Peculiar wetting behavior of nanodroplets comprising antagonistic alcohol-water mixtures on a graphene surface

Binary mixtures, consisting of partially and totally wetting liquids with respect to a specific substrate and known as an antagonistic mixture, are commonly encountered. Although the wetting behavior of antagonistic mixtures deviates from that of conventional liquids, studies exploring this phenomenon are scarce. At the nanoscale, the wetting behavior of nanodroplets containing such mixtures may diverge even further from our established understanding. Employing Nanoscale Molecular Dynamics simulations, we explore the wetting behavior of nanodroplets containing mixtures of water (partial wetting) and ethanol or isopropanol (total wetting) on a graphene surface. By adjusting the water fraction (x_w) in the antagonistic mixture, three distinct wetting states have been identified: (I) In total wetting regime (low x_w), spontaneous spreading is observed; (II) In plateau regime (medium x_w), alcohol leakage from the droplet occurs, with the apparent contact angle (CA) remaining constant; (III) In partial wetting regime (high x_w), the CA increases with x_w and alcohol molecules tend to accumulate at the contact line. Unexpectedly, the spreading coefficient (S) remains positive and turns negative only as x_w near unity. Furthermore, Young's equation proves to be inapplicable even when S < 0. These peculiar wetting phenomena are comprehensively discussed and elucidated.