Colloids in complex fluids

Colloidal particles dispersed in liquid crystals results in a diversity of self-assembled structures, such as linear chains or anisotropic clusters. In their turn, micro-inclusions can perturb the liquid crystal matrix thus enabling new control strategies. For instance, paramagnetic anisometric inclusions can be employed to reorient a nematic mesophase.

Anchoring conditions of the liquid crystal director on the surface of the inclusions lead to the orientation of the latter following the nematic director. Conversely, direct actuation on the inclusions could lead to new strategies for the control and the switching of liquid crystal alignment. As an example, we have employed paramagnetic polystyrene microellipsoids ( home-stretched from commercial spherical particles) embedded in a nematic matrix. Anchoring on the liquid crystal cells leads to a relatively rigid elastic medium that hinders the reorientation of the ellipsoids by means of modest magnetic fields. We have devised a strategy consisting on preparing a degenerate planar liquid crystal cell on which the director field glides on homeotropic boundary conditions at the cell plates.  Under these conditions, in-plane director field rotation incurs in modest elastic energy costs, thus enabling a direct actuation on the orientation of the director field using weak magnetic fields (lower than the Frederiks threshold) thanks to the mediation of the dispersed ellipsoidal paramagnetic inclusions.
Setup for the assembly of a colloidal cluster

Different from the passive role just exposed, liquid crystals provide new strategies for the transport of colloidal inclusions. When under a low-frequency AC electric field, ionic flow around inclusions is distorted by the defects generated on the liquid crystal matrix by the boundary conditions on the colloidal particle. Combined with the anisotropic ionic mobility in the mesogen, this can result in a net propulsion on the solid or liquid colloidal inclusions, provided the distortion in the liquid crystal director field has a dipolar structure. We have combined this phoretic propulsion with a photoelastic modulation of the liquid crystal director using a photosensitive, azobenzene-based, anchoring layer on the cell plate. With this, the AC electric field provides a global phoretic propulsion to the colloidal inclusions while light irradiation enables to locally steer the moving particles. By impringting radially symmetric irradiation patterns, colloidal particles assemble in clusters that can be reversibly moved as swarms along the liquid crystal layer simiply by scribing a path with light irradiation.

 Cluster of particles moving as a colloidal swarm