Non-equilibrium transport of colloidal soft matter: fromanomalous transport to self-propelled dynamics
M. Sano1, H.R. Jiang1, H. Wada2, and N. Yoshinaga2
1
The University of Tokyo, Japan2
Kyoto University, Japan
1. INTRODUCTION

Gradients of thermodynamic variables such as temperature, chemical potential, and osmotic pressurecause migration of molecules and small particles in fluids. Recently, novel methods to utilize phoretic
properties (electro-, thermo-, and diffusiophoresis) are proposed for transporting and screening particles such
as DNA molecules and colloids in lab on-chip. Several nontrivial transport properties of colloidal soft matterin non-equilibrium conditions have been found and studied in these efforts. Among them, it is a challenging
problem to understand and control the colloidal migration driven by a surface hydrodynamic flow which isinduced by the interaction between a colloidal surface and surrounding solute or charges, because the scalingof transport coefficients against particle size for such cases is different from the Einstein- Stokes relation[1].
We also encounter even richer non-equilibrium transport properties of colloids when an asymmetry is
introduced to each colloidal particle. We prepared asymmetric colloidal particles, so called Janus particle,
which exhibit self-propelled motion even in the uniform and symmetric external field owing to the surface
flow induced by the asymmetry of the particle itself. We found a way to promote self-organized collective
motions of Janus particles by controlling the interaction between Janus particles. When the interaction
becomes attractive, Janus particles self-organize a chain structure which spontaneously deforms its shape toexhibit snaking and spiraling motion. This provides an example of self-propelled deformablemicro-swimmer[2,3]. In the presentation, we will explain an example of nontrivial transport phenomena ofcolloids in the external fields, afterwards we demonstrate a rich variety of the transport phenomena of Janus
particles and their collective dynamics as micro-swimmers.
2. TRANSPORT IN TEMPERATURE GRADIENTIn thermophoresis, the speed and direction of migration along a temperature gradient are characterized bySoret coefficient, which is generally material-dependent. Diffusiophoresis is a similar phenomenon wherecolloid particles move along a gradient of solute molecules. In the presentation, we report experimental andtheoretical studies on a phoretic motion of colloidal particles in a polymer solution under a temperature
gradient. We find that a Soret coefficient of a colloid is sensitive to the polymer (PEG) concentration; asincreasing the amount of polymer, the Soret coefficient reverses its sign and its magnitude outweighs by far
its intrinsic value at the highest polymer concentration studied. The dependence of the Soret coefficient on
the polymer is experimentally determined and is corrobolated by our hydrodynamic calculations[1].
REFERENCES
1. H.-R. Jiang, H. Wada, N. Yoshinaga, and M. Sano, “Manipulation of Colloids by Nonequilibrium
Depletion Force in a Temperature Gradient”, Phys. Rev. Lett, 102, 208301 (2009).
2. T. Ohta and T. Ohkuma, Phys. Rev. Lett, 102, 154101 (2009).
3. T. Hiraiwa, T. Ohkuma, T. Ohta, M. Y. Matsuo and M. Sano, “Dynamics of a deformable self-propelleddomain”, preprint (2010).

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