COAGULATION OF HYDROPHOBIC AND HYDROPHILIC SOLIDS UNDER DYNAMIC CONDITIONS

The attachment of solid spherical particles to one another under dynam ic conditions has been investigated theoretically. The particles are a ssumed to be uniform with regard to their bulk properties; however, th eir surfaces have been altered so that one particle is hydrophobic, an other hydrophilic. The role of modification has been revealed in the ' 'apparent'' slippage of liquid over a solid hydrophobic surface, and i n an increase in the critical interlayer thickness at which the partic les coagulate. A problem similar to the Reynolds one has been solved f or the purpose of determining the force of hydrodynamic resistance to the approach of particles to one another (the difference between the p roblems consists only in the change of boundary conditions on the hydr ophobic surface, and in the spherical shape of interacting bodies). It has been shown that in the case under consideration the resistance to the motion cannot be determined by the Taylor formula. This formula l ater must be multiplied by a correction function dependent only on the ratio of the quadruple slippage coefficient to the interlayer thickne ss. The equation for the approach of bodies to one another under the e ffect of the hydrodynamic resistance force has been solved. An express ion has been derived for the particles approach velocity. On its basis , criteria for the attachment of particles to one another have been fo rmulated in the form of a condition for the Reynolds number. It has be en shown that in limit cases a hydrophobic particle behaves like a hyd rophilic one, or moves like a bubble. An intermediate case can also be realized, in which the motion velocity necessary for coagulation depe nds on the slippage coefficient value. (C) 1995 Academic Press, Inc.