Diffusion of Particles in Free-Standing Liquid Films

Self-diffusion and relative diffusion of hard spheres are measured in free-standing soap films using video imaging. We found the following: (1) Particle self-diffusion in the film is faster than in the three-dimensional fluid, in quantitative agreement with Saffman's calculation for diffusion in a thin sheet of fluid. (2) For a pair of particles, the average rate of separation between them appears to depend logarithmically on their initial distance. This suggests the presence of long-range hydrodynamic interaction in the film despite viscous damping at the liquid-air interfaces.