INFLUENCE OF DYNAMIC SURFACE-TENSION ON THE SPREADING OF SURFACTANT SOLUTION DROPLETS IMPACTING ONTO A LOW-SURFACE-ENERGY SOLID SUBSTRATE

We have investigated the impact of single droplets of various surfacta nt solutions on a low-surface-energy solid substrate using a high-freq uency visualization technique (one picture every 100 mu s) Whatever th e surfactant, the drop spreads and retracts in about 1 s under the act ion of inertia and capillarity, respectively. During retraction, the c apillary waves can be amplified and, in some cases, even yield droplet bouncing. Then, the droplet may slowly spread again due to gravity an d the unbalanced capillary forces at the contact line between the drop let and the substrate. During the fast spreading process (2-3 ms), the droplet surface increases by almost one order of magnitude since its shape changes from a sphere to a flat pancake; this causes a strong de viation from thermodynamic equilibrium. The relevant surface property is therefore the dynamic surface tension which we have evaluated using a maximum bubble pressure apparatus. We have shown that droplet retra ction is drastically influenced by the adsorption kinetics of the surf actant which limits the return to equilibrium surface tension. (C) 199 7 Academic Press.