DYNAMICS OF CONTACT LINES IN FOAM FILMS

Different factors can contribute to the value of the line tension acting at the periphery of a thin foam film. To study one of them, the motion of the contact line, a new experimental technique has been developed. It was established experimentally that slowly shrinking small air bubbles attached to a liquid - air interface form a nonequilibrium (dynamic) contact angles. Aqueous solution of sodium dodecyl sulfate (SDS) are used at two different concentrations of added electrolyte. A black thin liquid film is formed at the top of the bubble. The experimental cell used allows measurements with shrinking, expanding and quiescent ("stopped") bubbles. Hysteresis of the contact angle has been observed. The results show that the shrinking of the contact line (advancing meniscus) causes deviation of the contact angle from its equilibrium value. The data allow independent calculation of the film and line tensions, gamma and kappa. For a "stopped" bubble both gamma and kappa relax with time: kappa tends to zero and gamma tends to its equilibrium value for infinite film known from other experiments. The results suggest that the concept of dynamic line tension must be introduced. The latter was attributed to the motion of the contact line.