Synergy of two fatty acids as additives on lubricity of a nematic liquid crystal 5CB

Abstract

In examining the lubricity of a nematic liquid crystal (4-pentyl-4′-cyanobiphenyl [5CB]), synergy of 2 fatty acids (stearic acid [C18] and behenic acid [C22]) as additives was observed. In “friction measurements” using a ball-on-plate-type tribometer, the friction coefficient for the “mixed-additive” lubricant (5CB-C18-C22) was lower than those for the “single-additive” lubricants (5CB-C18 and 5CB-C22). This synergy mechanism was discussed with considering the results obtained in other tests: “orientation observations” in a stationary cell using polarising microscopy and “surface topography measurements” of adsorbed films using atomic force microscopy. As a result, it is believed that the microscopic patterned surface formed by the mixed additive strengthens the surface anchoring of 5CB, which leads to increase in the effective viscosity of 5CB and therefore to decrease in the macroscopic friction coefficient in the mixed lubrication regime.