Shear alignment of confined hydrocarbon liquid films

Shear-induced structural reordering in thin liquid films of the linear saturated alkane n-eicosane (C20H42) was investigated using a surface forces apparatus and freeze-fracture (atomic force) microscopy (AFM). By rapidly freezing a shearing film followed by splitting (cleaving) the films from the confining mica substrate surfaces, it was possible to obtain AFM images of the structures of the films during steady-state sliding, revealing striped domains approximately 2 A in height and a few nanometer wide whose structure depends on the sliding velocity and, most likely, also on the sliding distance and time. In contrast, confined but unsheared films yielded completely featureless images. To the best of our knowledge, the results are the first direct experimental measurement of shear-induced ordering in nano-confined films resulting in layering and domain formation, but any molecular-level alignment, if present, could not be established.