Berlin 2012 – wissenschaftliches Programm

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 34: Colloids and Complex Liquids II

CPP 34.8: Vortrag

Donnerstag, 29. März 2012, 17:15–17:30, C 130

Modeling swimming active droplets — •Maximilian Schmitt and Holger Stark — Institut für Theoretische Physik, TU Berlin

In a recent attempt to build an active microswimmer, a micron-sized droplet of bromine water was placed into a surfactant laden oil medium such that the surfactant molecules spontaneously assemble at the droplet interface [1]. Experiments revealed that due to a bromination reaction of the surfactant (mono-olein), the surface tension locally increases. At the same time these surfactants with a higher surface tension are readily replaced by surfactants from the surrounding oil phase. As a result, a steady gradient of surface tension on the droplet is created. Since an interface with higher surface tension pulls more strongly on a surrounding liquid than one with a lower surface tension, the surface tension gradient on the droplet generates the so-called Marangoni flow.

In order to understand the swimming mechanism, we establish a model based on a free energy functional for the droplet interface. The free energy accounts for the mixing entropy, interactions between the surfactant molecules, and adsorbtion/desorbtion of surfactants from/into the bulk medium. After also taking into account the bromination and the Maranogi flow, we arrive at a reaction-advection diffusion equation on a sphere. Numerical simulations are carried out in order to obtain the flow field at the interface and the swimming trajectory of the droplet.

[1] Shashi Thutupalli et al 2011 New J. Phys. 13 073021