NEAR-CONTACT THERMOCAPILLARY MOTION OF 2 NONCONDUCTING DROPS

The axisymmetric, thermocapillary-driven motion of a pair of non-condu cting, spherical drops in near contact is analysed for conditions of s mall Reynolds and Marangoni numbers. The pairwise motion and an associ ated contact force are computed by considering touching drops in point contact. Relative motion for nearly touching drops results from the c ontact force balanced by a lubrication resistance. A new, analytical s olution is obtained for the axisymmetric temperature field around an u nequal pair of non-conducting, tangent spheres embedded in an ambient temperature gradient. Numerical results for the pairwise migration vel ocity, contact force, and the relative and individual drop velocities are presented for all size ratios and a wide range of viscosity ratios , and asymptotic formulae are derived for small size ratios. For nearl y equisized drops, the ratio of the relative velocity for two drops in near contact to that for widely separated drops is similar for thermo capillary-driven and gravity-driven motion. For small and moderate siz e ratios, however, this ratio is much larger for thermocapillary-drive n relative motion than for gravity-driven relative motion, indicating that the former represents a more efficient coalescence mechanism. An explanation for this finding is provided in terms of the thermocapilla ry motion of the interface of the larger drop aiding the withdrawal of continuous phase from between the two drops.