Exploration of Thermocapillary Movement of Bubbles in Narrow Cylindrical Tubes

Abstract

This thesis documents the experimental investigations of thermocapillary motion for an air bubble in 5 cSt silicone oil confined to a cylindrical tube. Thermocapillary motion of a long bubble in a tube is due to an applied temperature difference that results in a surface tension gradient along the surface of the bubble. This surface tension gradient manifests as a thermocapillary stress which causes the bubble to move towards the region of higher temperature. These experiments assume low Reynolds and capillary numbers. The Bond number is on the order of 1, which results in deviation from current literature, where the assumption of a small Bond number is made. Then the Péclet number ranges from 10^-2 to 1, a similar range to other experimental investigations of the thermocapillary effect. This study explores thermocapillary motion of long bubbles for a variety of temperature gradients (β) ranging from 0.4 to 1.5 °C/cm. NTC thermistors in a voltage divider circuit are used to capture temperature measurements along the cylindrical tube. At the same time, photos are taken of the bubble progression to determine bubble position and length. The primary results are the bubble length and velocity over time which can be correlated to average temperature acting on the bubble at a given time step. The capillary number (Ca) and modified capillary number (∆σ^∗) are used to represent the experimental results. These parameters are the dimensionless representation of bubble velocity and temperature gradient respectively. The experimental results of this thesis are compared to that of theory and the order of magnitude for bubble velocity found experimentally is different than expected. The current hypothesis is that small deviations from the fluid regimes of theory result in very large deviations from literature. It is found that for large enough temperature gradients (β > 0.8 °C/cm), the bubble velocity is not constant as is typically seen in literature. Instead, as the bubble migrates towards the hotter region, the bubble begins to accelerate. Results also indicate that the average temperature acting on the bubble and fluid properties have been found to play a larger role in bubble velocity than previously accounted for in literature. Considerations for change in viscosity have been accounted for in the analysis of the data presented in this thesis.