| Abstract
| - We have used a new design of the mica surface force balance (SFB), with extreme sensitivity in measuringnormal and particularly shear or frictional forces between two surfaces sliding past each other, to measurethe forces between two mica surfaces across a confined 4-cyano-4‘-hexabiphenyl nematogen (6CB). In anearlier study (Langmuir1997, 13, 4466, paper 1 of the series) we investigated the normal force−distanceprofiles and the orientation of the confined liquid crystal (LC). Here we extend this to study the shear forcesFs between the sliding mica surfaces across the 6CB nematogen as a function of orientation of the confinedLC, the applied normal load Fn, the separation D of the mica surfaces, the shear velocity vs, and the relativeshear 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 thanin earlier studies, show that the highly confined nematogen (D in the range from 16 to ca. 100 Å) behavesunder shear in a quasi-solidlike fashion for all three orientations studied: planar, planar twisted, andhomeotropic. There is a linear relation between Fs and Fn for each of the three orientations, with theeffective friction coefficient (dFs/dFn) largest for the homeotropic orientation and lowest for the planartwisted one. Intriguingly, for the planar orientation a clear increase in the friction could be observed whenthe initial shear direction was changed by 90°. We attribute this to the effect of the initial shear in orientingthe confined nematogen layer in the initial direction, so that subsequent sliding motion at right anglesto this encounters greater resistance. We also find that within a range of some 40-fold in vs, there waslittle change in the shear force, in line with what is generally observed for solid−solid friction, and consistentwith the fact that little relaxation in Fs is observed over macroscopic times on applying a step strain tothe confined LC.
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