Effect of Temperature on the Shear-Thickening Behavior of Fumed Silica Suspensions

Shear thickening fluid (STF) in engineering applications and the potential of cork in STF-based composites

Shear thickening fluids (STFs) are a unique type of fluids that can quickly transform into a solid-like state when subjected to forces (rate dependent). These fluids are created by dispersing micro and nanoparticles within a medium. When the force is removed, they return to their original liquid state. Shear thickening fluids can absorb a significant amount of impact energy, making them useful for reducing vibrations and serving as a damper. This study provides a comprehensive and brief overview of existing literature on shear thickening fluids, including their properties, classification, and the rheological mechanisms behind the shear thickening behaviour. It also examines the use of these fluids in various applications, such as improving resistance to stabs and spikes, protecting against low- and high-velocity impacts, and as a new medium for energy dissipation in industries such as battery safety, vibration control and adaptive structures. Lastly, this work reviews the promising combination of STFs with cork. Given the sustainability of cork and its energy absorption capacity, cork-STF composites are a promising solution for various impact-absorbing applications. Overall, the paper underscores the versatility and potential of STFs, and advocates for further research and exploration.

Performance Enhancement of Silicone Rubber Using Superhydrophobic Silica Aerogel with Robust Nanonetwork Structure and Outstanding Interfacial Effect

The application of high-performance rubber nanocomposites has attracted wide attention, but its development is limited by the imbalance of interface and network effects caused by fillers. Herein, an ultrastrong polymer nanocomposite is successfully designed by introducing a superhydrophobic and mesoporous silica aerogel (HSA) as the filler to poly(methyl vinyl phenyl) siloxane (PVMQ), which increased the PVMQ elongation at break (∼690.1%) by ∼9.3 times and the strength at break (∼6.6 MPa) by ∼24.3 times. Furthermore, HSA/PVMQ with a high dynamic storage modulus (G'0) of ∼12.2 MPa and high Payne effect (ΔG') of ∼9.4 MPa is simultaneously achieved, which is equivalent to 2-3 times that of commercial fumed silica reinforced PVMQ. The superior performance is attributed to the filler-rubber interfacial interaction and the robust filler-rubber entanglement network which is observed by scanning electron microscopy. When the HSA-PVMQ entanglement network is subjected to external stress, both the HSA and bound-PVMQ chains are synergistically involved in resisting structural evolution, resulting in the maximized energy dissipation and deformation resistance through the desorption of the polymer chain and the slip/interpenetrating of the exchange hydrogen bond pairs. Hence, highly aggregated nanoporous silica aerogels may soon be widely used in the application of reinforced silicone rubber or other polymers shortly.

Construction of nano-silica particle clusters and their effects on the shear thickening properties of liquids

It is of great research significance to prepare a new shear thickening fluid (STF) with a simple process, remarkable thickening effect and excellent impact resistance from the properties of the particles. Inspired by the shear thickening mechanism, nano-silica particle clusters (SPC) with different morphological structures were prepared by the reaction of amino-modified silica with polyethylene glycol diglycidyl ether (PEGDGE), and the structure models of particle clusters were designed through theoretical analysis. The structure of SPC was affected by the degree of amination modification and the molecular weight of PEGDGE, which was analyzed by DLS and TEM. The shear thickening behavior of the fluid was evaluated by steady-state rheology and dynamic-state rheology analysis. The shear thickening behavior of the fluid composed of SPC also changed greatly with the influence of the degree of amination modification and the molecular weight of PEGDGE. In addition, compared with the STF contained original silica, the STF contained SPC could produce a faster and stronger shear thickening response. Therefore, silica particle clusters are not only a promising candidate for the preparation of high-performance shear thickening fluids, but can also be better applied to industrial and scientific fields such as impact protection and shock absorption.

Leveraging the Polymer Glass Transition to Access Thermally Switchable Shear Jamming Suspensions

Suspensions of polymeric nano- and microparticles are fascinating stress-responsive material systems that, depending on their composition, can display a diverse range of flow properties under shear, such as drastic thinning, thickening, and even jamming (reversible solidification driven by shear). However, investigations to date have almost exclusively focused on nonresponsive particles, which do not allow in situ tuning of the flow properties. Polymeric materials possess rich phase transitions that can be directly tuned by their chemical structures, which has enabled researchers to engineer versatile adaptive materials that can respond to targeted external stimuli. Reported herein are suspensions of (readily prepared) micrometer-sized polymeric particles with accessible glass transition temperatures (T _g) designed to thermally control their non-Newtonian rheology. The underlying mechanical stiffness and interparticle friction between particles change dramatically near T _g. Capitalizing on these properties, it is shown that, in contrast to conventional systems, a dramatic and nonmonotonic change in shear thickening occurs as the suspensions transition through the particles' T _g. This straightforward strategy enables the in situ turning on (or off) of the system's ability to shear jam by varying the temperature relative to T _g and lays the groundwork for other types of stimuli-responsive jamming systems through polymer chemistry.

Vibration Characteristics of Shear Thickening Fluid-Based Sandwich Structures

The vibration attenuation mechanism of shear thickening fluid- (STF-) filled sandwich structures was investigated in this study. Structural equivalent damping, stiffness, and mass increased simultaneously with the increase in the volume fraction of shear thickening fluid. However, the damping ratio decreased and natural frequency increased with the increase in structural mass. Thus, the damping ratio was not a monotonically increasing function of the volume fraction of STF. A modified shear strain model of the damping layer was developed based on the following conditions: (1) under the condition of small strain, shear thickening fluid was regarded as linear viscoelastic material, and (2) the warpage of the sandwich beam was considered during deformation and the influence of STF on the shear strain of sandwich beam. According to the modified shear strain model of the damping layer, the shear thickening occurred at 1 Hz to 20 Hz during vibration. Therefore, the resonance point of the structure shifted to the left. The predictions were in excellent agreement with the experimental results. The results demonstrated that shear thickening fluid improved the vibration damping performance of the sandwich structure, while the thickening ability was not the higher, the better.

Unexpected Method of High-Viscosity Shear Thickening Fluids Based on Polypropylene Glycols Development via Thermal Treatment

This study aimed to analyze the influence of the thermal treatment of shear thickening fluids, STFs, on their viscosity. For this purpose, shear thickening fluids based on polypropylene glycols PPG400 and PPG1000 and Aerosil^®200 were developed. The shear thickening behavior of obtained fluids was confirmed by using a parallel-plate rheometer. Next, thermogravimetric (TG) analyses were used to characterized thermal stability and weight loss of the STFs at a constant temperature. Finally, the thermal treatment of the STFs obtained was provided using the apparatus developed for this purpose. The received STFs exhibited a very high maximum viscosity up to 15 kPa. The rheology of the STFs measured after thermal treatment indicated that the proposed method allowed the development of STFs with a very high maximum viscosity. The maximum viscosity of the STFs increased twofold when thermal treatment of the STFs at elevated temperature for 210 min was performed. TG confirmed the convergence of the weight loss in the apparatus. Our results show that controlling the thermal treatment of STFs allows STFs to be obtained with high viscosity and a dilatation jump of the STFs by degradation of the liquid matrix.

Designing Stress-Adaptive Dense Suspensions Using Dynamic Covalent Chemistry

The non-Newtonian behaviors of dense suspensions are central to their use in technological and industrial applications and arise from a network of particle–particle contacts that dynamically adapt to imposed shear. Reported herein are studies aimed at exploring how dynamic covalent chemistry between particles and the polymeric solvent can be used to tailor such stress-adaptive contact networks, leading to their unusual rheological behaviors. Specifically, a room temperature dynamic thia-Michael bond is employed to rationally tune the equilibrium constant (K_eq) of the polymeric solvent to the particle interface. It is demonstrated that low K_eq leads to shear thinning, while high K_eq produces antithixotropy, a rare phenomenon where the viscosity increases with shearing time. It is proposed that an increase in K_eq increases the polymer graft density at the particle surface and that antithixotropy primarily arises from partial debonding of the polymeric graft/solvent from the particle surface and the formation of polymer bridges between particles. Thus, the implementation of dynamic covalent chemistry provides a new molecular handle with which to tailor the macroscopic rheology of suspensions by introducing programmable time dependence. These studies open the door to energy-absorbing materials that not only sense mechanical inputs and adjust their dissipation as a function of time or shear rate but also can switch between these two modalities on demand.

Linking intermolecular interactions and rheological behaviour in capillary suspensions

HYPOTHESIS

Capillary suspensions feature networks of particles connected by liquid bridges, which are obtained by adding a small amount of a second immiscible liquid to a suspension. It is possible to link the network formation as well as the rheological behaviour of capillary suspensions to the intermolecular interactions of their constituents.

EXPERIMENTS AND SIMULATIONS

Through a combination of experimental and numerical methods, we present a novel approach, based on Hansen solubility parameters computed from Molecular Dynamics (MD) simulations, to rationalize and predict the rheological behaviour of capillary suspensions. We investigated the formation of capillary suspensions for various combinations of bulk and secondary liquids mixed with hydrophilic silica particles. The predictions were confirmed experimentally by rheological analysis, interfacial tension measurements and microscopy (CLSM) imaging.

FINDINGS

Numerical and experimental results show that the Hansen solubility parameters theory allows to predict the formation of capillary suspensions, whose strength exponentially decays with decreasing intermolecular interactions between the secondary liquids and the dispersed particles. High immiscibility between the bulk and secondary liquid strengthens the gel up to a critical immiscibility point, above which the strength of the gel remains mostly affected by the affinity between the secondary liquids and the dispersed particles. Furthermore, we find that hydrogen-bonding and polar interactions control the formation of capillary suspensions. This simple approach can guide the selection of adequate solvents and immiscible secondary liquids, allowing an easy formulation of new particulate-based gels.

In Situ Observation of Shear-Induced Jamming Front Propagation during Low-Velocity Impact in Polypropylene Glycol/Fumed Silica Shear Thickening Fluids

Shear jamming, a relatively new type of phase transition from discontinuous shear thickening into a solid-like state driven by shear in dense suspensions, has been shown to originate from frictional interactions between particles. However, not all dense suspensions shear jam. Dense fumed silica colloidal systems have wide applications in the industry of smart materials from body armor to dynamic dampers due to extremely low bulk density and high colloid stability. In this paper, we provide new evidence of shear jamming in polypropylene glycol/fumed silica suspensions using optical in situ speed recording during low-velocity impact and explain how it contributes to impact absorption. Flow rheology confirmed the presence of discontinuous shear thickening at all studied concentrations. Calculations of the flow during impact reveal that front propagation speed is 3–5 times higher than the speed of the impactor rod, which rules out jamming by densification, showing that the cause of the drastic impact absorption is the shear jamming. The main impact absorption begins when the jamming front reaches the boundary, creating a solid-like plug under the rod that confronts its movement. These results provide important insights into the impact absorption mechanism in fumed silica suspensions with a focus on shear jamming.

Flow-Switched Bistability in a Colloidal Gel with Non-Brownian Grains

We show that mixing a colloidal gel with larger, non-Brownian grains generates novel flow-switched bistability. Using a combination of confocal microscopy and rheology, we find that prolonged moderate shear results in liquefaction by collapsing the gel into disjoint globules, whereas fast shear gives rise to a yield-stress gel with granular inclusions upon flow cessation. We map out the state diagram of this new "mechanorheological material" with varying granular content and demonstrate that its behavior is also found in separate mixture using different particles and solvents.

Fumed Silica-Based Suspensions for Shear Thickening Applications: A Full-Scale Rheological Study

Understanding shear thickening fluids (STFs) is critically important in a broad spectrum of fields ranging from biology to military. STFs are referred to the suspension of solid particles in an inert carrier liquid. Customizing the thickening behavior is vital for obtaining desired properties. Hence, comprehending shear thickening mechanisms is necessary to fully understand the factors affecting the shear thickening response of the STFs. Herein, we systematically investigate the effects of a wide range of parameters, from inherent properties of the constituents, including size and surface chemistry of the suspended particles, to practical conditions such as temperature and shear history, on the shear thickening behavior of fumed silica nanoparticles (NPs)-based suspensions in a polyethylene glycol (PEG) medium. Accordingly, increasing the hydrophobicity of the silica NPs or decreasing the NP size transforms the suspensions from sol to gel. The sol systems exhibit a strong shear thickening response, while shear thinning behavior is prominent in the strong gel systems. Hybridization of different silica NPs is also leveraged to tune the shear thickening behavior. In addition, we showcase the decisive role of operating temperature or shear history on the shear thickening behavior of suspensions. For instance, in terms of the shear history, above a critical value of preshear, the shear thickening behavior occurs at lower shear rates for STFs containing hydrophilic NPs. It is believed that the provided insights in this study can pave the way for developing advanced STFs with prescribed features.

Rheological and Technological Aspects in Designing the Properties of Shear Thickening Fluids

This work focuses on shear thickening fluids (STFs) as ceramic-polymer composites with outstanding protective properties. The investigation aims to determine the influence of raw material parameters on the functional properties of STFs. The following analyses were used to characterize both the raw materials and the STFs: scanning electron microscopy, dynamic light scattering, matrix-assisted laser desorption/ionization time-of-flight, chemical sorption analysis, rheological analysis, and kinetic energy dissipation tests. It was confirmed that the morphology of the solid particles plays a key role in designing the rheological and protective properties of STFs. In the case of irregular silica, shear thickening properties can be obtained from a solid content of 12.5 vol.%. For spherical silica, the limit for achieving shear thickening behavior is 40 vol.%. The viscosity curve analysis allowed for the introduction of a new parameter defining the functional properties of STFs: the technological critical shear rate. The ability of STFs to dissipate kinetic energy was determined using a unique device that allows pure fluids to be tested without prior encapsulation. Because of this, it was possible to observe even slight differences in the protective properties between different STFs, which has not been possible so far. During tests with an energy of 50 J, the dissipation factor was over 96%.

Supramolecular assembly inspired molecular engineering to dynamically tune non-Newtonian fluid:from quasi-static flowability-free to shear thickening

Shear thickening fluids (STFs) have been the research focus for decades because of the prospect as a damping ingredient. However, their inherent liquid character confines their practical applications. In this work, inspired by the assembly engineering, novel gelatinous shear thickening fluids (GSTFs) are fabricated by integrating low molecular weight gelators (LMWGs) into STFs and investigated by rheological experiments. The results show that the apparent performances of GSTFs are determined by the LMWGs content. LMWGs inside GSTFs can assemble into three-dimensional network that can constraint the flowability of liquid molecular and their content dominate the density and strength of assembly network. At a moderate content, GSTFs exhibit desired properties with restricted quasi-static flowability and almost undamaged dynamic shear thickening character. While a higher content will disappear shear thickening and a lower content cannot gelate STFs. Besides, three different LMWGs are employed to gelate STFs and all they can gelate STFs in spite of the distinct minimum gelation concentration, indicating the universality for GSTFs preparation and the superiority of a reasonable molecular structure of LMWGs. Further, the temperature sweep experiments suggest that GSTFs can endure higher temperature without flowing due to its higher gel-sol transition temperature. Basing on these advanced mechanical properties, we believe that the GSTFs with more expected characters have significance for the study of non-Newtonian fluids and will broaden the special application field of STFs.

Rheological and Ionic Transport Properties of Nanocomposite Electrolytes Based on Protic Ionic Liquids and Silica Nanoparticles

In this study, the effect of hydrophilic silica nanoparticle (AEROSIL 200) addition on the rheological and transport properties of several protic ionic liquids (PILs) consisting of protonated 1,8-diazabicyclo[5.4.0]undec-7-ene cation (DBU) was studied. Interactions between the surface silanol groups of the silica nanoparticles and the ions of these PILs affected the nature of particle aggregation and the hydrogen bonding environment, which was reflected in the nonlinear rheological behaviors and transport properties of their colloidal suspensions. In contrast to shear-thinning gels formed by colloidal suspensions of the silica nanoparticles in [DBU][TFSA] ([TFSA] = [N(SO₂CF₃)₂]), [DBU][TfO] ([TfO] = [CF₃SO₃]), and [DBU][TFA] ([TFA] = [CF₃CO₂]), a shear-thickening stable suspension was formed in the [DBU][MSA] ([MSA] = [CH₃SO₃]) system. A relatively strong interaction between the silanol groups and the ions of [DBU][MSA] and the ability of this PIL to form a thicker solvation layer through hydrogen bonding were assumed to be responsible for this unique behavior. Moreover, the [DBU][MSA]-silica system showed a large enhancement in the conductivity at a certain silica concentration. This enhancement was not observed in the other PIL-silica composites that exhibited shear-thinning behavior. Even though diffusion of ions was found to be restricted in the presence of silica, a preferentially stronger interaction between [MSA] anions and the silica surface resulted in an increase in the number of charge carriers.

A review of fibrous materials for soft body armour applications

A critical review on the factors affecting the impact resistance and various approaches adopted to enhance the performance of soft body armour materials is presented here.

Shear-thickening fluids in biologically relevant agents

BACKGROUND

The rheology of shear thickening fluids is well characterized for many physical applications, however the literature surrounding biologically or cryobiologically compatible shear thickening fluids is less well understood.

OBJECTIVE

This study examined fluids consisting of corn-derived hydroxyethyl starch with a variety of sugars and cryoprotectants to characterize their shear-rate viscosity relationship. The objective was to establish if cryobiologically relevant materials could be used to afford biologics protection through shear-thickening.

RESULTS

Fluids consisting of 50% hydroxyethyl starch by weight exhibited shear thickening with a variety of cryoprotectants. Lowering the temperature of the fluid both reduced critical shear rates and enhanced thickening magnitude. Starch derived from corn, wheat, and rice all exhibited non-Newtonian shear-dependent viscosity behaviour at 50% by weight in water. Between the starch sources however, the shear-rate viscosity relationship varied widely, with wheat-derived starch shear thinning, and the remaining starches forming shear thickening fluids. Different starch sources had different baseline viscosities, critical shear rates, and rates of viscosity increase.

CONCLUSIONS

This study established that shear thickening is compatible with cryobiologically relevant agents, particularly so at lower temperatures. This forms the basis for harnessing these phenomena in biological processes such as cryopreservation.

A general constitutive model for dense, fine-particle suspensions validated in many geometries

Fine-particle suspensions (such as cornstarch mixed with water) exhibit dramatic changes in viscosity when sheared, producing fascinating behaviors that captivate children and rheologists alike. Examination of these mixtures in simple flow geometries suggests intergranular repulsion and its influence on the frictional nature of granular contacts is central to this effect-for mixtures at rest or shearing slowly, repulsion prevents frictional contacts from forming between particles, whereas when sheared more forcefully, granular stresses overcome the repulsion allowing particles to interact frictionally and form microscopic structures that resist flow. Previous constitutive studies of these mixtures have focused on particular cases, typically limited to 2D, steady, simple shearing flows. In this work, we introduce a predictive and general, 3D continuum model for this material, using mixture theory to couple the fluid and particle phases. Playing a central role in the model, we introduce a microstructural state variable, whose evolution is deduced from small-scale physical arguments and checked with existing data. Our space- and time-dependent model is implemented numerically in a variety of unsteady, nonuniform flow configurations where it is shown to accurately capture a variety of key behaviors: 1) the continuous shear-thickening (CST) and discontinuous shear-thickening (DST) behavior observed in steady flows, 2) the time-dependent propagation of "shear jamming fronts," 3) the time-dependent propagation of "impact-activated jamming fronts," and 4) the non-Newtonian, "running on oobleck" effect, wherein fast locomotors stay afloat while slow ones sink.

Enhanced cluster order-disorder transition-induced dilatancy in silane-functionalized nanosilica colloids

Herein, we report a novel nanosilica-based shear-thickening fluid, whose shear-thickening performance has been largely augmented by surface functionalizing silica employing silane chains. The functionalized shear-thickening colloids were transparent; this suggested that they have promise for application. An enhancement in viscosity was observed by over an order of magnitude by the usage of functionalized particles, which could be explained on the basis of enhanced hydroclustering and an order-to-disorder transition of the particles due to physical bonding of the silane with the base polymer. It was also observed that the shear-thickening behavior was grossly modified due to the presence of the functionalized nanoparticles. Oscillatory analysis showed that the functionalized colloids exhibited an improved dynamic response, with enhanced elastic behavior under variant strain and frequency conditions. Additionally, impact resistance tests revealed that the thickening of the viscosity upon impact was augmented by over an order of magnitude; this established these functionalized colloids as excellent candidates for liquid armors. The viscoelastic behavior was modeled based on the Cox-Merz formalism. Additionally, three-element viscoelastic modeling was performed, and it was observed that while the silica-based colloids conformed to the predominantly viscous model, the functionalized system transited to a predominantly elastic model. The present article can have important implications for the design and engineering of shear-thickening fluids employing nanomaterials.

Controlling the shear thickening behavior of suspensions by changing the surface properties of dispersed microspheres

To investigate the effect of the surface properties of dispersed particles on the shear thickening behavior of their corresponding suspensions and further control this characteristic, three kinds of suspensions were prepared by mixing SiO₂, SiO₂-NH₂, and SiO₂-COOH microspheres with a poly(ethylene glycol) fluid medium, and their rheological behavior was analyzed carefully. Compared to the SiO₂ microsphere suspension, the SiO₂-NH₂ and SiO₂-COOH microsphere suspensions show a weaker thickening behavior and a greater critical shear rate due to the aggregation tendency caused primarily by the organic chains. Moreover, the rheological behavior of the three suspensions display different dependencies on the pH value, which is comprehensively determined by the interaction between the microspheres and the medium. Moreover, the critical shear stress of suspensions with different pH values could be predicted by the Wagner model, which basically proves that the interaction between the particles significantly influences the beginning of thickening. The thickening degree could be interpreted by friction theory. The critical volume fraction corresponding to the onset of discontinuous shear thickening is determined by the friction coefficient between the particles, which is greatly affected by the pH value.

Shear-Thickening Fluid Using Oxygen-Plasma-Modified Multi-Walled Carbon Nanotubes to Improve the Quasi-Static Stab Resistance of Kevlar Fabrics

The excellent mechanical property and light weight of protective materials are vital for practical application in body armor. In this study, O₂-plasma-modified multi-walled carbon nanotubes (M-MWNTs) were introduced into shear-thickening fluid (STF)-impregnated Kevlar woven fabrics to increase the quasi-static stab resistance and decrease the composite weight. The rheological test showed that the addition of 0.06 wt. % M-MWNT caused a marked increase in the peak viscosity from 1563 to 3417 pa·s and a decrease in the critical shear rate from 14.68 s-1 to 2.53 s-1. The storage modulus (G') and loss modulus (G″) showed a higher degree of abrupt increase with the increase of shear stress. The yarn pull-out test showed that the yarn friction of M-MWNT/STF/Kevlar fabrics was far superior to the original fabrics. Importantly, under similar areal density, the M-MWNT/STF/Kevlar fabrics could resist 1261.4 N quasi-static stab force and absorb 41.3 J energy, which were much higher than neat Kevlar fabrics. The results of this research indicated that quasi-static stab resistance was improved by M-MWNTs, which was attributed to the excellent shear-thickening effect and the high yarn friction. Therefore, M-MWNT/STF/Kevlar fabrics have a broad prospect in the fields of body protection.

Interparticle hydrogen bonding can elicit shear jamming in dense suspensions

Dense suspensions of hard particles in a liquid can exhibit strikingly counter-intuitive behaviour, such as discontinuous shear thickening (DST)^1-7 and reversible shear jamming (SJ) into a state where flow is arrested and the suspension is solid-like^8-12. A stress-activated crossover from hydrodynamic interactions to frictional particle contacts is key for these behaviours^2-4,6,7,9,13. However, in experiments, many suspensions show only DST, not SJ. Here we show that particle surface chemistry plays a central role in creating conditions that make SJ readily observable. We find the system's ability to form interparticle hydrogen bonds when sheared into contact elicits SJ. We demonstrate this with charge-stabilized polymer microspheres and non-spherical cornstarch particles, controlling hydrogen bond formation with solvents. The propensity for SJ is quantified by tensile tests¹² and linked to an enhanced friction by atomic force microscopy. Our results extend the fundamental understanding of the SJ mechanism and open avenues for designing strongly non-Newtonian fluids.

Effect of addition of silicone oil on the rheology of fumed silica and polyethylene glycol shear thickening suspension

Shear thickening fluids (STF) are stabilized and concentrated colloidal suspensions of hard nano-particles in a liquid medium (polymer) that, under the influence of impact forces, show non-Newtonian fluid behavior (shear thickening) dissipating the energy of impact. The viscosity of the dispersion medium should be optimum to lead to an increase in shear thickening, and at the same time, should also allow proper dispersion of the particles. Herein, an STF based on 20 wt% fractal nano-fumed silica particles of 11 nm suspended in a liquid medium of polyethylene glycol (PEG 200) with different concentrations of silicone oil was prepared. These systems were studied in terms of steady-state and dynamic-state rheological behavior under a wide range of temperature, shear rate, strain rate and frequency. The STF with replacement of up to only 20% of PEG with silicone oil as the liquid medium shows a large increase (about four times) in shear thickening parameters when compared with STF containing only PEG under the same processing conditions. It also shows more elastic behavior at high frequencies which are due to the high cross-linking property of silicone oil, contributing to much-improved properties, which are highly desirable from the view point of many applications.

Anisotropic Nanoparticles Contributing to Shear-Thickening Behavior of Fumed Silica Suspensions

Rheological characteristics of a concentrated suspension can be tuned using anisotropic particles having various shapes and sizes. Here, the role of anisotropic nanoparticles, such as surface-functionalized multiwall carbon nanotubes (MWNTs) and graphene oxide nanoplatelets (GONPs), on the rheological behavior of fumed silica suspensions in poly(ethylene glycol) (PEG) is investigated. In these mixed-particle suspensions, the concentrations of MWNTs and GONPs are much lower than the fumed silica concentration. The suspensions are stable, and hydrogen-bonded PEG solvation layers around the particles inhibit their flocculation. Fumed silica suspensions over the concentration range considered here display shear-thickening behavior. However, for a larger concentration of MWNTs and with increasing aspect ratios, the shear-thickening behavior diminishes. In contrast, a distinct shear-thickening response has been observed for the GONP-containing suspensions for similar mass fractions (MFs) of MWNTs. For these suspensions, shear thickening is achieved at a lower solid MFs compared to the suspensions consisting of only fumed silica. A significant weight reduction of shear-thickening fluids that can be achieved by this approach is beneficial for many applications. Our results provide guiding principles for controlling the rheological behavior of mixed-particle systems relevant in many fields.

Fumed and Precipitated Hydrophilic Silica Suspension Gels in Mineral Oil: Stability and Rheological Properties

Hydrophilic fumed silica (FS) and precipitated silica (PS) powders were suspended in mineral oil; increasing the silica volume fraction (φ in the suspension led to the formation of sol, pre-gel, and gel states. Gelation took place at lower φ values in the FS than the PS suspension because of the lower silanol density on the FS surface. The shear stresses and dynamic moduli of the FS and PS suspensions were measured as a function of φ. Plots of the apparent shear viscosity against shear rate depended on φ and the silica powder. The FS suspensions in the gel state exhibited shear thinning, followed by a weak shear thickening or by constant viscosity with an increasing shear rate. In contrast, the PS suspensions in the gel state showed shear thinning, irrespective of φ. The dynamic moduli of the pre-gel and gel states were dependent on the surface silanol density: at a fixed φ, the storage modulus G′ in the linear viscoelasticity region was larger for the FS than for the PS suspension. Beyond the linear region, the G′ of the PS suspensions showed strain hardening and the loss modulus G″ of the FS and PS suspensions exhibited weak strain overshoot.

Contribution of Surface Chemistry to the Shear Thickening of Silica Nanoparticle Suspensions

Shear thickening is a general process crucial for many processed products ranging from food and personal care to pharmaceuticals. Theoretical calculations and mathematical simulations of hydrodynamic interactions and granular-like contacts have proved that contact forces between suspended particles dominate the rheological characteristic of colloidal suspensions. However, relevant experimental studies are very rare. This study was conducted to reveal the influence of nanoparticle (NP) interactions on the rheological behavior of shear-thickening fluids (STFs) by changing the colloidal surface chemistries. Silica NPs with various surface chemical compositions are fabricated and used to prepare dense suspensions. Rheological experiments are conducted to determine the influence of NP interactions on corresponding dense suspension systems. The results suggest that the surface chemistries of silica NPs determine the rheological behavior of dense suspensions, including shear-thickening behavior, onset stress, critical volume fraction, and jamming volume fraction. This study provides useful reference for designing effective STFs and regulating their characteristics.

A facile one-step method to synthesize SiO2@polydopamine core–shell nanospheres for shear thickening fluid

An illustration of the synthesis of SiO₂@PDA core/shell nanospheres, in which the coating on the surface of the SiO₂nanospheres improves the rheological behavior of the resulting STFs.

Effects of ionic structures on shear thickening fluids composed of ionic liquids and silica nanoparticles

Shear thickening fluids (STFs) are energy dissipative materials and affected significantly by the properties of the dispersing media.