W. J. Tian

Saturated Orientational Polarization of Polar Molecules in Giant Electrorheological Fluids

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.

Polar-molecules-driven enhanced colloidal electrostatic interactions and their applications in achieving high active electrorheological materials

We have fabricated a class of colloidal electrorheological (ER) fluids, in which suspended TiO2 particles were synthesized by a sol-gel method and modified by 1,4-butyrolactone molecules with a permanent molecular dipole moment of 4.524 D. Compared with pure TiO2 ER fluids, the quasi-static yield stress of the polarmolecules-modified ER fluid is enhanced as high as 48.1 kPa when subjected to an external electric field of 5 kV/mm. Also, it possesses other attractive characters such as low current density (<14 A/cm) and low sedimentation. Based on a Green’s function method, we present a first-principles approach to investigate colloidal electrostatic interactions. Excellent agreement between experiment and theory has been shown for the enhancement ratio of quasi-static yield stress, which quantitatively reveals that enough polar molecules oriented within the field-directed gap between the colloidal particles can unexpectedly enhance the interactions, thus yielding the unusual enhancement. This shows a promising and flexible direction for achieving more highly active ER materials.

Electric-field-induced interaction between biological cells or colloidal particles

Biological cells can be treated as an inhomogeneous particle. In addition to biomaterials, inhomogeneous particles are also important in more traditional colloidal science. By using two energy methods that are based on Legendre polynomials and Green’s function, respectively, we investigate the interaction between biological cells or colloidal particles in the presence of an external electric field, in an attempt to investigate the effect of inhomogeneity on crossover frequencies across which the interaction force changes from attraction to repulsion or vice versa. The predictions by the two methods agree with each other very well. It is shown that there exist two kinds of crossover frequencies ωp, one from attraction to repulsion and the other from repulsion to attraction as external frequencies increase. The first ωp strongly depends on the degree of inhomogeneity, while the second ωp does not. This work has relevance to manipulation of biological cells or colloidal particles.

Scaling behaviours in settling process of fractal aggregates in water

We investigate the effect of permeant flow on the sedimentation of porous fractal- aggregates in water. Our theoretical analysis gives explicit calculations on the scaling behaviours of settling velocities, taking into account the fractality through a proper permeability for fractal aggregates. The calculated results for the scaling behaviours of settling velocities fit remarkably well with experimental data. The analytic expression for the settling velocity provides a criterion for determining fractal dimensions Df of aggregates from sedimentation experiments. Copyright c EPLA, 2007

On the Lorentz Local Electric Field in Soft-Matter Systems

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.

Electroinduced simple harmonic oscillation of a microparticle

We develop two energy methods to investigate interparticle forces in a chain of three microparticles in an electric field, two of which are fixed and symmetrically located in the two opposite sides of the third free microparticle. We reveal that, if the free microparticle is laterally dragged and released, it can oscillate perpendicular to the line joining the centers of the two fixed microparticles, being in simple harmonic oscillation with a fixed period for small oscillation amplitudes. This work demonstrates that microparticles can be used to record time, which makes it possible to design a new class of microparticulate clocks.

Nonlinear ac responses of erythrocyte suspensions: Experiment and theory

When a suspension consisting of electric particles having nonlinear characteristics is subjected to a sinusoidal alternating current (ac) electric field, the electric response will generally consist of ac fields at frequencies of higher-order harmonics. We experimentally report on harmonic generation by erythrocytes subjected to an ac electric field. We find that both even and odd harmonics are sensitive to cell shapes, conductivities, field frequencies, and field magnitude. Theoretical analysis based on a phenomenological model yield predictions that are in excellent agreement with the experiments. Thus, it becomes possible to detect nonlinear characteristics, shapes, and conductivities of erythrocytes by measuring such ac responses.

Alternating-current relaxation of a rotating metallic particle*

Based on a first-principles approach, we establish an alternating-current (AC) relaxation theory for a rotating metallic particle with complex dielectric constant . Here is the real part, the conductivity, ω 0 the angular frequency of an AC electric field, and . Our theory yields an accurate interparticle force, which is in good agreement with the existing experiment. The agreement helps to show that the relaxations of two kinds of charges, namely, surface polarized charges (described by ) and free charges (corresponding to ), contribute to the unusually large reduction in the attracting interparticle force. This theory can be adopted to determine the relaxation time of dynamic particles in various fields.