SURFACE-TENSION DRIVEN RANDOM MOTION OF A MERCURY DROP IN HNO3 AND K2CR2O7 SOLUTION

An irregular motion of a mercury drop in a petri dish filled with HNO3 and K2Cr2O7 solutions is studied experimentally. Larger mercury globu les are extended and broken into smaller droplets, while smaller ones translate or rotate randomly in an otherwise quiescent fluid owing to the non-uniform distribution of the surface tension associated with ch emical reaction. The translating mercury drop shows a peculiar form li ke a croissant with broad convex surface perpendicular to the directio n of motion. Although the direction and magnitude of the movement of t he drop change randomly, the average speed is almost independent of it s size if it satisfies a certain criterion. The fluctuation of velocit y shows a power spectrum proportional to 1/f(alpha), where alpha is ne arly equal to 1 in fairly wide frequency range. A simple analysis on t he basis of the Stokes approximation is made, which explains the relat ion between the average speed and the drop size.