Microscopy on thermal capillary waves in demixed colloid-polymer systems

Recently we have shown how to time length- and timescales in demixed colloid-polymer dispersions in such a way that thermal capillary waves at the free interface between demixed fluid phases can be studied directly by means of laser scanning confocal microscopy [Aarts, Schmidt and Lekkerkerker, Science 304, 847 (2004)]. Here, we focus on several static properties of the interface. We show that the probability of fluctuations of the local interface position around its equilibrium value is Gaussian. By comparing two-point correlations of these fluctuations as a function of distance with predictions from capillary wave theory, we obtain results for the interfacial tension and the capillary length. The presented technique enables us to measure also the probability distribution of the tilt angle of the local interface normal and the vertical direction.