Grand Séminaire d'Institut

Vendredi 29 Avril 2016 à 11h00.

Collective behavior and pattern formation in chemically active and actuated colloids

''Active systems generate motion due to energy consumption, usually associated to their internal internal metabolism. As a result, these systems are intrinsically out of equilibrium and their collective properties emerge from a balance between particle direct interactions and the indirect coupling to the medium in which they displace. In the case of active particles suspended in a liquid, their dynamics is affected by the liquid in which they swim. Moreover, usually active particles in the microscale (from molecular motors to microorganisms) move in constrained environments. Both the liquid and confinement interfere with the internal mechanisms that generate particle propulsion. Therefore, a consistent dynamical approach that accounts for the active particles and the liquid motion on the same footing is required to analyze particle motion and quantify their self-assembly and ability to generate intermediate and large-scale structures. Actuated and autocatalytic colloids constitute examples of such active syystems. As a result of their dynamic interactions, they can show a rich variety of self assembly scenarios. The observed self assembled structures make these systems very sensitive to external forcing, hence making actuated and active matter a fertile ground to explore and develop mechanically tunable materials. In this talk I will analyze the basic physical mechanisms that control the collective behavior of two kinds of colloidal particles that move in a liquid medium. On the one hand, confined magnetic colloids can rectify their motion when actuated with a rotating magnetic field, acting as a ydrodynamic conveyor belt. Self assembled chains of rotors propel faster than individual ones, until reaching a saturation speed at distances where induced-flow additivity vanishes. On the other hand, the development of Janus colloids has opened the possibility to create synthetic microrobots that can move due to the chemical reacions they catalyze on their heterogeneous surfaces. The motion of chemically powered colloids is intricate because the chemically active colloids perturb the spatial distribution of the chemical species and also the state of motion of the solvent. As a result, suspensions of chemically active colloids are characterized by long range, non-equilibrium interactions. These dynamic interactions have a strong impact in the collective behavior of these suspensions. I will describe the analogies and specificities in the hydrodynamic coupling that characterize these two types of systems and the different structures they spontaneously form B. Liebchen, D. Marenduzzo, I. Pagonabarraga, M.E. Cates, Phys. Rev. Lett. 115, 258301 (2015) F. Martinez-Pedrer, A. Ortiz-Ambriz, I. Pagonabarraga, P. Tierno, Phys. Rev. Lett. 115, 138301 (2015)''