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We used numerical simulations of a continuous model and the molecular dynamics model to understand the particle instability, formation of island-like structures and existence of one critical particle concentration of 1% (v/v) for formation of island-like structures in the suspension in a gradient AC electric field reported in Paper I. The simulations of the continuous model show that the critical concentration of 1% (v/v) is the concentration of which the particles of a suspension are just fully filling the lower field region finally. According to the MD simulations, the particles instability does exist in the corn oil in a gradient AC electric field, anisotropic polarization interactions among the particles are responsible for the particle instability and have memory, and the memory is still kept even when the particles are transported by a dielectrophoresis force. The island-like structures can be regarded as signature of the memory. We explored possibilities to apply our findings in biomedical fields. [ABSTRACT FROM AUTHOR]

Abstract: A distinct front, separating regions enriched with and depleted of particles, was recently observed in suspensions subjected to high-gradient ac electric fields and a set of theoretical model equations for the particle concentration, containing no fitting parameters, were developed [Kumar et al., 2004. Combined negative dielectrophoresis and phase separation in nondilute suspensions subject to a high-gradient ac electric field. Phys. Rev. E 69, 021402-1–10; Bennett et al., 2003. Combined field-induced dielectrophoresis and phase separation for manipulating particles in microfluidics. Appl. Phys. Lett. 83, 4866–4868]. Although the numerical solutions of these equations were found to be quantitatively consistent with the experimental observations, they did not provide sufficient information for elucidating the mechanism of the front formation due to the complexity of the equations. Here, we examine these equations analytically for the special case in which they admit a similarity solution and establish the existence of shock solutions to these equations. The shocks are shown to arise because of the rapid local growth of the suspension viscosity due to the field-driven particle accumulation in certain areas of the domain. [Copyright &y& Elsevier]

Some model problems are considered in order to investigate wetting failure in liquid-liquid systems. Three geometries are considered, two-dimensional two-phase shear flow, two-dimensional driven capillary rise, and both two- and three-dimensional two-phase driven cavity flow. In the first two cases, the two fluids are made equiviscous. The driven cavity flow is investigated for both equi- and non-equiviscous fluids. Three methods of analysis are used for the equiviscous case, an essentially exact Fourier series method, a quasi-parallel approximation and a phase-field model. The Fourier series validates the phase-field method in that they both give nearly identical results for onset of instability. At relatively large slip length divided by channel width (10-2), the capillary number at which onset of wetting failure (entrainment of the receding fluid in the advancing) occurs is highly dependent on the type of flow. This dependence, however, appears to diminish rapidly as the slip length becomes smaller. The capillary number for the onset of instability is moderately dependent on gravity level. Three-dimensional phase-field calculations are then discussed that show wetting failure through tipstreaming and splitting instabilities. Spot checks indicate that the onset points of two- and three-dimensional instabilities are very close. It is hypothesized that tipstreaming can be understood in part as a quasi-two-dimensional phenomenon. [ABSTRACT FROM AUTHOR]

The authors propose and experimentally demonstrate a conveyor-belt method appropriate for building large-scale microparticle structures by sequentially energizing electrodes to aggregate the particles into predetermined locations and then to translate them collectively to a work area for final assembly. This approach employs collective phenomena in a negatively polarized suspension exposed to a high-gradient strong ac electric field. [ABSTRACT FROM AUTHOR]

Experiments were conducted in microfluidics equipped with dielectrophoretic gates arranged perpendicular to the flow. Under the action of a high-gradient ac field and shear, flowing suspensions were found to undergo a phase separation and to form a distinct front between the regions enriched with and depleted of particles. We demonstrate that this many-body phenomenon, which originates from interparticle electrical interactions, provides a method for concentrating particles in focused regions and for separating biological and nonbiological materials. The evolution of the particle patterns formation is well described by a proposed electrohydrodynamic model. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]