Pockeling of Freshly Swept Surfaces of Solutions

Abstract

WHEN the surface of a dilute solution of surface active material is freed from insoluble contamination by sweeping with a movable barrier of metal or paper and the surface examined by lightly dusting with talc, both procedures originated by Miss Pockels, we have noted upon the surface a striking effect which is readily recorded with a motion picture camera. Here and there, from time to time, the talc suddenly withdraws from circular patches which may be up to a centimetre or more in diameter. After we had examined this phenomenon for some months, we found that it likewise had been described by Miss Pockels in 1917¹. She explained it by the unlikely hypothesis that all substances that lower the surface tension of water exist to a certain extent in the form of colloidal particles or micelles, even in very dilute solution, and that when these chance to approach the surface the ” pockel” appears. Alternative possibilities are that the ” pockels” arise not from micelles but from local concentrations within the liquid due to the fluctuations of ordinary molecular movement, or to dust particles (or possibly talc particles) which have sorbed material within the liquid, or to convection currents within the liquid which is no longer homogeneous because the solute must reconcentrate in the surface to satisfy the theorem of Gibbs. All of these suggestions seem quantitatively inadequate. Pockeling ceases when the system has come completely into equilibrium.