Polar-molecular And Ionic ER Fluid: A General Research

In electric-field-responsive soft-matter systems, the suspended particles respond to the Lorentz local field (LLF), yielding abundant important phenomena. Even though the particles can easily rotate, the LLF was conventionally adopted as a quantity that is independent of rotations in the literature. In contrast, here we design an experiment to measure the LLF between two metallic spheres, one of which is rotating, and report a rotation-driven reduction. Excellent agreement between our experiment and theory reveals the role of the relaxation of dipole moments. Its relevance to biophysics, colloidal physics, and nonlinear physics is also discussed.Many researches on polar-molecular electrorheological (PMER) fluids with giant electrorheological effects were reported in recent years. The particles of PMER fluids (PMER particles) are known to have a dielectric core with high dielectric constant and a shell of polar molecules. Our calculation of local electric fields using the finite element approach shows that the local electric field can cause an orientational polarization of the polar molecules. The saturation of the orientational polarization occurs on the outer shells of two nearby PMER particles. Then, it causes the strong outer shell-outer shell interaction between the two particles, and this kind of interaction is just responsible for the giant electrorheological effect. It is further realized that the PMER effect is mainly due to the interaction of the tail-head connected polar molecules within the two outer shells between the two PMER particles. Our theoretical results of static yield stresses are shown to be in excellent agreement with the reported experimental data by several groups. For general PMER fluids, the calculated static yield stress is nearly proportional to R^x-1. When h/R, the ratio between the thickness of shells and radius of PMER particles, changes from 0.05 to 0.5, the index x changes accordingly from 0.64 to 0.51. It is also found that particles with thinner thickness h and smaller radius R have larger electrorheological effects until the static yield stress shows a peak when R reaches about 10 nm.We fabricate novel TiO₂ nanoparticles modifid by a thin salt solution layer. The suspensions, formed by the nanoparticles in silicone oil, serve as model systems for exploring very general theories about interactions between colloidal particles with arbitrary ions confined on surfaces. At low DC electric fields we find that the static yield stress of the modified nanoparticles increases much faster than that of bare nanoparticles. The yield stress has a peak at a high field, not found before. The peak can be manipulated by tuning the amount of cations and anions confined in the salt solutionlayer. Excellent agreement between our experiment and theory reveals the mechanism concerning the competition between ions' polarized trapped state and conductively transmitted state, which is universal, independent of materials composed of nanoparticles. This work makes it possible to manipulate colloidal interactions by confining exogenous ions appropriately, and it is expected to have applications in colloidal science, materials engineering, and biotechnology.