Dynamics of Interfaces Between Drops With Miscible Liquids

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In the proposed research we will investigate the dynamics of interfaces between droplets of miscible liquids by experiment, theory and Computer simulations. This application is motivated by the recent experiment al observation of one of the applicants showing that sessile droplets of completely miscible liquids do unexpectedly not instantaneously coalesce after peripheral contact (if their contact angles are sufficiently small). Instead, after lateral contact, a microscopically thin liquid bridge connects the droplets and delays the droplet fusion. The droplets exchange liquid through this film, but they remain separated. Occasionally this film can destabilize into pulsating waves. Only after some time (up to minutes) the droplets finally merge into a single droplet and the fluid comes to rest. The coalescence dynamics is obviously influenced by surface (Marangoni effect) and bulk flow processes (which are induced by the liquid mixing). Droplets with high contact angles coalesce instantaneously. This shows that the neck topology in the moment of the first droplet contact is also an important parameter. In the framework of our application we want to investigate the coalescence behavior of droplets under defined conditions for different pairs of ifluids, different Substrates, different viscosities, to gain insight into the mechanisms, which govern the temporary non-coalescence. The experiments will be complemented by model calculations and simulations based on the lubrication and phase field theory. The understanding of the mixing behavior of different liquids, in particular that of small quantities (droplets) is of great technical importance (e.g. in micro-fluidics).