Re-entrant bimodality in spheroidal chiral swimmers in shear flow.

We use a continuum model to report on the behavior of a dilute suspension of chiral swimmers subject to externally imposed shear in a planar channel. Swimmer orientation in response to the imposed shear can be characterized by two distinct phases of behavior, corresponding to unimodal or bimodal distribution functions for swimmer orientation along the channel. These phases indicate the occurrence (or not) of a population splitting phenomenon changing the swimming direction of a macroscopic fraction of active particles to the exact opposite of that dictated by the imposed flow.

Population splitting of rodlike swimmers in Couette flow.

We present a quantitative analysis on the response of a dilute active suspension of self-propelled rods (swimmers) in a planar channel subjected to an imposed shear flow. To best capture the salient features of the shear-induced effects, we consider the case of an imposed Couette flow, providing a constant shear rate across the channel. We argue that the steady-state behavior of swimmers can be understood in the light of a population splitting phenomenon, occurring as the shear rate exceeds a certain threshold, initiating the reversal of the swimming direction for a finite fraction of swimmers from down- to upstream or vice versa, depending on the swimmer position within the channel.

Higher-order dielectrophoresis of nonspherical particles.

Higher-order terms of dielectrophoretic (DEP) force are commonly ignored by invoking the simplifying dipole approximation. Concurrently, the trend towards micro- and nano-electrode structures in DEP design is bringing about an increasing number of instances where the approximation is expected to lose reliability. The case is severe for nonspherical particles (the shape of many biological particles) due to the shape-dependent nature of dielectric polarization.

Continuous separation of colloidal particles using dielectrophoresis.

Dielectrophoresis is the movement of particles in nonuniform electric fields and has been of interest for application to manipulation and separation at and below the microscale. This technique has the advantages of being noninvasive, nondestructive, and noncontact, with the movement of particle achieved by means of electric fields generated by miniaturized electrodes and microfluidic systems. Although the majority of applications have been above the microscale, there is increasing interest in application to colloidal particles around a micron and smaller.