Marangoni convection at droplets and bubbles
Motivation and Background
Surface-active substances (surfactants) are either inherently present or purposefully added in many technological processes. In flotation, they serve as collectors or foaming agents. Surfactants lower surface tension with increasing concentration. Hence, a spatially inhomogeneous surfactant distribution causes surface tension gradients. The resulting shear stresses drive an interfacial flow which is called Marangoni convection. It is directed from regions of low to regions of high surface tension. This flow influences the behavior of rising bubbles or droplets and has the potential to strongly enhance mass transfer processes.
Interaction of Marangoni convection with other effects, like buoyancy-driven convection, can change the dynamics of the flow drastically. For droplets or bubbles placed in a vertical concentration gradient of a surface-active substance, we observed a temporal periodicity in the form of relaxation oscillations. In this process, a phase with active Marangoni convection consuming the driving concentration gradients alternates with a relaxation phase, where buoyancy convection and diffusion again restore the Marangoni driving force. If a droplet or a bubble is not isolated but located close to a neighbor, an intriguing coupling occurs: The relaxation oscillations of both droplets synchronize to a joint frequency. This interaction is particularly interesting for technological applications where droplets or bubbles appear as a collective system. Ongoing works in collaboration with S. Aland/M. Mokbel (HTW Dresden, Chair of Modelling and Simulation) show that the frequency coupling can even expand over arrays of multiple droplets. This opens a new way to propagate a signal, i.e. a frequency, along a defined path by convection and diffusion.
Schwarzenberger, K., Aland, S., Domnick, H., Odenbach, S., & Eckert, K. (2015). Relaxation oscillations of solutal Marangoni convection at curved interfaces. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 481, 633-643.
Schwarzenberger, K., Köllner, T., Linde, H., Boeck, T., Odenbach, S., & Eckert, K. (2014). Pattern formation and mass transfer under stationary solutal Marangoni instability. Advances in colloid and interface science, 206, 344-371.
Köllner, T., Schwarzenberger, K., Eckert, K., & Boeck, T. (2016). The eruptive regime of mass-transfer-driven Rayleigh–Marangoni convection. Journal of Fluid Mechanics, 791.