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Conductive organic polymers can be formulated with polymers that incorporate fine dispersed metallic particles. In this work, we present a general model for ferrogels which are chemically cross-linked polymer networks swollen with a ferrofluid. Our aim is to study the effect of the shape and/or material (conductivity) anisotropy on the effective electrical conductivity of the ferrogel in the presence of an external magnetic field. Our theory can reproduce the known results and provides a link between the particle properties and orientation distribution and the effective electrical conductivity. To this end, we find that material (conductivity) anisotropies are more important for yielding a high effective electrical conductivity than shape anisotropies, while magnetic fields can offer a correction. [ABSTRACT FROM AUTHOR]

Understanding the flow of liquids and particularly water in nanochannels is important for scientific and technological applications, such as for filtration and drug delivery. Here we perform molecular dynamics simulations to investigate the transfer of single-file water molecules across straight or nonstraight single-walled carbon nanotubes (SWCNTs). In contrast with the macroscopic scenario, the nonstraight nanostructure can increase the water permeation. Remarkably, compared with the straight SWCNT, the nonstraight SWCNT with the minimal bending angle of 35° in the simulations can enhance the water transport up to 3.5 times. This enhancement mainly originates from the Lennard-Jones interaction between water molecules and nonstraight nanostructures. Our work offers an additional freedom to design high-flux nanochannels by choosing nonstraight nanostructures and provides an insight into water flow across biological water nanochannels, which are often nonstraight since they are composed of integral membrane proteins.

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.

Dynamic behaviors are abundant in field-responsive colloidal suspensions. Being beyond the usual point-dipole approximation, we develop a multiple image method of dipoles for two dynamic unequal colloidal dielectric spherical particles, which can be perfectly reduced to those for two static conducting particles. The method is applied to investigate colloidal electric interparticle forces under various conditions of dynamics. As a result, we find that the force can be enhanced, reduced, or even changed from attraction to repulsion, or vice versa. Some other interesting results are also reported. Our theoretical results are compared favorably with existing experimental observations. Therefore, it becomes possible to achieve desired colloidal structures by adjusting colloidal interactions by choosing appropriate dynamic phenomena.

A rotating particle in electrorheological (ER) fluid leads to a displacement of its polarization charges on the surface which relax toward the external applied field E0, resulting in a steady-state polarization at an angle with respect to E0. This dynamic effect has shown to affect the ER fluid properties dramatically. In this paper, we develop a dynamic effective medium theory (EMT) for a system containing rotating particles of finite volume fraction. This is a generalization of established EMT to account for the interactions between many rotating particles. While the theory is valid for three dimensions, the results in a special two-dimensional configuration show that the system exhibits an off-diagonal polarization response in addition to a diagonal polarization response, which resembles the classic Hall effect. The diagonal response monotonically decreases with increasing rotational speed, whereas the off-diagonal response exhibits a maximum at a reduced rotational angular velocity omega0, compared to the case of isolated rotating particles. This implies a way of measurement on the interacting relaxation time. The dependencies of the diagonal and off-diagonal responses on various factors, such as omega0, the volume fraction, and the dielectric contrast, are discussed.

By using theoretical analysis and molecular dynamics simulations, we investigate the structure of colloidal crystals formed by nonmagnetic microparticles (or magnetic holes) suspended in ferrofluids (called inverse ferrofluids), by taking into account the effect of polydispersity in size of the nonmagnetic microparticles. Such polydispersity often exists in real situations. We obtain an analytical expression for the interaction energy of monodisperse, bidisperse, and polydisperse inverse ferrofluids. Body-centered tetragonal (bct) lattices are shown to possess the lowest energy when compared with other sorts of lattices and thus serve as the ground state of the systems. Also, the effect of microparticle size distributions (namely, polydispersity in size) plays an important role in the formation of various kinds of structural configurations. Thus, it seems possible to fabricate colloidal crystals by choosing appropriate polydispersity in size.

In reality, colloidal particles often possess an arbitrary inhomogeneity profile. We present a first-principles approach to dielectrophoresis of such an inhomogeneous colloidal particle under an inhomogeneous field of an oscillating electric dipole moment. For numerical simulations, we treat the inhomogeneous particle as a specific graded one with a physically motivated dielectric model and conductivity profile. We find that both multipolar interactions and spatial fluctuations inside the particle can affect the dielectrophoretic spectrum of the particle significantly.

It is known that macroscopic properties of colloidal suspensions are often determined by the microstructure of the particles in the suspensions, depending on the interparticle, Brownian, and hydrodynamic (if any) forces. We take electrorheological (ER) fluids as an example. By using a computer simulation and an experimental approach, we investigate the structure of ER fluids subjected to both an electric field and a shear flow. The microstructure evolution from random structure, to chains, and then to stable lamellar patterns, observed in the experiments, agrees very well with that obtained in the simulations. It is shown that the formation of such lamellar patterns originates from the difference between the dipole moment induced in the particles suspended in the ER fluids without shear and the one with shear. The results on the relaxation process of structural formation and the internal structure of layers are also presented. Thus, it seems possible to achieve various structures and hence desired macroscopic properties of colloidal suspensions by adjusting external fields and, simultaneously, a shear flow.

When a ferrogel, which is chemically cross-linked polymer networks swollen with a ferrofluid, consisting of magnetic particles having nonlinear characteristics is subjected to an alternating current (ac) magnetic field, the magnetic response will generally consist of ac fields at frequencies of the higher-order harmonics. By using a perturbation approach, we investigate nonlinear ac responses of ferrogels, under an ac magnetic field either coupled with a dc magnetic field or not. It is shown that it is possible to detect the volume fraction and shape of particles in ferrogels by measuring such ac responses. Our results are very well understood in spectral representation and are favorably compared with the experimental observations of suspensions being beyond ferrogels.

When a composite containing nonlinear dielectric particles suspended in a host medium is subjected to a sinusoidal alternating current (ac) electric field, the dielectric response of the composite will generally consist of ac fields at frequencies of higher order harmonics. For an electrorheological (ER) solid under structure transformations due to external fields, we apply the Ewald-Kornfeld formulation to derive the local electric fields and induced dipole moments explicitly, and then we perform the perturbation expansion method to extract their fundamental and third-order harmonics analytically. It is shown that the degree of anisotropy of the ER solid can affect these harmonics significantly. Our results are well understood in the spectral representation theory. Thus, it seems possible to perform a real-time monitoring of the structure transformation by measuring the nonlinear ac responses of ER solids.