MIGRATION OF AIR BUBBLES IN SILICONE OIL UNDER THE ACTION OF BUOYANCY AND THERMOCAPILLARITY

The buoyancy driven migration of air bubbles in dimethyl-polysiloxane and phenylmethyl-polysiloxane under isothermal conditions is studied. The radius of the migrating bubbles increases or decreases with time. The changes of the radii are attributed to an exchange of gas between the bubbles and polysiloxane. The direction of the gas transport depends of the state of saturation of the liquid with air. The velocity of migrating air bubbles as function of time in dimethyl-polysiloxane can be described by the Rybczynski-Hadamard relation using the instantaneous values of the velocity of migration and the instantaneous value of the radius of the bubbles. The analysis of corresponding the data obtained with air bubbles migrating in phenylmethyl-polysiloxane reveal that in that liquid the velocity of migration is described by the Stokes relation using the instantaneous values of the velocity and the radius of the bubbles. The bubbles behave like ''hard spheres with a time dependent radius''. The different behavior of air bubbles in the two types of polysiloxane is attributed to structural differences between macromolecules forming them. The observed thermocapillary motion of air bubbles in dimethyl-polysiloxane in the presence of buoyancy forces is described by the theory of Young, Goldstein, Block (J. Fluid. Mech. 6, 350 (1959)). No thermocapillary motion of air bubbles is found in phenylmethyl-polysiloxane. This is expected from the behaviour of air bubbles in phenylmethyl-polysiloxane under isothermal conditions.