|M.Sc Student||Liat Bialik-Rosenfeld|
|Subject||Thermocapillary Interaction between Drops Induced by a|
|Department||Department of Chemical Engineering||Supervisors||Professor Emeritus Nir Avinoam|
|Dr. Lavrentev Olga|
The spontaneous motion that may be induced when an exothermal surface reaction occurs on the interface of small drops suspended in an immiscible viscous fluid has been studied. The reactant diffuses toward the interface in the ambient fluid and reacts with a substance stored in the drops. The heat of reaction, which is proportional to the concentration of the reactants, results in elevation of temperature on the drops' interfaces and in the external fluid. If another drop is present in the vicinity, the concentration and temperature in the gap region become higher than those in the outer regions. The inhomogeneous distribution of concentration and temperature along the drops' interface induces surface tension variations, Marangoni type flow and migration of the fluid particles relative to each other.
A diffusion-controlled transport and creeping flow are assumed. The spontaneous motion of the drops in the absence of body forces is considered as well as their motion under the combined effect of gravity and Marangoni effect. Temperature, concentration and the velocity fields were constructed and the migration velocities of the drops were computed for a range of the governing parameters such as the drops' separation distance, ratios of material properties and non-dimensional parameters such as Thiele modulus, heat production intensity etc.
An interesting case appears when the variations of the interfacial tension due to inhomogeniety of temperature and of concentration of the reacting species are of the same order of magnitude. In this case the thermocapillarity induced by the heat emitted by the exothermic chemical reaction and the one induced by the local reduction of the concentration of the surface active substance due to its depletion by the reaction, can be opposite effects counteracting each other. At certain cases, the interplay of these two effects results in dynamic stationary states in which the thermocapillary induced flow proceeds while the drops remain motionless. These stationary configurations present a novel feature in spontaneous Marangoni interaction of droplets, which cannot be observed in previously studied cases.