By: Sabine Klapp
From: Technische Universität Berlin
At: Faculdade de Ciências, C8, 8.2.14
For more than a century, colloidal suspensions have been appreciated as theoretically and experimentally accessible "model" systems of condensed-matter phenomena such as condensation, freezing and nucleation. Recently, such systems are increasingly used to explore non-equilibrium phenomena such as pattern formation or synchronization. In the present talk we focus on colloidal particles under the combined influence of an external driving force and restricted geometry. Exploring the emerging collective effects and manipulating these effects through appropriate, open- or closed loop control strategies is a problem with strong relevance in diverse fields such as directed particle transport, sorting mechanisms and friction phenomena at the nanoscale.
In the main part of the talk we consider spatially confined suspensions under shear flow, analyzed by many-particle (Brownian Dynamics) computer simulations and effective one-particle models. By varying the externally applied shear rate ("open-loop" control) these colloidal films display a sequence of states characterized by pinning, shear-induced melting, laning, and moving crystalline order with synchronized oscillations of the particles. We also discuss the appearance of moving local density heterogeneities (kinks and antikinks) and relations to the behavior observed in driven colloidal monolayers. Finally, by adding an additional feedback equation of motion we are able to stabilise specific properties such as the degree of hexagonal ordering or the shear stress. This opens the route for a deliberate control of friction properties.
In the second part, we will discuss examples of controlled transport of interacting colloids in one- or two-dimensional "washboard" potentials, based on Fokker-Planck equations and Dynamical density functional theory. We show, in particular, that application of suitable control methods can lead to novel pattern formation effects, as well as to strong enhancement of transport through the freezing of density fluctuations.