The motion and deformation of a single bubble flowing through a contraction in a viscous liquid

Dr Ching-Hsien Chen, Dr B Hallmark, Prof. J. F. Davidson

Project status: ongoing

Polymeric foams consist of many gas bubbles contained within a small amount of highly viscous, or viscoelastic, liquid. When these foam systems flow through constrictions or other complex geometry, the bubbles are observed to have a rich variety of complex behaviour. A key to understanding this complexity is to understand how a single bubble contained in a viscous liquid moves and deforms as it flows through a contraction. The tranche of work described here systematically examined, and quantified, the behaviours of both a single air bubble, and clusters of air bubbles, contained within a Newtonian polybutene oil having a viscosity of 70 Pa s (70,000 times that of water). Experiments were carried out in the multi-pass rheometer and images captured using a solid-state high speed camera at a frame rate of 500 frames per second. 

Remarkable changes of bubble shape were observed when a single bubble passed through an orifice:

• as a bubble approached the orifice, the bubble elongated in the direction of flow;
• after a bubble had passed through the orifice, the liquid motion on the downstream side caused the bubble to take on the shape of a crescent moon;
• rather than undergoing break-up, the crescent-shaped bubble returned to its initially spherical shape significantly downstream of the orifice.

Some of these bubble shapes are shown in Figure 1: here, the air bubble is silhouetted in black, and the orifice constriction appears as two horizonal black bars in the centre of each image.

Figure 1.

Image analysis was used to quantify the shape changes and movement of the bubble, which allowed direct comparison to both flow simulation using computational fluid dynamics (CFD), detailed on the next page, and to a newly-developed geometric theory. The geometric theory assumed that the bubble would behave in a similar manner to a 'blob' of liquid due to the viscous forces dominating the surface tension forces in the flow (high capillary number). An illustration of bubble deformation predicted by geometric theory is shown in Figure 2.

Figure 2: prediction of bubble shape and motion from geometric theory.

The findings from this section of research have been submitted to the journal Chemical Engineering Science for their consideration to publish. A preprint of this paper can be found here. The quality and quantity of experimental work in this area was such that it was able to be used as a benchmark against which to compare CFD studies. This study is decribed on the next page.