Structure and flow conditions through a colloidal packed bed formed under flow and confinement.

When a colloidal suspension flows in a constriction, particles deposit and are able to clog it entirely, this fouling process being followed by the accumulation of particles. The knowledge of the dynamics of formation of such a dense particle assembly behind the clog head and its structural features is of primary importance in many industrial and environmental processes and especially during filtration. While most studies concentrate on the conditions under which pore clogging occurs, i.e., the pore narrowing up to its complete obstruction, this paper focuses on the accumulation of particles that follows pore obstruction. We determine the relative contribution of the confinement dimensions, the ionic strength and the flow conditions on the permeability and particle volume fraction of the resultant accumulation. In high confinement the irreversible deposition of particles on the channel surfaces controls the structure of the accumulation and the flow through it, irrespective of the other conditions, leading to a Darcy flow. Finally, we show that contrarily to the clog head, in which there is cohesion between particles, those in the subsequent accumulation are held together by the fluid and form a dense suspension of repulsive hard spheres.

Dynamics of progressive pore clogging by colloidal aggregates.

The flow of a suspension through a bottleneck often leads to its obstruction. Such a continuous flow to clogging transition has been well characterized when the constriction width to particle size ratio, W/D, is smaller than 3-4. In such cases, the constriction is either blocked by a single particle that is larger than the constriction width (W/D < 1), or there is an arch formed by several particles that try to enter it together (2 < W/D < 4). For larger W/D ratios, 4 < W/D < 10, the blockage of the constriction is presumed to be due to the successive accumulations of particles. Such a clogging mechanism may also apply to wider pores. The dynamics of this progressive obstruction remains largely unexplored since it is difficult to see through the forming clog and we still do not know how particles accumulate inside the constriction. In this paper, we use particle tracking and image analysis to study the clogging of a constriction/pore by stable colloidal particles. These techniques allow us to determine the shape and the size of all the objects, be they single particles or aggregates, captured inside the pore. We show that even with the rather monodisperse colloidal suspension we used individual particles cannot clog a pore alone. These individual particles can only partially cover the pore surface whilst it is the very small fraction of aggregates present in the suspension that can pile up and clog the pore. We analyzed the dynamics of aggregate motion up to the point of capture within the pore, which helps us to elucidate why the probability of aggregate capture inside the pore is high.

Clogging transition induced by self filtration in a slit pore.

Particles that flow through porous environments like soils, inside a filter or within our arteries, often lead to pore clogging. Even though tremendous efforts have been made in analysing this to circumvent this issue, the clog formation and its dynamics remain poorly understood. Coupling two experimental techniques, we elucidate the clogging mechanism at the particle scale of a slit pore with its height slightly larger than the particle diameter. We identify all the particle deposition modes during the clog formation and accurately predict the corresponding deposition rate. We show how the geometrical features of the pores and the competition between deposition modes can profoundly change the clog morphology. We find that the direct capture of particles by the pore wall is rather limited. The clog formation is more closely related to the short range hydrodynamic interaction between flowing particles and those which are already immobilized within the pore. Finally we demonstrate that all the clogging regimes can be gathered on a single phase diagram based on the flow conditions and the filter design.

Structural Transition in a Fluid of Spheroids: A Low-Density Vestige of Jamming.

A thermodynamically equilibrated fluid of hard spheroids is a simple model of liquid matter. In this model, the coupling between the rotational degrees of freedom of the constituent particles and their translations may be switched off by a continuous deformation of a spheroid of aspect ratio t into a sphere (t=1). We demonstrate, by experiments, theory, and computer simulations, that dramatic nonanalytic changes in structure and thermodynamics of the fluids take place, as the coupling between rotations and translations is made to vanish. This nonanalyticity, reminiscent of a second-order liquid-liquid phase transition, is not a trivial consequence of the shape of an individual particle. Rather, free volume considerations relate the observed transition to a similar nonanalyticity at t=1 in structural properties of jammed granular ellipsoids. This observation suggests a deep connection to exist between the physics of jamming and the thermodynamics of simple fluids.

Particle-size effects in the formation of bicontinuous Pickering emulsions.

We demonstrate that the formation of bicontinuous emulsions stabilized by interfacial particles (bijels) is more robust when nanoparticles rather than microparticles are used. Emulsification via spinodal demixing in the presence of nearly neutrally wetting particles is induced by rapid heating. Using confocal microscopy, we show that nanospheres allow successful bijel formation at heating rates two orders of magnitude slower than is possible with microspheres. In order to explain our results, we introduce the concept of mechanical leeway, i.e., nanoparticles benefit from a smaller driving force towards disruptive curvature. Finally, we suggest that leeway mechanisms may benefit any formulation in which challenges arise due to tight restrictions on a pivotal parameter, but where the restrictions can be relaxed by rationally changing the value of a more accessible parameter.

Different mechanisms for dynamical arrest in largely asymmetric binary mixtures.

Using confocal microscopy we investigate binary colloidal mixtures with large size asymmetry, in particular the formation of dynamically arrested states of the large spheres. The volume fraction of the system is kept constant, and as the concentration of small spheres is increased we observe a series of transitions of the large spheres to different arrested states: an attractive glass, a gel, and an asymmetric glass. These states are distinguished by the degree of dynamical arrest and the amount of structural and dynamical heterogeneity. The transitions between two different arrested states occur through melting and the formation of a fluid state. While a space-spanning network of bonded particles is found in both arrested and fluid states, only arrested states are characterized by the presence of a space-spanning network of dynamically arrested particles.

Inter-particle correlations in a hard-sphere colloidal suspension with polymer additives investigated by Spin Echo Small Angle Neutron Scattering (SESANS).

Using a neutron scattering technique that measures a statistically-averaged density correlation function in real space rather than the conventional reciprocal-space structure factor, we have measured correlations between poly(methyl-methacrylate) (PMMA) colloidal particles of several sizes suspended in decalin. The new method, called Spin Echo Small Angle Neutron Scattering (SESANS) provides accurate information about particle composition, including the degree of solvent penetration into the polymer brush grafted on to the PMMA spheres to prevent aggregation. It confirms for particles, between 85 nm and 150 nm in radius that inter-particle correlations closely follow the Percus-Yevick hard-sphere model when the colloidal volume-fraction is between 30% and 50% provided the volume-fraction is used as a fitted parameter. No particle aggregation occurs in these systems. When small amounts of polystyrene are added as a depletant to a concentrated suspension of PMMA particles, short-range clustering of the particles occurs and there is an increase in the frequency of near-neighbor contacts. Within a small range of depletant concentration, near-neighbor correlations saturate and large aggregates with power law density correlations are formed. SESANS clearly separates the short- and long-range correlations and shows that, in this case, the power-law correlations are visible for inter-particle distances larger than roughly two particle diameters. In some cases, aggregate sizes are within our measurement window, which can extend out to 16 microns in favorable cases. We discuss the advantages of SESANS for measurements of the structure of concentrated colloidal systems and conclude that the method offers several important advantages.

Structure and interactions in fluids of prolate colloidal ellipsoids: comparison between experiment, theory, and simulation.

The microscopic structure of fluids of simple spheres is well known. However, the constituents of most real-life fluids are non-spherical, leading to a coupling between the rotational and translational degrees of freedom. The structure of simple dense fluids of spheroids - ellipsoids of revolution - was only recently determined by direct experimental techniques [A. P. Cohen, E. Janai, E. Mogilko, A. B. Schofield, and E. Sloutskin, Phys. Rev. Lett. 107, 238301 (2011)]. Using confocal microscopy, it was demonstrated that the structure of these simple fluids cannot be described by hard particle models based on the widely used Percus-Yevick approximation. In this paper, we describe a new protocol for determining the shape of the experimental spheroids, which allows us to expand our previous microscopy measurements of these fluids. To avoid the approximations in the theoretical approach, we have also used molecular dynamics simulations to reproduce the experimental radial distribution functions g(r) and estimate the contribution of charge effects to the interactions. Accounting for these charge effects within the Percus-Yevick framework leads to similar agreement with the experiment.

Transient dynamics in dense colloidal suspensions under shear: shear rate dependence.

A combination of confocal microscopy and rheology experiments, Brownian dynamics (BD) and molecular dynamics (MD) simulations and mode coupling theory (MCT) have been applied in order to investigate the effect of shear rate on the transient dynamics and stress-strain relations in supercooled and glassy systems under shear. Immediately after shear is switched on, the microscopic dynamics display super-diffusion and the macroscopic rheology a stress overshoot, which become more pronounced with increasing shear rate. MCT relates both to negative sections of the generalized shear modulus, which grow with increasing shear rate. When the inverse shear rate becomes much smaller than the structural relaxation time of the quiescent system, relaxation through Brownian motion becomes less important. In this regime, larger stresses are accumulated before the system yields and the transition from localization to flow occurs earlier and more abruptly.

Fluid suspensions of colloidal ellipsoids: direct structural measurements.

A fluid of spheroids, ellipsoids of revolution, is among the simplest models of the disordered matter, where positional and rotational degrees of freedom of the constituent particles are coupled. However, while highly anisometric rods, and hard spheres, were intensively studied in the last decades, the structure of a fluid of spheroids is still unknown. We reconstruct the structure of a simple fluid of spheroids, employing direct confocal imaging of colloids, in three dimensions. The ratio t between the polar axis and the equatorial diameter for both our prolate and oblate spheroids is not far from unity, which gives rise to a delicate interplay between rotations and translations. Strikingly, the measured positional interparticle correlations are significantly stronger than theoretically predicted, indicating that further theoretical attention is required, to fully understand the coupling between translations and rotations in these fundamental fluids.

A self-quenched defect glass in a colloid-nematic liquid crystal composite.

Colloidal particles immersed in liquid crystals frustrate orientational order. This generates defect lines known as disclinations. At the core of these defects, the orientational order drops sharply. We have discovered a class of soft solids, with shear moduli up to 10(4) pascals, containing high concentrations of colloidal particles (volume fraction φ ≳ 20%) directly dispersed into a nematic liquid crystal. Confocal microscopy and computer simulations show that the mechanical strength derives from a percolated network of defect lines entangled with the particles in three dimensions. Such a "self-quenched glass" of defect lines and particles can be considered a self-organized analog of the "vortex glass" state in type II superconductors.

Passive and active microrheology of hard-sphere colloids.

We performed passive and active microrheology using probe particles in a bath of well-characterized, model hard-sphere colloids in the fluid state over the whole range of volume fractions below the glass transition. The probe and bath particles have nearly the same size. Passive tracking of probe particles yields short-time self-diffusion coefficients. Comparison with literature data demonstrates that the interaction between probe and bath particles is hard-sphere-like. The short-time diffusivities yield one set of microviscosities as a function of volume fraction, which agrees with previous macrorheological measurements of the high-frequency viscosity of hard-sphere colloids. Using optical tweezers, we measure the force on a trapped probe particle as the rest of the sample is translated at constant velocity. This yields a second set of microviscosities at high Péclet numbers. These agree with previous macrorheological measurements of the high-shear viscosity of similar colloids, at shear-rates below the onset of shear-thickening.

Structure, dynamics, and rheology of colloid-polymer mixtures: from liquids to gels.

We investigate the structural, dynamical, and viscoelastic properties of colloid-polymer mixtures at intermediate colloid volume fraction and varying polymer concentrations, thereby tuning the attractive interactions. Within the examined range of polymer concentrations, the samples varied from fluids to gels. In the liquid phase, an increasing correlation length of the density fluctuations when approaching the gelation boundary was observed by static light scattering and microscopy, indicating clustering and formation of space-spanning networks. Simultaneously, the correlation function determined by dynamic light scattering decays completely, indicating the absence of dynamical arrest. Clustering and formation of transient networks when approaching the gelation boundary is supported by significant changes in the viscoelastic properties of the samples. Upon increasing the polymer concentration beyond the gelation boundary, the rheological properties changed qualitatively again, now they are consistent with the formation of colloidal gels. Our experimental results, namely, the location of the gelation boundary as well as the elastic (storage) and viscous (loss) moduli, are compared to different theoretical models. These include consideration of the escape time as well as predictions for the viscoelastic moduli based on scaling relations and mode coupling theories.

Probing the equilibrium dynamics of colloidal hard spheres above the mode-coupling glass transition.

We use dynamic light scattering and computer simulations to study equilibrium dynamics and dynamic heterogeneity in concentrated suspensions of colloidal hard spheres. Our study covers an unprecedented density range and spans seven decades in structural relaxation time, tau(alpha0, including equilibrium measurements above phi(c), the location of the glass transition deduced from fitting our data to mode-coupling theory. Instead of falling out of equilibrium, the system remains ergodic above phi(c) and enters a new dynamical regime where tau(alpha) increases with a functional form that was not anticipated by previous experiments, while the amplitude of dynamic heterogeneity grows slower than a power law with tau(alpha), as found in molecular glass formers close to the glass transition.

Bicontinuous emulsions stabilized solely by colloidal particles.

Recent large-scale computer simulations suggest that it may be possible to create a new class of soft solids, called 'bijels', by stabilizing and arresting the bicontinuous interface in a binary liquid demixing via spinodal decomposition using particles that are neutrally wetted by both liquids. The interfacial layer of particles is expected to be semi-permeable; hence, if realized, these new materials would have many potential applications, for example, as micro-reaction media. However, the creation of bijels in the laboratory faces serious obstacles. In general, fast quench rates are necessary to bypass nucleation, so that only samples with limited thickness can be produced, which destroys the three-dimensionality of the putative bicontinuous network. Moreover, even a small degree of unequal wettability of the particles by the two liquids can lead to ill-characterized, 'lumpy' interfacial layers and therefore irreproducible material properties. Here, we report a reproducible protocol for creating three-dimensional samples of bijel in which the interfaces are stabilized by essentially a single layer of particles. We demonstrate how to tune the mean interfacial separation in these bijels, and show that mechanically, they indeed behave as soft solids. These characteristics and their tunability will be of great value for microfluidic applications.

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.

Hypersonic acoustic excitations in binary colloidal crystals: big versus small hard sphere control.

The phononic band structure of two binary colloidal crystals, at hypersonic frequencies, is studied by means of Brillouin light scattering and analyzed in conjunction with corresponding dispersion diagrams of the single colloidal crystals of the constituent particles. Besides the acoustic band of the average medium, the authors' results show the existence of narrow bands originating from resonant multipole modes of the individual particles as well as Bragg-type modes due to the (short-range) periodicity. Strong interaction, leading to the occurrence of hybridization gaps, is observed between the acoustic band and the band of quadrupole modes of the particles that occupy the largest fractional volume of the mixed crystal; the effective radius is either that of the large (in the symmetric NaCl-type crystalline phase) or the small (in the asymmetric NaZn(13)-type crystalline phase) particles. The possibility to reveal a universal behavior of the phononic band structure for different single and binary colloidal crystalline suspensions, by representing in the dispersion diagrams reduced quantities using an appropriate length scale, is discussed.

Spinodal decomposition in a model colloid-polymer mixture in microgravity.

We study phase separation in a deeply quenched colloid-polymer mixture in microgravity on the International Space Station using small-angle light scattering and direct imaging. We observe a clear crossover from early-stage spinodal decomposition to late-stage, interfacial-tension-driven coarsening. Data acquired over 5 orders of magnitude in time show more than 3 orders of magnitude increase in domain size, following nearly the same evolution as that in binary liquid mixtures. The late-stage growth approaches the expected linear growth rate quite slowly.

Stable jets of viscoelastic fluids and self-assembled cylindrical capsules by hydrodynamic focusing.

We demonstrate formation of long-lived cylindrical jets of a viscoelastic fluid using hydrodynamic focusing. A solution of polyacrylamide in water is driven coaxially with immiscible oil and subjected to strong extensional flow. At high flow rates, the aqueous phase forms jets that are 4-90 microm in diameter and several centimeters long. The liquid surfaces of these jets are then used as templates for assembly of microspheres into novel rigid and hollow cylinders.

Stability of the binary colloidal crystals AB2 and AB13.

Suspensions of binary mixtures of hard-sphere poly-methylmethacrylate colloidal particles were studied at six different size ratios alpha. The main aim was to determine the range of size ratios over which the binary colloidal crystals AB2 and AB13 are stable. Combining these results with those of earlier work, we found stability of AB2 for 0.60 approximately > alpha approximately > 0.425, in good agreement with theoretical predictions by computer simulation and cell model methods. AB13 was observed for 0.62 approximately > alpha approximately > 0.485, the lower limit being significantly smaller than the theoretical prediction of about 0.525. Rough measurements of crystallization rates showed that AB2 tended to crystallize fastest at small size ratios, whereas the opposite was true for AB13. These findings should provide a guide to the optimum conditions for materials applications of these binary colloidal crystals.