Phase separation produces an emulsion of liquid mixtures with several components - an effect that can also be observed in the Greek liqueur ouzo, which becomes cloudy when water is added. In addition, phase separation occurs in many other areas, such as cloud formation in the sky or molecular condensation in biological cells. Normally, phase separation requires contact with solid particles, since wetting their surfaces lowers the amount of energy required for this purpose. Scientists at the Max Planck Institute for Dynamics and Self-Organization (MPI-DS) led by Stefan Karpitschka have now found that not only does wetting drive phase separation, but also, conversely, phase separation drives wetting. Normally, classical forces such as gravity or capillarity cause the spreading of liquids. But phase separation also actively drives surface wetting, sometimes much faster than classical forces. "This makes it technically possible to influence thin films of complex liquids on surfaces, for example in manufacturing processes that involve a phase change," explains Karpitschka, group leader at MPI-DS.
Phase separation influences liquid spreading
To determine the effects of phase separation on liquid spreading, the researchers studied droplets of liquid mixtures on solid surfaces. Youchuang Chao, lead author of the study, describes the main findings, "We observed an unexpected phenomenon compared to the known dispersion laws for liquids made of only one component. Phase separation occurs at the edge of the droplet, which allows the liquid components to concentrate in this specific region, thus controlling chemical processes." These results demonstrate the strong coupling between dynamic wetting and phase separation at the molecular level. The study's new findings could thus spur the development of new strategies for surface processing and finishing. These include oil recovery from surfaces and applications in the production of microchips and semiconductors.