Pitch-dependent behavior of colloids in confined cholesteric liquid crystals*

Confined soft matter provides unique opportunities to impose energy landscapes to address and control colloid dynamics. In this context, the geometry of bounding surfaces can be employed to mold an energy landscape. Within this landscape, colloids assemble into reconfigurable, hierarchically organized structures, a leading challenge in material science. Example soft matter systems include liquid crystals. For instance, when nematic liquid crystals (NLCs) are confined in a vessel, bend and splay distortions can be used to position particles.
Here we study cholesteric liquid crystals (ChLCs) in a vessel with undulated boundaries. ChLCs have an “intrinsic” twist distortion which adds to the ones imposed by the solid boundaries. The cholesteric pitch competes with the other length scales in the system (colloid radius, vessel thickness, wavelength of boundaries undulations), enriching the possible configurations. Depending on the pitch-to-radius and pitch-to-thickness ratios the interaction can be attractive or repulsive, alternatively. By tuning the pitch (i.e. changing the concentration of the chiral dopant), it is possible to selectively promote or inhibit particle trapping at the docking sites.