Max Goldman

Sonoluminescence: An alternative “electrohydrodynamic” hypothesis

A mechanism is proposed for the sonoluminescence (SL) arising from a single bubble maintained in levitation by an acoustic field. This proposal follows from a plasma diagnostic analysis which reveals that a single Ar bubble is characterized by a sparklike plasma (electron temperature and density: 20u2009000 K and 1025 m−3, respectively). The theoretical scenario (based on four hypotheses) is as follows. During its expansion and the major part of its collapse, a levitating bubble is governed by Rayleigh–Plesset dynamics. Several ns before maximum collapse the bubble interface becomes unstable and needlelike jets (the number of which is thought to be about 10) invade the bubble more or less symmetrically. Each jet (radius≈65 nm) reaches a distance equal to half of the radius of the bubble and releases a droplet (radius≈150 nm), so that an intracavity spray is released about 3 ns (perhaps less) before the time of collapse. It is implicitly proposed that the acoustic pressure at which inward jets (likely to disin...

Surface modification of LDPE by a DC corona discharge generated in a point-to-grid system: the influence of geometric parameters of the system on modification power

Abstract We examined the influence of the geometric parameters of the system on the modification power as determined by the contact angle on the surface of the treated low-density polyethylene (LDPE). We have found that (1) with a constant electric energy to generate a corona discharge, the modification power decreases as the distance from the center of surface (the point on the film immediately below the point electrode) increases and that the corona discharge in a point-to-grid system can modify the film surface over a wider area than in a point-to-LDPE system without grid; (2) with a constant discharge current, the modification power on the center of surface decreases when the point-to-grid gap in negative corona treatment increases, but increases in positive corona treatment; (3) the modification power compared to the electric energy used to generate a corona discharge (the yield) is inversely proportional to the point-to-grid gap. However, in a positive corona discharge, the yield did not reach zero when the point-to-grid gap was extrapolated to infinity, possibly because the streamer reduces the effective point-to-grid gap and produces neutral activated species along the streamer; and (4) in a negative corona, the modification power as measured by the temperature increases at the plain electrode (anode) and varies with the energy dissipated by neutral activated species.

Ozone in wire-to-plane electrode systems

There is a lack of knowledge on the role of the gas chemistry in the ageing of materials in air insulated electrical equipment. This paper aims to provide data on this subject through results obtained on one of the gaseous products (O/sub 3/) generated by corona discharges, using a wire-to-plane geometry in ambient air, under continuous or alternating voltage. It is shown how its production depends both on the electrode material and on the gas humidity, and how results can be partly analysed in terms of plasma catalytic phenomena.

Picosecond sonoluminescence: An alternative electrohydrodynamic hypothesis

A mechanism is proposed for the sonoluminescence (SL) of single bubble in levitation in an acoustic field. The theoretical scenario is that during expansion and the major part of its collapse, a bubble is governed by Rayleigh-Plesset (RP) dynamics. Several ns before maximum collapse the bubble interface becomes unstable and /spl sim/10 to 20 needlelike jets are thought to invade the bubble symmetrically. Each of them releases a submicronic droplet. An intracavity spray is shed about 3 ns before the time of collapse. Because of the distortion of the electrical double layer surrounding the bubble, both jets and droplets are electrically (but oppositely) charged. The electrical field is so high that it is thought that electron emission occurs to such an extent that the jets are ablated and that a microplasma highly charged with energy is released within <50 ps (T/spl ges/30000 K; N/sub e/: 10/sup 26/ m/sup -3/). The mechanism proposed is showed to apply to the case of bubbles in non-aqueous solvents, too.

Influence of water layers on the electrical behaviour of polymer foils submitted to AC air coronas

Since any solid surface exposed to a gaseous atmosphere is covered with a water layer, interactions between a corona discharge and a polymer surface have to proceed through the water layer which already initially exists on that surface. At the beginning of corona exposure, forces acting on the water layer are predominantly repulsive forces which are exerted by the corona induced aerodynamic flow, and which lead to evaporation processes but, after a certain corona exposure duration, attractive forces, due to the dipoles grafted in increasing number on the surface oxidized by the corona discharge, prevail. The paper focuses on the transition phenomena which bring the first stage to the second one.