Recent development of electro-responsive smart electrorheological fluids

Vivid-Colored Electrorheological fluids with simultaneous enhancements in color clarity and Electro-Responsivity

HYPOTHESIS

Surface modification of dielectric materials changes the dipole-dipole interactions under electric fields, thereby controlling the electrorheological (ER) response. The introduction of metal oxides onto mica templates and further coating of dyes is expected to simultaneously improve the color clarity and ER performance.

EXPERIMENTS

Dye-coated TiO2 platelets on mica are synthesized for high-performance colorful ER fluids. A sol-gel method is utilized to grow TiO2 on mica to prepare precursor light-colored mica/TiO2 materials, which are coated with appropriate dyes to enhance the vividness as determined by the Commission Internationale de clairage L*a*b* color system. The color expression and color clarity improvement are explained via the light interference effect and the presence of chromophores.

FINDINGS

The uniform TiO2 layers can be obtained under low pH conditions with controlled nucleation kinetics. The addition of dyes to TiO2 increases the surface area and porosity of ER materials and introduces heteroatoms that act as positive factors. In practical ER applications, dye-coated TiO2-based ER fluids exhibit higher ER performances compared with the corresponding light-colored TiO2-based ER fluids. The vivid-colored ER fluids could provide an easy selection for a wide range of rheological systems requiring a specific magnitude of stress by confirming the color.

A Study on Enhanced Electrorheological Performance of Plate-like Materials via Percolation Gel-like Effect

The use of plate-like materials to induce a percolation gel-like effect in electrorheological (ER) fluids is sparsely documented. Hence, we dispersed plate-like materials, namely natural mica, synthetic mica, and glass, as well as their pulverized particles, in various concentrations in silicone oil to form ER fluids. Subsequently, the rheological properties of the fluids were evaluated and compared to identify the threshold concentration for percolating a gel-like state. The shear stress and viscoelastic moduli under zero-field conditions confirmed that plate-like materials can be used to induce percolation gel-like effects in ER fluids. This is because of the high aspect ratio of the materials, which enhances their physical stability. In practical ER investigations, ER fluids based on synthetic mica (30.0 wt%) showed the highest yield stress of 516.2 Pa under an electric field strength of 3.0 kV mm-1. This was attributed to the formation of large-cluster networks and additional polarization induced by the ions. This study provides a practical approach for developing a new type of gel-like ER fluid.

A Feedback Control Sensing System of an Electrorheological Brake to Exert a Constant Pressing Force on an Object

The paper presents the application of a strain gauge sensor and a viscous brake filled with an electrorheological (ER) fluid, which is a smart material with controlled rheological properties, by an electric field to the fluid domain. For experimental tests, a cylindrical viscous brake was designed. The tests were carried out on a test stand especially prepared for this purpose and suitable for the examination of the impact of the rotational speed of the input shaft and the value of the electric voltage supplied to the viscous brake on pressing forces, taking into account the ER fluid temperature and brake fluid filling level. On the basis of the experimental research results, a viscous brake control system to exert constant pressing forces with feedback from a strain gauge sensor, based on the programmable logic controller, was designed and implemented. This system, using its own control algorithm, ensured a control pressing force within the assumed range, both during the constant and follow-up control. The measurement results obtained during the tests of the viscous brake designed to exert a force were presented in the form of time courses, showing the changes of the pressing force, the electric voltage applied to the brake and the rotational speed of the brake input shaft. The developed ER fluid brake control system with feedback was tested for constant and follow-up control, taking into account the impact of the working fluid temperature. During the test it was possible to obtain a maximum pressing force equal to 50 N for an electric voltage limited to 2.5 kV. The resultant error was lower than 1 N, wherein the adjustment time after changing the desired value of the force was around 1.5 s. The correct operation of both the brake and the control system, as well as the compatibility of the pressing force value and time adjustment, were determined. The main technical contribution described in this article is the design of a new type of DECPF and a new method for its control with the use of a specifically programmed programmable logic controller which simulates the proportional-integral controllers' operation.

CO2-responsive gels

CO₂-responsive materials undergo a change in chemical or physical properties in response to the introduction or removal of CO₂. The use of CO₂ as a stimulus is advantageous as it is abundant, benign, inexpensive, and it does not accumulate in a system. Many CO₂-responsive materials have already been explored including polymers, latexes, surfactants, and catalysts. As a sub-set of CO₂-responsive polymers, the study of CO₂-responsive gels (insoluble, cross-linked polymers) is a unique discipline due to the unique set of changes in the gels brought about by CO₂ such as swelling or a transformed morphology. In the past 15 years, CO₂-responsive gels and self-assembled gels have been investigated for a variety of emerging potential applications, reported in 90 peer-reviewed publications. The two most widely exploited properties include the control of flow (fluids) via CO₂-triggered aggregation and their capacity for reversible CO₂ absorption-desorption, leading to applications in Enhanced Oil Recovery (EOR) and CO₂ sequestration, respectively. In this paper, we review the preparation, properties, and applications of these CO₂-responsive gels, broadly classified by particle size as nanogels, microgels, aerogels, and macrogels. We have included a section on CO₂-induced self-assembled gels (including poly(ionic liquid) gels).

Enhanced Electrorheological Polishing Efficiency of Alumina-Doped Titanium Dioxide Particles

Electrorheological (ER) polishing is a novel polishing technology having flexible and tunable characteristics. At present, ER polishing uses ER particles to drive abrasive particles to polish the material surface. Under the action of high-speed centrifugation, the abrasive particles are easily separated from ER particles due to their significantly different ER effect, and this can easily cause the degradation of polishing ability. In this work, alumina-doped titanium dioxide ER polishing particles were developed via a sol-gel method. As a classical abrasive, alumina has higher hardness and can improve the ER effect of titanium dioxide by doping. Thus, alumina-doped titanium dioxide particles simultaneously possess high ER effect and high hardness. No phase separation appears in the polishing process and the result shows that alumina-doped titanium dioxide has a good polishing efficiency for materials with Mohs hardness of 3 and below.

Self-Reporting Joule Heating Modulated Stiffness of Polymeric Nanocomposites for Shape Reconfiguration

Shape reconfigurable devices, e.g., foldable phones, have emerged with the development of flexible electronics. But their rigid frames limit the feasible shapes for the devices. To achieve freely changeable shapes yet keep the rigidity of devices for user-friendly operations, stiffness-tunable materials are desired, especially under electrical control. However, current such systems are multilayer with at least a heater layer and a structural layer, leading to complex fabrication, high cost, and loss of reprocessability. Herein, we fabricate covalent adaptable networks-carbon nanotubes (CAN-CNT) composites to realize Joule heating controlled stiffness. The nanocomposites function as stiffness-tunable matrices, electric heaters, and softening sensors all by themselves. The self-reporting of softening is used to regulate the power control, and the sensing mechanism is investigated by simulating the CNT-polymer chain interactions at the nanoscale during the softening process. The nanocomposites not only have adjustable mechanical and thermodynamic properties but also are easy to fabricate at low cost and exhibit reprocessability and recyclability benefiting from the dynamic exchange reactions of CANs. Shape and stiffness control of flexible display systems are demonstrated with the nanocomposites as framing material, where freely reconfigurable shapes are realized to achieve convenient operation, wearing, or storage, fully exploiting their flexible potential.

Multiscale nature of electric-field-induced structural formations in non-colloidal suspensions

Non-colloidal suspensions undergoing dipolar interactions in an electric field have been extensively studied and are also known as smart materials as they share similarities with electrorheological (ER) fluids. Although the macroscopic responses are well-documented, the multiscale nature of such suspensions is still lacking. In this study, a large-scale Stokesian dynamics simulation is used to investigate the structural formation of such suspensions in an electric field up to highly concentrated regimes across different length scales: from particle-level (microscale) to particle cluster-level (mesoscale) and stress response-level (macroscale). It is observed that at a volume fraction of ϕ ≈ 30%, the steady-state structures are the most isotropic at the microscale, but at the macroscale, their normal stress fields are the most anisotropic. Interestingly, these structures are also the most heterogeneous at both the microscale and mesoscale. Furthermore, the effects of confinement on the multiscale responses are explored, revealing that there could be a strong link between the mesoscale and macroscale. This multiscale nature can offer the potential for precisely controlling or designing ER fluids in practical applications.

Efficient and stable electrorheological fluids based on chestnut-like cobalt hydroxide coupled with surface-functionalized carbon dots

Intrinsically polarized electrorheological fluids (ERFs) have better thermal stability than ERFs with polar molecules, so they have a broader application prospect. However, the electrorheological efficiency of the common intrinsically polarized ERF is still lower than 1500, which is related to the poor wettability between polarized materials and the continuous phase. Carbon dots (CDs) exhibit good stability, semiconductor properties and low toxicity. We prepared biomimetic chestnut-like cobalt hydroxide coupled with surface-functionalized CD particles (Co(OH)₂@CDs) by a simple hydrothermal method. Then we prepared an ERF by mixing Co(OH)₂@CDs with silicone oil and studied the effect of CDs on its rheology and electrorheology properties. The synergistic effect of the lipophilic groups on the surface of CDs and the biomimetic chestnut-like structure makes Co(OH)₂@CDs exhibit good wettability with silicone oil, and the optimal zero-field viscosity of Co(OH)₂@CDs-ERF is only 0.46 Pa s (particle mass fraction of 40%). Exceptional electrorheological efficiency (about 10 000, shear rate 0.1 s^-1, 5 kV mm^-1) and dynamic shear stress stability of optimal Co(OH)₂@CDs-ERF can be attributed to the dielectric enhancement of the biomimetic chestnut-like structure coupled with the semiconductor properties of CDs. In addition, Co(OH)₂@CDs-ERF has excellent anti-settling performance, outstanding thermal stability and low current density.

Experimental Confirmation of a Controllable Transmission/Braking Element Consisting of a Functional Elastomer Pasted on a Winding Surface

Electoadhesive (EA) gel can be applied to various mechatronics devices because it provides more degrees of freedom in mechanical design than other functional materials. While case studies have revealed much about these devices, the knowledge attained is still not integrated. We assembled new structured and controllable transmission and braking elements by fixing EA gel on plain and/or winding surfaces. This paper summarizes the characteristics of these devices from experimental findings on the initial pressure on the EA gel surface. The results showed that it is important for the initial pressure to be stable at approximately 0.5–1 kPa.

Field-Induced Assembly and Propulsion of Colloids

Electric and magnetic fields have enabled both technological applications and fundamental discoveries in the areas of bottom-up material synthesis, dynamic phase transitions, and biophysics of living matter. Electric and magnetic fields are versatile external sources of energy that power the assembly and self-propulsion of colloidal particles. In this Invited Feature Article, we classify the mechanisms by which external fields impact the structure and dynamics in colloidal dispersions and augment their nonequilibrium behavior. The paper is purposely intended to highlight the similarities between electrically and magnetically actuated phenomena, providing a brief treatment of the origin of the two fields to understand the intrinsic analogies and differences. We survey the progress made in the static and dynamic assembly of colloids and the self-propulsion of active particles. Recent reports of assembly-driven propulsion and propulsion-driven assembly have blurred the conceptual boundaries and suggest an evolution in the research of nonequilibrium colloidal materials. We highlight the emergence of colloids powered by external fields as model systems to understand living matter and provide a perspective on future challenges in the area of field-induced colloidal phenomena.

Low-filled suspensions of α-chitin nanorods for electrorheological applications

Highly anisometric α-chitin nanoparticles isolated by TEMPO-oxidation were investigated as filler for electrorheological fluids. The dimensions of rod-like particles were determined by AFM and cryo-TEM methods. The rheological behavior of α-chitin nanoparticles in polydimethylsiloxane changes from viscous to elastic under electric field. The yield stress reaches about 220 Pa at 7 kV/mm for 1.0 wt% fluid. Despite the nanosize of particles, the suspensions sedimentation ratio was found to be low (~23%). The electrorheological behavior of the fluids was discussed in terms of the Mason numbers. The stability of fluids response under switching electric field was shown. The activation energy of polarization processes in suspensions was calculated as 58 ± 2 and 64 ± 1 kJ/mol for 0.5 and 1.0 wt% filler content from the impedance spectra. The high aspect ratio (~70) and dielectric permittivity result in high electrorheological activity of α-chitin suspensions at extremely low concentrations (≤1.0 wt%).

Enhanced response of titanium doped iron(ii) oxalate under electric field

Electrorheological (ER) fluid, containing polarized particles within an insulating liquid, represents a smart material, the mechanical properties of which can be altered mainly by an electric field.

Bio-Inspired Soft Grippers Based on Impactive Gripping

Grasping and manipulation are fundamental ways for many creatures to interact with their environments. Different morphologies and grasping methods of "grippers" are highly evolved to adapt to harsh survival conditions. For example, human hands and bird feet are composed of rigid frames and soft joints. Compared with human hands, some plants like Drosera do not have rigid frames, so they can bend at arbitrary points of the body to capture their prey. Furthermore, many muscular hydrostat animals and plant tendrils can implement more complex twisting motions in 3D space. Recently, inspired by the flexible grasping methods present in nature, increasingly more bio-inspired soft grippers have been fabricated with compliant and soft materials. Based on this, the present review focuses on the recent research progress of bio-inspired soft grippers based on impactive gripping. According to their types of movement and a classification model inspired by biological "grippers", soft grippers are classified into three types, namely, non-continuum bending-type grippers, continuum bending-type grippers, and continuum twisting-type grippers. An exhaustive and updated analysis of each type of gripper is provided. Moreover, this review offers an overview of the different stiffness-controllable strategies developed in recent years.

Core-Shell Structured Magnetite-Poly(diphenylamine) Microspheres and Their Tunable Dual Response under Magnetic and Electric Fields

Core-shell type poly(diphenylamine)-coated magnetite (Fe₃O₄-PDPA) microspheres were designed and adopted as a novel actively tunable smart material which is responsive under both electric and magnetic fields. Their morphology, chemical structure, crystalline structure, and thermal properties were characterized using scanning electron microscopy, transmission electron microscope, Fourier transform-infrared spectroscopy, X-ray diffraction, and a thermal gravimetric analyzer. Their magnetic and dielectric properties were determined using vibrating-sample magnetometer and an LCR meter, respectively. They were dispersed in silicone oil and their electrorheological (ER) and magnetorheological (MR) responses under the electric and magnetic fields, respectively, were examined. The formation of chain structure of Fe₃O₄-PDPA based E/MR fluid under the application of electric field or magnetic field was observed by an optical microscopy and the sedimentation stability was observed by a Turbiscan optical analyzer system. It was observed that the yield stress, ER efficiency, and leakage current density increased with an increase in the particle concentration, while the slope of the electric field-dependent yield stress decreased. Several models such as the Bingham model, Herschel-Bulkley model, and Cho-Choi-Jhon equations were used to describe the shear stress curves of the ER fluid; the curves fitted well. For the dielectric properties, the two types of ER fluids tested displayed the same relaxation time and distribution; however, the one with the higher concentration had a higher dielectric constant and polarizability. The Fe₃O₄-PDPA based MR fluid (10 vol %) exhibited typical MR properties. In addition, the Herschel-Bulkley model matched well with the shear stress curves under a magnetic field.

1D and 2D hybrid polymers based on zinc phenylphosphates: synthesis, characterization and applications in electroactive materials

The synthesis, structure and properties of three hybrid polymers based on zinc arylphosphates are described in this study. Zinc bis(diphenylphosphate) (ZnDPhP) was obtained as needle-like crystals containing hexagonally packed, homochiral 1 ∞[Zn(DPhP)2/2] helical chains. The XRD and DSC studies revealed that upon heating, ZnDPhP undergoes a reversible thermal transition at ca. 160 °C with expansion mainly perpendicular to its c-axis. Zinc phenylphosphate hydrate (ZnMPhP-H) formed plate-like particles with an average thickness of less than 1 μm and much thinner nanolayers with a basal spacing of 15.5 Å. ZnMPhP-H was easily and reversibly dehydrated to its anhydrous form, ZnMPhP-A, which exhibited a somewhat larger basal spacing of 16.5 Å and the capacity for amine intercalation. The thermal decomposition of ZnDPhP or ZnMPhP-A began around 250 °C, resulting in the formation of solid mixtures of zinc phosphates and electron-conducting carbonaceous phases. The bulk electrical conductivities of the poly(vinylidene fluoride)-based composites containing the ZnDPhP pyrolyzates reached 0.1-0.2 S cm-1. Upon mixing with silicone oil, all the synthesized hybrid polymers formed fluids that exhibit significant negative electrorheological effects and have potential for application in electroresponsive smart materials. The application of an electric field during the crosslinking of such systems affected the viscoelastic properties of the resultant solid composites, while the cured systems showed rather small electrorheological effects.

Enhanced electrorheological activity of porous chitosan particles

Novel porous filler for electrorheological fluids was fabricated from chitosan via freeze drying technique. An exceptional electrorheological effect was discovered in suspensions of polydimethylsiloxane (silicone oil) filled by highly porous chitosan particles. The electrorheological activity was studied by rotational rheometry and visualized by optical microscopy. High porosity of the filler allows preparing highly efficient electrorheological fluids at rather low (< 1 wt%) concentration of dispersed phase. The mechanism of chain-like structure formation was considered. The electrorheological behavior of suspensions and the filler structural organization at different concentration were comprehended in terms of dielectric properties. The rheological data were approximated by Bingham and Cho-Choi-Jhon equations. The sedimentation stability of chitosan suspensions in polydimethylsiloxane was significantly affected by particles porosity.

Electrorheological Properties of Polydimethylsiloxane/TiO2-Based Composite Elastomers

Electrorheological elastomers based on polydimethylsiloxane filled with hydrated titanium dioxide with a particle size of 100-200 nm were obtained by polymerization of the elastomeric matrix, either in the presence, or in the absence, of an external electric field. The viscoelastic and dielectric properties of the obtained elastomers were compared. Analysis of the storage modulus and loss modulus of the filled elastomers made it possible to reveal the influence of the electric field on the Payne effect in electrorheological elastomers. The elastomer vulcanized in the electric field showed high values of electrorheological sensitivity, 250% for storage modulus and 1100% for loss modulus. It was shown, for the first time, that vulcanization of filled elastomers in the electric field leads to a significant decrease in the degree of crosslinking in the elastomer. This effect should be taken into account in the design of electroactive elastomeric materials.

Influence of Tethered Ions on Electric Polarization and Electrorheological Property of Polymerized Ionic Liquids

Polymerized ionic liquids (PILs) show potential to be used as new water-free polyelectrolyte-based electrorheological (ER) material. To direct ER material design at the molecular level, unveiling structure-property relationships is essential. While a few studies compare the mobile ions in PILs there is still a limited understanding of how the structure of tethered counterions on backbone influences ER property. In this study, three PILs with same mobile anions but different tethered countercations (e.g., poly(dimethyldiallylammonium) P[DADMA]⁺, poly(benzylethyl) trimethylammonium P[VBTMA]⁺, and poly(1-ethyl-4-vinylimidazolium hexafluorophosphate) P[C₂VIm]⁺) are prepared and the influence of tethered countercations on the ER property of PILs is investigated. It shows that among these PILs, P[DADMA]⁺ PILs have the strongest ER property and P[C₂VIm]⁺ PILs have the weakest one. By combining dielectric spectra analysis with DFT calculation and activation energy measurement, it can clarify that the influence of tethered counterions on ER property is mainly associated with ion-pair interaction energy that is affecting ionic conductivity and interfacial polarization induced by ion motion. P[DADMA]⁺ has the smallest ion-pair interaction energy with mobile ions, which can result in the highest ionic conductivity and the fastest interfacial polarization rate for its strongest ER property.

Design Principles for Aqueous Interactive Materials: Lessons from Small Molecules and Stimuli-Responsive Systems

Interactive materials are at the forefront of current materials research with few examples in the literature. Researchers are inspired by nature to develop materials that can modulate and adapt their behavior in accordance with their surroundings. Stimuli-responsive systems have been developed over the past decades which, although often described as "smart," lack the ability to act autonomously. Nevertheless, these systems attract attention on account of the resultant materials' ability to change their properties in a predicable manner. These materials find application in a plethora of areas including drug delivery, artificial muscles, etc. Stimuli-responsive materials are serving as the precursors for next-generation interactive materials. Interest in these systems has resulted in a library of well-developed chemical motifs; however, there is a fundamental gap between stimuli-responsive and interactive materials. In this perspective, current state-of-the-art stimuli-responsive materials are outlined with a specific emphasis on aqueous macroscopic interactive materials. Compartmentalization, critical for achieving interactivity, relies on hydrophobic, hydrophilic, supramolecular, and ionic interactions, which are commonly present in aqueous systems and enable complex self-assembly processes. Relevant examples of aqueous interactive materials that do exist are given, and design principles to realize the next generation of materials with embedded autonomous function are suggested.

Nonmonotonic Influence of Size of Quaternary Ammonium Countercations on Micromorphology, Polarization, and Electroresponse of Anionic Poly(ionic liquid)s

The size influence of quaternary ammonium countercations in poly[4-styrenesulfonyl(trifluoromethylsulfonyl)imide][tetraalkylammonium] (P[STFSI][Nnnnn], n = 1, 2, and 3) poly(ionic liquid)s on dielectric polarization and the stimuli-responsive electrorheological effect is investigated by dielectric spectroscopy and rheology, and the microstructure-level understanding behind the influence is analyzed by Raman and X-ray scattering spectra. The size influence of quaternary ammonium cations is found to be nonmonotonic. The largest electrorheological effect accompanied by best polarization properties is demonstrated in P[STFSI][N2222]. Raman spectra and activation energy measurements demonstrate that the nonmonotonic influence originates from the fact that, compared to small N1111⁺ and large N3333⁺, intermediate N2222⁺ as countercations can contribute a higher mobile ion number and lower activation energy barrier of ion dissociation and motion. But the experimental values of activation energy are not consistent with theoretically calculated values by considering the ion pair electrostatic potential and elastic force contribution of the matrix. By X-ray scattering and diffraction characterizations, it is clarified that the nonmonotonic influence and the inconsistency of activation energy originate from the size influence of Nnnnn⁺ on the micromorphology of P[STFSI][Nnnnn]. Compared to the semicrystalline structure of P[STFSI][N1111] and the ionic aggregation structure of P[STFSI][N3333], the relatively uniform amorphous structure of P[STFSI][N2222] may be responsible for its lower activation energy barrier of ion motion. This study further provides insights into the design and preparation of future poly(ionic liquid)-based electrorheological materials by considering not only molecular structure but also micromorphology.

A single-component photorheological fluid with light-responsive viscosity

Viscoelastic fluids whose rheological properties are tunable with light have the potential to deliver significant impact in fields relying on a change in flow behavior, such as in-use tuning of combined efficient heat-transfer and drag-reduction agents, microfluidic flow and controlled encapsulation and release. However, simple, single-component systems must be developed to allow integration with these applications. Here, we report a single-component viscoelastic fluid, capable of a dramatic light-sensitive rheological response, from a neutral azobenzene photosurfactant, 4-hexyl-4'butyloxymonotetraethylene glycol (C₆AzoOC₄E₄) in water. From cryo-transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS) and rheology measurements, we observe that the photosurfactant forms an entangled network of wormlike micelles in water, with a high viscosity (28 Pa s) and viscoelastic behaviour. UV irradiation of the surfactant solution creates a less dense micellar network, with some vesicle formation. As a result, the solution viscosity is reduced by four orders of magnitude (to 1.2 × 10^-3 Pa s). This process is reversible and the high and low viscosity states can be cycled several times, through alternating UV and blue light irradiation.

Electric Field Enhances Shear Resistance of Polymer Melts via Orientational Polarization in Microstructures

In this paper, we studied the alteration of viscoelastic properties of a neat poly(methyl methacrylate) (PMMA), induced by an applied external electric field. The rheological properties of PMMA are measured using a rotational rheometer at elevated temperatures. The electric field effect on the shear resistance of the polymer was studied by examining rheological responses under difference experimental scenarios. We find that the external electric field can remarkably enhance shear resistance and prevent flow of PMMA melt, enabling it to behave more predictably at high temperatures. Dynamic rheological analysis illustrates that the external electric field speeds up the recovery of mechanical properties of the PMMA melt after large deformations, whereas the PMMA melt exhibits thixotropic behaviors. The recovery velocity is influenced by the strength of the electric field, specifically, and is found to be proportional to the electric field strength. Our experimental characterization may provide new evidence on the tuning mechanical properties of polymer melts via controlling segmental polarization.

Highly stable and efficient electrorheological suspensions with hydrophobic interaction

HYPOTHESIS

Electrorheological fluid (ERF) is a kind of suspension or colloid composed of fine particles and insulating oil as continuous phase. The second miscible liquid phase with less affinity to the particles than the continuous phase is expected to influence particles aggregation, assembly and spanning mesostructures. Hence, it should be possible to tune the rheological and electrorheological properties and stability by the addition of second miscible liquid with different chain length and substituents.

EXPERIMENTS

We developed a giant ERF (GERF) with a binary liquid phase (BLP) by the addition of alkane to the silicone oil continuous phase. We studied the shear stress and viscosity under different shear rates, thixotropy and particle size distributions of these suspensions and characterized the concentration variation of GERFs under quiescent conditions by measuring the backscattering light intensity variation through vertical scanning.

FINDINGS

The dispersed particle size distribution is broadened, which produces higher static yield stress and lower zero-field viscosity than those of a single-liquid-phase GERF. The ER efficiency is much higher with the addition of alkane, reaching 10656, which is 1.8 times larger than that of single-liquid-phase suspensions. We performed 100-day stability testing and found that the GERF with 1-phenyldodecane showed excellent stability and performance.

Surfactant-Switched Positive/Negative Electrorheological Effect in Tungsten Oxide Suspensions

The electrorheological (ER) effect was experimentally observed in dielectric suspensions containing tungsten oxide (WO₃) modified with surfactant molecules (sodium dodecyl sulfate (SDS) and dodecylamine (DDA)) in electric fields up to several kilovolts per millimeter. The dielectric properties of WO₃ suspensions in silicone oil were analyzed, depending on the frequency of the electric field, in the range from 25 to 10⁶ Hz. Unmodified WO₃ suspensions, as well as suspensions modified with sodium dodecyl sulfate, were shown to exhibit a positive electrorheological effect, whereas suspensions modified with dodecylamine demonstrated a negative electrorheological effect. The quantitative characteristics of the negative electrorheological effect in the strain–compression and shear regimes were obtained for the first time. Visualization experiments were performed to see the chain structures formed by WO₃ particles modified with sodium dodecyl sulfate, as well as for dynamic electroconvection in electrorheological fluids containing WO₃ modified with dodecylamine. The negative electrorheological effect was shown to be associated with the processes of phase separation in the electric field, which led to a multiplicative effect and a strong electroconvection of the suspension at field strengths above 1 kV/mm.