Reversal in Thermally Driven Rotation of Chiral Liquid Crystal Droplets

Abstract

Thermomechanical coupling in chiral liquid crystals enables the direct conversion of heat current into mechanical rotation, providing a promising mechanism for the utilisation of low-grade thermal energy and heat-driven soft micro-actuation. However, the physical origin governing the direction and magnitude of this coupling remains elusive. Here, we demonstrate that cholesteric liquid crystal droplets undergo a reversal of their rotational direction with changing droplet size and temperature, even under a fixed temperature gradient and unchanged molecular chirality. This previously unrecognised behaviour cannot be accounted for by the conventional description of the thermal Leslie effect. Systematic investigations of droplet size, temperature and molecular structure, together with phenomenological analysis, reveal that thermomechanical coupling is strongly influenced by molecular orientational order and may even reverse sign with changes in the scalar order parameter. These findings identify molecular ordering as an active design parameter, rather than merely a structural descriptor, for controlling thermomechanical energy conversion. More broadly, our results suggest that thermomechanical coupling should be understood as a property emerging from molecular ordering, providing a new framework for designing heat-to-motion energy conversion in chiral soft matter.