Catalytic Janus Micromotors

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Resumo: Autonomous motion at the micro- and nano-scale is indispensable for the development of new generations of devices for modern bio-analytical or chemical assays, performed either on a chip, or in a biological environment. Man-made synthetic micro- and nano-motors capable of moving cargo loads within a liquid medium are of significant interest for applications such as targeted drug delivery, biosensing, or shuttle-transport of living cells. One promising approach is the use of catalytically active Janus micro-particles. As a general principle, these artificial micromotors use an asymmetric decoration of their surface with a catalyst (usually platinum) which promotes a specific chemical reaction in the surrounding liquid and leads to concentration gradients along the surface of the particle. Depending on the systems, various propulsion mechanisms emerge, such as interfacial tension gradients, bubble propulsion, self-electrophoresis, or self-diffusiophoresis. In this talk we consider only the last mechanism, i.e. propulsion due to self-generated electrically neutral solute gradients.First, we discuss the self-diffusiophoretic motion of a spheroidal particle which is covered by a catalyst 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 [Eur. Phys. J. E 31, 351 (2010)]. 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 [EPL 95, 28004 (2011)]. 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. The subtlety of such carriers is underscored by the observation that a spherical particle completely covered by catalyst, which is motionless when isolated, acts as a carrier once attached to a cargo.