Diverse wetting behavior of a binary mixture of antagonist liquids: Nanodroplet with finite precursor film and leak-out phenomenon

The binary mixtures containing the partial and total wetting liquids corresponding to a specified substrate are commonly seen. However, studies of their wetting behavior, particularly the leak-out phenomenon, are very scarce. The wetting state of a nanodroplet of the binary mixture on a smooth substrate is investigated by Many-body dissipative particle dynamics. The influences of the molecular size of the partial wetting liquid and the affinity between the two liquids on the leak-out phenomenon are examined. Dependent on the mixture composition (ϕ_p), three wetting regimes of the nanodroplet can be identified: (I) total wetting, (II) partial wetting with precursor film, and (III) partial wetting. The binary droplet always spreads spontaneously in regime (I), while its contact angle (CA) changes with the composition monotonously in regime (III). However, in regime (II), the CA remains constant regardless of the composition. The droplet morphology exhibits the fried-egg shape, a spherical cap surrounded by a precursor film. The compositions in both egg yolk and egg white regions remain unaltered for varying ϕ_p in the plateau regime (II). Although the affinity between the two liquids is strong, some of the total wetting liquid always escape from the spherical cap to precursor film in regime (II), the so-called leak-out phenomenon. The outcome can be beneficial to the application of nanotechnologies such as lithography and microfluidics.