Shear of molecularly confined liquid crystals. 2. Stress anisotropy acrossa model nematogen compressed between sliding solid surfaces

We have used a new design of the mica surface force balance (SFB), with ext reme sensitivity in measuring normal and particularly shear or frictional f orces between two surfaces sliding past each other, to measure the forces b etween two mica surfaces across a confined 4-cyano-4'-hexabiphenyl nematoge n (6CB). In an earlier study (Langmuir 1997, 13, 4466, paper 1 of the serie s) we investigated the normal force-distance profiles and the orientation o f the confined liquid crystal (LC). Here we extend this to study the shear forces F-s between the sliding mica surfaces across the 6CB nematogen as a function of orientation of the confined LC, the applied normal load F-n, th e separation D of the mica surfaces, the shear velocity nu (s), and the rel ative shear direction of the confining surfaces (which may be varied using the new SFB design). Our results, where the shear forces are measured down to levels that are some orders of magnitude more sensitive than in earlier studies, show that the highly confined nematogen (D in the range from 16 to ca. 100 Angstrom) behaves under shear in a quasi-solidlike fashion for all three orientations studied: planar, planar twisted, and homeotropic. There is a linear relation between F-s and F-n for each of the three orientation s, with the effective friction coefficient (dF(s)/dF(n)) largest for the ho meotropic orientation and lowest for the planar twisted one. Intriguingly, for the planar orientation a clear increase in the friction could be observ ed when the initial shear direction was changed by 90 degrees. We attribute this to the effect of the initial shear in orienting the confined nematoge n layer in the initial direction, so that subsequent sliding motion at righ t angles to this encounters greater resistance. We also find that within a range of some 40-fold in nu (s), there was little change in the shear force , in line with what is generally observed for solid-solid friction, and con sistent with the fact that little relaxation in F-s is observed over macros copic times on applying a step strain to the confined LC.