Moving Mercury Drop by Chemical Reaction

1. Beating of a Mercury Drop Mercury, or quicksilver, is the only material among the metals that is in the liquid state under room temperature. The surface tension of that material is 475 [dyne/cm] at 20◦C, which is quite large compared with that of the water (72.75 [dyne/cm]). In general, the effect of the gravity force becomes smaller for smaller drop compared with the effect of the surface tension, which explains why the shape of a small mercury drop is nearly spherical, and rolls down over a slightly tilted plane. Spontaneous beating of a mercury drop in sulfuric acid H2SO4 associated with potassium permanganate KMnO4 or associated with potassium dichromate K2Cr2O7 has been widely known (Freundlich, 1930; Alyea and Dutton, 1960; Keiser et al., 1979; Kai, 1986; Avnir, 1989), and its mechanism is attributed to an electro-capillary phenomenon, in which surface tension between two kinds of fluid varies by applying a potential difference and charges (Lippmann, 1873; Bard and Faulkner, 1980). Similarly, in “beating mercury heart” system, the potential difference between the mercury and the tungsten or iron tip in the presence of an acidic solution as well as a corroding electrode is essential to electron transfer processes, which changes the magnitude of surface tension. By an appropriate positioning of the electrode tip with respect to the mercury drop, a kind of electric circuit is switched on and off, which leads to a mechanical oscillation. Sometimes the formation of film on the surface of the mercury seems to be involved. The center of gravity of the drop, however, remains fixed in these cases.