Thermocapillary deformations of a two-layer system of liquids under laser beam heating

Abstract

Heat and mass transfer in a bilayer system of viscous incompressible liquids with interfaces have been studied theoretically and experimentally. Thermocapillary convection and interface deformations have been induced by an intense thermal exposure on the free boundary due to the action of a laser beam. The evolution of the interfaces and liquid behavior have been experimentally investigated. The static profile of the liquid – liquid interface and free surface have been measured using the laser-sheet technique. The scheme of the experiment and parameters of the physical system have been presented. The theoretical study of the system dynamics includes the development of a correct mathematical model in the framework of continuum mechanics and creation of an effective numerical algorithm, validated with the help of new experimental results. An outline of the numerical method has been given. The proposed approach allows one to describe the characteristics of the arising flows, to predict the shape of thermocapillary deformations of the liquid interfaces, and to analyze the interaction of the thermal gravitational and thermocapillary convection. The mechanisms causing the occurrence of the decaying oscillations that were first discovered by the authors have been discussed. The results of the numerical study for a two-layer system of the “silicone oil – glycerin” type are in good agreement with the experimental data, and enable to estimate the influence of the upper layer thickness on the features of fluid motion and to forecast the rupture time of the top liquid under local heating at the free boundary. The rupture time depends on the rate at which the threshold temperature difference in the top layer sets in. For systems with relatively large thickness of the upper layer the appearance of a stable profile of the thermocapillary flexure on the free surface without layer breakdown is characteristic.