Many-Body Hydrodynamic Interactions in Charge-Stabilized Suspensions

Abstract

In this joint experimental-theoretical work we study hydrodynamic interaction effects in dense suspensions of charged colloidal spheres. Using x-ray photon correlation spectroscopy we have determined the hydrodynamic function Hq, for a varying range of electrosteric repulsion. We show that Hq can be quantitatively described by means of a novel Stokesian dynamics simulation method for charged Brownian spheres, and by a modification of a many-body theory developed originally by Beenakker and Mazur. Very importantly, we can explain the behavior of Hq for strongly correlated particles without resorting to the controversial concept of hydrodynamic screening, as was attempted in earlier work by Riese et al. [Phys. Rev. Lett. 85, 5460 (2000)]. DOI: 10.1103/PhysRevLett.96.138303 PACS numbers: 82.70.Dd, 83.10.Mj, 87.15.Vv Suspensions of colloidal particles undergoing Brownian motion in a low-molecular-weight solvent are ubiquitous in chemical industry, biology, food science, and in medical and cosmetic products. For polar solvents like water, the particles are usually charged. At long to intermediate distances these particles interact electrostatically by an exponentially screened Coulomb repulsion originating from the overlap of the neutralizing electric double layers. A considerable effort has been devoted over the past years to study the dynamics of colloidal model suspensions of charged spheres at the microscopic level [1,2]. The dynamics is determined by a subtle interplay of direct interactions and solvent-flow mediated hydrodynamic interaction (HI). The latter dynamic interaction plays a pivotal role not only in unconfined colloidal systems, but also in microfluidic devices where narrow wall confinements or channels are present [3], and in sedimenting dispersions of large nonBrownian particles [4]. HI in unconfined suspensions is very long ranged. It decays with interparticle distance r like 1=r. An account of HI effects in theoretical and computer simulation studies is quite challenging due to its many-body nature, which must be accounted for in nondilute suspensions.An important measure of the strength of HI with regard to short-time particle diffusion caused by local density gradients is given by the hydrodynamic function [2]which is the sum of a wave-number-independent self-part and a q-dependent distinct part. Here, D s is the short-time self-diffusion coefficient, and D 0 is the particle diffusion coefficient at infinite dilution. In the limit of large q, Hq reduces to D s =D 0 . Without HI, Hq 1 and any variation in Hq is a hallmark of HI. It has a direct physical meaning [5] as the (reduced) mean particle sedimentation velocity for a suspension subject to a weak periodic force field, collinear to the wave vector q, and oscillating like cosq r. Experimentally, Hq can be determined as a function of wave number through a combination of static and dynamic scattering experiments [6,7]. Experimental findings [6] for the Hq of highly charged colloidal spheres at low volume fractions (typically <0:05) are in e...

Hydrodynamic and electrokinetic effects on the dynamics of charged colloids and macromolecules

“…For systems with added salt, the values of these short-time properties are located in between the zero-salt and infinite-salt (zero-charge) limits. Whereas the majority of experimentally available scattering data on charge-stabilized suspensions is in agreement with the ordering relations in equations (8) [22,23], a violation of these relations has been purportedly observed in certain scattering experiments [25,26]. Colloidal hard spheres have a common static and hydrodynamic length scale given by the particle radius a.…”

Surface and Interface Analyses of Polymer Brushes by Synchrotron Radiation

“…Taking this advantage, the studies of concentrated colloidal suspensions have been energetically conducted, and the understanding of hydrodynamic interactions of diffusive motion has been developed. [36][37][38][39][40] Though the DLS experiments are conducted with full coherent light source, visible laser, the light source of XPCS is not usually fully coherent. Here, we will consider about the coherence of the X-ray obtained from the third generation synchrotron radiation source, which is a chaotic source with Gaussian intensity distribution.…”

Dynamical Density Functional Theory with Hydrodynamic Interactions and Colloids in Unstable Traps

“…Apart from the stochastic Brownian motion of the colloidal particles due to their kicks with the solvent molecules, hydrodynamic interactions between colloidal particles arising from the induced solvent flow field are getting relevant for concentrated suspensions. It has been shown by experiments [3,4], computer simulations [5,6] and theory [7,8,9,10] that hydrodynamic interactions can lead to qualitatively different behavior in the bulk transport properties and in colloidal sedimentation as compared to simple Brownian motion valid at very low volume fractions.…”