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"Thermocapillary Flow and Aggregation of Bubbles on a Solid Wall"

Hiroki Kasumi (Advisor: Paul J. Sides)

During the electrolytic evolution of oxygen bubbles forming on a vertically oriented transparent tin oxide electrode, bubbles were found to be mutually attractive [1, 2].  The mechanism of the aggregation had never been explained satisfactorily until Guelcher et al. [3] attributed it to thermocapillary flow.  The gradient of surface tension of the liquid at the bubble’s surface, which was established because of reaction heat and ohmic heat loss at the electrode wall, drives flow of the liquid adjacent to each bubble; the bubble "pumps" fluid along its surface away from the wall.  Fluid flows toward the bubble to conserve mass and entrains nearby bubbles in the flow pattern.  The same logic would apply when two bubbles of equal size are adjacent to each other on a warm wall.  Each bubble drives thermocapillary flow and hence entrains the other in its flow pattern, which drives the aggregation.  Our objective here is to perform experiments where the temperature gradient at the wall is well known and controlled.  The theory can be quantitatively tested by studying aggregation of bubble pairs of equal size, and by varying system parameters such as temperature gradient, bubble size and fluid viscosity. 

Two types of silicone oil (h = 0.02 and 0.50 Pa s) were used as a fluid and two equal sized air bubbles were injected into the cell with a syringe.  The center-to-center distance of bubbles was observed through a microscope.  Bubble radius ranged from 0.40 mm to 0.65 mm and the temperature gradients along with the cell ranged from 1400 to 5000 K/m.  The bubbles aggregated when heat flows from the wall to the fluid.  The velocities of bubbles were in the range of 1 – 10 mm/s.  The separation r decreased more quickly when the temperature gradient was higher, bubble size was larger, and the oil viscosity was lower.  r decreased more rapidly as the bubbles approached each other.  Below is a plot of scaled data by appropriate time scale and bubble radius.  Dimensionless time was arbitrarily set to be zero when the dimensionless center-to-center distance between the bubbles was 4.  All the bubble trajectories fall onto one line, especially in the range of dimensionless distance from 4 to 3.  This means the relative movement of the bubble pair is proportional to the temperature gradient and bubble size and it is inversely proportional to the viscosity of the oil. This result strongly suggests that the thermocapillary flow-based aggregation mechanism is correct.

REFERENCES

[1]. Sides, P. J., Ph.D. Thesis, University of California, Berkeley, California (1981).

[2]. Sides, P. J., and C. W. Tobias, J. Electrochem. Soc., 132, 583 (1985).

[3]. Guelcher, S. A., Y. E. Solomentsev, P. J. Sides, and J. L. Anderson, J. Electrochem. Soc., 145(6), 1848 (1998).

[4]. Kasumi, H., S. A. Guelcher, Y. E. Solomentsev, P. J. Sides, and J. L. Anderson, J. Colloid. Interfacial Sci., 232, 111 (2000).