MEASUREMENT OF THE DRAG COEFFICIENT OF SPHERICAL-PARTICLES ATTACHED TO FLUID INTERFACES

The drag coefficient, beta, of spherical particles attached to a pure air-water interface is determined, The method is based on the measurem ent of the particle velocity V, under the action of a well-defined lat eral capillary force F. The capillary force is created by controlled d eformation of the water surface by means of a Teflon barrier whose ver tical position can be precisely adjusted. The magnitude of the force i s calculated by means of the theory of capillary interaction between a sphere and a vertical wall (Kralchevsky ct al., J. Colloid Interface Sci. 167, 47, 1994). The drag coefficient is calculated from the ratio beta = F/V at small Reynolds numbers, The dependence of the drag coef ficient on the particle size and the three-phase contact angle is dete rmined. For small spheres, which do not create substantial deformation of the fluid interface, beta is always smaller than the Stokes coeffi cient, beta(S) = 6 pi eta a (eta is the water viscosity and a is the p article radius). For large spheres, however, beta can be greater than beta(S). This higher hydrodynamic resistance can be explained by the p resence of a curved meniscus around heavier particles, The measured va lues of beta are compared with theoretical calculations and very good agreement is reached. It is demonstrated that the method is sensitive to the presence of adsorbed surfactants and that it can be used for th e determination of the surface viscosity Of adsorbed layers. (C) 1995 Academic Press, Inc.