Drops behaving badly

By Miro Cafolla, Postgraduate and International Tutor

St John’s College Café Scientifique LRC Wednesday 7 October 2019

During Freshers’ Week, one of the main cultural highlights has been undoubtedly the Cafe Scientifique talk delivered by Professor Colin Bain. Professor Bain is a full Professor in the Chemistry Department and Pro-Vice Chancellor for Research. He presented us, in a very accessible and friendly way, with some of his latest breakthrough research on fluid dynamics and how the properties of macroscopic thin film is deeply influenced by the micro and nano-scopic details of its constituent molecules. 

Professor Bain moved to some mind-blowing examples and showed unexpected experimental observations where a good understanding remains to be developed- and in some cases, magic things may even happen!

Professor Bain started explaining the relevance of such fundamental research for future industrial and technological applications. This field is indeed crucial for a number of areas such as lubrication, food processing, printing and coating technology, process engineering and pathology. If we take lubrication as an example, it is a very thin film of molecules at the boundary of two sliding surfaces to determine how good a lubricant is. Similarly, in order to improve our printing technology, we would need a better control on the single ink molecules and their interactions with the extrusion jets and the paper substrate.

After highlighting the importance of studying fluid dynamics at the nanoscale, Professor Bain moved to some mind-blowing examples and showed unexpected experimental observations where a good understanding remains to be developed- and in some cases, magic things may even happen!

The first example was about tracer particles routinely used to visualise flows. When the fluid flow is not turbulent, tracer particles are expected to simply follow the streamlines. However, if we watch what happens to the drying of an ink composed of a solvent mixture, tracer particles tend to quickly move across streamlines in order to create a particle-rich phase which is reminiscent of the shape of a mill stone.

In the presence of surfactants, the oil drops undergo a remarkable process: a water droplet grows within an oil drop until it bursts through the oil droplet under gravity

Professor Bain then moved to an oil-water system. A simple experimental method for measuring the affinity of an oil to water is to measure the thickness of a captive droplet of oil within a glass cuvette full of water.  We can add surfactants to the oil-water mixture. Surfactants are molecules which have affinity to both oil and water. In the presence of surfactants, the oil drops undergo a remarkable process: a water droplet grows within an oil drop until it bursts through the oil droplet under gravity. This process starts again and keeps on for about an hour.

The last example was not less fascinating. When in water, oil droplets may be deformed with optical tweezers. When a single drop is pulled apart, there is still a connection between the two daughter droplets. There is indeed an invisible thread of oil which can be arbitrarily long and does not undergo any instability. Interestingly, we can use laser beams as suction pumps and move the liquid!

The talk was highly stimulating and inspiring and even if those who knew anything about fluid mechanics, were deeply intrigued and fascinated by the videos

The audience was amazed by such unexpected experimental observations and almost every participants suggested a number of possible explanations. As Professor Bain suggested, however, further studies are definitely needed to fully understand the physics and chemistry behind the previous observations.

The talk was highly stimulating and inspiring and even if those who knew anything about fluid mechanics, were deeply intrigued and fascinated by the videos and will never look at any ink or milk drops in the same way again!

Photographs by Miro Cafolla

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