Colloidal Shear-Thickening Fluids Using Variable Functional Star-Shaped Particles: A Molecular Dynamics Study

Complex colloidal fluids, depending on constituent shapes and packing fractions, may have a wide range of shear-thinning and/or shear-thickening behaviors. An interesting way to transition between different types of such behavior is by infusing complex functional particles that can be manufactured using modern techniques such as 3D printing. In this paper, we perform 2D molecular dynamics simulations of such fluids with infused star-shaped functional particles, with a variable leg length and number of legs, as they are infused in a non-interacting fluid. We vary the packing fraction (ϕ) of the system, and for each different system, we apply shear at various strain rates, turning the fluid into a shear-thickened fluid and then, in jammed state, rising the apparent viscosity of the fluid and incipient stresses. We demonstrate the dependence of viscosity on the functional particles’ packing fraction and we show the role of shape and design dependence of the functional particles towards the transition to a shear-thickening fluid.

Switchable electrorheological activity of polyacrylonitrile microspheres by thermal treatment: from negative to positive

The study focuses on the effect of thermal deformation degree of polyacrylonitrile (PAN) particles on the electrorheological (ER) properties of their suspensions. The heat-treated PAN particles are manufactured as ER materials using a thermocatalytic processes. The molecular structures of ER materials are analyzed to confirm a stabilization or a carbonization degree. We categorized the prepared ER particles into three basic types according to their deformation degree: Thermal dried PAN, stabilized PAN, and pre-carbonized PAN. The raw, stabilized, and pre-carbonized PAN particle-dispersed suspensions showed positive ER properties. The ER properties are enhanced as the heat-treatment temperature increases due to improved dielectric property. However, the thermal dried PAN particle ER suspensions showed negative ER behavior though the particles have higher conductivity and dielectric constants than those of the host fluid, which is contrary to the conduction model. XRD results indicate that the ER materials could show contradictory ER behavior even if they have the same molecular structures due to their crystallinity. This discovery is expected to boost the development of both positive ER and negative ER suspensions based on carbonaceous ER materials.

Electrohydrodynamic fibrillation governed enhanced thermal transport in dielectric colloids under a field stimulus

Electrorheological (ER) fluids are known to exhibit enhanced viscous effects under an electric field stimulus. The present article reports the hitherto unreported phenomenon of greatly enhanced thermal conductivity in such electro-active colloidal dispersions in the presence of an externally applied electric field. Typical ER fluids are synthesized employing dielectric fluids and nanoparticles and experiments are performed employing an in-house designed setup. Greatly augmented thermal conductivity under a field's influence was observed. Enhanced thermal conduction along the fibril structures under the field effect is theorized as the crux of the mechanism. The formation of fibril structures has also been experimentally verified employing microscopy. Based on classical models for ER fluids, a mathematical formalism has been developed to predict the propensity of chain formation and statistically feasible chain dynamics at given Mason numbers. Further, a thermal resistance network model is employed to computationally predict the enhanced thermal conduction across the fibrillary colloid microstructure. Good agreement between the mathematical model and the experimental observations is achieved. The domineering role of thermal conductivity over relative permittivity has been shown by proposing a modified Hashin-Shtrikman (HS) formalism. The findings have implications towards better physical understanding and design of ER fluids from both 'smart' viscoelastic as well as thermally active materials points of view.

Large electrorheological phenomena in graphene nano-gels

Large-scale electrorheology (ER) response has been reported for dilute graphene nanoflake-based ER fluids that have been engineered as novel, readily synthesizable polymeric gels. Polyethylene glycol (PEG 400) based graphene gels have been synthesized and a very high ER response (∼125 000% enhancement in viscosity under influence of an electric field) has been observed for low concentration systems (∼2 wt.%). The gels overcome several drawbacks innate to ER fluids. The gels exhibit long term stability, a high graphene packing ratio which ensures very high ER response, and the microstructure of the gels ensures that fibrillation of the graphene nanoflakes under an electric field is undisturbed by thermal fluctuations, further leading to mega ER. The gels exhibit a large yield stress handling caliber with a yield stress observed as high as ∼13 kPa at 2 wt.% for graphene. Detailed investigations on the effects of graphene concentration, electric field strength, imposed shear resistance, transients of electric field actuation on the ER response and ER hysteresis of the gels have been performed. In-depth analyses with explanations have been provided for the observations and effects, such as inter flake lubrication/slip induced augmented ER response. The present gels show great promise as potential ER gels for various smart applications.

Research and Development of Functional Fluid Mechatronics, Rehabilitation Systems, and Mechatronics of Flexible Drive Systems

[abstFig src='/00280001/01.jpg' width=""260"" text='PLEMO-P3 Developed by Furusho Lab at Osaka Univ.' ]We conducted many research and development activities on functional fluid mechatronics, rehabilitation systems, and servo drive systems. In this review, studies on the development of magnetorheological fluid devices, electrorheological effects of liquid crystalline polymers on one-sided pattern electrodes, and vibration control using control theory and liquid crystalline polymer are introduced. In addition, applications of rehabilitation systems for upper and lower extremities employing functional fluids for individuals suffering from stroke, cerebellar ataxia, and Guillain-Barre syndrome are also introduced.

Enhanced Electrorheological Properties of Elastomers Containing TiO₂/Urea Core-Shell Particles

Polar molecule-coated core-shell particles have been used to prepare electrorheological (ER) fluids with high performance. Inspired by those studies, TiO2/urea core-shell structured particles were fabricated and used to prepare novel ER elastomers, whose properties were compared with the ER elastomers with bare TiO2 particles. Particles characterization results illustrate the TiO2/urea particles present little change in size, morphology and crystal structure with respect to the bare amorphous TiO2 particles, while clear core-shell structure is observed. Compared with the bare TiO2 particles filled elastomer, the TiO2/urea particles filled elastomer presents higher dielectric constant, indicating enhanced polarization. The viscoelastic properties of the two elastomers under different strain amplitude, frequency and electric field were tested. The results indicate that the TiO2/urea particles filled elastomer shows higher storage modulus G' and higher relative ER effect within the low field strength region from 0 to 2 kV/mm. Coating polar molecules is an effective method to improve the ER performance for ER elastomers.

Dielectric relaxation in concentrated nonaqueous colloidal suspensions

In this work we report on the permittivity of suspensions of elongated goethite particles in silicone oils of different viscosities. In spite of the low conductivity of the systems, the electrode polarization is significant. To correct this phenomenon, the procedure chosen is the one called logarithmic derivative of the real part of the permittivity, and it proves to efficiently reduce the effect of electrodes to the extent that the spectra of pure liquids are flat in the accessible frequency range (20 Hz-1 MHz). In our suspensions, we observe the presence of a dielectric relaxation for frequencies in the range 4-40 kHz. In principle, such relaxations might be ascribed to the Maxwell-Wagner (MW) polarization. However, it is found that both the characteristic frequency and the relaxation amplitude of the suspensions increase with volume fraction, something unexpected for an MW relaxation. Such discrepancy can be explained by considering the Frenkel-Trukhan model, which reproduces the Maxwell-Wagner results in conditions of thin electrical double layers (which it is not our case). An excellent agreement is found between our data and the model predictions, using only the particle surface charge as a parameter.

Electro-osmosis of electrorheological fluids

Electrorheological fluids are suspensions that are characterized by a strong functional dependence of the constitutive behavior of the fluids on the electric field. In this work, we consider electro-osmosis of an electrorheological fluid through a channel where a transverse, nonuniform electric field is spontaneously induced due to the presence of an electric double layer that is manifested due to surface charge density at the channel wall. We reveal a nonlinear interplay between the applied electric field, the induced electric field, and the observed flow profiles, which is fundamentally distinctive from other types of nonlinear electrokinetic effects that have been extensively discussed in the literature, in a sense that here an interaction between the applied electric field, the induced electric field, and the dependence of the rheology on the resultant electric field happens to be the focal source of nonlinearity in the observed phenomena. We analyze the electro-osmotic flow control through the exploitation of a combined nonlinear interplay of the driving electrokinetic forces and the resistive viscous interactions, which gives rise to distinctive flow regimes as compared to those realized in cases of either Newtonian fluids or non-Newtonian fluids having electric-field-independent flow rheology.

Silica-graphene oxide hybrid composite particles and their electroresponsive characteristics

Silica-graphene oxide (Si-GO) hybrid composite particles were prepared by the hydrolysis of tetraethyl orthosilicate (TEOS) in the presence of hydrophilic GO obtained from a modified Hummers method. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images provided visible evidence of the silica nanoparticles grafted on the surface of GO, resulting in Si-GO hybrid composite particles. Energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) spectra indicated the coexistence of silica and GO in the composite particles. The Si-GO hybrid composite particles showed better thermal stability than that of GO according to thermogravimetric analysis (TGA). The electrorheological (ER) characteristics of the Si-GO hybrid composite based ER fluid were examined further by optical microscopy and a rotational rheometer in controlled shear rate mode under various electric field strengths. Shear stress curves were fitted using both conventional Bingham model and a constitutive Cho-Choi-Jhon model. The polarizability and relaxation time of the ER fluid from dielectric spectra measured using an LCR meter showed a good correlation with its ER characteristics.

Phosphorylation of Potato Starch and Its Electrorheological Suspension

Highly substituted potato starch phosphate (HPSP) particles were synthesized via an esterification process of potato starch with a mixture of several different concentrations of disodium hydrogen phosphate and sodium hydrogen phosphate. These particles were characterized via thermogravimetric analysis, scanning electron microscope, and inductively coupled plasma mass spectrometer. The electrorheological (ER) fluid was prepared by dispersing these HPSP particles in nonconducting silicone oil, and their ER properties were investigated. The HPSP particle-based ER fluid exhibited typical ER responses with a nonzero yield stress under an applied electric field. We examined the yield stresses for the potato starch-based anhydrous ER system by varying the degree of phosphate substitution and found that the higher polarization induced by the external electric field strength resulted in the higher values of yield stresses and shear stresses.

Electrorheological properties and microstructure of silica suspensions

We present experimental and theoretical results on the electrorheological response and microstructure of colloidal suspensions composed of silica nanoparticles dispersed in a silicon oil, as a function of electric field strength and silica water content. Using small-angle neutrons scattering experiments, we determined the evolution of the static structure factor of the suspensions when an electric field is applied. Experimental data were fitted with model calculations using the Percus-Yevick solution for Baxter's hard-sphere adhesive potential. The obtained stickiness parameter is directly related to the polarization interactions that depend on the water content of silica particles. The influence of the polarization interparticle potential on the rheology of the silica dispersions was investigated in a second time. A microscopic theory for the shear viscosity of adhesive hard-sphere suspensions was successfully used which describes the steady shear viscosity of suspension in terms of the fractal concept.

Electrorheological suspensions

The objective of this article is to give a review of electrorheological (ER) suspensions whose rheological properties can abruptly change under an external electric field. Attention is given to the physical backgrounds behind ER phenomena reported recently. The criteria on how to design a high performance ER fluid and mechanisms explaining how an ER suspension displays the ER effect are focused upon. We begin with a brief historic introduction, ER materials, followed by positive ER effect, negative ER effect and photo-ER effect discussions. The physical parameters that can substantially affect the ER effect are discussed thereafter, and physical processes occurring in ER suspensions under an electric field are reviewed. The mechanisms of the ER effect proposed before are summarized. A future outlook on the ER material development and ER fluid applications is given.

A Surfactant Bridge Model for the Nonlinear Electrorheological Effects of Surfactant-Activated ER Suspensions

In surfactant-activated electrorheological (ER) suspensions it is observed that the ER response shows linear ER behavior (F~E(2)) at small surfactant concentrations and nonlinear ER behavior (F~E(n), n approximately 1) at large surfactant concentrations. Here, a surfactant bridge model is developed to explain the nonlinear ER behavior of surfactant-activated ER suspensions. The model shows that the formation and size of a surfactant bridge depend on various variables, especially the electric field strength, the surfactant surface tension, and the initially adsorbed amount of surfactants on particles. The predicted dependence of the formation and size of a surfactant bridge on the electric field strength and the initially adsorbed amount of surfactants is consistent with the observations. Also, the model indicates that there is a critical minimum electric field E(crit) for the formation of a surfactant bridge, and the estimated E(crit) shows good agreement with the observations. The force acting between particles is composed of the electrostatic force and force associated with surface tension. However, it is found that the contribution of the force associated with surface tension can be ignored and the electrostatic force is dominant regardless of the formation of surfactant bridges between particles. When surfactant bridges are formed between particles, the predicted force shows nonlinear ER behavior (F~E(n), n approximately 1), consistent with the observed nonlinear ER behavior at large surfactant concentrations. When no surfactant bridge is formed, the predicted force is proportional to the electric field squared (F~E(2)), consistent with the interfacial polarization. The model can successfully predict the nonlinear ER behavior at large surfactant concentrations, confirming that the nonlinear ER behavior of surfactant-activated ER suspensions arises from the observed formation of surfactant bridges between particles. Copyright 2001 Academic Press.

Phase diagrams of electric-field-induced aggregation in conducting colloidal suspensions

To develop a theory for electric-field-driven phase transitions in concentrated suspensions, we extended our microscopic theory [Phys. Rev. E 52, 1669, (1995); 54, 5428, (1996)] beyond the dilute regime. Based on the model of the Maxwell-Wagner interfacial polarization of colloids, our theory overcomes the limitations of Brillouin's formula for the electric energy of conducting materials which is applicable only for negligibly small energy dissipation and slow time variations of the field. We found that the phase diagrams of "the particle concentration vs the electric field strength" for colloids are similar to the phase diagrams for the first-order phase separation in quenched conventional binary systems with a high-temperature miscibility gap. This explains why a variety of colloids exhibit similar field-induced aggregation patterns. Our theory provides a reasonable interpretation of the available experimental data on field-induced aggregation phenomena in electrorheological fluids and aqueous suspensions, whereas currently used theoretical models are in variance with many of the data. The theoretical results enable one to trace how the variations of the electrical properties of the constituent materials influence the topology of the suspension phase diagram and to evaluate the effects of the field strength and frequency on the particle aggregation.