BROWNIAN-MOTION AT LIQUID-GAS INTERFACES .3. EFFECT OF INSOLUBLE SURFACTANTS

The drag force of a translating sphere half immersed on a flat liquid- gas interface in the presence of insoluble surfactants is analyzed. Sp ecial attention is paid to some singularities of the perturbed adsorpt ion GAMMA and of the drag force. It is shown that in the frame of Stok es hydrodynamics the steady-state perturbation GAMMA does not vanish a t infinity but tends to constant and the drag force due to the Marango ni effect is infinite. These discrepancies are explained considering t he incorrectness of the Stokes velocity field far from the sphere. The Oseen approximation is proposed as a better hydrodynamic model and, a s a result, a finite force proportional to the logarithm of the Reynol ds number is obtained.