arXiv subjects

Joshua A. Dijksman

Publications and source records attributed to Joshua A. Dijksman.

At least 19 recordsLinked to original sources

arXiv2026

The Paper-Stack Trampoline: Acoustic Restitution in Layered Media

A ball bouncing on a thicker cushion should bounce less. We show that this is not always so . Measuring the coefficient of restitution $e$ of a steel ball dropped on a stack of $N$ sheets of standard A4 printer paper, we find that $e$ first drops, then rises to a pronounced maximum, and only then decreases towards a minimum. The maximum occurs where the acoustic round trip through the stack matches the contact time of the impact. For a contact set by the stack itself, one recovers the classical bar-impact condition of a stack-to-ball mass ratio of order one. Bar impact, however, predicts only a weak recovery, insensitive to the surrounding gas. For paper stacks, the recovery is large and no maximum is resolved when the stack is evacuated: the air trapped between the sheets, not the paper alone, makes the stack a trampoline.

cond-mat.soft↗

arXiv2026

The motion of intruders through soft solids

We present the results of an experimental investigation into the motion of large buoyant rigid spheres rising through highly concentrated collections of hydrated hydrogel particles. The concentration of the soft highly poro-elastic particles is such that the mechanical properties of the material are that of a soft solid which can also flow very slowly. Despite the established time-dependent, non-Newtonian character of hydrogel packings, we findthat when the upper surface of the material is free, an immersed buoyant sphere travels through the material at a constant speed. Qualitatively distinct behavior is found when a rigid lid is placed on the surface of the material. In these cases, sublinear time-dependent motion of the sphere is found. The effects of the motion are generally observed to be highly localised around the sphere in all cases. However, when the translational speed of the sphere is constant, it is accompanied by significant flow at the surface of the sample whereas surface movement is suppressed when a lid is present. When the stress exerted on the material is changed by varying the mass of the sphere, its terminal velocity is found to depend exponentially on buoyancy. We use these observations to support a hypothesis which links the exponential stress dependence of the drag coefficient induced by the material to the effects of the boundary conditions on the kinematics of the intruder.

cond-mat.soft↗

arXiv2026

Proximity to Jamming Governs Acoustic Attenuation in Damped Packings

We use particle-based numerical simulations to address a longstanding question regarding the origins of the linear frequency dependence of attenuation in fluid-saturated granular media. We study both the acoustic modes and wave propagation in damped, disordered particle packings. We calculate the damped vibrational modes of packings as a function of frequency, pressure, and grain-contact dissipation. The spatial structure and dissipation of these modes show a clear transition at a pressure-dependent critical frequency from viscous-like continuum behavior to more localized, scattering modes. We also measure how wavespeed and spatial attenuation rate depend on these same parameters. At the same critical frequency, wave propagation also shifts from coherent motion, where attenuation scales quadratically with frequency and linearly with contact damping, to much more incoherent particle-scale motion, where attenuation scales linearly with frequency and sublinearly with contact damping. All of these features, including the transition frequency, are consistent with a large collection of experimental data, which has not been explained by any framework based on grain-scale physics. We refer to this approach as ``Jammed-Network Scattering'' (JNS), and propose it as a grain-scale framework for understanding the acoustics of fluid-saturated granular media.

cond-mat.soft↗

arXiv2026

Pressure sensitivity in non-local flow behaviour of dense hydrogel particle suspensions

Slowly sheared particulate media like sand and suspensions flow heterogeneously as they yield via shear bands, in which most strain accumulates. Understanding shear band localization from microscopics is still a major challenge. One class of so-called non-local theories identified that the width of the shearing zone should depend on the stress field, in particular through the local distance to the yield point of the material. We explicitly test this stress sensitivity picture by using a uniquely stress-tunable suspension while probing its flow behavior in a classic geometry in which shear bands are known to scale nontrivially with local stress: the Split-Bottom Shear Cell. The stress-tunable suspension is composed of mildly polydisperse soft, slippery hydrogel spheres submersed in water. We measure their flow profiles and rheology while controlling the confinement stress via both hydrostatic effects and compression. Unique for these soft particles is that we can probe flow fields under confining normal stresses that reach about 1\% of their elastic modulus. We determine the average angular velocity profiles in the quasi-static flow regime using Magnetic Resonance Imaging based particle image velocimetry and discrete element method simulations. We explicitly match a pressure-sensitive non-local granular fluidity (NGF) model to observed flow behavior. We find that shear bands for this type of suspension become extremely broad under the low confining stresses from the almost density-matched fluid particle mixture, while collapsing to a narrow shear zone under finite, externally imposed compression levels. The DEM and NGF results match the observations qualitatively, confirming the conjectured pressure sensitivity for suspensions and its role in the NGF model. Our results indicate that pressure sensitivity should be part of non-local flow rules to describe slow flows of granular media.

cond-mat.soft↗

arXiv2025

Paste extrusion generates a surface lubrication layer

Dense particle-fluid mixtures, or \emph{pastes}, are encountered in the production of various materials, including animal feed, human food, pharmaceuticals, and biomass for bioenergy. The flow behavior of such dense deformable particulate media is poorly understood, as the interplay between applied stresses, particle deformability and interstitial fluids can be very complex. One challenging context is high pressure pipe flow, encountered in extrusion. Despite its widespread use, many questions remain about how during high pressure flow of the paste, the particle-fluid mixture behaves and interacts with boundaries. We show how high pressure paste extrusion induces the formation of a fluid boundary thinner than the particle size. The induced fluid layer emerges from a pressure-induced phase segregation process. The fluid layer is sufficiently thin to affect particle-wall contacts, making the paste friction coefficient tunable. Our results so offer potential pathways for reducing energy consumption and even extrusion product composition and failure.

cond-mat.soft↗

arXiv2025

A temperature dependent framework to predict and control physical pellet quality in biomass extrusion

Pellet manufacturing of biomass (food, feed, bioenergy) presses powders or particles into dense pellets with improved nutritional, calorific, and handling properties. This process upgrades industrial co-products from agriculture, forestry, and bioenergy into higher-value products. However, processing particulate streams raises the scientific question: Under which conditions do loose particles bind to form rigid, durable pellets? This work answers this question for biomass extrusion. Systematic experiments reveal how steam conditioning temperature, production rate, and die geometry interact to determine pellet quality. We propose an overarching framework introducing the stickiness temperature ($T^*$), marking the onset of enthalpic reactions required for particle agglomeration. $T^*$ serves as the boundary for inter-particle bond formation and is reached through a combination of steam conditioning and friction, both controllable via process parameters. Results highlight the combined role of pellet temperature and die residence time in optimizing pellet durability while lowering specific energy use (J/kg). Validation with experiments and literature confirms that this framework offers practical guidance to enhance efficiency and sustainability of pelleting. By providing operational parameters to control bonding and energy input, this work supports a more circular economy through efficient conversion of diverse biomass streams into valuable products while reducing energy consumption and greenhouse gas emissions.

cond-mat.soft↗

arXiv2025

Strain dependent viscous response describes the mechanics of cohesionless soft granular materials

Granular materials are ubiquitous in nature and are used extensively in daily life and in industry. The modeling of these materials remains challenging; therefore, finding models with acceptable predictive accuracy that at the same time also reflect the complexity of the granular dynamics is a central research theme in the field. Soft particle packings present additional modeling challenges, as it has become clear that soft particles also have particle-level relaxation timescales that affect the packing behavior. We construct a simple one-dimensional, one-timescale model that replicates much of the essence of compressed hydrogel packing mechanics. We verify the model performance against both 3D and 2D packings of hydrogel particles, under both controlled strain and stress deformation conditions. We find that the modification of a Standard Linear Solid model with a strain dependent prefactor for the relaxation captures the time-history and rate dependence, as well as the necessary absence of cohesion effectively. We also indicate some directions of future improvement of the modeling.

cond-mat.soft↗

arXiv2024

Confinement controls the creep rate in soft granular packings

Flow in soft materials encompasses a wide range of viscous, plastic and elastic phenomena which provide challenges to modelling at the microscopic level. To create a controlled flow, we perform falling ball viscometry tests on packings of soft, frictionless hydrogel spheres. Systematic creep flow is found when a controlled driving stress is applied to a sinking sphere embedded in a packing. Here, we take the novel approach of applying an additional global confinement stress to the packing using an external load. This has enabled us to identify two distinct creep regimes. When confinement stress is small, the creep rate is independent of the load imposed. For larger confinement stresses, we find that the creep rate is set by the mechanical load acting on the packing. In the latter regime, the creep rate depends exponentially on the imposed stress. We can combine the two regimes via a rescaling onto a master curve, capturing the creep rate over five orders of magnitude. Our results indicate that bulk creep phenomena in these soft materials can be subtly controlled using an external mechanical force.

cond-mat.soft↗

arXiv2024

Viscoelastic material properties determine contact mechanics of hydrogel spheres

Granular materials are ubiquitous in nature and industry; their mechanical behavior has been of academic and engineering interest for centuries. One of the reasons for their rather complex mechanical behavior is that stresses exerted on a granular material propagate only through contacts between the grains. These contacts can change as the packing evolves. This makes any deformation and mechanical response from a granular packing a function of the nature of contacts between the grains and the material response of the material the grains are made of. We present a study in which we isolate the role of the grain material in the contact forces acting between two particles sliding past each other. We use hydrogel particles and find that a viscoelastic material model, in which the shear modulus decays with time, coupled with a simple Coulomb friction model captures the experimental results. The results suggest that the particle material evolution itself may play a role in the collective behavior of granular materials.

cond-mat.soft↗

arXiv2023

Elongated particles discharged with a conveyor belt in a two-dimensional silo

The flow of elliptical particles out of a 2-dimensional silo when extracted with a conveyor belt is analyzed experimentally. The conveyor belt - placed directly below the silo outlet - reduces the flow rate, increases the size of the stagnant zone, and it has a very strong influence on the relative velocity fluctuations as they strongly increase everywhere in the silo with decreasing belt speed. In other words, instead of slower but smooth flow, flow reduction by belt leads to intermittent flow. Interestingly, we show that this intermittency correlates with a strong reduction of the orientational order of the particles at the orifice region. Moreover, we observe that the average orientation of the grains passing through the outlet is modified when they are extracted with the belt, a feature that becomes more evident for large orifices.

cond-mat.soft↗

arXiv2022

Speciation in a MacArthur model predicts growth, stability and adaptation in ecosystems dynamics

Ecosystems dynamics is often considered as driven by a coupling of species' resource consumption and its population size dynamics. Such resource-population dynamics is captured by MacArthur-type models. One biologically relevant feature that would also need to be captured by such models is the introduction of new and different species. Speciation introduces a stochastic component in the otherwise deterministic MacArthur theory. We describe here how speciation can be implemented to yield a model that is consistent with current theory on equilibrium resource-consumer models, but also displays readily observable rank diversity metric changes. The model also reproduces a priority effect. Adding speciation to a MacArthur-style model so provides an attractively simple extension to explore the rich dynamics in evolving ecosystems.

q-bio.PE↗

arXiv2022

Quantitative coarse graining of laminar fluid flow penetration in rough boundaries

The interaction between a fluid and a wall is described with a certain boundary condition for the fluid velocity at the wall. To understand how fluids behave near a rough wall, the fluid velocity at every point of the rough surface may be provided. This approach requires detailed knowledge of, and likely depends strongly on the roughness. Another approach of modeling the boundary conditions of a rough wall is to coarse grain and extract a penetration depth over which on average the fluid penetrates into the roughness. In this work we show that for a broad range of periodic roughness patterns and relative flow velocities, a universal penetration depth function can be obtained. We obtain these results with experiments and complementary numerical simulations. Our results show that wall roughness boundary conditions can be captured with an average ``slip length'' and so indicate that surface patterning yields extensive control over wall slip.

physics.flu-dyn↗

arXiv2021

Darcy-Reynolds forces during intrusion into granular-fluid beds

We experimentally study intrusion into fluid-saturated granular beds by a free-falling sphere, varying particle size and fluid viscosity. We test our results against Darcy-Reynolds theory, where the deceleration of the sphere is controlled by Reynolds dilatancy and the Darcy flow resistance. We find the observed intruder dynamics are consistent with Darcy-Reynolds theory for varied particle size. We also find that our experimental results for varied viscosity are consistent with Darcy-Reynolds theory, but only for a limited range of the viscosity. For large viscosities, observed forces begin to decrease with increasing viscosity, in contrast with the theoretical prediction.

cond-mat.soft↗

arXiv2021

Viscous-like forces control the impact response of shear-thickening dense suspensions

We experimentally and theoretically study impacts into dense cornstarch and water suspensions. We vary impact speed as well as intruder size, shape, and mass, and we characterize the resulting dynamics using high-speed video and an onboard accelerometer. We numerically solve previously proposed models, most notably the added-mass model as well as a class of {viscous-like} models. In the {viscous-like models}, the intruder dynamics are dominated by {large, viscous-like forces} at the boundary of the jammed front {where large shear rates and accompanying large viscosities are present.} We find that our experimental data are consistent with this class of models and inconsistent with the added mass model. Our results strongly suggest that the added-mass model, which is the dominant model for understanding the dynamics of impact into shear-thickening dense suspensions, should be updated to include these viscous-like forces.

cond-mat.soft↗

arXiv2021

Non-local effects in the shear banding of a thixotropic yield stress fluid

We observe a novel type of shear banding in the rheology of thixotropic yield-stress fluids that is due to the coupling of both non-locality and thixotropy. The latter is known to lead to shear banding even in homogeneous stress fields, but the bands observed in the presence of non-local effects appear different as the shear rate varies continuously over the shear band. Here, we introduce a simple non-local model for the shear banding (NL-SB), and we implement it for the analysis of micron-scale rheo-MRI velocimetry measurements of a milk microgel suspension in a cone-and-plate geometry. The proposed NL-SB model accurately quantifies the cooperativity length and yields values in the order of the aggregate size in the microgel.

cond-mat.soft↗

arXiv2020

Sheared Amorphous Packings Display Two Separate Particle Transport Mechanisms

Shearing granular materials induces non-affine displacements. Such non-affine displacements have been studied extensively, and are known to correlate with plasticity and other mechanical features of amorphous packings. A well known example is shear transformation zones as captured by the local deviation from affine deformation, $D^2_{min}$, and their relevance to failure and stress fluctuations. We analyze sheared frictional athermal disk packings and show that there exists at least one additional mesoscopic transport mechanism that superimposes itself on top of local diffusive motion. We evidence this second transport mechanism in a homogeneous system via a diffusion tensor analysis and show that the trace of the diffusion tensor equals the classic $D^2_{min}$ when this second mesoscopic transport is corrected for. The new transport mechanism is consistently observed over a wide range of volume fractions and even for particles with different friction coefficients and is consistently observed also upon shear reversal, hinting at its relevance for memory effects.

cond-mat.soft↗

arXiv2020

Spatially correlated rotational dynamics reveals strain dependence in amorphous particle packings

Microstructural dynamics in amorphous particle packings is commonly probed by quantifying particle displacements. While rigidity in particle packings emerges when displacement of particles are hindered, it is not obvious how the typically disordered displacement metrics connect to mechanical response. Particle rotations, in contrast, are much less sensitive to confinement effects, while still sensitive to the mechanics of the packing. So far, little attention has been paid to connect microscopic rotational motion to mechanics of athermal amorphous packings. We demonstrate through experimental measurements that particle packing mechanics can be directly linked to the rotational motion of even round particles in a sheared packing. Our results show that the diffusive nature of rotational dynamics is highly strain sensitive. Additionally, there is substantial spatial correlation in rotation dynamics that is a function of the particle friction and packing density. Analysis of our measurements reveals that particle rotation dynamics plays an essential role in amorphous material mechanics.

cond-mat.soft↗

arXiv2020

Direct evidence of stress-induced chain proximity in a macromolecular complex

The mechanical properties of many supramolecular materials are often determined by non-covalent interactions that arise from an interplay between chemical composition and molecular microstructural organization. The reversible nature of non-covalent interactions gives supramolecular materials responsive properties that are otherwise difficult to obtain, such as becoming rigid as a response to mechanical stress. How exactly non-covalent interactions emerge from microstructure, how they might change in response to applied force or deformation is not understood. Here we combine Nuclear Magnetic Resonance (NMR) and rheology to directly probe the role of chain proximity in polymer complexes. We observe an increase in chain proximity in response to imposed flow, which we hypothesize to originate from enhanced hydrogen bonding. The chain proximity is directly correlated to rod climbing and shear banding. Flow persists only when applied stresses are low, suggesting a stress-induced thickening mechanism. We verify that hydrogen bond disruptors can turn off both the non-trivial flow behavior and the spectroscopic evidence of chain proximity. The combined rheo-NMR approach shows that it is possible to directly observe the molecular origins behind supramolecular mechanics, paving the way for further study into mechano-chemical properties of supramolecular materials.

cond-mat.soft↗