Crystallization of condensation droplets on a liquid surface

Highly ordered microporous two-dimensional membranes have been obtained fro m polymer solutions (Widawski et al. (1994) Nature 369. 397-399). Recently, a mechanism for the formation of such membranes was proposed, involving wa ter vapour condensation (induced by the rapid evaporation of the volatile s olvent) onto the surface of solutions and the formation of floating water d roplets. Unfortunately, the droplets growth process was not observed, and c onsequently only qualitative information was reported. In the present paper , results of light-scattering experiments with this system are reported. Th e formation of water droplets growing at the surface of the solution has be en observed and the evolution with time of the mean droplet radius has been found to be described by a power law with an exponent of 1/3, proving that no coalescence processes occur. This particular behaviour is attributed to the precipitation of the polymer at the water/solution interface and to th e formation of a mechanically resistant polymer layer encapsulating each dr oplet. In this way, water droplets behave like solid particles, allowing co mpact sheets to be formed. The presence of important surface currents is be lieved to promote the formation of "polycrystal" and "monocrystal" patterns .