Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session F26: Soft Particles IRecordings Available
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Sponsoring Units: DSOFT GSNP GMED Chair: Joshua Dijksman, Wageningen University Room: McCormick Place W-187B |
Tuesday, March 15, 2022 8:00AM - 8:12AM |
F26.00001: Clogging of soft particles in 2D hoppers Ran Tao, Eric R Weeks We experimentally study the flow of soft particles through quasi-two-dimensional hoppers. We examine the clogging probability for particles flowing out as a function of the hopper exit width. We find that particle softness plays a critical role in clogging, using different types of particles with varying softness. Clogging is harder for softer particles. The clogging process is caused by the arch formation at the hopper exit. We investigate how the gravitational force, exit width, and particle softness affect the arch size and the number of particles remaining in the hopper when a clog occurs. The experimental results for soft, low-friction, hydrogel particles agree well with previously published simulation data. However, experiments with harder and/or more frictional particles reveal interesting differences. |
Tuesday, March 15, 2022 8:12AM - 8:24AM Withdrawn |
F26.00002: Simulations of arch formation in hopper flows of soft spherical particles Cameron J Lerch, Yuxuan Cheng, John D Treado, Eric R Weeks, Mark D Shattuck, Corey S O'Hern The flow of soft particles is observed in a variety of systems, from pressure-driven suspension flows in microfluidic channels to gravity-driven flows of granular materials in silos and hoppers. In these systems, flow-induced jamming (or clogging) can occur when collections of particles flow into a confined region and those particles jam before exiting the region. Clogging occurs in particulate materials over a wide range of particle sizes, shapes, and interactions, as well as driving and boundary conditions. During a clogging event, particles collect at the hopper opening, forming an arch and preventing outflow. Understanding the mechanical properties of arches is necessary to develop a predictive model for the flow and clogging behavior of soft particles. We carry out soft-particle simulations of hopper flows for a range of orifice widths and driving forces. To quantify the clogging behavior, we characterize arch formation by measuring the size and shape of the arches, as well as the particle dynamics leading up to a clogging event. |
Tuesday, March 15, 2022 8:24AM - 8:36AM |
F26.00003: Impact of particle friction and stiffness on silo discharge Tivadar Pongó, Viktória Stiga, János Török, Sára Lévay, Ralf Stannarius, Raúl Cruz Hidalgo, Tamás Börzsönyi The silo discharge process was studied experimentally and numerically, varying the particle stiffness and friction. We obtained that lowering the friction coefficient of soft grains (relative deformation in the order of 10%) leads to a gradual change in the discharge curve: the flow rate becomes filling height dependent, decreasing during the discharge process. Hard grains with a maximum relative deformation of 0.05% showed a similar but less pronounced trend. The force acting at the silo bottom revealed a gradual transition from Janssen to hydrostatic-like behavior when decreasing friction. In addition, we computed the macroscopic density, velocity, and stress tensor fields by applying coarse-graining to the DEM simulation data. The analysis of the pressure fields revealed significant differences in the spatial stress transmission, when examining soft or hard particles. We proposed a phenomenological formulation based on the momentum balance that predicts the linear decrease of the flow rate with decreasing filling height for soft low-friction particles. |
Tuesday, March 15, 2022 8:36AM - 8:48AM |
F26.00004: Flow and clogging of explicitly deformable particles Yuxuan Cheng, John D Treado, Cameron J Lerch, Eric R Weeks, Mark D Shattuck, Corey S O'Hern Flow-induced jamming, or clogging, is observed across a wide range of systems, from flows of granular materials in silos to flows of emulsion droplets through microfluidic chambers. While there have been significant advances in the understanding of clogging in granular materials with hard particles, little is known about clogging when the particles can change their shapes in response to applied stress. To understand flow and clogging in systems composed of deformable particles, we carry out discrete element method (DEM) simulations of shape-changing particles flowing through hoppers under the influence of a body force. We compare these results to simulations of soft particles with fixed reference shapes flowing through hoppers. We measure the clogging probability as a function of the orifice opening and particle stiffness, characterize the multi-particle structures that give rise to clogs, and correlate changes in particle shape to particle motion that relieves temporary clogs. In addition, we compare the particle flow rate and shape distribution in the simulations to experimental studies of emulsion droplets flowing through quasi-2D microfluidic channels. |
Tuesday, March 15, 2022 8:48AM - 9:00AM |
F26.00005: Silo flow of soft particles in varying gravity Abigail Tadlock, Kerstin Nordstrom We present results of LAMMPS Molecular Dynamics simulations of 2D gravity-driven flows of ~30,000 soft uniform spheres through a vertical silo. We model the flow while varying both the gravitational field strength of the system (g) and the elastic modulus of the particles (E). We find that the empirical Beverloo equation captures the data adequately for many trials, but fails for extreme combinations of g and E. We additionally present results of velocity profiles, granular temperatures, and local rearrangements in these systems. We introduce a dimensionless combination of g, E, the density (ρ), and radius (r) of the spheres, which collapses these measurements. |
Tuesday, March 15, 2022 9:00AM - 9:12AM |
F26.00006: Stochastic jetting and dripping in confined soft granular flows Michał Bogdan, Andrea Montessori, Adriano Tiribocchi, Fabio Bonaccorso, Marco Lauricella, Sauro Succi, Jan Guzowski We report new dynamical modes in confined soft granular flows, such as stochastic jetting and dripping, with no counterpart in continuum viscous fluids. The new modes emerge from an avalanche-like dynamics of a close-packed monodisperse emulsion entering a narrow orifice. We observe formation of remarkably stable single-file granular jets which occasionally break resulting in non-Gaussian distribution of cluster sizes. We find that the sequences of droplet rearrangements that lead to the formation of such chains resemble unfolding of cancer cell clusters in narrow capillaries, overall demonstrating that the microfluidic emulsion systems could serve to model certain aspects of tissue dynamics. |
Tuesday, March 15, 2022 9:12AM - 9:24AM |
F26.00007: Printing of soft-granular threads and clusters. Jan Guzowski, Jonathan Pullas Navarrete, Ronald Terrazas Mallea We use microfluidics to generate monodisperse emulsion droplets at high volume fraction (>80%) and extrude such droplets one-by-one using either external flow [1] or direct deposition ('printing') at a substrate. Upon extrusion, the continuous phase forms capillary bridges between the droplets which leads to their capillary arrest and stabilizes the emerging droplet patterns. In the printing experiments, depending on the applied types of fluids, the interaction with the substrate (wetting/non-wetting etc.) and on the printing speed, we find various patterns ranging from seperate clusters to granular threads including single-file chains of droplets. We study long-term relaxation of such patterns including coalescence of quasi-2D granular clusters which proceeds via avalanches of rearrangements associated with transitions between different metastable states. Upon fine-tuning of the printing speed we also observe generation of chains which randomly self-fold resulting in the formation of regular yet stochastic super-patterns along the chains. We discuss the perspective of using of such patterns to encode information, e.g., for the purpose of droplet 'labeling', potentially useful in applications such as high-throughput screening [2]. |
Tuesday, March 15, 2022 9:24AM - 9:36AM |
F26.00008: Coiling and buckling Instabilities in moving chains of droplets impacting an interface Carmen L Lee, Kari Dalnoki-Veress We produce a chain of microscopic monodisperse droplets in an aqueous bath, which rises due to the buoyancy of the droplets. There is an attractive interaction between the droplets and if the droplets are produced quickly, such that one droplet is produced and contacts the next, they adhere. Producing many droplets in this fashion allows us to create a chain of sticky droplets. Tuning the rate of droplet production results in coiling and buckling instabilities in the rising chain as it moves through the aqueous bath and eventually hits a static interface. |
Tuesday, March 15, 2022 9:36AM - 9:48AM |
F26.00009: Creeping through a rigidity transition in packings of soft spheres Joshua A Dijksman, Tom Mullin The ability of soft granular packings to have both liquid and solid-like behavior has widespread fundamental and applied significance. We probe this transition in packings of soft spheres with spherical and cylindrical intruder tests. We find power law creep sinking behavior, with diffusive dynamics. The creep amplitude is exponentially dependent on both applied stress and the packing density of spheres. At the highest densities, a transition to constant sinking speeds is observed. Results are independent of intruder shape. Our experiments provide benchmark results for existing frameworks of particle packing mechanics and point towards relevance of particle contact level physics in understanding the role of fluctuations in soft structures. |
Tuesday, March 15, 2022 9:48AM - 10:00AM |
F26.00010: Effects of Polydispersity on the Nonaffine Behaviors of Dense 2D Granular Systems Under Shear Yonglun Jiang, Eric R Weeks We study simulations of highly polydisperse emulsions under shear flow where the largest droplet diameters being as much as ten times the smallest diameters. We quantify non-affine motion in these highly polydisperse samples -- motion where droplets move with displacements distinctly different from the overall imposed shear flow. The largest droplets typically move affinely, as if they are in a simple effective fluid formed by the other droplets. In contrast, the smallest droplets are often forced to move non-affinely by the larger droplets. We quantify how the non-affine motion diminishes as a function of droplet size and distance from the larger particles and how it depends on the position in the system. We further show how the behavior depends on the overall droplet size distribution; in general, the largest droplets are always the troublemakers. Our main conclusion is that highly polydisperse samples behave qualitatively differently than weakly polydisperse samples. |
Tuesday, March 15, 2022 10:00AM - 10:12AM |
F26.00011: Stick-slip dynamics of an intruder pulled through granular matter Lou Kondic, Rituparna Basak, Joshua Socolar, Luis Pugnaloni, Manuel Carlevaro, Miroslav Kramara, Hu Zhang, Ryan H Kozlowski We will discuss computational and experimental results for the setup |
Tuesday, March 15, 2022 10:12AM - 10:24AM |
F26.00012: Unconventional rheological properties in systems of deformable particles Anshuman Pasupalak We demonstrate in this work, the existence of an unconventional rheological and memory property in a Voronoi-based model of deformable particles such as foams, tissues, etc. whose energy depends upon their shape. These systems experience an unconventional "shear-weakening" transition when subjected to large shear strain, which is characterized by a sudden increase in the mechanical energy along with a drastic drop in the shear stress. This transition stems from the emergence of a short-ranged tetratic order in the system. In these shear weakened states, the contact network for the system evolves reversibly when subject to a reversal of applied strain, keeping the memory of its initial state, whereas the microscopic dynamics remain irreversible. |
Tuesday, March 15, 2022 10:24AM - 10:36AM |
F26.00013: Investigation of the Form factor of Faceted Particles using Statistical Simulations Tom Höfken Scattering techniques are a powerful tool to characterize the shape and architecture of colloids and, using neutron scattering with contrast variation, the response of soft colloids in crowded environments. Microgels, deformable polymeric crosslinked networks often used as model system for soft colloids, have been investigated in crowded environment and their scattered intensities fitted with sphere-based models to gain information on their response in size and shape to crowding. For these systems, but also for nanoemulsions, an increase in the fitting parameter describing the size-polydispersity was observed and considered to be due to the deformation of the particles. This study aims to investigate this hypothesis by systematically deforming model particles and computing the corresponding form factors in a computer simulation. The model shape of a faceted sphere is introduced. Our simulations show that the computed averaged form factors of faceted particles can be fitted with a spherical model with in an increased parameter describing the size-polydispersity, which is consistent with the experimental observations. Finally we compare our calculated form factors with experimental data. |
Tuesday, March 15, 2022 10:36AM - 10:48AM Withdrawn |
F26.00014: Crystallization of hollow microgels assisted by regular ones Alexander Petrunin, Andrea Scotti, Walter Richtering Microgels are colloidal polymer networks swollen by a good solvent. Due to their compressibility, they represent an excellent model system to study phase behavior and flow properties of soft spheres. However, hollow microgels with a solvent-filled cavity in the center, unlike regular ones, do not form crystals at any volume fractions even when the polydispersity is below the limit for crystallization of hard spheres (12%). Here, we show that hollow microgels can be successfully co-crystallized with regular ones of the same size. Both the regular and hollow microgel we use are relatively monodisperse and virtually of the same size, as probed using light and small-angle neutron scattering. The phase diagram of the binary mixture is obtained as a function of number of hollow microgels in the suspension. Surprisingly, we found that crystals can still form at fractions of hollow microgels as high as 50%. The evolution of the crystalline structures, obtained using both small-angle X-ray scattering and UV-Vis spectroscopy, suggests that hollow microgels gradually inhibit crystallization. Therefore, they can be considered a new type of "defect" that has no counterpart in the atomic crystals since they suppress crystallization not due to size mismatch but a different internal architecture |
Tuesday, March 15, 2022 10:48AM - 11:00AM |
F26.00015: Determination of the packing fraction of soft colloids using SANS with contrast variation Andrea Scotti The volume occupied by colloids in a suspension – the volume fraction – is the thermodynamic variable that determines the phase behavior of these systems. While for hard incompressible spheres this quantity is well defined, for soft compressible colloids such as microgels – polymeric crosslinked networks swollen in a good solvent – the determination of the real volume occupied by these particles in solution is particularly challenging. This fact depends on two aspects: first the surface and, therefore, the volume of the microgels is hard to define properly given their external fuzziness; second, microgels can osmotically deswell or deform, i.e. change their shape and size depending on the solution concentration. Here, the form factors of few hydrogenated microgels embedded in a matrix of deuterated but otherwise identical microgels are measured using small-angle neutron scattering with contrast variation. From the analysis of the scattering data, the variation of the volume of the microgels as a function of concentration is obtained and used to calculate the real microgel volume fraction in solution. Soft neutral microgels are shown to facet already at low concentrations while in contrast, harder microgels maintain their shape and change their volume. |
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