Spreading Transition of a Liquid Film

Liquids of poly(phenylmethylsiloxane) spread at the air-water interface undergo, with increasing temperature, a transition from the spread to the nonspread state. This was manifested as a vanishing of the surface pressure, π. It was also visualized. In spreading from a droplet of macroscopic size, i.e., from an essentially infinite reservoir, π vanished at 55 ± 2°C for a liquid with number-average molecular weight M_n = 2240 g·mol^-1. However, when the amount spread was limited to 2 and 1 mg·m^-2, the temperatures at which the surface pressure vanished were 42 and 37 ± 2°C, respectively. The same pattern held, but at higher transition temperatures, for a liquid species with M_n = 1470 g·mol^-1. The dependence on the amount spread might be owing to molecular weight distribution in the polymer fractions but might also reflect increased equilibrium film thickness when the spreading coefficient is small (a "pancake"), as predicted by Joanny and de Gennes but apparently not previously observed. In this system, it is possible by varying the temperature to tune the spreading coefficient so that it passes continuously from positive to negative.