Clyde Orr

Aerosol size and relative humidity

Abstract A theoretical and experimental investigation was made of the gain and loss of water with humidity change for particles of NaCl, (NH 4 ) 2 SO 4 , CaCl 2 ·6H 2 O, AgI, PbI 2 , and KCl having radii between 0.01 and 0.1 micron. Predictions were made using a combination of adsorption theory, the Kelvin equation, Ostwalds theory, and other thermodynamic considerations. Where actual data were nonexistent, empirical relations were employed to arrive at reasonable approximations. The indicated behavior of hygroscopic particles exposed to increasing humidity is to adsorb moisture amounting to a few molecular layers at low relative humidities; to dissolve as humidity increases, becoming saturated droplets and at the same time undergoing an abrupt size increase; and thereafter, as humidity increases still further, to grow larger and more dilute. The indicated behavior for decreasing humidity is for the droplet size to decrease as humidity decreases and, at a humidity considerably lower than that at which the particle initially dissolved, to recrystallize, undergoing an abrupt size decrease. For nonhygroscopic compounds, no abrupt solution or recrystallization irregularities are predicted. The predictions are in satisfactory agreement with experimental findings.

Application of mercury penetration to materials analysis

Abstract The technique and apparatus for determining the void spaces and pores in a porous material by use of a penetration porosimeter are described. From the volume of mercury penetrating the porous material as a function of pressure, other characteristics, such as the particle size distribution and specific surface, can be determined. In addition, information regarding the shape and structure of pores can be obtained from the volume of mercury expelled from the pores as a function of decreasing pressure.

An evaluation of pore structure by mercury penetration

Abstract The pore structure of graded series of controlled pore glasses and Nuclepore membranes—both materials having pores with right-cylinder characteristics—were assessed by electron microscopy and mercury penetration. From comparisons of results, parameters in mercury penetration theory were evaluated, the correction for compressibility was examined, the cause of hysteresis was explored, and the general accuracy of the technique was appraised.

Capillary hydrodynamic chromatography

Abstract Liquid chromatographic separation in capillary tubing of particles ranging from 0.5 to 30 μm is reported. Elution volumes are inversely related to particle diameter for materials of diverse composition such as latex, pollen, bacterial spores, silica, and whole cells. Relative elution times of particles are dependent on column diameter and both the velocity and viscosity of the mobile phase. Viscosity appears to affect the relative retention of small particles more than particles over 1 μm in diameter. Addition of ethyleen glycol to aqueous mobile phases diminishes peak trailing wiht latex particles. The practical limit on column length is approximately 300 ft.

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.

The photophoretic force

Abstract Photophoresis has many times been described qualitatively but quantitative information is scarce. Photophoretic-force measurements have been made with an apparatus that, in essence, compares the force with that of gravity. To the present, only light beams sufficiently intense to develop measurable forces with carbon black particles have been achieved. Unfortunately, these particles were actually agglomerates of smaller particles, making the formers properties such as density, considerably uncertain. Nevertheless, the results, in agreement with radiometer theory, show that the photophoretic force increases exponentially with particle radius. A gas pressure dependence, undetected by several previous investigators, was demonstrated also.

Rapid, automatic particle size analysis in the subsieve range

Abstract A new instrument is described for rapidly and automatically determining the size distribution of subsieve particles. The concentration of particles remaining suspended in a liquid at various sedimentation depths is determined as a function of settling time by a finely collimated beam of X-rays. The time required for a determination is minimized by continuously changing the effective sedimentation depth, a test of diameters from 50 to 0.75 μm requiring, for example, only 22.5 min for particles having a specific gravity of 2.6. An analysis of errors inherent in the technique and the instrument itself is included, showing that an overall accuracy near ±1% is attainable with a great many materials.

Thermal Precipitator for Continuous Aerosol Sampling

Thermal precipitation is attractive for airborne particle collection because of the high collection efficiencies which may be attained and the convenient examination of the deposit. Most thermal precipitators, however, have very low sampling capacities or flow rates. To take fullest advantage of the desirable features of thermal precipitation, a new precipitator was designed and constructed. It operated continuously, and deposited particles upon a moving tape or substrate. Complete collection of particles resolvable with an electron microscope was attained in a 3‐in. diameter precipitating zone at a flow rate of 1 l/min.

The aggregation of aerosols

Aggregation of the individual particles composing an aerosol is a phenomenon of primary importance in the aging of the aerosol. The influence of electrification and foreign vapours on the aggregation process has been investigated. Results indicate that electrification amounting to a few electron charges per particle has little effect on the rate of aggregation, but has a decided influence on the shapes of the aggregates. In the case of aerosols whose particles have a significant vapour pressure, certain substances which lower this vapour pressure have been found to increase the rate of aggregation, presumably by reducing the vapour cushion surrounding the particles. When the particles are readily soluble in a substance whose vapour is present, the rate of aggregation also appears to be increased because of reduction of the vapour layer about the particles.