Back in August, I spent 2 weeks with Durham University’s volcanology research group, devising and conducting some experiments. In a nutshell, I was looking at how bubbles behave in a non-Newtonian fluid called hydroxyethyl cellulose solution (HEC solution hereafter), and seeing how the behaviour of the bubbles changed as I varied the rate at which gas flowed through the solution.
These experiments were very different to what I normally do for my PhD, so it was fun to gain experience in something different! Durham is also where I did my undergraduate degree so it was great to go back and see my past lecturers.
The experiments were based on a paper called “Continuous chain of bubbles in concentrated polymeric solutions” by Kliakhandler (2002). The paper described a phenomena called bubble chains. In everyday life, we are used to seeing bubbles travelling as individual, separate spheres, such as in fizzy drinks. Kliakhandler (2002) described bubbles forming a stable, continuous, and slowly rising chain of linked bubbles, with a similar appearance to beads on a string – see Picture 1. These bubble chains would only form in concentrated polymer solutions, in this case, a solution of cellulose, the polymer which forms leaves and stems.
Picture 1: Picture of bubble chains in HEC solution from Kliakhandler (2002).
More recent papers by Divoux et al. (2009) and Divoux et al. (2011) described slightly different bubble chains which were made up of narrower bubbles or were more tube- or flue-like. Interestingly, it was not clear what the relationship between the bubble chains presented by the various authors was – this was our motivation for conducting more experiments.
Another intriguing aspect is whether or not these bubble chains could be related to volcanic degassing. The mechanisms by which volcanoes degas is still not certain, providing another motivation for these experiments.
For my experiments, I used a Perspex tube full of HEC solution, at various concentrations. I then attached a nozzle which injected gas into the base of the solution. I varied the rate at which gas was injected into the solution using a flowmeter. The overall aims of the experiments were:
- To form the bubble chains first described by Kliakhandler (2002).
- To attempt to form different bubble chains, with rounder bubbles, narrower bubbles, and so on.
- If we could form different bubble chains, to quantify when the different bubble chains would form.
Overall, the experiments were a great success! I was able to recreate the bubble chains described by the papers that motivated these experiments. I observed several types of bubbles chains: chains of round bubbles (Picture 2); chains of narrow bubbles (Picture 3); and a more tube-like structure which the gas travelled through (Picture 4).
Picture 2: simple chain of connected bubbles in HEC solution, within the Perspex tube. Scale bar shown on the right hand side.
Picture 3: another type of bubble chain, with more elongate bubbles, which formed within the HEC solution at a different flow rate to the bubble chains shown in Picture 2.
Picture 4: an interesting structure, where the gas travelled through the HEC solution as a tube, rather than as a chain of bubbles, as seen in Pictures 2 and 3.
With some additional work, I hope to produce a paper from these experiments, which details the specific parameters needed to produce each of these bubble chain morphologies. HEC solution is commonly used as an analogue for magma in many experiments, so I think characterising the material would be very useful. Further work in this field may also help volcanologists pin down the mechanisms for volcanic degassing.