Interaction of two drops at different temperatures: The role of thermocapillary convection and surfactant

Abstract

The interaction of two drops is studied experimentally: a small droplet falls on a large sessile drop located on a hot wall. The temperature of the falling droplet is 20 °C, and the wall temperature is 80 °C. It is well known that the surfactant suppresses the Marangoni flow (Ma). This paper considers the influence of surfactant in the presence of several key factors – dynamic and thermal ones. When a droplet impacts a liquid layer, the heat and mass transfer is associated with both the dynamic factor (inertial forces and pressure “jumps” inside the droplet) and the thermal factor (an increase in the surface temperature gradient). The novelty of this work is that it for the first time shows that at the moment of extremely short-term interaction of drops, there is 7–8 times increase of velocity inside the drop. In this case, the determining effect on the convection enhancement is associated with thermocapillary convection, and the role of the dynamic factor is insignificant. The influence of graphite particles and surfactant of sodium dodecyl sulphate (SDS) has been investigated. The velocity “jump” resulting from the fall of the water droplet with surfactant is 4 times smaller as compared to the fall of surfactant-free droplet. The instantaneous velocity fields inside the sessile drop have been experimentally studied using Micro Particle Image Velocimetry (Micro PIV). The obtained results are of great importance for the correct modeling of heat and mass transfer and have a wide application for sprays, as well as for the multiphase flows.