Distinct coalescence behaviors of hot and cold drops in the presence of a surrounding viscous liquid

Abstract

Coalescence of a millimeter-sized drop initially touching a pool of the same liquid in the presence of another surrounding viscous liquid is studied in this work, wherein the drop may be hotter or colder than its surroundings. Moreover, the effect of the outer fluid viscosity on the coalescence dynamics and thermal convection is examined. An axisymmetric numerical model is employed to investigate the drop merger dynamics, wherein the drop and pool are modeled as water fluid, and the surroundings are modeled as silicone oils of different viscosities. The coalescence behaviors of hot and cold drops are found to be significantly different, especially at higher temperature differences. An otherwise partial coalescence for an isothermal system turns into a case of total coalescence when the drop is made colder than its surroundings, whereas the behavior in the case of a hot drop does not depart qualitatively from that of a corresponding isothermal system. Thermal convection has been examined in terms of the penetration depth of hot or cold fluid into the pool. Hot drops are found to have a greater penetration depth as compared to cold drops for higher viscosities of the surrounding fluid. The penetration depth is also related to the size of the leading vortex ring and the maximum vorticity magnitude.