CORRESPONDENCE
WILLIAM R. WlLCOX
Inclusion Movement Evidence t h a t inclusions sometimes m o v e a w a y f r o m t h e h e u t source rather than towurd
it d u r i n g t e m p e r a t u r e g r a d i e n t processes
f o r r e m o v i n g gas a n d liquid inclusions f r o m crystals n a recent review ( 5 ) , it was pointed out that solvent
I and gaseous inclusions frequently can be removed from
chemicals and crystals by application of a temperature gradient. When solubility or vapor pressure increases with temperature, the inclusions move toward the heat source. However, evidence was obtained which indicated that certain inclusions can move away from the heat source (5). Aqueous-grown crystals of ADP (NH4H2P04) and AgN03 were placed on a horizontal hot plate. As expected the aqueous inclusions migrated down and out of the bottom of the crystals, leaving exit pits there. I n addition, exit pits were noted on top of the crystals, and it was hypothesized that these pits were caused by inclusions containing both solvent and an air bubble. T h e hypothesis was that solvent evaporated from the bottom (hotter) part of the bubble leaving behind crystalline material. T h e vapor then condensed on the upper (cooler) part of the bubble and dissolved fresh material. For the bubble to have continued to move, the solution must have flown back down to the hot portion of the inclusion. I t was thought that gravity provided the necessary driving force for this Aow. T h e purpose of the studies described here was to test this new inclusionmovement hypothesis in general and in detail. Artificial inclusions in sodium chloride crystals were used for most of the experiments. T h e inclusions were formed by drilling a 0.75-mm-diam hole several milli-
R. Witcox is associated with the Departments of Materials Science and Chemical Engineering at the University of Southern California, Los Angeles, Calq. 90007. W h e n this correspondence was written, he was with T h e Aerospace Gorp., El Segundo, Calf. T h e author is grateful to R. Teviotdale for preparation .f the artgcial inclusions and the organic cells. T h i s work was supported by the U S . Air Force under contract No. F04701-68-C-0200. AUTHOR William
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meters into a cube of salt cleaved from a melt-grown single crystal. T h e cube was then placed in a saturated salt solution and crystal growth allowed to proceed until the hole was completely sealed over. T h e result was a rough cylindrical inclusion containing both solution and an air bubble. Experiments were performed with crystals heated from below, from the side, and from above. I n all cases, the bulk of the solution in the inclusions migrated toward the heat source, while the air bubble and some of the solution traveled in the opposite direction. Thus it is clear that gravity is not required for gas-liquid inclusions to move in the "wrong" direction. T h e rates of movement of the bubbles were much greater than those for purely liquid inclusions. For example, in a horizontal gradient of about 60 'C/cm, at an average temperature of 85 "C, the bubble moved at about 0.4 rnm/hr, which was an order of magnitude faster than for the liquid. With a mean temperature of 160 "C and the bottom of the inclusion heated, a bubble and a large liquid inclusion moved at nearly the same rate in opposite directions (Figure I ) . With the crystal heated on top to an average temperature of 100 "C and a gradient of 98 "C/cm, the inclusion moved down at about 0.6 mm/hr. I t was also discovered how bubbles are introduced into inclusions when none are initially present. FVhen the liquid inclusions reached the surface of the crystal, the liquid very slowly drained out. Bubbles of air replaced the liquid, and each slowly moved awal- from the heat. A rough-walled tube resulted from the bubble movement. Crystals heated from the side and from the bottom exhibited horizontal and vertical tubes, respectively. The tubes in top-heated crystals curved downward at an appreciable angle to the vertical (Figure 2). Similar inclusion behavior was noted i n a 1270 camphor-anthracene mixture subjected to zone melting ( 4 ) . Bubble-containing melt inclusions moved into the solid contained in a horizontal cell made of large micro-
scope cover glasses. Foreign particles were moving rapidly around and spinning on the surface of several bubbles, indicating considerable fluid motion. A foreign particle was observed similarly moving on the surface of a bubble in one of the salt experiments. Discussion
The experimental evidence shows that gravity is not required for the solution to return from the cool to the hot side of the bubble. It is proposed that surface tension supplies the driving force for this circulation. Due to the evaporation-condensation process, the salt concentration is higher on the hot side of the bubble. Since surface tension increases with salt concentration, a surface tension gradient is produced. The gradient in surface tension then causes circulation from the low- to the high-surface tension region and also causes a net force on the bubble directed toward the low-surface tension region (6). Thus, for example, heating the crystal from above causes the bubble to move down the inclusion and produces dissolution at the bottom. The above mechanism must also be responsible for bubbles moving away from the heat source in liquid inclusions of natural crystals ( 3 ) . Related behavior has been noted in upward drilling of ceramics by molten salts with no temperature gradient applied ( I , 2). Where bubbles are present the ceramic is dissolved much faster than elsewhere. Again, a gradient in composition around the bubble is thought to lead to vigorous circulation and hence to rapid dissolution. T h e author has noted rotation of the fluid around bubbles on dissolving natural sodium chloride. (The bubbles and the particles marking the fluid motion were both present as inclusions in the crystals.) Addition of a detergent stopped the motion. A solution of half water and half denatured alcohol caused the bubbles to vibrate vigorously, to “pump” the solution, and to cause rapid dissolution in their vicinity. Pure denatured alcohol caused no bubble motion. (It would be interesting to observe bubble-liquid inclusion behavior in a temperature gradient with the inclusion containing an alcohol-water mixture.)
Figure 1. Separated bubble (on top) and liquid in NaCI crystal heated from below f o r 26 hr on a hot plate at 175 ‘C
Conclusions
We have shown that inclusions containing both liquid and gas can split in a temperature gradient, with most of the liquid moving toward the heat source and the bubble and some liquid moving in the opposite direction. This phenomenon depends not on gravity but on a surface tension gradient due to the concentration gradient caused by evaporation-condensation across the bubble. Bubbles are introduced by leakage of liquid from a n inclusion when it reaches a surface, with replacement of the liquid by the ambient gas. REFERENCES (1) Busby, T. S., and Barkcr, J., J . Am. Ccrarn. Soc., 49,441 (1966). (2) Preston, F. W., and Turnbull, J. C., Am. 3.SCL, 239, 72 (1941). (3) Roedder E., Private Communication, US. Geological Survey, Washington, D.C.(176f). (4) Wilcox, W. R “Fractional Solidification Phenomena ” TR-0200(9250-03)-2, The Aerospace dorp., El Segundo, Calif. (November 19i8). (5) Wilcox, W.R.,IND.ENG.CHEM., 60 (3), 13 (1968). (6) Young, N. O.,Goldstein, J. S., and Block, M. J., J . Fluid Mcch., 6,350 (1959).
Figure 2. Bubbles moving down inclusion in NaCl crystal heated on top to a gradient of 98 “C/cm at a mean temperature of 100 “C VOL.
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