Michael Cates

Jamming, Force Chains, and Fragile Matter

Consider a concentrated colloidal suspension of hard particles under shear [Fig. 1(a)]. Above a certain threshold of stress, this system may jam [1]. (To observe such an effect, stir a concentrated suspension of cornstarch with a spoon.) Jamming apparently occurs because the particles form “force chains” along the compressional direction [1]. Even for spherical particles the lubrication films cannot prevent contacts; once these arise, an array or network of force chains can support the shear stress indefinitely [2]. By this criterion, the material is a solid. In this Letter, we propose some simple models of jammed systems like this, whose solidity stems directly from the applied stress itself. We argue that such materials may show fundamentally new mechanical properties, very different from those of conventional (elastic or elastoplastic) bodies. We start from a simple model of a force chain: a linear string of rigid particles in point contact. Crucially, this chain can only support loads along its own axis[Fig. 2(a)]: successive contacts must be collinear, with the forces along the line of contacts, to prevent torques on particles within the chain [3]. (Neither friction at the contacts nor particle aspherity can obviate this.) Let us now model a jammed colloid by an assembly of such force chains, characterized by a director n ,i n a sea of “spectator” particles, and incompressible solvent. (We ignore for the moment any “collisions” between force chains or deflections caused by weak interaction with the spectators.) In static equilibrium, with no body forces acting, the pressure tensor pijs› 2sijd is then

Kinetics of the Micelle-to-Vesicle Transition: Aqueous Lecithin-Bile Salt Mixtures

Important routes to lipid vesicles (liposomes) are detergent removal techniques, such as dialysis or dilution. Although they are widely applied, there has been only limited understanding about the structural evolution during the formation of vesicles and the parameters that determine their properties. We use time-resolved static and dynamic light scattering to study vesicle formation in aqueous lecithin-bile salt mixtures. The kinetic rates and vesicle sizes are found to strongly depend on total amphiphile concentration and, even more pronounced, on ionic strength. The observed trends contradict equilibrium calculations, but are in agreement with a kinetic model that we present. This model identifies the key kinetic steps during vesicle formation: rapid formation of disk-like intermediate micelles, growth of these metastable micelles, and their closure to form vesicles once line tension dominates bending energy. A comparison of the rates of growth and closure provides a kinetic criterion for the critical size at which disks close and thus for the vesicle size. The model suggests that liposomes are nonequilibrium, kinetically trapped structures of very long lifetime. Their properties are hence controlled by kinetics rather than thermodynamics.

Phase separation and rotor self-assembly in active particle suspensions.

Adding a nonadsorbing polymer to passive colloids induces an attraction between the particles via the “depletion” mechanism. High enough polymer concentrations lead to phase separation. We combine experiments, theory, and simulations to demonstrate that using active colloids (such as motile bacteria) dramatically changes the physics of such mixtures. First, significantly stronger interparticle attraction is needed to cause phase separation. Secondly, the finite size aggregates formed at lower interparticle attraction show unidirectional rotation. These micro-rotors demonstrate the self-assembly of functional structures using active particles. The angular speed of the rotating clusters scales approximately as the inverse of their size, which may be understood theoretically by assuming that the torques exerted by the outermost bacteria in a cluster add up randomly. Our simulations suggest that both the suppression of phase separation and the self-assembly of rotors are generic features of aggregating swimmers and should therefore occur in a variety of biological and synthetic active particle systems.

Topological effects in ring polymers: A computer simulation study

Unconcatenated, unknotted polymer rings in the melt are subject to strong interactions with neighboring chains due to the presence of topological constraints. We study this by computer simulation using the bond-fluctuation algorithm for chains with up to N=512 statistical segments at a volume fraction \ensuremath{\Phi}=0.5 and show that rings in the melt are more compact than Gaussian chains. A careful finite-size analysis of the average ring size R\ensuremath{\propto}

Hard spheres: crystallization and glass formation

{\mathit{N}}^{\ensuremath{\nu}}

Fluctuating lattice Boltzmann

yields an exponent \ensuremath{\nu}\ensuremath{\approxeq}0.39\ifmmode\pm\else\textpm\fi{}0.03 in agreement with a Flory-like argument for the topological interactions. We show (using the same algorithm) that the dynamics of molten rings is similar to that of linear chains of the same mass, confirming recent experimental findings. The diffusion constant varies effectively as

Comparative Simulation Study of Colloidal Gels And Glasses

{\mathit{D}}_{\mathit{N}}

When are active Brownian particles and run-and-tumble particles equivalent? Consequences for motility-induced phase separation

\ensuremath{\propto}

Arrested phase separation in reproducing bacteria creates a generic route to pattern formation

{\mathit{N}}^{\mathrm{\ensuremath{-}}1.22(3)}

Bijels: a new class of soft materials

and is slightly higher than that of corresponding linear chains. For the ring sizes considered (up to 256 statistical segments) we find only one characteristic time scale