Experimental observation of the solutocapillary migration of air bubbles in inhomogeneous liquid solutions

Abstract

The ability of air bubbles in a liquid to move spontaneously toward a decrease in the surface tension [1] can have a significant effect on various technological processes occurring in multicomponent media. This is especially important under conditions when the gravitational mechanisms responsible for motion are weak or absent (for example, in microgravity) [2]. Usually, this motion arises in nonuniformly heated liquids due to the temperature dependence of the surface tension coefficient σ. In this case, the effect is called thermocapillary migration of bubbles [3]. Meanwhile, in nonuniform-composition media (in particular, in binary solutions of liquids), the other reason for changing the surface tension σ can exist. We imply the dependence of surface tension on the concentration of a dissolved substance. In such a situation, the solutocapillary migration, i.e., motion of bubbles toward the more surfactant of a solution, will be observed. Experimental examination of the phenomenon is much hampered by both subsidiary effects associated with the presence of gravity and difficulties in producing a stationary concentration gradient. It should also be noted that the methods of measuring such gradients on the free surface still remain poorly developed. Therefore, the solutocapillary migration of bubbles, which is predicted theoretically, was not previously observed in experiments.