Van der Waals-Zeeman Colloquium

Tuesday, 12 Dec 2006, 16:00h

The Casimir effect and surface forces in wetting

Dr. Salima Rafai
Van der Waals-Zeeman Instituut, Universiteit van Amsterdam

Thin films are of great importance in many applications, and an understanding and controlling their thickness is of great importance in many processes. I will present experiments on the equilibrium thicknesses of wetting films, and the forces that govern these. After a general introduction on surface forces, I will show how a thin wetting film can be used as an analogue of a vacuum cavity for Casimir forces.

In 1948, Hendrik Casimir predicted that the so-called zero-point fluctuations give rise to an attractive force between objects separated by a vacuum. This force from 'nothing' is a particularly striking consequence of the quantum theory of electrodynamics. In analogy, when confining concentration fluctuations of a two-component liquid mixture between two walls (i.e., a thin wetting film) an analogue force should exist. We show experimentally that this purely entropic force can be either attractive or repulsive, depending on the boundary conditions for the fluctuations: for symmetrical boundary conditions an attractive force is found while asymmetrical ones lead to a repulsive force, leading to a thinning and thickening of the wetting film, respectively.