  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.
