Carol Wood

Laser-induced breakdown in large transparent water droplets

Recent experiments on the laser-induced breakdown (LIB) of large transparent liquid droplets are reviewed. A physical model of LIB processes is presented with the aim of integrating the following recent results: (1) the internal and near-field distributions for large transparent spheres; (2) the location of LIB initiation based on spatially resolved plasma emission spectroscopic techniques; (3) spatially resolved but time-averaged density of the plasma plumes and temperature of the atomic species within the plasma; (4) the plasma front propagation velocities inside and outside the droplet; and (5) the fate of the remaining superheated droplet and the expelled material.

Propagation velocity of laser-induced plasma inside and outside a transparent droplet

The supersonic propagation velocity of the emission front of plasma produced by laser-induced breakdown of a micrometer-sized transparent droplet flowing in a gas was measured with a streak camera at three intensity levels. At low input intensity, the plasma velocities in the gas away from and toward the shadow face were determined. At medium input intensity, the plasma velocities in the gas outside the shadow face and within the liquid (traveling toward the illuminated face) were measured. At high input intensity, the plasma velocities in the gas outside the shadow face, within the liquid, and in the gas outside the illuminated face were deduced.

Explosive vaporization of a large transparent droplet irradiated by a high intensity laser

Shadowgraph studies of the explosive vaporization of a transparent water droplet after irradiation by a high intensity beam show that dielectric breakdown occurs within the droplet shadow face and generates a dense plasma, which absorbs the laser pulse. The convective forces expel the vapor from the droplet shadow face. We have deduced (1) the vapor propagation velocities, (2) the recoil velocity of the remaining droplet, and (3) the deformation rate of the illuminated face. Droplets are noted to eject fingerlike material from the surface facing the single laser-vaporized droplet when the asymmetrical vapor intercepts the neighboring droplets.

Time-resolved shadowgraphs of large individual water and ethanol droplets vaporized by a pulsed CO 2 laser

Carbon dioxide laser-induced explosive vaporization of water and ethanol droplets at high laser fluence has been observed with time-resolved shadowgraphs. The asymmetry seen in the droplet vaporization can be qualitatively explained by comparison to the internal-field intensity distribution. A central green spot observed in the shadowgraph is attributed to the near-field distribution just outside the shadow face of the droplet when the droplet is illuminated by a visible laser. This spot can be used to probe the shape deformation and optical inhomogeneity of the droplet. The energy dependence of the explosive vaporization of water was also studied. Increasing the CO(2) laser fluence increases the rate of explosive vaporization.

Fluorescence imaging of CO 2 laser-heated droplets

Distortion, ejection, shattering, and propulsion of water and ethanol droplets containing Rhodamine 6G dye have been photographed at different time delays after initiation of a CO(2) laser pulse, whichcauses explosive vaporization of the droplets. We have developed a fluorescence imaging technique to photograph the liquid-phase portion of the ejected material and the parent droplet after irradiation by the CO(2) laser pulse.

Prime model extensions for differential fields of characteristic p ≠ 0

The main purpose of this paper is to show that there exists a prime differentially closed extension over each differentially perfect field. We do this in a roundabout manner by first giving new and simple axioms for the theory of differentially closed fields (in the manner of Blum [1] for characteristic 0) and by proving that this theory is the model completion of the theory of differentially perfect fields. This paper can be read independently from [10], where we gave more complicated axioms for the same theory (in the manner of Robinson [6] for characteristic 0). I am indebted to E. R. Kolchin for answering many questions and for making the manuscript of his forthcoming book [2] available to me.

On homogeneous nilpotent groups and rings

We give a new framework for the construction of homogeneous nilpotent groups and rings which goes a long way toward unifying the two cases, and enables us to extend previous constructions, producing a variety of new examples. In particular we find ingredients for the manufacture of 2 N0 homogeneous nilpotent groups «in nature»