Contact-line instabilities in liquids spreading on solid substrates

An analysis of contact-line instabilities of spontaneous spreading of nonionic surfactant solutions is presented. The solutions were prepared using C12En surfactants in ethylene and diethylene glycol as solvents. The experiments were performed in two different configurations: drops on horizontal substrates and planar fronts on vertical substrates. The spreadings are produced on hydrophylic substrates (silicon wafers) in a controlled humidity chamber. It is found that for low humidities (RH <30%) the spreading are stable while for higher humidity and intermediate surfactant concentration (ranging from 0.1 up to twice the critical micellar concentration), contact line instabilities due to Marangoni effect are observed. A systematic study of these instabilities is presented and a qualitative explanation based on diffusion of surfactants on the interfaces is proposed.