arXiv subjects

Eric R. Weeks

Publications and source records attributed to Eric R. Weeks.

At least 37 records · Page 2Linked to original sources

arXiv2021

Details of soft particle clogging in two-dimensional hoppers

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.

cond-mat.soft↗

arXiv2021

Direct observation of crystal nucleation and growth in a quasi-two-dimensional nonvibrating granular system

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.

cond-mat.soft↗

arXiv2021

Neglecting polydispersity degrades propensity measurements in supercooled liquids

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.

cond-mat.soft↗

arXiv2020

Visualizing Free Energy Landscapes for Four Hard Disks

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.

cond-mat.dis-nn↗

arXiv2020

Brownian motion of ellipsoidal particles on a granular magnetic bath

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.

cond-mat.soft↗

arXiv2019

Isomorph invariance of dynamics of sheared glassy systems

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.

cond-mat.dis-nn↗

arXiv2019

The role of deformability in determining the structural and mechanical properties of bubbles and emulsions

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 $ρ$, instead of the reduced number density $ϕ$. We show that near jamming onset the excess contact number $Δz=z-z_J$ and shear modulus ${\cal G}$ scale as $Δρ^{0.5}$ in the large system limit for both the SP and DP models, where $Δρ= ρ-ρ_J$ and $z_J \approx 4$ and $ρ_J \approx 0.842$ are the values at jamming onset. $Δz$ and ${\cal G}$ for the SP and DP models begin to differ for $ρ\gtrsim 0.88$. In this regime, $Δz \sim {\cal G}$ can be described by a sum of two power-laws in $Δρ$, i.e. $Δz \sim {\cal G} \sim C_0Δρ^{0.5} +C_1Δρ^{1.0}$ 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 $ρ$. We find that, unlike the SP model, the DP model is able to describe the formation of Plateau borders as the system approaches $ρ= 1$. We further show that the results for $z$ and the shape factor ${\cal A}$ versus $ρ$ for the DP model agree with recent experimental studies of compressed foams and emulsions.

physics.app-ph↗

arXiv2019

Rearrangement of 2D aggregates of droplets under compression: signatures of the energy landscape from crystal to glass

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.

cond-mat.soft↗

arXiv2019

Random packing of rods in small containers

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.

cond-mat.soft↗

arXiv2018

Spatiotemporal intermittency and localized dynamic fluctuations upon approaching the glass transition

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.

cond-mat.dis-nn↗

arXiv2017

Clogging of soft particles in 2D hoppers

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.

cond-mat.soft↗

arXiv2017

Aging near rough and smooth boundaries in colloidal glasses

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.

cond-mat.soft↗

arXiv2017

Decoupling of translational and rotational diffusion in quasi-2D colloidal fluids

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 $ϕ$ of the particles varying from dilute to nearly glassy. At low $ϕ$ rotational and translational diffusion have a ratio set by the dimer size, as expected. At higher $ϕ$ 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 $ϕ$ dependence. Longer dimers do not exhibit fast rotations, and we find translational and rotational diffusion stay coupled for all $ϕ$. Our results bridge prior results that used spheres (very fast rotation) and long ellipsoids (very slow rotation).

cond-mat.soft↗

arXiv2016

An introduction to the colloidal glass transition

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.

cond-mat.soft↗

arXiv2016

Long Wavelength Fluctuations and the Glass Transition in 2D and 3D

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.

cond-mat.soft↗

arXiv2016

Energy barriers, entropy barriers, and non-Arrhenius behavior in a minimal glassy model

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.

cond-mat.soft↗

arXiv2016

Predicting the size of droplets produced through Laplace pressure induced snap-off

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.

physics.flu-dyn↗

arXiv2015

Snap-off production of monodisperse droplets

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 $μ$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.

cond-mat.soft↗