A survey is provided of a variety of problems where a passive or an active drop experiences directed motion consequence of the action of an external or internal agent or a combination of both. An active drop is capable of reacting by engendering autonomous, self-propelled motion in favor or against the agent. The phenomena involved offer diverse complexity but one way or another the drop motion finally rests on thermo- or soluto-capillarity hence on interfacial tension gradients. Accordingly, here a minimal mathematical framework underlying such drop motions is provided when direct external temperature or solute gradients, illumination, internal heat generation or surface chemical reaction are incorporated into the physico-chemical-hydrodynamics.
Keywords: Bubbles; Drops; Laser; Marangoni effect; Soluto-capillarity; Thermo-capillarity.
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