Atypical wetting behavior of binary mixtures of partial and total wetting liquids: leak-out phenomena

The wetting behavior of simple binary mixtures containing both partial and total wetting liquids is investigated on the commonly used substrates. The variation of the contact angle (CA) of the binary liquid with the composition (x_P) on different solid surfaces is obtained. It is interesting to find that for the droplet which can exhibit self-propulsion, four different regimes can be identified in the CA-x_P plot. Among them, the plateau regime where the CA remains a constant value in a certain concentration range is most intriguing. In contrast, for the droplet which cannot self-propel, the CA grows monotonously with the concentration of the partial wetting liquid. The leak-out phenomenon in which the droplet is surrounded by the precursor film is believed to occur in the plateau regime, and it is examined by observing the encounter of two miscible droplets by optical microscopy. The apparently noncontact repulsion between the moving and static droplets reveals the presence of the precursor film leaked out from the self-propelled droplet.