Bipolar excitation of circular surface waves on dielectric-supported oil films

Surface waves on a thin oil film placed on top of a ground-supported dielectric sheet has recently been observed to exist when the oil film has been precharged adequately and when a sharp grounded electrode is suddenly introduced into the vicinity of the oil layer [Lee and Phan 1995]. This experimental work investigates the circumstances leading to the generation of analogous wavelike phenomena through a new, bipolar scheme of DC corona charging of the separate sides of a dielectric sheet and susequent triggering by externally short-circuiting the opposing points. In advance preparation, both sides were coated with a thin film of oil by smear. Results revealed that, consistently in all cases, an ensemble composed of two circularly expanding waves one on each side, was generated. Wave fronts appeared locked in and expanded in decaying unison. With eventual weakening at larger radii, two new features were spotted: (a) Ridge of each wave front would evolve into periodic structure associated with an apparent annular rim instability and (b) In some situations, the two co-moving wave fronts might become uncoupled in the process.