Foam free drainage and bubbles size for surfactant concentrations below the CMC

Abstract

Abstract Foams produced with surfactant concentration below the Critical Micelle Concentration (CMC) are usually moderately stable due to high drainage rates and intense bubbles coarsening/coalescence. Aim of this work is to examine how such low surfactant concentration affects foam destabilization and elucidate the interplay between free drainage and bubble size variation. Foam destabilization experiments are conducted at varying SDS concentrations (below the CMC) where the evolution of liquid fraction and bubbles size is registered simultaneously. Instantaneous volume measurements of the drained liquid and the remaining foam yield the evolution of the global liquid fraction and drainage rate in the foam. Continuous electrical conductance measurements give the local liquid fraction and drainage rate in the foam. Microphotographs allow estimation of bubble size distribution and bubble population at regular time intervals. The present data show that the lifetime of moderately stable foams depend largely on surfactant concentration below the CMC but this effect does not scale linearly with surfactant concentration. Furthermore, measurements are fitted to semi-empirical expressions and are compared to a modified Leonard and Lemlich (L–L) drainage model that has been expanded to incorporate bubble size evolution. The latter is a rough approximation based on certain assumptions but it is a fair approach given the excessive difficulty of detailed numerical calculations. The comparison reveals the significant role of bubble size on the free drainage of moderately stable foams. Interestingly, incorporation of global liquid fraction data into the modified L–L model yields surface shear viscosity values in agreement with literature.