By: Mykola Tasinkevych
From: Max Planck Institute for Intelligent Systems
At: Instituto de Investigação Interdisciplinar, Anfiteatro
Synthetic micro-motors which can move cargo in a well controlled way through a liquid environment are of significant interest for applications such as targeted drug delivery, biosensing, and shuttle-transport of living cells. One promising approach is to use catalytically active Janus colloidal particles as model micro-motors. This type of particles harvest âchemicalâ free energy from the surrounding liquid environment and then transform it into mechanical energy. Due to an asymmetric decoration of their surface with a catalyst, which promotes a specific chemical reaction in the surrounding liquid, concentration gradients of the reaction products develop along the surface of the particle. Depending on the systems, various self-propulsion mechanisms emerge, such as bubble propulsion, self-electrophoresis, or self-diffusiophoresis, which in some systems can be activated by light. In this talk only the last mechanism, i.e. self-propulsion due to self-generated electrically neutral solute gradients, shall be considered.
First, we discuss the self-difusiophoretic motion of a spheroidal particle, which is covered by acatalyst over a cap-like region centered at one of the poles of the particle . We describe how the self-phoretic velocity depends on the aspect ratio of the polar and the equatorial diameters of the particle and on the fraction of the particle surface contributing to the chemical reaction. Next we show that such particles can be used as micro-carriers . As a simple model for a carrier-cargo system we consider a catalytically active particle connected by a thin rigid rod to a catalytically inert cargo particle. We show that the velocity of the composite strongly depends on the relative orientation of the carrier-cargo link. Accordingly, there is an optimal configuration for the linkage. Finally we show that a catalytically active Janus particle moving near a wall reveals a very rich behavior, including reflection, steady sliding and hovering states . We argue that the sliding states could provide a starting point to establish a stable and predictable motion of swimmers in microdevices. Hovering particles create recirculating regions of flow, and could be used to mix fluid or to trap other particles.
 M. N. Popescu, S. Dietrich, M. Tasinkevych, and J. Ralston, Eur. Phys. J. E 31, 351 (2010).
 M. N. Popescu, M. Tasinkevych, and S. Dietrich, EPL 95, 28004 (2011).
 W. E. Uspal, M. N. Popescu, S. Dietrich, and M. Tasinkevych, http://arxiv.org/abs/1407.3216