Nonequilibrium Depletion Force in Temperature Gradient: A New Micromanipulation
Masaki Sano
Department of Physics, University of Tokyo

Gradients of thermodynamic variables such as temperature, chemical potential, and osmotic pressure cause migration of molecules or small particles both in simple and complex fluids. For example in biological cell, couplings between two gradients are often used to promote molecular transport against one of the gradients as in chemiosmosis. In physics and chemistry, novel methods to utilize thermodynamic gradients (electro-, thermo-, and diffusiophoresis) are proposed for, such as transport and screening particles in lab-on-chip or designing self-propelling particles. In the seminar, starting from explaining typical out of equilibrium phenomena, I will demonstrate how the coupling of two gradients can efficiently control migration and accumulation of colloids using nonequilibrium forces by phoretic effects. This new effect based on entropic repulsion in nonequilibrium does not rely on a specific character of particles, and thus provides a new micromanipulation for a diverse range of particles.

Bio:
Masaki Sano received his Ph.D in Engineering from Tohoku University at Sendai in Japan and later was postdoctoral fellow in Physics at University of Chicago. Currently, he is a professor at Department of Physics of the University of Tokyo.
His current research interest is searching fundamental laws in systems far from equilibrium both at macroscopic and microscopic level. The methodology is an experimental approach combined with theoretical analysis based on statistical mechanics, hydrodynamics, and nonlinear dynamics. As a macroscopic level approach, he is working on problems in fluid dynamics (Rayleigh-Benard convection, universalities in turbulence transition, flow-defect interaction in liquid crystals, instabilities in soft matter). At microscopic and mesoscopic level, he works on problems in bio-soft matter physics, such as mechanical responses of DNA and proteins, measuring nonequilibrium fluctuations, dynamics of artificial gene networks, dynamics of cell migration, searching for physical principles underlying the design of biological systems.

Dynamical Responses of Soft Matter to Temperature Gradients
Hong-Ren Jiang
Department of Physics, University of Tokyo

Many interesting dynamics of soft matter can be observed under temperature gradients. Even a simple thermophoretic property would cause many unexpected results, which provide a new way to understand the nature of soft matter. In the beginning, I will discuss the osmotic pressure induced by temperature gradient. Although osmosis and osmotic pressure are involved in many microscopic processes, the ways to control local osmotic pressure are limited. A polymer distribution controlled by thermophoresis gives a new way to overcome this problem. Different kinds of deformations and transformations of cells and vesicles are observed. The elasticity of a membrane also can be measured. This method also leads to a novel optical trap for different objects including DNA. Later I will present the special self-propelling motion based on thermophoresis to demonstrate how the phoretic motion can change into self-propelling motion.

Interactions between Charged Colloidal Particles at an Aqueous Interface
Wei Chen
Department of Physics, The Hong Kong University of Science & Technology

Like-charge attraction in aqueous solutions has been reported in a number of experiments. Despite the intensive experimental and theoretical investigations, a satisfying explanation for the origin of the attraction still remains illusive. In a series of experiments, we studied the colloidal interactions in a monolayer of micron-sized particles (polystyrene and silica spheres) at a water-air interface. In the experiment, the interaction potential U(r) between the colloidal particles is obtained from the measured pair correlation function by optical microscopy. Attractive interactions are found among the interfacial particles. Phase images obtained by atomic force microscopy show patchy domains of size ~100 nm on the particle surface, indicating that the surface charge distribution is not uniform as is commonly believed. Such patchy charges can introduce fluctuating in-plane dipoles, leading to an attraction at intermediate inter-particle separations.