Edward Y. H. Keng

Photophoretic Effects in the Stratosphere

Abstract Photophoresis, the movement of small particles under the influence of light, may exert a significant effect on stratospheric processes. The photophoretic behavior of salt and metallic particles from 0.1 to 2.0 microns in radius under pressures of from 4 to 50 mm Hg with artificial light of known intensity was investigated. The velocities expected to be imparted to particles by sunlight at altitudes of from 22 to 40 km were then calculated from typical measured force values. It is concluded that, in the absence of wind and other effects, some particles may actually be caused to rise against the force of gravity while others are induced to fall considerably more rapidly than they would under gravity alone.

Thermal precipitation and particle conductivity

Abstract Aerosol particles in a thermal gradient experience a force tending to drive them in the direction of the lower temperature. The force has been described both as strongly dependent and as almost independent of particle thermal conductivity. An analysis of new precipitation data involving both high and low conductivity particles reveals thermal forces to be of almost identical magnitudes regardless of conductivity.

Particle dynamics in centrifugal fields

Abstract The trajectories in centrifugal force fields of particles with diameters from one to 50 microns, were examined both theoretically and experimentally. Equations were developed and solved by analog computer for the non-steady, two-dimensional motion of particles in viscous fluids. An experimental centrifugal classifier was developed that permitted initial conditions and types and strengths of centrifugal fields to be varied widely. Experimental results are shown to be in excellent agreement with theoretical predictions for a wide range of particle diameters when a function generator fit of experimental drag coefficient data is employed.

Aerosols produced by X-rays☆

Abstract Small proportions of organic vapors in air produce aerosols under the influence of X-rays. The particulates are generally spherical in shape and less than a micron in diameter. Those formed from acetylene and benzene vapor appear yellow and brown, respectively. They are essentially nonvolatile at room temperature and are very stable even when exposed to the high vacuum of an electron microscope. However, a substantial size reduction was attained by several hours of heating at 250°C under atmospheric pressure. The size and quantity of the particulates formed under Xirradiation depend on the organic vapor, its concentration, the humidity of the air, the intensity of the X-rays, irradiation time, and aging time after exposure. The growth and formation of particulates do not stop immediately after exposure ceases. Infrared spectrum analysis provides information on the chemical structure of the particulates.

Particle size and the rate of radiant heat transfer to gas-suspended particles

Abstract Rates of radiant heat transfer to clouds composed of well-dispersed, non-luminous, solid particles of known concentrations were measured. In agreement with theoretical predictions, clouds of the smaller particles absorbed more radiant energy than those having the same mass concentration of larger particles. The absorption coefficient was found to be directly related to the cross-sectional area of the particles per unit volume of cloud. Equations are given describing the rate of radiant heat transfer to particle clouds in terms of the mean beam length. particle concentration, particle density and particle size.