Flow Properties of a Twist-bend Nematic Liquid Crystal

We present the first shear alignment studies and rheological measurements in the twist-bend nematic ($N_{tb} )$ liquid crystal phase of odd numbered flexible dimer molecules. It is found that the $N_{tb} $ phase is strongly shear-thinning. At shear stresses below $1Pa$ the apparent viscosity of $N_{tb} $ is $1000$ times larger than in the nematic phase. At stress above $10Pa$ the $N_{tb} $ viscosity drops by two orders of magnitude and the material exhibits Newtonian fluid behavior. This is consistent with the heliconic axis becoming normal to the shear plane via shear-induced alignment. From measurements of the dynamic modulus we estimate the compression modulus of the pseudo-layers to be $B\sim 2kPa$; this value is discussed within the context of a simple theoretical model based upon a coarse-grained elastic free energy.