Spreading dynamics of liquids and surfactant solutions on partially wettable hydrophobic substrates

The drop spreading of water and aqueous solutions of ethanol and nonionic surfactant on hydrophobic substrates (alkylsilane treated glass) have been investigated. For the low viscous liquids and solutions, the spreading on the surface of hydrophobic glass rod was also studied and compared to the drop spreading experiment. In both experiments, care was taken to ensure a minimum impact of inertial forces. The results for the aqueous systems show rapid initial spreading processes that abruptly halts after less than 30 ms, as the interfacial tension forces are balanced. In the case of surfactants solutions, this is followed by slower adsorption driven drift towards equilibrium conditions. During the initial spreading phase, the wetting front exhibits ∼ t1/2 spreading law. Two more viscous liquids, ethylene glycol and glycerol, were also examined and found to show a weaker time-dependence in the whole spreading regime. An ∼ t1/10 scaling of the drop radius versus time was for these liquids observed in the asymptotic long-time regime. For the surfactant solution, a slow relaxation towards equilibrium was observed following the initial fast spreading phase. The rate-limiting process in this regimes was in the drop spreading experiment found to be surfactant adsorption from the bulk to the expanding liquid-vapour interface, whereas surface diffusion at the liquid-vapour interface appeared rate-determining in the rod experiment. The reason for this is the differences in aspect ratio between relative expansion of the liquid-vapour and solid-liquid interfaces during spreading in the two experiments. In the study of surfactant solution spreading, the importance of surface relaxation prior to contact of the solution and the solid was also pointed out