  We study colloidal particles in a nematic-liquid-crystal-filled microfluidic
channel and show how elastic interactions between the particle and the channel
wall lead to different particle dynamics compared with conventional
microfluidics. For a static particle, in the absence of a flow field, the
director orientation on the particle surface undergoes a rapid transition from
uniform to homeotropic anchoring as a function of the anchoring strength and
the particle size. In the presence of a flow field, in addition to fluid
viscous stresses on the particle, nematic-induced elastic stresses are exerted
as a result of the anchoring conditions. The resulting forces are shown to
offer the possibility of size-based separation of individual particles. For
multi-particle systems, the nematic forces induce inter-particle attraction
induces particle attraction, following which the particles aggregate and
reorientate. These results illustrate how coupled nematic-hydrodynamic effects
can affect the mobility and spatial reorganization of colloidal particles in
microfluidic applications.
