Crystallization in thin liquid films induced by shear

It is known that the thin-film structure of confined fluids and solids can be changed when the confining surfaces are sheared. Positional and orientational short- or long-range reordering can occur that often have no bulk counterparts. These multilayer, monolayer, or even sub-monolayer effects are important for understanding adhesion and friction processes, but they have proved difficult to measure, partly due to a lack of experimental techniques and partly to their apparent subtle dependence on many experimental parameters. Here we report the use of shear measurements and "optical absorption spectroscopy" in the surface forces apparatus to measure a shear-induced phase transition of an anisotropic (dye) molecule confined between two shearing mica surfaces in aqueous solution. Our studies on the shear-induced ordering and friction forces of highly anisotropic cyanine dye molecules in thin water films show only a weak effect of molecular anisotropy on shear-induced ordering, friction forces, and the onset of shear-induced crystallization, although dramatic changes do occur when the confined molecules ultimately crystallize.