Tailoring of phononic band structures in colloidal crystals

We report an experimental study of the elastic properties of a two-dimensional (2D) colloidal crystal subjected to light-induced substrate potentials. In agreement with recent theoretical predictions [H.H. von Gruenberg and J. Baumgartl, Phys. Rev. E 75, 051406 (2007)] the phonon band structure of such systems can be tuned depending on the symmetry and depth of the substrate potential. Calculations with binary crystals suggest that phononic band engineering can be also performed by variations of the pair potential and thus opens novel perspectives for the fabrication of phononic crystals with band gaps tunable by external fields.Comment: 4 pages, 4 figures, to appear in Physical Review Letter

Relaxation of a Colloidal Particle into a Nonequilibrium Steady State

We study the relaxation of a single colloidal sphere which is periodically driven between two nonequilibrium steady states. Experimentally, this is achieved by driving the particle along a toroidal trap imposed by scanned optical tweezers. We find that the relaxation time after which the probability distributions have been relaxed is identical to that obtained by a steady state measurement. In quantitative agreement with theoretical calculations the relaxation time strongly increases when driving the system further away from thermal equilibrium

Characterizing Potentials by a Generalized Boltzmann Factor

Based on the concept of a nonequilibrium steady state, we present a novel method to experimentally determine energy landscapes acting on colloidal systems. By measuring the stationary probability distribution and the current in the system, we explore potential landscapes with barriers up to several hundred \kT. As an illustration, we use this approach to measure the effective diffusion coefficient of a colloidal particle moving in a tilted potential

Thermodynamics of a Colloidal Particle in a Time-Dependent Non-Harmonic Potential

We study the motion of an overdamped colloidal particle in a time-dependent non-harmonic potential. We demonstrate the first law-like balance between applied work, exchanged heat, and internal energy on the level of a single trajectory. The observed distribution of applied work is distinctly non-Gaussian in good agreement with numerical calculations. Both the Jarzynski relation and a detailed fluctuation theorem are verified with good accuracy

Criticality and phase separation in a two-dimensional binary colloidal fluid induced by the solvent critical behavior

We present an experimental and theoretical study of the phase behavior of a binary mixture of colloids with opposite adsorption preferences in a critical solvent. As a result of the attractive and repulsive critical Casimir forces, the critical fluctuations of the solvent lead to a further critical point in the colloidal system, i.e. to a critical colloidal-liquid--colloidal-liquid demixing phase transition which is controlled by the solvent temperature. Our experimental findings are in good agreement with calculations based on a simple approximation for the free energy of the system.Comment: 5 pages, 5 figures, to be published in Europhysics Letter

The Einstein relation generalized to non-equilibrium

The Einstein relation connecting the diffusion constant and the mobility is violated beyond the linear response regime. For a colloidal particle driven along a periodic potential imposed by laser traps, we test the recent theoretical generalization of the Einstein relation to the non-equilibrium regime which involves an integral over measurable velocity correlation functions

Noninvasive Measurement of Dissipation in Colloidal Systems

According to Harada and Sasa [Phys. Rev. Lett. 95, 130602 (2005)], heat production generated in a non-equilibrium steady state can be inferred from measuring response and correlation functions. In many colloidal systems, however, it is a nontrivial task to determine response functions, whereas details about spatial steady state trajectories are easily accessible. Using a simple conditional averaging procedure, we show how this fact can be exploited to reliably evaluate average heat production. We test this method using Brownian dynamics simulations, and apply it to experimental data of an interacting driven colloidal system

Theory of orientational ordering in colloidal molecular crystals

Freezing of charged colloids on square or triangular two-dimensional periodic substrates has been recently shown to realize a rich variety of orientational orders. We propose a theoretical framework to analyze the corresponding structures. A fundamental ingredient is that a non spherical charged object in an electrolyte creates a screened electrostatic potential that is anisotropic at any distance. Our approach is in excellent agreement with the known experimental and numerical results, and explains in simple terms the reentrant orientational melting observed in these so called colloidal molecular crystals. We also investigate the case of a rectangular periodic substrate and predict an unusual phase transition between orientationnaly ordered states, as the aspect ratio of the unit cell is changed.Comment: 4 pages, to appear in Phys. Rev. Let

Formation, compression and surface melting of colloidal clusters by active particles

We demonstrate with experiments and numerical simulations that the structure and dynamics of a suspension of passive particles is strongly altered by adding a very small (<1%) number of active particles. With increasing passive particle density, we observe first the formation of dynamic clusters comprised of passive particles being surrounded by active particles, then the merging and compression of these clusters, and eventually the local melting of crystalline regions by enclosed active particles. © The Royal Society of Chemistry 2015

Hysteresis and re-entrant melting of a self-organized system of classical particles confined in a parabolic trap

A self-organized system composed of classical particles confined in a two-dimensional parabolic trap and interacting through a potential with a short-range attractive part and long-range repulsive part is studied as function of temperature. The influence of the competition between the short-range attractive part of the inter-particle potential and its long-range repulsive part on the melting temperature is studied. Different behaviors of the melting temperature are found depending on the screening length ($\kappa$) and the strength ($B$) of the attractive part of the inter-particle potential. A re-entrant behavior and a thermal induced phase transition is observed in a small region of ($\kappa,B$)-space. A structural hysteresis effect is observed as a function of temperature and physically understood as due to the presence of a potential barrier between different configurations of the system.Comment: 8 pages, 6 figure