  In our daily lives, after shaking a salad dressing, we see the coarsening of
oil droplets suspended in vinegar. Such a demixing process is observed
everywhere in nature and also of technological importance. For a case of high
droplet density, domain coarsening proceeds with interdroplet collisions and
the resulting coalescence. This phenomenon has been explained primarily by the
so-called Brownian coagulation mechanism: stochastic thermal forces exerted by
molecules induce random motion of individual droplets, causing accidental
collisions and subsequent interface-tension driven coalescence. Contrary to
this, we demonstrate that the droplet motion is not random, but
hydrodynamically driven by the composition Marangoni force due to an
interfacial tension gradient produced in each droplet as a consequence of
composition correlation among droplets. This alters our physical understanding
of droplet coarsening in immiscible liquid mixtures on a fundamental level.
