  Sorting and separation of microparticles is a challenging problem of
interdisciplinary nature. Existing technologies can differentiate
microparticles by their bulk properties, such as size, density, electric
polarizability, etc. The next level of challenge is to separate particles that
show identical bulk properties and differ only in subtle surface features, such
as functionalization with ligands. In this work, we propose a technique to sort
and separate particles and fluid droplets that differ in surface properties. As
a dispersive medium, we use a nematic liquid crystal (LC) rather than an
isotropic fluid, which allows us to amplify the difference in surface
properties through distinct perturbations of LC order around the dispersed
particles. The particles are placed in a LC cell with spatially distorted
molecular orientation subject to an alternating current electric field. The
gradients of the molecular orientation perform two functions. First, elastic
interactions between these pre-imposed gradients and distortions around the
particles separate the particles with different surface properties in space.
Second, these pre-imposed patterns create electro-osmotic flows powered by the
electric field that transport the sorted particles to different locations thus
separating them. The demonstrated unique sorting and separation capability
opens opportunities in lab-on-a-chip, cell sorting and bio-sensing
applications.
