An experimental analysis of the adiabatic bubble formation in pure liquids at constant flow rates is presented here. The role played by various controlling parameters on the transient behavior of the bubble ebullience from a vertically submerged capillary orifice in the isolated bubble regime is studied. The dynamics of the growing bubble is visualized using a high speed digital camera which captures the events at mili-scale level. An image processing software was used to post process the visual data obtained to determine the formation time as well as the shape and size of the bubbles. To ascertain the effects of fluid properties (surface tension and viscosity), experiments were conducted with four pure liquids; the test fluids (water, ethanol, propylene glycol and glycerol) selected were such that they have vastly different fluid properties. Constant air flow into the orifice was varied over a wide range for the same fluid to understand the influence of gas flow on the phenomenon. Also, capillary tubes of 3 different diameters (0.32 mm, 1.0mm and 1.76 mm) were used to analyze the role of orifice diameter on bubble formation.
The results show that changes in gas flow rate, orifice diameter, liquid viscosity and surface tension significantly affect the transient behavior. In the case of pure liquids, for a fixed flow rate and orifice diameter, lower surface tension results in the formation of smaller bubbles with least formation time. Likewise, a high value of viscosity resulted in larger bubbles. Also, use of a larger orifice produced bubbles of bigger sizes for a particular fluid. Two regimes of bubble formation were observed depending on the flow rate employed. Up to a certain value of flow rate, the bubble diameter was independent of the flow rate and was depended only on the orifice diameter and the properties of the fluid used. Beyond the threshold value, bubble diameter increased with increase in flow rate. The presence of the two regimes was a strong function of orifice diameter, which was evident when a smaller diameter was employed where the second regime was almost non existent in most cases.
Experiments were also performed at high flow rates beyond the single bubble regime to study the aperiodic bubble formation characterized by multiple bubble formations. The role of gas flow rate in determining the various regimes of irregular bubble formation was studied and the influence of orifice geometry in determining the phenomenon has been underlined. With the increase in flow rates beyond single bubble regime, the wake of a departing bubble started influencing the subsequent growing bubble resulting in a premature release of the growing bubble. Depending on the orifice size, the non-linear characteristics like bubble coalescence comes into play where two or more departed bubbles move up as a group. These groups of bubbles dispersed into small groups further away from the orifice as it translates to the liquid surface. However, with the use of a smaller diameter, the bubbles exhibited a different growth behavior with the trailing bubble merging with the leading bubble near the orifice tip resulting in packets of bubbles of volume equal to the sum of the bubbles joined together and rising up to the surface of the liquid column as a single unit.
A sample study was carried out with aqueous SDS solution to observe the transient bubble dynamics of air bubbles in the presence of surface active chemicals. The bubble formation appeared to follow a trend similar to those observed with pure liquids, though the necessity of a much detailed investigation was evident to completely understand the effect of dynamic surface tension reduction on the complex phenomenon of bubble ebullience.