Spontaneous spreading of surfactant solutions on hydrophilic surfaces: CnEm in ethylene and diethylene glycol

The spontaneous spreading of solutions of nonionic CnEm surfactants in ethy lene and diethylene glycol on hydrophilic surfaces is investigated. The par ameters of the study are the relative concentration of surfactant, defined as being the ratio between the bulk concentration and the critical micellar concentration, and the relative humidity (RH) of the surrounding atmospher e. At low RH, RH < 30%, normal spreading is observed, i.e., the radius R of the drops scales like t(1/10), t being the time. For RH > 40%, an accelera tion of spreading rate and the emergence of contact-line instabilities are observed in an increasing range around the cmc. A systematic study of these spreading laws and instabilities is reported. We propose a qualitative mod el of the processes, where the mobility of the surfactant on the solid surf ace controls the start of a Marangoni flow at the free liquid surface. Once star;ted, this flow dominates the spreading.