"Experimental techniques are developed for observing droplet evaporation in the submicroscopic size range. An expansion-compression cloud chamber with an atmosphere of tank helium saturated with water vapor is employed. This work was undertaken with the aim of examining submicroscopic droplet behavior and the existence of re-evaporation nuclei, a form of memory effect.
The theories of evaporation and nucleation on submicroscopic drop residues were re-examined, and alterations which appear to be consistent with the results of this work were incorporated into the evaporation theory.
The observed continually decreasing evaporation rate with diminishing droplet size confirms the existence of re-evaporation nuclei, but these observed terminal evaporation rates appear to be too small to be compatible with existing theory. The results do support the viewpoint that surface effects predominate in controlling the terminal rate of evaporation.
When the critical supersaturations required for renucleation on the residues were analyzed with Fletcher's heterogeneous nucleation theory by assuming a value for m, the experimental points agreed well with theory which postulated that the evaporation coefficient varied directly as the area of the drop after the drop reached a certain size. It appears that this work could lead to a verification of Fletcher's heterogeneous nucleation theory, providing that the drop contaminant(s) can be determined and an independent measurement of size can be made.
An independent mobility measurement was attempted, but yielded null results. These results are consistent with the existence of an insoluble surface layer on the drop. Such a layer is consistent with sizes determined by using Fletcher's heterogeneous nucleation theory for values of m less than 0.95.
Calculations which assumed the drops to be pure water predicted complete evaporation in 0.37 seconds after the beginning of the cloud chamber compression. The experimental results indicate that the drops approach a stable size. Determination of radii with the Kelvin equation for pure water yielded mean radii of 112 Å and 91 Å for respective times of 0.57 and 0.88 seconds after the beginning of the compression.
Evaporation theory, assuming the presence of a solute in the drop, predicts a stable size for the drop in 0.38 seconds. Experimental results disagree by showing a finite evaporation rate much later in the compression. The Kelvin equation, amended for solute content, predicts mean drop radii of about 70 Å and 60 Å for the respective times 0.57 and 0.88 seconds.
Corresponding radii determination using Fletcher's heterogeneous nucleation theory for an assumed m = 0.95 yielded radii of 80 Å and 218 Å respectively. These latter values are consistent with the null result of the mobility experiment"--Abstract, pages ii-iv.
Kassner, James L.
Rivers, Jack L.
Carson, Ralph S.
Lund, Louis H., 1919-1998
Skitek, G. G. (Gabriel G.)
Johnson, Charles A.
Anderson, Richard A.
Ph. D. in Physics
National Science Foundation (U.S.)
University of Missouri at Rolla
ix, 119 pages
© 1966 James G. Smith, All rights reserved.
Dissertation - Open Access
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Smith, James G., "Re-evaporation nuclei and evaporation in a Wilson cloud chamber" (1966). Doctoral Dissertations. 442.