Cichos, Frank

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The Open-Access Journal for the Basic Principles of Diffusion Theory, Experiment and Application Fig. 1: Electron mi-croscopy image of a Janus particle made out of polystyrene (1µm) with a half-side  50 nm gold coating. From Hot Brownian Motion to Self-Propelled Particles Frank Cichos University of Leipzig, Molecular Nanophotonics Group, Linnéstraße 5,  04103 Leipzig, Germany, E-Mail: cichos@physik.uni-leipzig.de 1. Introduction  Thermal fluctuations drive the motion of particles and molecules in solvents. This er-ratic motion of Brownian particles can be understood as a continuous interconversion of solvent thermal energy into particle kinetic energy and back mediated by the viscous friction. Accordingly, a fluctuation dissipation relation known as the Stokes-Einstein relation expresses the relation of ther-mal energy and viscous friction in Brownian motion. As this random jittering motion rules all motions of small particles in liquids, manipulation of particles in liquids on the nanoscale requires beating Brownian motion.  2. Heated particles at a Nanoscale Here we describe our efforts to manipulate Brownian mo-tion by varying the local temperature as well as the local viscous friction around the diffusing particle. This is imple-mented by heating gold nanoparticles with focused laser beams. The temperature and viscosity field around the parti-cle modify the conditions of the fluctuation dissipation rela-tion mentioned above and introduce effective temperatures as well as effective friction coefficients [1]. While this concept of Hot Brownian motion of heated nanoparticles relies on radially symmetric temperature and viscosity profiles, Janus type particles with half-side gold coating break the symmetry to cause a net directed transport of particles along the axis of the Janus particle. We demonstrate how these effects can be used to prepare microscale self-propelled swimmers to overcome the randomization of Brownian motion. 3. Conclusion In summary, we show how to employ local heat sources in liquids to manipulate and overcome Brownian motion of particles, which is a prerequisite for future developments of thermally driven nano-machines. References [1] Rings, D, R Schachoff, M Selmke, F Cichos, and K Kroy, Phys. Rev. Lett. 105 (2010) 090604.  © 2011, Frank Cichos diffusion-fundamentals.org 16 (2011) 23, pp 11

From hot Brownian motion to self-propelled particles

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From hot Brownian motion to self-propelled particles

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