Liquid mixing, sculpting and stability using light-actuated photosurfactants
Demetrios Papageorgiou (Imperial)
A theoretical model is developed and used to study the utilisation of photo-surfactants and light illumination in the generation of Marangoni interfacial forces that can be useful in microfluidic applications. We begin with a brief overview of surfactants and their effect on flows (in many cases detrimental), and then introduce two-species systems that contain so-called trans and cis molecules. These can be switched from one state to another by the use of different wavelength light (the effect is essentially isothermal). The physics opens the possibility of light-actuation as a control mechanism. After describing the models and studying their basic properties, we present two applications motivated by experiments: (i) the use of a “laser pointer” to create a converging or diverging flow for particle trapping/motion – the so-called Marangoni tweezer experiment, and (ii) the use of non-uniform light gradients to generate mixing on the microscale. An inverse problem will also be described and solved that calculated the light gradient needed to produce a desired interfacial shape (or indeed a desired bulk flow). Finally (and time permitting) the effect of light actuation on the classical capillary instability of a liquid thread will be described.
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