Simultaneous investigation of sedimentation and diffusion of a single colloidal particle near an interface

Abstract

We describe here a new procedure for the simultaneous investigation of sedimentation and diffusion of a colloidal particle in close proximity to a solid, planar wall. The measurements were made using the optical technique of total internal reflection microscopy, coupled with optical radiation pressure, for dimensionless separation distances (gap width/radius of particle) ranging from 0.01 to 0.05. In this region, the hydrodynamic mobility and diffusion coefficient are substantially reduced below bulk values. The procedure involved measuring the mean and the variance of vertical displacements of a Brownian particle settling under gravity toward the plate. The spatially varying diffusion coefficient was calculated from the displacements at small times (where diffusive motion was dominant). The mobility relationship for motion normal to a flat plate was tested by measuring the average distance of travel versus time as the particle settled under the constant force of gravity. For the simple Newtonian fluid used here (aqueous salt solution), the magnitude of the diffusion coefficient and mobility, plus their dependence on separation distance, showed excellent agreement with predictions. This new technique could be of great value in measuring the mobility and diffusion coefficient for near-contact motion in more complex fluids for which the hydrodynamic correction factors are not known a priori, such as shear-thinning fluids.

The effects of hindered mobility and depletion of particles in near-wall shear flows and the implications for nanovelocimetry

“…In the vicinity of a solid wall, a colloidal particle will experience an anisotropic 'hindered' Brownian motion due to an increase of its hydrodynamic drag. This reduced mobility has been reported both theoretically (Brenner 1961;Goldman, Cox & Brenner 1967a,b;Chaoui & Feuillebois 2003) and experimentally (Bevan & Prieve 2000;Lin, Yu & Rice 2000;Banerjee & Kihm 2005;Oetama & Walz 2006;Huang & Breuer 2007a). In most cases, such a spatially dependent hindered mobility leads to difficulties in determining measurement uncertainties, although a few researchers have managed to turn the disadvantages into merits by proposing usage of the hindered Brownian motion to reconstruct near-wall shear flow velocity profile and to probe the no-slip boundary condition (Lauga & Squires 2005).…”

Shape‐Tailored Polymer Colloids on the Road to Become Structural Motifs for Hierarchically Organized Materials

“…Hence, gravitational forces are influencing the motion of microparticles. Moreover, the diffusion characteristics might differ from the bulk behavior because the microscope slide can bring strong wall effects on the diffusion …”

Effect of small particles on the near-wall dynamics of a large particle in a highly bidisperse colloidal solution

“…Examples include dairy products, polymeric gels and nearly all complex fluids in biological organisms. Since the dynamics of colloidal systems is often influenced by their confining boundaries [1,2], the confining effects are important in many emerging applications, e.g., in self-assembly technology involving colloidal crystals [3,4] and in chemical deposition processes on patterned wall [5]. Hence, it is necessary to understand how polydisperse suspensions behave in the proximity of a bounding surface.…”