spherical colloidal dispersion system based on these seven factors.

All this seven factors can be summarized in figure 19. When the stable dispersion is achieved, isoelectric point of the normal and colloidal charged is determined by using pH titration technique, also known as acid- base titration method using pH meter. The detailed about this titration is presented in the chapter 3.


Therefore to maintain the stability of the dispersion system, the repulsive force must be dominant. There are two basic mechanisms that affected dispersion stability (figure 20):-

Steric repulsion - this involves the introduction of polymer into the system absorbing on the particle surface and preventing the particle surface coming into close contact. The barrier that created from the particle surfaces mainly designed for primary stabilizing system in the suspension system. The good steric system related to the thickness of the barrier. The surface potential and molecular weight of the absorbing polymer affected the performance of the steric system in dispersion system especially for colloidal stability (Ion steric effects on electrophoresis of a colloidal particle by By Aditya S. Khair and Todd M. Squires).

Electrostatic repulsion - this is effect of the particle interaction due to the contribution charged to the colloidal dispersion. The designing of the electrostatic system involved surfactant such as ionic or cationic surfactant and also non-ionic surfactants. By creating interfacial layers called diffused layer, particle separation can be controlled by adjusting the electrolytes ionic strength and type od surfactant also affected the particle separation and dispersion stability (Zetasizer Nano series technical note).


The colloidal particles basically are surrounding by two barriers: the inner region (stern layer) when the ions strongly bound and an outer region (diffuse layer) where they are less associated. When particles moves (eg. due to the gravity), ions within barrier move it. Those ions beyond the barrier stay in bulk dispersant (Zetasizer Nano series technical note). The potential at this barrier is the zeta potential (figure 21).


Each mechanism has its benefits for particular system. Steric repulsion system is simple, requiring only the addition of a polymer such as polyethylene glycol and polyvinyl alcohol. However, polymer is expensive and in some cases the polymer is undesirable eg. Application at high temperature such as ceramic and titanium casting process involving cast and sintered process at high temperature, making polymer has to be burn out due to the degradation temperature of polymer itself. This causes shrinkage and can leads to defects [???].

Electrostatic repulsion system has the benefits to stabilizing the dispersion by altering concentration of the ions in the system. Also, by altering pH value which is closely related to the electrostatic repulsion system, the rheological behavior of the system also changing such as the formation of macrosturcutre of the particles dispersed or suspended will be decreases or increases due to the altering of the attraction force in the dispersion or suspension. Nongkhran Chaiwong claimed by varying the stochiometric behavior of the clay suspension (table 2.1), the floc size also gave different results during experiment using different electrolytes and polymers. The formation of floc size are closely related with viscosity of the dispersion or suspension, whether its tends to flocculated (shear thinning) or deflocculated (shear thickening)

Therefore, by monitoring that main parameter in colloid technology such as steric and electrostatic repulsion system, the formation of the floc size can be control via polymer or stochiometric which is closely related to the final rheological behavior of the Shear Thickening Fluid (STF).

2.4.3 Isoelectric point (iep)

Isoelectric point is a particle whereas the surface potential is normal or uncharged. At the isoelectric point of solid particles is zero, zeta potential is zero. This is found by adjusting the value of pH to appropriate value during pH titration experiment (acid-base). The oxide surface responds by becoming more positive as the pH is loader by undergoing the following reaction:

If the pH is raised it becomes more negative:

Normal (uncharged) and colloidal particles are very important to the STF, therefore, determine the value of isoelectric point is a critical. Table below shows the exact value for some common oxides.

2.5 Applications

2.5.1 Personal body armor

In conventional body armor, in order to achieve those superior properties, many aspects must not be neglected like the cost of system, the total weight of the system, and the flexibility of the system. Usually, the conventional system consists of hard and soft body armor used ceramic composite as an insert, which is it are very heavy and bulky, and also stiff in order to achieve specific detailed of desired properties. In average, total weight of conventional system is around 30 kilograms per vest. Other disadvantages of the conventional system are the mechanical properties of ceramic composite, which is normally brittle in the nature.

Other disadvantages of the conventional system are the total protective area and the effect of bullet trauma after the bullet impact. First, total protective areas in the conventional system are limited because of the rigidity and total mass of the body armor system. Not all the body of person wear it is protected; ceramic plate only covered the critical and the main organs like heart. The bullet trauma effect after all is the main key of all the armor system and all designer should tackle down these main factors. Ceramic will stop the bullet penetration from through the system but ceramic has poor impact absorption of force from bullet to the all system efficiently. The consequences of this failure are the soldier's life will be in jeopardy because of the hidden force from the impact between bullet and ceramic plate (Lee et.al 2003).

Usually stab and puncture resistance are neglected because the manufacturer of body armor system always assumed that, Kevlar itself are good in ballistic impact properties therefore, Kevlar should have no problem at all with the stab and puncture resistance.

But all that assumptions is not as expected because neat Kevlar has critical weakness for stab and puncture resistance. Neat Kevlar are combination of Kevlar fibers at different orientation. Because of the different orientation, neat Kevlar have a gap (interphase) between the Kevlar fibers (waft and weft). Therefore, the spike or knife can easily penetrate through the gap between the fibers by pushing away the fibers. At the end, spike and knife easily penetrate Kevlar fabric.

2.5.2 Material development for personal body armor

Body armor is essential equipment for police and protection system. Currently, body armor is fielded only in specific high-risk scenarios, and is typical limited to chest and head protection. However, a significant percentage of battlefield injuries occur to the extremities, including arms, legs, hands, and neck. Armor for these extremities must offer protection from fragment and ballistic threats, without significantly limiting soldier mobility and dexterity (G. Fritz, B. J. Maranzano et. al).

Conventional body armor material is typically comprised of many layers of polyaramide fabrics with optional ceramic tile inserts. These materials are too bulky and stiff for application in extremities protection. A material is needed which can offer the equivalent ballistic performance of existing body armor materials, but with significantly more compactness and flexibility.

The general features of containment fibers for use in energy dissipating fabrics are high tenacity and high tensile modulus. These materials example Kevlar and spectra are also considered ballistic materials. At the same time, in more application, it may be desirable the utilize fabrics having the benefits of relative low bulk and flexibility. To achieve such properties, polymeric fibers may be used. The fibers which may be preferred include aramide fibers, ultra-high molecular weight polymers like polyethylene and polypropylene.

Typically, polymer fibers having high tensile strength and a high modulus are highly oriented, thereby resulting in very smooth fiber surfaces exhibiting a low coefficient of friction. Such fibers, when formed into a fabric network, exhibit poor energy transfer to neighboring fibers during an impact event. This lack of energy transfer may correlate to a reduced efficiency in dissipating the kinetic energy of a moving object thereby necessitating the use of more material to achieve full dissipation. The increase in material is typically achieved through the addition of more layers of material which has the negative consequence of adding to the bulk and weight of the overall structure.

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