Direct visual observation of thermal capillary waves, Science 304:847

We studied the free fluid-fluid interface in a phase-separated colloid-polymer dispersion with laser scanning confocal microscopy and directly observed ther-mally induced capillarywaves at the interface in real space. Experimental results for static and dynamic correlation functions validate the capillary wave model down to almost the particle level. The ultralow interfacial tension, the capillary length, and the capillary time are found to be in agreement with independent measurements. Furthermore, we show that capillary waves induce the spon-taneous breakup of thin liquid films and thus are of key importance in the process of droplet coalescence. At rest, the free interface between any two fluids, like that between a liquid and its vapor, appears to be smooth. Yet thermal motion inevitably gives rise to statistical fluctuations of the local interface position, leading to a rough interface. This phenom-enon was first predicted by von Smolu-chowski in 1908 (1); 5 years later, Mandel-stam quantitatively described the interface roughness in terms of thermally excited capillary waves (2). These capillary waves have been studied with light (3) and x-ray scattering (4–8). They play an important role in modern theories of interfaces (9–11) and have been argued to be essential in the rupture of thin liquid films (12, 13), as occurs in droplet coalescence. We show how to tune length and time scales with the use of colloidal suspensions, such that the fluctuating fluid-fluid interfaces can be seen directly in real space with a resolution comparable to the particle size. Moreover, the droplet coalescence event can be ob-served down to the scale of capillary fluc-tuations in quasi slow motion. In the capillary wave spectrum, each Fou-rier component hk of the interface displace-ment contributes according to the equiparti-tion theore