On the origin of permeative flows in cholesteric liquid crystals

Abstract

Permeative flows, known for the explanation of the anomalous viscosity (10^5 Poise) in cholesterics at low shear rates, are still under debate due to the difficulty of experiments. Here we use the Surface Force Balance, in which uniform domains with regular circular defects are formed, to probe the forces generated by compression in the direction of the helical axis. At the quasi-static speed of the surface approach, the measured forces are shown to be elastic (not dissipative), arising from the twist elastic deformation when the planar anchoring at the walls is strong. A mechanism involving frictional surface torque under strong planar surface anchoring will be proposed. The results indicate that the strong resistance to flow observed, previously interpreted as an enormous apparent viscosity, may in fact originate from the intrinsic non-linear increase of elasticity when the molecules are rotated away from equilibrium. The system is found to store energy (the force is reversible), without dissipation, as long as the applied stress is below the threshold for nucleating new defects. Our study underpins the importance of boundary conditions that may dramatically change the rheology of other viscoelastic materials and sheds light on the rational design of strain-stiffening materials, nanomotors, and artificial muscles involving helical architectures.