Berlin 2018 – scientific programme

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DY: Fachverband Dynamik und Statistische Physik

DY 39: Wetting, Microfluidics and Confined Liquids I (joint session CPP/DY)

DY 39.9: Talk

Wednesday, March 14, 2018, 12:00–12:15, C 264

Feedback-Control of Photoresponsive Fluid Interfaces — •Josua Grawitter and Holger Stark — Institut für Theoretische Physik, Technische Universität Berlin, 10623 Berlin, Deutschland

New microfluidic devices promise powerful applications in diagnosing and combatting illness around the world. Conventional, pressure-driven devices rely on static channel geometries designed for specific flow patterns. Light-driven devices instead use photosensitive surfactants to produce surface tension gradients that drive Marangoni currents. By changing their light patterns in response to fluid flow, light-driven devices add feedback control methods to the microfluidic toolbox.

To explore their potential, we develop a diffusion-advection-reaction equation for photosensitive surfactants and calculate Marangoni currents at fluid-fluid interfaces. We then study how the interface responds when illuminated by spots of light. Switching on a single light spot, the density of the switched surfactant spreads in time and assumes an exponentially decaying profile in steady state. Simultaneously, the induced radial Marangoni flow reverses its flow direction from inward to outward. We use this feature to set up specific feedback rules, which couple advection velocities sensed at the light spots to their intensities. As a result two neighboring spots switch on and off alternately. When we arrange more light spots on regular polygons, regular and irregular oscillations in light intensity emerge for certain numbers of spots. This demonstrates how light-driven feedback control may be used to create responsive and versatile microfluidic devices.