Surface Tension Driven Random Motion of a Mercury Drop in HNO₃ and K₂Cr₂O₇ Solution

Abstract

An irregular motion of a mercury drop in a petri dish filled with HNO₃ and K₂Cr₂O₇ solutions is studied experimentally. Larger mercury globules 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 chemical reaction. The translating mercury drop shows a peculiar form like a croissant with broad convex surface perpendicular to the direction of motion. Although the direction and magnitude of the movement of the drop change randomly, the average speed is almost independent of its size if it satisfies a certain criterion. The fluctuation of velocity shows a power spectrum proportional to 1/f^α , where α is nearly equal to 1 in fairly wide frequency range. A simple analysis on the basis of the Stokes approximation is made, which explains the relation between the average speed and the drop size.