Splashing of water drops on solid and wetted surfaces: hydrodynamics and charge separation

Abstract

The charges separated by splashes on solid and wetted surfaces are shown to be carried on the liquid fragments ejected from the crown. Experimental observations on the nature of these splashes are presented and mechanisms for the formation and break-up of the crown are proposed. The surface on which a splash occurs is generally left with a positive charge, the magnitude of which decreases with increasing concentration of solute in the drop. This is explained by the disruption of the electrical double layer. When drops containing ammonium hydroxide in excess of 2.5 x 10-5 mol 1_1 splash on ice at — 1 °C the ice receives a positive charge. In this case, freezing potentials appear to play a role in the charging. In an applied electric field the charges separated by splashing increase as the field is increased, in agreement with an induction mechanism of charging. Also, in an applied electric field, the charges initially increase as the impact speed is increased but beyond a certain impact speed the charges decrease. This is explained in terms of the relative magnitudes of the time of contact of the ejected liquid fragments with the crown and the relaxation time of charge carriers in the liquid. The role of charge separation due to splashing in thunderstorm electrification and in the build-up of space charges during rainfall is discussed.