Translational and rotational near-wall diffusion of spherical colloids studied by evanescent wave scattering Maciej Lisicki, * a Bogdan Cichocki, a Simon A. Rogers, b Jan K. G. Dhont cd and Peter R. Lang c In this article we extend recent experimental developments [Rogers et al., Phys. Rev. Lett., 2012, 109, 098305] by providing a suitable theoretical framework for the derivation of exact expressions for the first cumulant (initial decay rate) of the correlation function measured in Evanescent Wave Dynamic Light Scattering (EWDLS) experiments. We focus on a dilute suspension of optically anisotropic spherical Brownian particles diffusing near a planar hard wall. In such a system, translational and rotational diffusion are hindered by hydrodynamic interactions with the boundary which reflects the flow incident upon it, affecting the motion of colloids. The validity of the approximation by the first cumulant for moderate times is assessed by juxtaposition to Brownian dynamics simulations, and compared with experimental results. The presented method for the analysis of experimental data allows the determination of penetration-depth-averaged rotational diffusion coefficients of spherical colloids at low density. 1 Introduction Rotational diffusion plays a crucial role in a number of physical, chemical, and biological processes occurring in a variety of systems. Notable examples include microrheology, in which frequency-dependent viscoelastic shear moduli can be investi- gated by measuring rotational diffusion of a tracer sphere; 1 random reorientation of biomacromolecules in membranes (like proteins in human erythrocyte membrane, 2 or rhodopsin chromophores 3 ); rotational-diffusion controlled chemical reac- tivity; 4–6 and gaseous combustion models, where rotational diffusion is of importance for the interpretation of coherent anti-Stokes Raman spectroscopy data. 7 Much attention has been devoted over the last decade to rotational diffusion of bulk systems, particularly in the context of macromolecules. Similar systems in geometrical connement are, however, much less understood, and are becoming a very active eld of research. This is motivated by the fundamental importance of the effects of connement for macromolecular solutions, which are most pronounced in the small-scale channel ows which are an inherent feature of micro-, 8 nano- 9 and optouidics. 10 An illustrative example may be given in the rapidly growing “lab- on-a-chip” applications, in which a single colloid might be used as a micropump, 11 or by investigation of swimming microor- ganisms, 12 nutrition of which is strongly inuenced by their hydrodynamic interactions; 13 conned geometry plays a key role also in chip-based capillary electrophoresis 14 and sorting of white blood cells. 4 To investigate the effects of connement on rotational diffusion of Brownian particles, we have employed Evanes- cent Wave Dynamic Light Scattering 15 (EWDLS), which is a technique that probes the near-wall dynamics of submicron- sized particles. In the experiments, only the region of the sample close to the boundary is illuminated, as the electric eld strength of an evanescent wave decays with distance z away from the wall as exp(kz/2). The characteristic length scale 2/k, called the penetration depth, is typically of the order of several hundred nanometers. Using this feature, one can infer information on the effects of hydrodynamic interactions with the surface on the dynamics of suspended colloids. By changing the scattering vector q, the system is probed on different length scales. Starting with the pioneering work by Lan and Ostrowsky, 15 EWDLS has been employed frequently to investigate the near surface dynamics of so matter. The translational diffusion of colloids has been studied in dilute solutions 16–18 and in suspensions with volume fractions up to 45 percent. 19,20,21 The dynamics of stiff polymers adsorbed to the interface 22 were investigated as well as the collective motion of end-graed polymer brushes. 23,24 With a setup that allows independent a Institute of Theoretical Physics, Faculty of Physics, University of Warsaw, ul. Ho˙ za 69, 00-681 Warsaw, Poland. E-mail: mklis@fuw.edu.pl b Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716, USA c ICS-3, Institute of Complex Systems, Forschungszentrum J¨ ulich, D-52425 J¨ ulich, Germany d Heinrich-Heine University, Department of Physics, D¨ usseldorf, Germany Cite this: Soft Matter, 2014, 10, 4312 Received 7th January 2014 Accepted 9th April 2014 DOI: 10.1039/c4sm00148f www.rsc.org/softmatter 4312 | Soft Matter, 2014, 10, 4312–4323 This journal is © The Royal Society of Chemistry 2014 Soft Matter PAPER Published on 14 April 2014. Downloaded by University of Warsaw on 30/05/2014 09:41:59. View Article Online View Journal | View Issue