Instability of a two-dimensional colloidal crystal at a water-air interface

Abstract

The experimental observation of the sudden collapse of a two-dimensional colloidal crystal at a water-air interface is reported and explained here for the first time. A thin film of an aqueous suspension of polystyrene particles of 0.53 mu m diameter is confined between a smooth glass plate and an air bubble. A single layer of a 2D colloidal crystal is observed in this thin film using an optical reflection microscope equipped with a video camera and an image processor. On gradual draining of water from this thin film, particles forming a 2D colloidal crystal suddenly emerge to the water-air interface. At the interface they flocculate to form a compact 2D sheet of particles in which the particles touch each other, forming hexagonal and square 2D structures. This phenomenon is explained on the basis of the interfacial surface energies and the interparticle interaction potential at the water-air interface.