Colloidal particles with site-specific directional interactions, so called "patchy particles", are promising candidates for bottomup assembly routes towards complex structures with rationally designed properties. Here we present an experimental realization of patchy colloidal particles based on material independent depletion interaction and surface roughness. Curved, smooth patches on rough colloids are shown to be exclusively attractive due to their different overlap volumes. We discuss in detail the case of colloids with one patch that serves as a model for molecular surfactants both with respect to their geometry and their interactions. These one-patch particles assemble into clusters that resemble surfactant micelles with the smooth and attractive sides of the colloids located at the interior. We term these clusters "colloidal micelles". Direct Monte Carlo simulations starting from a homogeneous state give rise to cluster size distributions that are in good agreement with those found in experiments. Important differences with surfactant micelles originate from the colloidal character of our model system and are investigated by simulations and addressed theoretically. Our new "patchy" model system opens up the possibility for self-assembly studies into finite-sized superstructures as well as crystals with as of yet inaccessible structures.anisotropic colloids | depletion interactions | Monte-Carlo simulations N ature has mastered the self-assembly of simple basic subunits into complex, functional structures with outstanding precision. Examples include biological membranes and viruses, which exhibit excellent control over the assembled structures with respect to their functionalities, shapes or sizes. However, the interactions between the building blocks, in the case of viruses, the protein subunits, are often complex and it remains challenging to identify the key elements for guiding and controlling the selfassembling process. By mimicking such self-assembly processes on a colloidal scale, insights into the paramount elements that control the assembly can be obtained in situ and applied to build up superstructures with new and desirable properties.Colloidal particles with site-specific directional interactions, so called "patchy particles", are promising candidates for bottom-up assembly routes towards such complex structures with rationally designed properties (1-3). The size and geometry of the patches together with the shape of the interparticle potential are expected to determine the formed structures and phases, which may range from empty liquids (4) and crystals (5-7) to finite-sized clusters (1, 2, 8-11), and lead to novel collective behavior (12).Recent experimental approaches to assemble colloidal particles at specific sites include hydrophobic-hydrophilic interactions (6,7,(13)(14)(15), and lock-and-key recognition mechanisms (16). With a wide variability of colloidal shapes available today, the ultimate challenge is to identify general methods to render specific areas of the colloids attractive or ...

Over the last number of years several simulation methods have been introduced to study rare events such as nucleation. In this paper we examine the crystal nucleation rate of hard spheres using three such numerical techniques: molecular dynamics, forward flux sampling, and a Bennett-Chandler-type theory where the nucleation barrier is determined using umbrella sampling simulations. The resulting nucleation rates are compared with the experimental rates of Harland and van Megen [Phys. Rev. E 55, 3054 (1997)], Sinn et al. [Prog. Colloid Polym. Sci. 118, 266 (2001)], Schätzel and Ackerson [Phys. Rev. E 48, 3766 (1993)], and the predicted rates for monodisperse and 5% polydisperse hard spheres of Auer and Frenkel [Nature 409, 1020 (2001)]. When the rates are examined in units of the long-time diffusion coefficient, we find agreement between all the theoretically predicted nucleation rates, however, the experimental results display a markedly different behavior for low supersaturation. Additionally, we examined the precritical nuclei arising in the molecular dynamics, forward flux sampling, and umbrella sampling simulations. The structure of the nuclei appears independent of the simulation method, and in all cases, the nuclei contains on average significantly more face-centered-cubic ordered particles than hexagonal-close-packed ordered particles.

Shear thickening is a widespread phenomenon in suspension flow that, despite sustained study, is still the subject of much debate. The longstanding view that shear thickening is due to hydrodynamic clusters has been challenged by recent theory and simulations suggesting that contact forces dominate, not only in discontinuous, but also in continuous shear thickening. Here, we settle this dispute using shear reversal experiments on micron-sized silica and latex particles to measure directly the hydrodynamic and contact force contributions to shear thickening. We find that contact forces dominate even continuous shear thickening. Computer simulations show that these forces most likely arise from frictional interactions.

The rheology of suspensions of Brownian, or colloidal, particles (diameter d ≲ 1 µm) differs markedly from that of larger grains (d ≳ 50 µm). Each of these two regimes has been separately studied, but the flow of suspensions with intermediate particle sizes (1 µm ≲ d ≲ 50 µm), which occur ubiquitously in applications, remains poorly understood. By measuring the rheology of suspensions of hard spheres with a wide range of sizes, we show experimentally that shear thickening drives the transition from colloidal to granular flow across the intermediate size regime. This insight makes possible a unified description of the (non-inertial) rheology of hard spheres over the full size spectrum. Moreover, we are able to test a new theory of friction-induced shear thickening, showing that our data can be well fitted using expressions derived from it.Complex fluids, polymers, colloids and surfactant solutions find wide applications, partly because of their highly tuneable behavior under deformation and in flow. The success of the mean-field 'tube' model for polymers [1], which describes how each chain is constrained by thousands of neighbours, means it has long been possible to predict ab initio their linear and non-linear rheology from the molecular topology with very few free parameters. In particular, a scaling description is available of the dependence of rheology on molecular weight.However, progress in suspension rheology has been more difficult [2]. The small number of nearest neighbours (order 10) rules our any mean-field description: local details matter. It is now possible to predict the lowshear viscosity of a suspension of Brownian hard spheres (HS, diameter d ≲ 1 µm) up to volume fractions of φ ≲ 0.6, and the rheology of granular HS (d ≳ 50 µm) is increasingly being studied. Surprisingly, however, how the rheology of HS changes over the whole size spectrum remains unknown, because the behavior in the industriallyubiquitous intermediate size regime, 1 ≲ d ≲ 50 µm, has not been systematically explored. We offer such an exploration in this Letter, and show that the physics bridging the colloidal and the granular regimes is shear thickening.The rheology of colloidal HS is well known [3][4][5]: the viscosity is determined by the particle volume fraction, φ, and the dimensionless shear rate, or Péclet number, Pe (= τ Bγ , the shear rateγ non-dimensionalised by the Brownian time, τ B , needed for a free particle to diffuse its own radius). At Pe ≪ 1 the flow is Newtonian; the viscosity becomes immeasurably large at φ g ≈ 0.58 [5,6]. Shear thinning starts at Pe ≲ 1, reaching a second Newtonian regime at Pe ≫ 1 with a viscosity that diverges at random close packing [2], φ RCP ≈ 0.64, the densest amorphous packing for lubricated (frictionless) HS.Since τ B scales as d 3 , granular HS inhabit the Pe ≫ 1 regime at all practical shear rates. Extrapolating naïvely from the above description of colloidal flow, one expects Newtonian behaviour with a viscosity diverging at φ RCP . Experiments do find a Newtonian viscosity, but i...

We present experimental results on dense corn-starch suspensions as examples of non-Brownian, nearly-hard particles that undergo continuous and discontinuous shear thickening (CST and DST) at intermediate and high densities respectively. Our results offer strong support for recent theories involving a stress-dependent effective contact friction among particles. We show however that in the DST regime, where theory might lead one to expect steady-state shear bands oriented layerwise along the vorticity axis, the real flow is unsteady. To explain this, we argue that steady-state banding is generically ruled out by the requirement that, for hard non-Brownian particles, the solvent pressure and the normal-normal component of the particle stress must balance separately across the interface between bands. (Otherwise there is an unbalanced migration flux.) However, long-lived transient shear-bands remain possible