Regensburg 2013 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 41: Colloids and Complex Liquids I

CPP 41.6: Talk

Thursday, March 14, 2013, 11:30–11:45, H40

Swimming trajectory of an active droplet — •Max Schmitt and Holger Stark — Institut für Theoretische Physik, TU Berlin

In recent experiments a spherical microswimmer was realized by placing a micron-sized droplet of water and bromine into a surfactant rich oil medium [1]. This active droplet then started to swim in a random direction. However it did not swim in a straight line but rather changed its swimming direction on a timescale much shorter than that of thermal rotational diffusion.

The swimming motion arises due to a chemical reaction of the bromine with the surfactant monolayer at the droplet interface. The reaction product is a surfactant with a higher surface tension. As a consequence, local gradients in surface tension will lead to a fluid flow in the adjacent fluid inside and outside of the droplet. Due to this so-called Marangoni effect, the resting state of the droplet becomes unstable and the droplet starts to move. Simulations of a model in axisymmetric geometry, based on a free energy functional for the droplet interface, confirm the basic swimming motion of the droplet [2].

In a next step we want to explain the increased rotational diffusion relative to the simple thermal diffusion of a sphere. For this we include thermal fluctuations in the surfactant mixture at the interface and omit the axisymmetric constraint. The aim is to connect thermal fluctuations at the interface to the dynamics of the swimming direction of the whole droplet.

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

[2] Schmitt M. and Stark H. 2012 arXiv 1210.2560