Statistical Mechanics out of thermal equilibrium
Our information machine is a machine capable of moving a particle against flow without applying direct force on the particle. Specifically our experimental realization of the information machine includes a colloidal particle driven by flow in a channel against an array of repelling optical beams forming a blocking wall. The particle's diffusion is recorded by sequential measurements at a given constant rate. After each measurement a decision is made; if the particle's distance from the wall (due to diffusive motion) is large enough the wall is moved closer to the particle (without direct contact). Otherwise no action is taken. This imaging and feedback cycle continues until the system reaches a steady state. This periodic process drives the particle upstream performing work. Since the wall is created by light it does not interact with the particle via hydrodynamic interactions. The amount of direct work done on the particle due to optical potential changes upon wall motion is controlled and is orders of magnitude smaller than the machine's output.
In the movie the laser light wall's position is indicated by the green light and the trajectory of the particle is drawn in blue. Once the laser is turned off the particle moves with the fluid flow. Tamir Admon,Saar Rahav, and Yael Roichman, Physical Review Letters 121, 180601 (2018).
Particles on a ring
Colloidal spheres driven through water along a circular path by an optical ring trap display unexpected dynamical correlations. Together with Prof. Haim Diamant we used Stokesian Dynamics simulations and a simple analytical model to demonstrate that the path's curvature breaks the symmetry of the two-body hydrodynamic interaction, resulting in particle pairing. The influence of this effective nonequilibrium attraction diminishes as either the temperature or the stiffness of the radial confinement increases. We found a well-defined set of dynamically paired states whose stability relies on hydrodynamic coupling in curving trajectories. Y. Sokolov et al., Phys. Rev. Lett., 2011.
Active in-vitro cytoskeleton networks
Actomyosin active networks have been studied extensively as a minimal model for cytoskeletal behaviour in living cells. In live cells, the concentrations of the networks' constituents is regulated and changes to enable processes such as cell migration and cell division. Together with Prof. Anne Bernheim-Groswasser we performed a detailed study of the effect of constituent concentration on the morphology and dynamics of these gels as they undergo self organization. We show that myosin II exhibits additional functionality such as network nucleator, active bundling and cross-linking protein, and as an actin turnover regulating agent. In addition, we were able to show how the concentration and size of the myosin II clusters affect network evolution, morphology and dynamics.
We examine the response of a quasi-two-dimensional colloidal suspension to a localized circular driving induced by optical tweezers. This approach allows us to resolve over 3 orders of magnitude in the Péclet number (Pe) and provide a direct observation of a sharp spatial crossover from far- to near-thermal-equilibrium regions of the suspension. In particular, particles migrate from high to low Pe regions and form strongly inhomogeneous steady-state density profiles with an emerging length scale that does not depend on the particle density and is set by Pe≈1. We show that the phenomenological two phase fluid constitutive model is in line with our results.