Friction of the liquid crystal 8CB as probed by the surface forces apparatus

The surface forces apparatus (SFA) was used to study the lubricating properties of an 8CB liquid crystal film confined between mica surfaces. After displacement of most of the liquid crystal by compression, a 10.2 Angstrom thick layer of 8CB was found between mica surfaces in the range of applied pressures from 1 to 20 MPa. Under these conditions, we found that friction depends linearly on contact area in the 1-1.5 MPa range. We found that the critical shear stress was anisotropic, with values of 0.28 MPa for shear along the 8CB molecular axis and 0.42 MPa for shear in the perpendicular direction. This anisotropy was observed through all the ranges of load where slip occurred. At higher loads (1.5-2.7 MPa), the critical shear stresses increase to 0.6 and 1.2 MPa for parallel and perpendicular shear, respectively.