Thermally reconfigurable monoclinic nematic colloidal fluids

Abstract

Fundamental relationships are believed to exist between the symmetries of building blocks and the condensed matter phases that they form¹. For example, constituent molecular and colloidal rods and disks impart their uniaxial symmetry onto nematic liquid crystals, such as those used in displays^1,2. Low-symmetry organizations could form in mixtures of rods and disks^3,4,5, but entropy tends to phase-separate them at the molecular and colloidal scales, whereas strong elasticity-mediated interactions drive the formation of chains and crystals in nematic colloids^{6,7,8,9,10,11}. To have a structure with few or no symmetry operations apart from trivial ones has so far been demonstrated to be a property of solids alone¹, but not of their fully fluid condensed matter counterparts, even though such symmetries have been considered theoretically^12,13,14,15 and observed in magnetic colloids¹⁶. Here we show that dispersing highly anisotropic charged colloidal disks in a nematic host composed of molecular rods provides a platform for observing many low-symmetry phases. Depending on the temperature, concentration and surface charge of the disks, we find nematic, smectic and columnar organizations with symmetries ranging from uniaxial^1,2 to orthorhombic^{17,18,19,20,21} and monoclinic^12,13,14,15. With increasing temperature, we observe unusual transitions from less- to more-ordered states and re-entrant²² phases. Most importantly, we demonstrate the presence of reconfigurable monoclinic colloidal nematic order, as well as the possibility of thermal and magnetic control of low-symmetry self-assembly^2,23,24. Our experimental findings are supported by theoretical modelling of the colloidal interactions between disks in the nematic host and may provide a route towards realizing many low-symmetry condensed matter phases in systems with building blocks of dissimilar shapes and sizes, as well as their technological applications.