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Drops Behaving Badly - Colin Bain

Durham University, UK

The coupling of interfacial properties to fluid flows leads to often complex and unexpected behaviour. This talk will tell three stories from our work on microfluidic and nanofluidic flows.

(i) Emulsion droplets with ultralow interfacial tensions can be manipulated with light. When an oil droplet is pulled apart with optical tweezers the two daughter droplets remain connected by a stable oil nanothread that does not break up by a Plateau-Rayleigh instability. Liquid flows through these threads in a direction opposed to capillary flow, driven by optical pressure on the trapped droplets.

(ii) Inkjet droplets of ethanol-water mixtures exhibit vigorous Marangoni flows as they dry. Sub-micron tracer particles do not follow the streamlines as expected, but collect into an aggregate in the centre of the droplet in the shape of a mill stone. The physical origin of the cross-stream migration remains uncertain.

(iii) Captive oil drops in a SDS/butanol/brine solution exhibit a remarkable cyclical behaviour lasting more than an hour in which a water droplet grows inside the oil drop and then bursts through the oil under gravity before the cycle repeats itself. A qualitative physical model driven by butanol dissolution in the oil is proposed.


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