Rotational Diffusion in a Chain of Particles.pdf

a r X i v : c o n d - m a t / 0 5 1 0 6 3 1 v 1 [ c o n d - m a t .s o f t ] 2 4 O c t 2 0 0 5 Rotational Diffusion in a Chain of Particles Holger Stark?, Michael Reichert?, and Je?rome Bibette? ? Universita?t Konstanz, Fachbereich Physik, D-78457 Konstanz, Germany ? Laboratoire Colloides et Mate?riaux Divise?s, UMR 7612, ESPCI, 10 rue Vauquelin, 75005 Paris, France E-mail: Holger.Stark@uni-konstanz.de Abstract. We study the coupled rotational diffusion in a two-particle chain on the basis of a Smoluchowski equation and calculate time-correlation functions that are measurable in an experiment. This might be used to explore hydrodynamic interactions in the limit where lubrication theory is valid. PACS numbers: 05.45.Xt, 47.15.Gf, 47.85.Np, 82.70.Dd, 82.60.Yz 1. Introduction Colloids are widely considered as models for atomic systems [1, 2]. However, a specific signature of colloidal suspensions are hydrodynamic interactions [3, 4]. Moving colloids interact with each other through the flow fields they create. This is a true multi-body problem which can only be handled by approximate methods such as multipole expansions for large particle distances (see, e.g., Refs. [4, 5]) and lubrication theory when they come close to each other [6]. Conventionally, hydrodynamic interactions are monitored through their effect on self and collective diffusion in colloidal suspensions [1, 4, 7] but recent experiments with optical tweezers on a pair of particles [8] also allow a controlled exploration of hydrodynamic interactions as a function of particle separation confirming standard approaches due to Oseen and Rotne-Prager [3, 4]. Recent work also studied the rotational diffusion of tracer particles [9, 10, 11] or colloids trapped in optical tweezers [12]. An experimental system introduced by Bibette et al. [13] suggests a possibility to directly measure the effect of hydrodynamic interactions on the rotational diffusion in the limit where lubrication theory is valid. Charged