Average negative heat in a non-Markovian bath

We experimentally study the motion of a colloidal particle, translated back and forth within a viscoelastic, i.e. non-Markovian bath. The particle starts in equilibrium before the forward motion, but only partially relaxes at the turning point. During the backward motion, we measure a systematic (negative) heat flow from the bath to the particle. Our observations are in good agreement with a simple model that describes the time-delayed response of the fluid. We expect our results to be important for the realization and optimization of novel types of micro-engines in non-Markovian surroundings.Comment: 5 pages, 5 figure

Sedimentation of self-propelled Janus colloids: polarization and pressure

We study experimentally-using Janus colloids-and theoretically-using Active Brownian Particles- the sedimentation of dilute active colloids. We first confirm the existence of an exponential density profile. We show experimentally the emergence of a polarized steady state outside the effective equilibrium regime, i.e. when v_s is not much smaller than the propulsion speed. The experimental distribution of polarization is very well described by the theoretical prediction with no fitting parameter. We then discuss and compare three different definitions of pressure for sedimenting particles: the weight of particles above a given height, the flux of momentum and active impulse, and the force density measured by pressure gauges

Universal symmetry of optimal control at the microscale

Optimizing the energy efficiency of driving processes provides valuable insights into the underlying physics and is of crucial importance for numerous applications, from biological processes to the design of machines and robots. Knowledge of optimal driving protocols is particularly valuable at the microscale, where energy supply is often limited. Here we investigate experimentally and theoretically the paradigmatic optimization problem of moving a potential carrying a load through a fluid, in a finite time and over a given distance, in such a way that the required work is minimal. An important step towards more realistic systems is the consideration of memory effects in the surrounding fluid, which are ubiquitous in real-world applications. Therefore, our experiments were performed in viscous and viscoelastic media, which are typical environments for synthetic and biological processes on the microscale. Despite marked differences between the protocols in both fluids, we find that the optimal control protocol and the corresponding average particle trajectory always obey a time-reversal symmetry. We show that this symmetry, which surprisingly applies here to a class of processes far from thermal equilibrium, holds universally for various systems, including active, granular, and long-range correlated media in their linear regimes. The uncovered symmetry provides a rigorous and versatile criterion for optimal control that greatly facilitates the search for energy-efficient transport strategies in a wide range of systems. Using a machine learning algorithm, we demonstrate that the algorithmic exploitation of time-reversal symmetry can significantly enhance the performance of numerical optimization algorithms.Comment: 16 pages with 10 figures, accepted for publication in PR

Critical Casimir interactions of colloids in micellar critical solutions

We study the temperature-dependence of critical Casimir interactions in a critical micellar solution of the nonionic surfactant C12E5 dissolved in water. Experimentally, this is achieved with total internal reflection microscopy (TIRM) which measures the interaction between a single particle and a flat wall. For comparison, we also studied the pair interactions of a two dimensional layer of colloidal particles in the identical micellar system which yields good agreement with the TIRM results. Although, at the surfactant concentration considered here, the fluid forms a dynamical network of wormlike micelles whose structure is considerably more complex than that of simple critical molecular fluids, the temperature-dependence of the measured interactions is - for surface-to-surface distances above 160 nm - in excellent quantitative agreement with theory. Below 160 nm, deviations arise which we attribute to the adsorption of micelles to the interacting surfaces.publishe