Condensed-Matter Physics & Materials Science Seminar

In quantum electrodynamics, Casimir forces (a special case of van der Waals forces) arise due to zero point fluctuations in the electromagnetic field. In the presence of boundaries, the fluctuations are modified, however, leading to a correction to the free energy that depends on the distance between the boundaries, and hence leading to an effective force that acts on the boundaries. The force is either attractive bringing the boundaries closer together or repulsive pushing the boundaries apart. A completely analogous situation arises when a liquid film is adsorbed on a solid surface. There are fluctuations in the liquid, but the fluctuations cannot extend past the vapor or solid interfaces, and so there appears a fluctuation-induced "Casimir" force between the vapor and solid interfaces. It has been predicted that the equilibrium thickness of a wetted film is affected by these Casimir forces. We have indeed suceeded in confirming the existence of the Casimir force in liquid helium films that wet a metal surface. In this case, the Casimir forces in question are due to fluctuations associated with superfluidity in the film. It has also been predicted that Casmir forces associated with thermal fluctuations in liquids near room
temperature are so strong that they can determine whether or not certain films will wet a surface. We are, at present, trying to confirm this prediction.