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

The drop spreading of water and aqueous solutions of ethanol and nonionic s urfactant on hydrophobic substrates (alkylsilane treated glass) have been i nvestigated. For the low viscous liquids and solutions, the spreading on th e surface of hydrophobic glass rod was also studied and compared to the dro p spreading experiment. In both experiments, care was taken to ensure a min imum impact of inertial forces. The results for the aqueous systems show ra pid 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 equil ibrium conditions. During the initial spreading phase, the wetting front ex hibits similar to t(1.2) spreading law. Two more viscous liquids, ethylene glycol and glycerol, were also examined and found to show a. weaker time-de pendence in the whole spreading regime. An similar to t(1/10) scaling of th e drop radius versus time was for these liquids observed in the asymptotic long-time regime. For the surfactant solution, a slow relaxation towards eq uilibrium was observed following the initial fast spreading phase. The rate -limiting process in this regimes was in the drop spreading experiment foun d to be surfactant adsorption from the bulk to the expanding liquid-vapour interface, whereas surface diffusion at the liquid-vapour interface appeare d rate-determining in the rod experiment. The reason for this is the differ ences in aspect ratio between relative expansion of the liquid-vapour and s olid-liquid interfaces during spreading in the two experiments. In the stud y of surfactant solution spreading, the importance of surface relaxation pr ior to contact of the solution and the solid was also pointed out. (C) 2001 Elsevier Science B.V. All rights reserved.