Interfacial dynamics in demixing systems with ultralow interfacial tension

We report measurements on fluid-fluid phase separation in a colloid polymer mixture, which can be followed in great detail due to the ultralow interfacial tension. The use of the real-space technique, laser-scanning confocal microscopy, leads to clear, well-defined images making quantitative comparisons to theory possible and being highly instructive. Simple scaling arguments are given why, in experiment, three steps of the phase separation can be observed: an interfacial-tension-driven coarsening, gravity-driven flow and finally the interface formation. All these processes are observed in a single experiment. The first stage can be quantitatively described by viscous hydrodynamics. Coarsening occurs through pinch-off events. The second stage begins at a typical size of similar to 2 pi times the capillary length reminiscent of the Rayleigh-Taylor instability. The liquid phase breaks up and becomes discontinuous. There is strong directional flow in the system, but the Reynold's number remains much smaller than unity. Finally, the macroscopic interface is formed, growing upwards, with a velocity comparable to the coarsening velocity in the initial stage. Again, viscous hydrodynamics apply with a characteristic velocity of the interfacial tension over the viscosity.