| dc.description |
Drops and bubbles are ubiquitous. Stokes, Rybczynskii, Hadamard, Boussinesq and others provided the drag law experienced by a drop. Young, Goldstein and Block and others studied the motion due to interfacial stresses induced on a drop immersed in a medium where an imposed thermal gradient exists. They demonstrated the possibility of its levitation in the presence of gravity. Levich, Sanfeld and others pointed out the role played by surfactants in affecting the drag law and the possible fluid motion inside the drop. All those works refer to passive drops, i.e. drops experiencing at most the interfacial stresses due to variation of interfacial tension with temperature or surfactant concentration in the surrounding fluid near the drop surface. After providing a succinct account of the results of the earlier theories and some relevant experiments, we consider the behaviour of an active drop, i.e. a drop with chemical reaction at its surface or with an internal heat generation source, etc. Attention is focused on the case of a drop immersed in a homogeneous surrounding when due to surface stresses (the Marangoni effect) and, consequently, due to thermo/soluto-hydrodynamic instability there is spontaneous breaking of the radial symmetry of the temperature and/or concentration distributions, hence overcoming the drag and originating self-sustained translational drop motion. Moreover, the autonomous motion may offer a multiplicity of steady values for a given external (weak) force like buoyancy, and levitation is possible for multiple (weak) buoyancy levels. |
