The strength of long-range forces across thin liquid films

We present experimental results on the instability of thin liquid polydimet hylsiloxane films intercalated between a solid substrate and a liquid surro unding medium. The instability is induced by long-range van der Waals force s across the film. From the characteristic wavelength of the instability an d from the apparent contact angle of the equilibrium structures, we deduce an effective Hamaker constant (A(132)) about 2-3 orders of magnitude higher than what is expected from calculations based on the individual Hamaker co nstants. With air as the surrounding medium, we obtain the expected values. (C) 1999 Academic Press.