Rut Besseling

Three-Dimensional Imaging of Colloidal Glasses under Steady Shear

Using fast confocal microscopy we image the three-dimensional dynamics of particles in a yielded hard-sphere colloidal glass under steady shear. The structural relaxation, observed in regions with uniform shear, is nearly isotropic but is distinctly different from that of quiescent metastable colloidal fluids. The inverse relaxation time tau(alpha)(-1) and diffusion constant D, as functions of the local shear rate gamma*, show marked shear thinning with tau(alpha)(-1) proportional to D proportional to gamma*(0.8) over more than two decades in gamma*. In contrast, the global rheology of the system displays Herschel-Bulkley behavior. We discuss the possible role of large scale shear localization and other mechanisms in generating this difference.

Shear Banding and Flow-Concentration Coupling in Colloidal Glasses

We report experiments on hard-sphere colloidal glasses that show a type of shear banding hitherto unobserved in soft glasses. We present a scenario that relates this to an instability due to shear-concentration coupling, a mechanism previously thought unimportant in these materials. Below a characteristic shear rate γ(c) we observe increasingly nonlinear and localized velocity profiles. We attribute this to very slight concentration gradients in the unstable flow regime. A simple model accounts for both the observed increase of γ(c) with concentration, and the fluctuations in the flow.

Quantitative imaging of colloidal flows.

We present recent advances in the instrumentation and analysis methods for quantitative imaging of concentrated colloidal suspensions under flow. After a brief review of colloidal imaging, we describe various flow geometries for two and three-dimensional (3D) imaging, including a confocal rheoscope. This latter combination of a confocal microscope and a rheometer permits simultaneous characterization of rheological response and 3D microstructural imaging. The main part of the paper discusses in detail how to identify and track particles from confocal images taken during flow. After analyzing the performance of the most commonly used colloid tracking algorithm by Crocker and Grier extended to flowing systems, we propose two new algorithms for reliable particle tracking in non-uniform flows to the level of accuracy already available for quiescent systems. We illustrate the methods by applying it to data collected from colloidal flows in three different geometries (channel flow, parallel plate shear and cone plate rheometry).

Shear zones and wall slip in the capillary flow of concentrated colloidal suspensions

We image the flow of a nearly random close packed, hard-sphere colloidal suspension (a paste) in a square capillary using confocal microscopy. The flow consists of a plug in the center while shear occurs localized adjacent to the channel walls, reminiscent of yield-stress fluid behavior. However, the observed scaling of the velocity profiles with the flow rate strongly contrasts yield-stress fluid predictions. Instead, the velocity profiles can be captured by a theory of stress fluctuations originally developed for chute flow of dry granular media. We verified this both for smooth and rough walls.

Differential dynamic microscopy: a high-throughput method for characterizing the motility of microorganisms.

We present a fast, high-throughput method for characterizing the motility of microorganisms in three dimensions based on standard imaging microscopy. Instead of tracking individual cells, we analyze the spatiotemporal fluctuations of the intensity in the sample from time-lapse images and obtain the intermediate scattering function of the system. We demonstrate our method on two different types of microorganisms: the bacterium Escherichia coli (both smooth swimming and wild type) and the biflagellate alga Chlamydomonas reinhardtii. We validate the methodology using computer simulations and particle tracking. From the intermediate scattering function, we are able to extract the swimming speed distribution, fraction of motile cells, and diffusivity for E.xa0coli, and the swimming speed distribution, and amplitude and frequency of the oscillatory dynamics for C.xa0reinhardtii. In both cases, the motility parameters were averaged over ∼10(4) cells and obtained in a few minutes.

Slip and Flow of Hard-Sphere Colloidal Glasses

We study the flow of concentrated hard-sphere colloidal suspensions along smooth, nonstick walls using cone-plate rheometry and simultaneous confocal microscopy. In the glass regime, the global flow shows a transition from Herschel-Bulkley behavior at large shear rate to a characteristic Bingham slip response at small rates, absent for ergodic colloidal fluids. Imaging reveals both the solid microstructure during full slip and the local nature of the slip to shear transition. Both the local and global flow are described by a phenomenological model, and the associated Bingham slip parameters exhibit characteristic scaling with size and concentration of the hard spheres.

Wall slip and flow of concentrated hard-sphere colloidal suspensions

We present a comprehensive study of the slip and flow of concentrated colloidal suspensions using cone-plate rheometry and simultaneous confocal imaging. In the colloidal glass regime, for smooth, nonstick walls, the solid nature of the suspension causes a transition in the rheology from Herschel–Bulkley (HB) bulk flow behavior at large stress to a Bingham-like slip behavior at low stress, which is suppressed for sufficient colloid-wall attraction or colloid-scale wall roughness. Visualization shows how the slip-shear transition depends on gap size and the boundary conditions at both walls and that partial slip persist well above the yield stress. A phenomenological model, incorporating the Bingham slip law and HB bulk flow, fully accounts for the behavior. Microscopically, the Bingham law is related to a thin (subcolloidal) lubrication layer at the wall, giving rise to a characteristic dependence of slip parameters on particle size and concentration. We relate this to the suspension’s osmotic pressure an...

Velocity Oscillations in Microfluidic Flows of Concentrated Colloidal Suspensions

We study the pressure-driven flow of concentrated colloids confined in glass microchannels at the single-particle level using fast confocal microscopy. For channel to particle size ratios 2a/D[over ] less, similar30, the flow rate of the suspended particles shows fluctuations. These turn into regular oscillations for higher confinements (2a/D[over ] approximately 20). We present evidence to link these oscillations with the relative flow of solvent and particles (permeation) and the effect of confinement on shear thickening.

Slip of gels in colloid–polymer mixtures under shear

We investigate the time-dependent rheology and slip behaviour of colloidal gels formed under polymer-induced depletion attraction. The shape of the flow curves at low applied shear rates is suggestive of slip, which we confirm using confocal imaging. Time-dependent linear viscoelastic measurements show an unexpected drop of the elastic modulus below the viscous one after a critical time. We present a dynamic phase diagram characterizing the dependence of slip on polymer concentration and colloid volume fraction. Confocal imaging links slip to the restructuring of clusters with time, which leads to a reduction of the number of contacts between the colloidal network and the rheometer surfaces. Such behaviour is shear rate dependent and correlated to changes in the gel structure, which changed from independent small aggregates at high shear rates to percolated clusters at low shear rates.

Experimental studies of the flow of concentrated hard sphere suspensions into a constriction

Interesting flow properties are observed when a concentrated suspension of colloidal particles flows into a geometrical constriction. We present here a description of two different experimental techniques used to study the pressure driven flow of dense suspensions of micron- sized hard spheres into glass capillaries. The first one involves the analysis of the driving pressure during the flow, the other one is based on fast confocal microscopy. Technical details are given, together with a selection of preliminary results.