Journal of Colloid Science最新文献

The coagulation rate of uncharged NaCl aerosols with mean particle radii of 25 and 45 A. was measured by means of a flow method: the aerosol was passed through a pipe and the ratio of particle concentrations at the inlet and the outlet of the pipe was measured nephelometrically after “developing” the particles by condensing DBP vapor on them. Corrections for particle losses due to diffusion to the walls of the pipe were estimated in separate experiments with diluted aerosols. The absolute value of the particle concentration was determined by ultramicroscopic counting of developed aerosols. The average residence time of the aerosols in the pipe, i.e., the coagulation time, was measured by means of a coarse oil mist. Experimental values 13.4 × 10⁻¹⁰ and 15.0 × 10⁻¹⁰ cm.³/sec. were obtained for the coagulation constant of the aerosols with r = 25 and 45 A., respectively. The values calculated by means of the formula for the number of collisions between gas molecules, taking into account the molecular attraction forces between the particles, are 10.5 × 10⁻¹⁰ and 14.1 × 10⁻¹⁰ cm.³/sec., respectively.

Theories of the thermal force acting on aerosols are reviewed. Experimental measurements of the thermal force on spherical sodium chloride aerosols in argon are reported. The experimental measurements in the slip flow region are compared with slip flow theories, including a second-order theory presented here.

The detection of dust particles in air by an acoustic phenomenon is described. The particles are passed through a sensor in which they are gradually accelerated to about 100 m./sec. and then the particles are suddenly projected into a wide exit cavity. At this point a pressure pulse is generated by a particle and gives an audible click. The sound pulse lasts 2 to 20 milliseconds, depending on the entrance design, and has an optimum signal-to-noise ratio of $\text{50}\text{1}$. This sensor in its present state detects particles down to 5 microns with no change in signal amplitude with size. It has been applied in the laboratory to count ice crystals in supercooled clouds.

An aerosol spectrometer was used to determine the size distribution of sulfur-containing particulate materials in urban areas of northern and southern California. Based on a limited number of summer daytime samples, a typical Menlo Park size distribution had a mass median diameter of about 0.3 μ with upper and lower quartiles at about 0.9 and 0.1 μ, respectively. A typical Pasadena size distribution would be about one-tenth micron larger. The diameters given are for equivalent unit-density spheres of the same settling velocities as the sampled particles.

An apparatus used to study collision and coalescence of liquid aerosols and some of the physical quantities enhancing or hindering these processes is described. The relative trajectories, collision, and coalescence of two oppositely charged water droplets are shown for the droplets approaching at right angles. The droplets were 96 μ and 79 μ in radius and had velocities at impact of 330 cm./sec. and 110 cm./sec., respectively. It was found extremely difficult to make the two droplets collide when both were highly charged with the same sign of charge. However, droplets highly charged with the opposite sign of charge had a high collision rate.

The development of the theory for collision and coagulation of aerosols in Brownian motion is reviewed. Theoretical conclusions resulting from the application of continuum theory are compared with calculations based on the kinetic theory of gases. These two approaches lead to different results which are applicable only to charged dilute aerosol clouds, in a limited range of the ratio of the mean free path of the suspending medium λ to the particle radius R. The present theories suggest that (a) the rate of coagulation of aerosols reaches a maximum between the ratio λ/R = 5 and 10, (b) charging of particles should have an increasing effect on coagulation rates as the ratio of the average charge per particle to the particle radius increases. There is a substantial region of aerosol behavior for which the present theory and experimental work are incomplete, particularly in accounting for the effects of charged particles. It is likely that a completion of the theory for coagulation of aerosols must await new developments in the physics of ionized dense gases.

A method of measuring the thickness of soap bubbles from the interference effects in transmitted monochromatic and white light is described. With monochromatic light it is based on the observation of the maxima and minima in the transmitted intensity; with white light the film thickness is obtained from the colors of the transmitted light. These interferences are most pronounced in the region where the angle of incidence is close to 90°, i.e., along the outline of the bubble.