Research into Multi-scale Examination of MultiPHase physIcs in flowS (MEMPHIS)

Posted by Nina Kovalchuk | February - 24 - 2017 | 0 Comment

Microfluidics enables formation of emulsions and foams with drops/bubbles of a submillimetre size and a narrow size distribution. Surfactants are usually used in emulsification/foaming processes to protect drops/bubbles against coalescence through creating protective adsorption layers on the phase interfaces. However, a recent study performed at the University of Birmingham has shown that a protective surfactant layer does not always work when drops are moving in the confined geometry of microchannel. Frequent coalescence events were in particular observed in the channels with their height smaller than their width for surfactant-laden drops larger than the channel height. An example of such coalescence is shown in the video below. The dispersed phase (water-glycerol mixture) contains 50 mM of ionic surfactant dodecyltrimethylammonium bromide insoluble in the continuous phase (silicone oil). The concentration of the surfactant is 2.5 times higher than the critical micelle concentration and according to performed estimation the surfactant adsorption process is completed before the coalescence takes place. A further study is in progress to reveal the mechanism causing the coalescence. One of the hypotheses assumes the polarisation of the drops due to surfactant redistribution caused by the shear stress near the channel walls.

Coalescence of surfactant-laden drops in a microchannel.

The channel width is 0.390 mm. Recording was made at 2000 frames per second, playback is at 30 fps.

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