Two-dimensional observation of drainage and layering transitions in confined liquids

We used a surface forces apparatus (SFA) to investigate the drainage of thin films of undecanol from a pore consisting of two atomically smooth mica sheets. Time-resolved images of the contact area during drainage show an initial elastohydrodynamic deformation of the substrates. In the late stage, the expulsion of a molecularly thin layer of alcohol is imaged in two dimensions. We analyze the dynamics of the expulsion process and discuss the physical mechanisms in terms of a model that includes the hydrodynamic flow in the two-dimensional liquid layer and the elastic properties of the substrates. We also discuss the optical resolution required to observe layering transitions in two dimensions and present a new design of a SFA optimized for these purposes.