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Photosynthesis is vital for the survival of entire ecosystems on Earth. While light is fundamental to this process, excessive exposure can be detrimental to plant cells. Chloroplasts, the photosynthetic organelles, actively move in response to light and self-organize within the cell to tune light absorption. These disk-shaped motile organelles must balance dense packing for enhanced light absorption under dim conditions with spatial rearrangements to avoid damage from excessive light exposure. Here, we reveal that the packing characteristics of chloroplasts within plant cells show signatures of optimality. Combining measurements of chloroplast densities and three-dimensional cell shape in the water plant Elodea densa, we construct an argument for optimal cell shape versus chloroplast size to achieve two targets: dense packing into a two-dimensional monolayer for optimal absorption under dim light conditions and packing at the sidewalls for optimal light avoidance. We formalize these constraints using a model for random close packing matched with packing simulations of polydisperse hard disks confined within rectangular boxes. The optimal cell shape resulting from these models corresponds closely to that measured in the box-like plant cells, highlighting the importance of particle packing in the light adaptation of plants. Understanding the interplay between structure and function sheds light on how plants achieve efficient photo adaptation. It also highlights a broader principle: how cell shape relates to the optimization of packing finite and relatively small numbers of organelles under confinement. This universal challenge in biological systems shares fundamental features with the mechanics of confined granular media and the jamming transitions in dense active and passive systems across various scales and contexts.

We study the aging of colloidal gels using light microscopy movies of depletion gels from the International Space Station. Under such microgravity conditions, we observe a slowdown in particle dynamics consistent with gel aging. Stronger attractive forces promote the formation of thicker gel strands over time. The samples are bidisperse, composed of particles with a size ratio 1.2. Larger particles experience stronger depletion forces, which lead to a large first-neighbor peak in the pair correlation function $g(r)$ due to the prevalence of large-large particle contacts. As the gel ages, small mobile particles are incorporated into the gel structure. The changes in gel structure correlate with a slow power-law decay in particle motion, observed over nearly two orders of magnitude of time scales in microgravity experiments. Additionally, through complementary ground-based experiments, we compare two-dimensional (2D) and three-dimensional (3D) images of depletion colloidal gels. While microgravity gel data are limited to 2D projections, ground-based data establish a correspondence between the 2D and 3D $g(r)$ peak heights. Our results provide new insights into how colloidal gels age in the absence of gravitational collapse., Comment: Study of microscopy movies from the NASA PSI database (originally from T. Kodger and M. Lynch)

Capillary droplets form due to surface tension when two immiscible fluids are mixed. We describe the motion of gravity-driven capillary droplets flowing through narrow constrictions and obstacle arrays in both simulations and experiments. Our new capillary deformable particle model recapitulates the shape and velocity of single oil droplets in water as they pass through narrow constrictions in microfluidic chambers. Using this experimentally validated model, we simulate the flow and clogging of single capillary droplets in narrow channels and obstacle arrays and find several important results. First, the capillary droplet speed profile is nonmonotonic as the droplet exits the narrow orifice, and we can tune the droplet properties so that the speed overshoots the terminal speed far from the constriction. Second, in obstacle arrays, we find that extremely deformable droplets can wrap around obstacles, which leads to decreased average droplet speed in the continuous flow regime and increased probability for clogging in the regime where permanent clogs form. Third, the wrapping mechanism causes the clogging probability in obstacle arrays to become nonmonotonic with surface tension $\Gamma$. At large $\Gamma$, the droplets are nearly rigid and the clogging probability is large since the droplets can not squeeze through the gaps between obstacles. With decreasing $\Gamma$, the clogging probability decreases as the droplets become more deformable. However, in the small-$\Gamma$ limit the clogging probability increases, since the droplets are extremely deformable and cause clogs as they wrap around the obstacles. The results from these studies are important for developing a predictive understanding of capillary droplet flows through complex and confined geometries.

We use confocal microscopy to image colloidal gels formed from highly polydisperse particles. We suspend our polydisperse particles in a density matched solvent, and let the particles spontaneously aggregate through the van der Waals force. The particle size distribution $P(R)$ is roughly log-normal, with the largest particles more than 15 times the size of the smallest particles. The pairing of nearest neighbor particles is consistent with a null hypothesis that pairings are made randomly, that is, any two particle sizes have a probability of being neighbors consistent with their proportionality in $P(R)$. That being said, as expected, larger particles have more nearest neighbors than small ones. This leads to an over-representation of large particles in tetrahedral structures where four particles are mutually nearest neighbors, showing that large particles help provide rigidity to the gel structure. We discuss the implications of how other size distributions $P(R)$ would affect the gel structure., Comment: Fig. 1 has a pretty confocal microscope image

Circles of a single size can pack together densely in a hexagonal lattice, but adding in size variety disrupts the order of those packings. We conduct simulations which generate dense random packings of circles with specified size distributions, and measure the area fraction in each case. While the size distributions can be arbitrary, we find that for a wide range of size distributions the random close packing area fraction $\phi_{rcp}$ is determined to high accuracy by the polydispersity and skewness of the size distribution. At low skewness, all packings tend to a minimum packing fraction $\phi_0 \approx 0.840$ independent of polydispersity. In the limit of high skewness, $\phi_{rcp}$ becomes independent of skewness, asymptoting to a polydispersity-dependent limit. We show how these results can be predicted from the behavior of simple, bidisperse or bi-Gaussian circle size distributions., Comment: 12 pages, 9 figures, 1 appendix, 1 supplemental data

Distributions of strictly positive numbers are common and can be characterized by standard statistical measures such as mean, standard deviation, and skewness. We demonstrate that for these distributions the skewness $D_3$ is bounded from below by a function of the coefficient of variation (CoV) $\delta$ as $D_3 \ge \delta-1/\delta$. The results are extended to any distribution that is bounded with minimum value $x_{\rm min}$ and/or bounded with maximum value $x_{\rm max}$. We build on the results to provide bounds for kurtosis $D_4$, and conjecture analogous bounds exists for higher statistical moments., Comment: 14 pages, 2 figures. Awaiting publication in PLoS One

We simulate a two-dimensional array of droplets being compressed between two walls. The droplets are adhesive due to an attractive depletion force. As one wall moves toward the other, the droplet array is compressed and eventually induced to rearrange. The rearrangement occurs via a fracture, where depletion bonds are quickly broken between a subset of droplets. For monodisperse, hexagonally ordered droplet arrays, this fracture is preceded by a maximum force exerted on the walls, which drops rapidly after the fracture occurs. In small droplet arrays a fracture is a single well-defined event, but for larger droplet arrays, competing fractures can be observed. These are fractures nucleated nearly simultaneously in different locations. Finally, we also study the compression of bidisperse droplet arrays. The addition of a second droplet size further disrupts fracture events, showing differences between ideal crystalline arrays, crystalline arrays with a small number of defects, and fully amorphous arrays. These results are in good agreement with previously published experiments., Comment: 15 pages, 15 figures, Supplemental Movies are available, url in the bibliography

As amorphous materials get jammed, both geometric and dynamic heterogeneity are observed. We investigate the correlation between the local geometric heterogeneity and local rearrangements in a slowly compressed bidisperse quasi-two-dimensional emulsion system. The compression is driven by evaporation of the continuous phase, and causes the area packing fraction to increase from 0.88 to 0.99. We quantify the structural heterogeneity of the system using the radical Voronoi tessellation following the method of [Rieser et al., Phys. Rev. Lett. 116, 088001 (2016)]. We define two structural quantities characterizing local structure, the first which considers nearest neighbors and the second of which includes information from second nearest neighbors. We find that droplets in heterogeneous local regions are more likely to have local rearrangements. These rearrangements are generally T1 events where two droplets converge toward a void, and two droplets move away from the void to make room for the converging droplets. Thus the presence of the voids tends to orient the T1 events. The presence of a correlation between the structural quantities and the rearrangement dynamics remains qualitatively unchanged over the entire range of packing fractions observed., Comment: Latest paper in our series of quasi-2D emulsion experiments

Magnetorheological fluids (MRF) are smart materials of increasing interest due to their great versatility in mechanical and mechatronic systems. As main rheological features, MRFs must present low viscosity in the absence of a magnetic field (0.1 - 1.0 Pa.s) and high yield stress (50 - 100 kPa) when magnetized, in order to optimize the magnetorheological effect. Such properties, in turn, are directly influenced by the composition, volume fraction, size, and size distribution (polydispersity) of the particles, the latter being an important piece in the improvement of these main properties. In this context, the present work aims to analyze, through experiments and simulations, the influence of polydispersity on the maximum packing fraction, on the yield stress under field (on-state), and on the plastic viscosity in the absence of field (off-state) of concentrated MRF (phi = 48.5 vol.%). Three blends of carbonyl iron powder in polyalphaolefin oil were prepared. These blends have the same mode, but different polydispersity indexes, ranging from 0.46 to 1.44. Separate simulations show that the random close packing fraction increases from about 68% to 80% as the polydispersity index increases over this range. The on-state yield stress, in turn, is raised from 30 +/- 0.5 kPa to 42 +/- 2 kPa (B ~ 0.57 T) and the off-state plastic viscosity, is reduced from 4.8 Pa.s to 0.5 Pa.s. Widening the size distributions, as is well known in the literature, increases packing efficiency and reduces the viscosity of concentrated dispersions, but beyond that, it proved to be a viable way to increase the magnetorheological effect of concentrated MRF. The Brouwers model, which considers the void fraction in suspensions of particles with lognormal distribution, was proposed as a possible hypothesis to explain the increase in yield stress under magnetic field.

We have studied shear deformation of binary Lennard-Jones glasses to investigate the extent to which the transient part of the stress strain curves is invariant when the thermodynamic state point is varied along an isomorph. Shear deformations were carried out on glass samples of varying stability, determined by cooling rate, and at varying strain rates, at a state point deep in the glass. Density changes up to and exceeding a factor of two were made. We investigated several different methods for generating isomorphs but none of the previously developed methods could generate sufficiently precise isomorphs given the large density changes and non-equilibrium situation. Instead, the temperatures for these higher densities were chosen to give state points isomorphic to the starting state point by requiring the steady state flow stress for isomorphic state points to be invariant in reduced units. In contrast to the steady state flow stress, we find that the peak stress on the stress strain curve is not invariant. The peak stress decreases by a few percent for each ten percent increase in density, although the differences decrease with increasing density. Analysis of strain profiles and non-affine motion during the transient phase suggests that the root of the changes in peak stress is a varying tendency to form shear bands, with the largest tendency occurring at the lowest densities. We argue that this reflects the effective steepness of the potential; a higher effective steepness gives a greater tendency to form shear bands.

We study particle-scale motion in sheared highly polydisperse amorphous materials, in which the largest particles are as much as ten times the size of the smallest. We find strikingly different behavior from the more commonly studied amorphous systems with low polydispersity. In particular, analysis of the nonaffine motion of particles reveals qualitative differences between large and small particles: the smaller particles have dramatically more nonaffine motion, which is induced by the presence of the large particles. We characterize the crossover in nonaffine motion from the low- to high-polydispersity regime, and demonstrate a quantitative way to distinguish between "large" and "small" particles in systems with broad distributions of particle sizes.

We study a quasi-two-dimensional macroscopic system of magnetic spherical particles settled on a shallow concave dish under a temporally oscillating magnetic field. The system reaches a stationary state where the energy losses from collisions and friction with the concave dish surface are compensated by the continuous energy input coming from the oscillating magnetic field. Random particle motions show some similarities with the motions of atoms and molecules in a glass or a crystal-forming fluid. Because of the curvature of the surface, particles experience an additional force toward the center of the concave dish. When decreasing the magnetic field, the effective temperature is decreased and diffusive particle motion slows. For slow cooling rates we observe crystallization, where the particles organize into a hexagonal lattice. We study the birth of the crystalline nucleus and the subsequent growth of the crystal. Our observations support non-classical theories of crystal formation. Initially a dense amorphous aggregate of particles forms, and then in a second stage this aggregate rearranges internally to form the crystalline nucleus. As the aggregate grows, the crystal grows in its interior. After a certain size, all the aggregated particles are part of the crystal and after that, crystal growth follows the classical theory for crystal growth., Comment: Continuing our collaboration from arXiv:1910.05887, this time studying crystallization. 10 pages

We study the outflow of soft particles through quasi-two-dimensional hoppers with both experiments and simulations. The experiments utilize spheres made with soft hydrogel, silicon rubber and glass. The hopper chamber has an adjustable exit width and tilt angle (the latter to control the magnitude of gravitational forcing). Our simulation mimics the experiments using purely two-dimensional soft particles with viscous interactions but no friction. Results from both simulations and experiments demonstrate that clogging is easier for reduced gravitational force or stiffer particles. For particles with low or no friction, the average number of particles in a clogging arch depends only on the ratio between hopper exit width and particle's diameter. In contrast for the silicon rubber particles with larger frictional interactions, arches are larger than the low friction case. Additionally, an analysis of the number of particles left in the hopper when clogging occurs provides evidence for a hydrostatic pressure effect that is relevant for the clogging of soft particles, but less so for the harder (glass) or frictional (silicon rubber) particles., Comment: Followup to arXiv:1512.02500 (Hong et al, PRE 2017); v2 fixed bug with Gompertz distribution fits

We conduct molecular dynamics simulations of a bidisperse Kob-Andersen (KA) glass former, modified to add in additional polydispersity. The original KA system is known to exhibit dynamical heterogeneity. Prior work defined propensity, the mean motion of a particle averaged over simulations reconstructing the initial positions of all particles but with randomized velocities. The existence of propensity shows that structure and dynamics are connected. In this paper, we study systems which mimic problems that would be encountered in measuring propensity in a colloidal glass former, where particles are polydisperse (they have slight size variations). We mimic polydispersity by altering the bidisperse KA system into a quartet consisting of particles both slightly larger and slightly smaller than the parent particles in the original bidisperse system. We then introduce errors into the reconstruction of the initial positions that mimic mistakes one might make in a colloidal experiment. The mistakes degrade the propensity measurement, in some cases nearly completely; one no longer has an isoconfigurational ensemble in any useful sense. Our results show that a polydisperse sample is suitable for propensity measurements provided one avoids reconstruction mistakes., Comment: Implications for hypothetical colloidal experiment to test propensity

We study the rheology of monodisperse and bidisperse emulsions with various droplet sizes (1 $\mu$m -- 2 $\mu$m diameter). Above a critical volume fraction $\phi_c$, these systems exhibit solid-like behavior and a yield stress can be detected. Previous experiments suggest that for small thermal particles, rheology will see a glass transition at $\phi_c = \phi_g =0.58$; for large athermal systems, rheology will see a jamming transition at $\phi_c = \phi_J =0.64$. However, simulations point out that at the crossover of thermal and athermal regimes, the glass and jamming transitions may both be observed in the same sample. Here we conduct an experiment by shearing four oil-in-water emulsions with a rheometer. We observe both a glass and a jamming transition for our smaller diameter droplets, and only a jamming transition for our larger diameter droplets. The bidisperse sample behaves similarly to the small droplet sample, with two transitions observed. Our rheology data are well-fit by both the Herschel-Bulkley model and the Three Component model. Based on the fitting parameters, our raw rheological data would not collapse onto a master curve. Our results suggest that liquid-solid transitions in dispersions may not be universal, but depend on particle type.

We present a simple model system with four hard disks moving in a circular region for which free energy landscapes can be directly calculated and visualized in two and three dimensions. We construct several energy landscapes for our system and explore the strengths and limitations of each in terms of understanding system dynamics, in particular the relationship between state transitions and free energy barriers. We also demonstrate the importance of distinguishing between system dynamics in real space and those in landscape coordinates, and show that care must be taken to appropriately combine dynamics with barrier properties to understand the transition rates. This simple model provides an intuitive way to understand free energy landscapes, and illustrates the benefits free energy landscapes can have over potential energy landscapes., Comment: Followup to arXiv:1112.5128 with 33% more disks. V2 adds ballistic dynamics

We study the Brownian motion of ellipsoidal particles lying on an agitated granular bath composed of magnetic particles. We quantify the mobility of different floating ellipsoidal particles using the mean square displacement and the mean square angular displacement, and relate the diffusion coefficients to the bath particle motion. In terms of the particle major radius $R$, we find the translational diffusion coefficient scales roughly as $1/R^2$ and the rotational diffusion coefficient scales as roughly $1/R^4$; this is consistent with the assumption that diffusion arises from random kicks of the bath particles underneath the floating particle. By varying the magnetic forcing, the bath particles diffusivity changes by a factor of ten; over this range, the translational and rotational diffusion of the floating particles change by a factor of fifty. However, the ratio of the two diffusion constants for the floating particles is forcing-independent. Unusual aspects of the floating particle motion include non-Gaussian statistics for their displacements., Comment: Moctezuma and Donado's studies of quasi-2D excited granular media, crossed with Weeks's interest in rotational and translational diffusion

We study hidden scale invariance in the glassy phase of the Kob-Andersen binary Lennard-Jones system. After cooling below the glass transition, we generate a so-called isomorph from the fluctuations of potential energy and virial in the NVT ensemble -- a set of density, temperature pairs for which structure and dynamics are identical when expressed in appropriate reduced units. To access dynamical features we shear the system using the SLLOD algorithm coupled with Lees-Edwards boundary conditions, and study the statistics of stress fluctuations and the particle displacements transverse to the shearing direction. We find good collapse of the statistical data showing that isomorph theory works well in this regime. The analysis of stress fluctuations, in particular the distribution of stress changes over a given strain interval, allows us to identify a clear signature of avalanche behavior in the form of an exponential tail on the negative side. This feature is also isomorph invariant. The implications of isomorphs for theories of plasticity are discussed briefly., Comment: Submitted to Phys. Rev. E

We study signatures of the energy landscape's evolution through the crystal-to-glass transition by compressing 2D finite aggregates of oil droplets. Droplets of two distinct sizes are used to compose small aggregates in an aqueous environment. Aggregates range from perfectly ordered monodisperse single crystals to disordered bidisperse glasses. The aggregates are compressed between two parallel boundaries, with one acting as a force sensor. The compression force provides a signature of the aggregate composition and gives insight into the energy landscape. In particular, crystals dissipate all the stored energy through single catastrophic fracture events whereas the glassy aggregates break step-by-step. Remarkably, the yielding properties of the 2D aggregates are strongly impacted by even a small amount of disorder., Comment: 13 pages, 9 figures

We perform computational studies of jammed particle packings in two dimensions undergoing isotropic compression using the well-characterized soft particle (SP) model and the deformable particle (DP) model that we developed for compressed bubbles and emulsions. In the SP model, circular particles are allowed to overlap, generating purely repulsive forces. In the DP model, particles minimize their perimeter, while deforming at fixed area to avoid overlap during compression. We directly compare the structural and mechanical properties of jammed particle packings generated using the SP and DP models as a function of the true packing fraction $\rho$, instead of the reduced number density $\phi$. We show that near jamming onset the excess contact number $\Delta z=z-z_J$ and shear modulus ${\cal G}$ scale as $\Delta \rho^{0.5}$ in the large system limit for both the SP and DP models, where $\Delta \rho = \rho-\rho_J$ and $z_J \approx 4$ and $\rho_J \approx 0.842$ are the values at jamming onset. $\Delta z$ and ${\cal G}$ for the SP and DP models begin to differ for $\rho \gtrsim 0.88$. In this regime, $\Delta z \sim {\cal G}$ can be described by a sum of two power-laws in $\Delta \rho$, i.e. $\Delta z \sim {\cal G} \sim C_0\Delta \rho^{0.5} +C_1\Delta \rho^{1.0}$ to lowest order. We show that the ratio $C_1/C_0$ is much larger for the DP model compared to to that for the SP model. We also characterize the void space in jammed packings as a function of $\rho$. We find that, unlike the SP model, the DP model is able to describe the formation of Plateau borders as the system approaches $\rho = 1$. We further show that the results for $z$ and the shape factor ${\cal A}$ versus $\rho$ for the DP model agree with recent experimental studies of compressed foams and emulsions., Comment: 12 pages, 11 figures, 9 main figures, and 2 supplemental figures

We conduct experiments and simulations to study the disordered packing of rods in small containers. Experiments study cylindrical rods with aspect ratio ranging from 4 to 32; simulations use of spherocylinders with similar aspect ratios. In all cases, rods pack randomly in cylindrical containers whose smallest dimension is larger than the rod length. Packings in smaller containers have lower volume fractions than those in larger containers, demonstrating the influence of the boundaries. The volume fraction extrapolated to infinite container size decreases with increasing aspect ratio, in agreement with previous work. X-ray tomography experiments show that the boundary effects depend on the orientation of the boundary, indicating a strong influence of gravity, whereas the simulation finds boundary effects that are purely geometric. In all cases, the boundary influence extends approximately half a particle length into the interior of the container., Comment: Bulk experiments, x-ray tomography, simulations

We introduce a new and robust approach for characterizing spatially and temporally heterogeneous behavior within a system based on the evolution of dynamic fuctuations once averaged over different space lengths and time scales. We apply it to investigate the dynamics in two canonical systems as the glass transition is approached: simulated Lennard-Jones liquids and experimental dense colloidal suspensions. We find that in both cases the onset of glassines is marked by spatially localized dynamic fluctuations originating in regions of correlated mobile particles. By removing the trivial system size dependence of the fluctuations we show that such regions contain tens to hundreds of particles for time scales corresponding to maximally non-Gaussian dynamics., Comment: New analysis technique introduced, and applied to previously published data

We use confocal microscopy to study the aging of a bidisperse colloidal glass near rough and smooth boundaries. Near smooth boundaries, the particles form layers, and particle motion is dramatically slower near the boundary as compared to the bulk. Near rough boundaries, the layers nearly vanish, and particle motion is nearly identical to that of the bulk. The gradient in dynamics near the boundaries is demonstrated to be a function of the gradient in structure for both types of boundaries.Our observations show that wall-induced layer structures strongly influence aging., Comment: 8 pages, 7 figures

We observe the translational and rotational diffusion of dimer tracer particles in quasi-2D colloidal samples. The dimers are in dense samples of two different sizes of spherical colloidal particles, with the area fraction ${\phi}$ of the particles varying from dilute to nearly glassy. At low ${\phi}$ rotational and translational diffusion have a ratio set by the dimer size, as expected. At higher ${\phi}$ dimers become caged by their neighboring particles, and both rotational and translational diffusion slow. For short dimers we observe rapid reorientations so that the rotational diffusion is faster than translational diffusion: the two modes of diffusion are decoupled and have different ${\phi}$ dependence. Longer dimers do not exhibit fast rotations, and we find translational and rotational diffusion stay coupled for all ${\phi}$. Our results bridge prior results that used spheres (very fast rotation) and long ellipsoids (very slow rotation).

Colloids are suspensions of small solid particles in a liquid, and exhibit glassy behavior when the particle concentration is high. In these samples, the particles are roughly analogous to individual molecules in a traditional glass. This model system has been used to study the glass transition since the 1980's. In this Viewpoint we summarize some of the intriguing behaviors of the glass transition in colloids, and discuss open questions., Comment: Review article which is more concise than Hunter & Weeks, Rep. Prog. Phys. 2012

Phase transitions significantly differ between two-dimensional and three-dimensional systems, but the influence of dimensionality on the glass transition is unresolved. We use microscopy to study colloidal systems as they approach their glass transitions at high concentrations, and find differences between 2D and 3D. We find that in 2D particles can undergo large displacements without changing their position relative to their neighbors, in contrast with 3D. This is related to Mermin-Wagner long-wavelength fluctuations that influence phase transitions in 2D. However, when measuring particle motion only relative to their neighbors, 2D and 3D have similar behavior as the glass transition is approached, showing that the long wavelength fluctuations do not cause a fundamental distinction between 2D and 3D glass transitions.

Laplace pressure driven snap-off is a technique that is used to produce droplets for emulsions and microfluidics purposes. Previous predictions of droplet size have assumed a quasi-equilibrium low flow limit. We present a simple model to predict droplet sizes over a wide range of flow rates, demonstrating a rich landscape of droplet stability depending on droplet size and growth rate. The model accounts for the easily adjusted experimental parameters of geometry, interfacial tension, and the viscosities of both phases.

Using experiments and simulations, we study the flow of soft particles through quasi-two-dimensional hoppers. The first experiment uses oil-in-water emulsion droplets in a thin sample chamber. Due to surfactants coating the droplets, they easily slide past each other, approximating soft frictionless disks. For these droplets, clogging at the hopper exit requires a narrow hopper opening only slightly larger than the droplet diameter. The second experiments use soft hydrogel particles in a thin sample chamber, where we vary gravity by changing the tilt angle of the chamber. For reduced gravity, clogging becomes easier, and can occur for larger hopper openings. Our simulations mimic the emulsion experiments and demonstrate that softness is a key factor controlling clogging: with stiffer particles or a weaker gravitational force, clogging is easier. The fractional amount a single particle is deformed under its own weight is a useful parameter measuring particle softness. Data from the simulation and hydrogel experiments collapse when compared using this parameter. Our results suggest that prior studies using hard particles were in a limit where the role of softness is negligible which causes clogging to occur with significantly larger openings., Comment: v03 has new experiments using hydrogel particles, along with new coauthors who did those experiments

We study glassy dynamics using a simulation of three soft Brownian particles confined to a two-dimensional circular region. If the circular region is large, the disks freely rearrange, but rearrangements are rarer for smaller system sizes. We directly measure a one-dimensional free energy landscape characterizing the dynamics. This landscape has two local minima corresponding to the two distinct disk configurations, separated by a free energy barrier which governs the rearrangement rate. We study several different interaction potentials and demonstrate that the free energy barrier is composed of a potential energy barrier and an entropic barrier. The heights of both of these barriers depend on temperature and system size, demonstrating how non-Arrhenius behavior can arise close to the glass transition., Comment: 9 pages, 10 figures

We introduce a novel technique to produce monodisperse droplets through the snap-off mechanism. The methodology is simple, versatile, and requires no specialized or expensive components. The droplets produced have polydispersity <1% and can be as small as 2.5 $\mu$m radius. A convenient feature is that the droplet size is constant over a 100-fold change in flow rate, while at higher flows the droplet size can be continuously adjusted., Comment: to be published in Eur. Phys. J. E as a "Tips and Tricks" article

We experimentally and computationally study the flow of a quasi-two-dimensional emulsion through a constricting hopper shape. Our area fractions are above jamming such that the droplets are always in contact with one another and are in many cases highly deformed. At the lowest flow rates, the droplets exit the hopper via intermittent avalanches. At the highest flow rates, the droplets exit continuously. The transition between these two types of behaviors is a fairly smooth function of the mean strain rate. The avalanches are characterized by a power law distribution of the time interval between droplets exiting the hopper, with long intervals between the avalanches. Our computational studies reproduce the experimental observations by adding a flexible compliance to the system. The compliance results in continuous flow at high flow rates, and allows the system to clog at low flow rates leading to avalanches. The computational results suggest that the interplay of the flow rate and compliance controls the presence or absence of the avalanches., Comment: Final pre-publication version

We observe tracer particles diffusing in soap films to measure the two-dimensional (2D) viscous properties of the films. We make soap films with a variety of water-glycerol mixtures and of differing thicknesses. The single-particle diffusivity relates closely to parameters of the film (such as thickness $h$) for thin films, but the relation breaks down for thicker films. Notably, the diffusivity is faster than expected for thicker films, with the transition at $h/d = 5.2 \pm 0.9$ using the tracer particle diameter $d$. This indicates a transition from purely 2D diffusion to diffusion that is more three-dimensional. Additionally, we measure larger length scale flow fields from correlated particle motions and find good agreement with what is expected from theory of 2D fluids for all our films, thin and thick. We measure the effective 2D viscosity of a soap film using single-particle diffusivity measurements in thin films, and using the two-particle correlation measurements in all films.

We present experimental confirmation of dynamic facilitation in monodisperse and bidisperse colloidal suspensions near the glass transition volume fraction. Correlations in particle dynamics are seen to exist not only in space (clusters and strings) but also as bubbles in space-time. Quantitatively, highly mobile particles are more likely (than immobile particles) to have nearest neighbors that were highly mobile in immediately preceding times. The interpretation is that a particle's mobility enables or facilitates the subsequent motion of its neighbors. Facilitation is most pronounced at the relaxation time that corresponds with cage-breaking, when dynamic heterogeneity is also maximized., Comment: 4 pages, 4 figures

We use colloidal suspensions encapsulated in emulsion droplets to model confined glass-forming liquids with tunable boundary mobility. We show dynamics in these idealized systems are governed by physical interactions with the boundary. Gradients in dynamics are present for more mobile boundaries, whereas for less mobile boundaries gradients are almost entirely suppressed. Motions in a system are not isotropic, but have a strong directional dependence with respect to the boundary. These findings bring into question the ability of conventional quantities to adequately describe confined glasses., Comment: 8 pages, 7 figures. Submitted to Phys. Rev. Lett

We study how local rearrangements alter droplet stresses within flowing dense quasi-two-dimensional emulsions at area fractions $\phi \geq 0.88$. Using microscopy, we measure droplet positions while simultaneously using their deformed shape to measure droplet stresses. We find that rearrangements alter nearby stresses in a quadrupolar pattern: stresses on neighboring droplets tend to either decrease or increase depending on location. The stress redistribution is more anisotropic with increasing $\phi$. The spatial character of the stress redistribution influences where subsequent rearrangements occur. Our results provide direct quantitative support for rheological theories of dense amorphous materials that connect local rearrangements to changes in nearby stress., Comment: 5 pages, 5 figures

The densest amorphous packing of rigid particles is known as random close packing. It has long been appreciated that higher densities are achieved by using collections of particles with a variety of sizes. The variety of sizes is often quantified by the polydispersity of the particle size distribution: the standard deviation of the radius divided by the mean radius. Several prior studies quantified the increase of the packing density as a function of polydispersity. Of course, a particle size distribution is also characterized by its skewness, kurtosis, and higher moments, but the influence of these parameters has not been carefully quantified before. In this work, we numerically generate many packings with different particle radii distributions, varying polydispersity and skewness independently of one another. We find two significant results. First, the skewness can have a significant effect on the packing density and in some cases can have a larger effect than polydispersity. Second, the packing fraction is relatively insensitive to the value of the kurtosis. We present a simple empirical formula for the value of the random close packing density as a function of polydispersity and skewness., Comment: 5 Pages, 3 Figures

We examine the response of a dense colloidal suspension to a local force applied by a small magnetic bead. For small forces, we find a linear relationship between the force and the displacement, suggesting the medium is elastic, even though our colloidal samples macroscopically behave as fluids. We interpret this as a measure of the strength of colloidal caging, reflecting the proximity of the samples' volume fractions to the colloidal glass transition. The strain field of the colloidal particles surrounding the magnetic probe appears similar to that of an isotropic homogeneous elastic medium. When the applied force is removed, the strain relaxes as a stretched exponential in time. We introduce a model that suggests this behavior is due to the diffusive relaxation of strain in the colloidal sample., Comment: long-delayed followup paper to Habdas et al., EPL 67, 477-483 (2004)

We study bidisperse colloidal suspensions confined within glass microcapillary tubes to model the glass transition in confined cylindrical geometries. We use high speed three-dimensional confocal microscopy to observe particle motions for a wide range of volume fractions and tube radii. Holding volume fraction constant, we find that particles move slower in thinner tubes. The tube walls induce a gradient in particle mobility: particles move substantially slower near the walls. This suggests that the confinement-induced glassiness may be due to an interfacial effect., Comment: Submitted to AIP conference proceedings for "Slow Dynamics in Complex Systems" (Sendai, Japan, Dec. 2012)

We experimentally study the jamming of quasi-two-dimensional emulsions. Our experiments consist of oil-in-water emulsion droplets confined between two parallel plates. From the droplet outlines, we can determine the forces between every droplet pair to within 8% over a wide range of area fractions $\phi$. We study three bidisperse samples that jam at area fractions $\phi_c \approx 0.86$. Our data show that for $\phi > \phi_c$, the contact numbers and pressure have power-law dependence on $\phi-\phi_c$ in agreement with the critical scaling found in numerical simulations. Furthermore, we see a link between the interparticle force law and the exponent for the pressure scaling, supporting prior computational observations. We also observe linear-like force chains (chains of large inter-droplet forces) that extend over 10 particle lengths, and examine the origin of their linearity. We find that the relative orientation of large force segments are random and that the tendency for force chains to be linear is not due to correlations in the direction of neighboring large forces, but instead occurs because the directions are biased towards being linear to balance the forces on each droplet., Comment: 14 pages, 13 figures, 4 tables

We recently reviewed the experimental determination of the volume fraction, $\phi$, of hard-sphere colloids, and concluded that the absolute value of $\phi$ was unlikely to be known to better than $\pm 3$-6%. Here, in a second part to that review, we survey effects due to softness in the interparticle potential, which necessitates the use of an {\em effective} volume fraction. We review current experimental systems, and conclude that the one that most closely approximates hard spheres remains polymethylmethacrylate spheres sterically stabilised by polyhydroxystearic acid `hairs'. For these particles their effective hard sphere diameter is around 1-10% larger than the core diameter, depending on the particle size. We argue that for larger colloids suitable for confocal microscopy, the effect of electrostatic charge cannot be neglected, so that mapping to hard spheres must be treated with caution., Comment: 11 pages

We study dense and highly polydisperse emulsions at droplet volume fractions \phi >= 0.65. We apply oscillatory shear and observe droplet motion using confocal microscopy. The presence of droplets with sizes several times the mean size dramatically changes the motion of smaller droplets. Both affine and nonaffine droplet motions are observed, with the more nonaffine motion exhibited by the smaller droplets which are pushed around by the larger droplets. Droplet motions are correlated over length scales from one to four times the mean droplet diameter, with larger length scales corresponding to higher strain amplitudes (up to strains of about 6%)., Comment: 6 pages

We investigate cage breaking in dense hard disk systems using a model of three Brownian disks confined within a circular corral. This system has a six-dimensional configuration space, but can be equivalently thought to explore a symmetric one-dimensional free energy landscape containing two energy minima separated by an energy barrier. The exact free energy landscape can be calculated as a function of system size. Results of simulations show the average time between cage breaking events follows an Arrhenius scaling when the energy barrier is large. We also discuss some of the consequences of using a one-dimensional representation to understand dynamics in a multi-dimensional space, such as diffusion acquiring spatial dependence and discontinuities in spatial derivatives of free energy., Comment: 11 pages, 15 figures

Often experimentalists study colloidal suspensions that are nominally monodisperse. In reality these samples have a polydispersity of 4-10%. At the level of an individual particle, the consequences of this polydispersity are unknown as it is difficult to measure an individual particle size from microscopy. We propose a general method to estimate individual particle radii within a moderately concentrated colloidal suspension observed with confocal microscopy. We confirm the validity of our method by numerical simulations of four major systems: random close packing, colloidal gels, nominally monodisperse dense samples, and nominally binary dense samples. We then apply our method to experimental data, and demonstrate the utility of this method with results from four case studies. In the first, we demonstrate that we can recover the full particle size distribution {\it in situ}. In the second, we show that accounting for particle size leads to more accurate structural information in a random close packed sample. In the third, we show that crystal nucleation occurs in locally monodisperse regions. In the fourth, we show that particle mobility in a dense sample is correlated to the local volume fraction., Comment: 7 pages, 5 figures

We describe a novel method of tracking the rotational motion of clusters of colloidal particles. Our method utilizes rigid body transfor- mations to determine the rotations of a cluster and extends conventional proven particle tracking techniques in a simple way, thus facilitating the study of rotational dynamics in systems containing or composed of colloidal clusters. We test our method by measuring dynamical properties of simulated Brownian clusters under conditions relevant to microscopy experiments. We then use the technique to track and describe the motions of a real colloidal cluster imaged with confocal microscopy., Comment: 14 pages, 6 figures. Submitted to Optics Express

We use confocal microscopy to study a random close packed sample of colloidal particles. We introduce an algorithm to estimate the size of each particle. Taking into account their sizes, we compute the compressibility of the sample as a function of wave vector $q$, and find that this compressibility vanishes linearly as $q \rightarrow 0$. The particle sizes must be considered to calculate the compressibility properly. These results also suggest that the experimental packing is hyperuniform., Comment: 4 pages, 4 figures