arXiv subjects

Joel T. Clemmer

Publications and source records attributed to Joel T. Clemmer.

14 recordsLinked to original sources

arXiv2026

Merons Mediate Re-Ordering of Curved Rods Under Shear Flow

Bent-core liquid crystals are a canonical example of a soft matter system whose behavior is controlled by a local preference for order that cannot be universally achieved. This geometric frustration, arising from the rod's curved shape, has been shown to stabilize a variety of equilibrium phases, such as the helically ordered nematic twist-bend phase ($N_{\rm TB}$). Unlike a traditional nematic, the twist-bend state has 1D translational order arising from a periodic rotation of bend orientation along the helical axis. Here, we use molecular dynamics simulations to study the effect of shearing the $N_{\rm TB}$ phase along directions parallel and perpendicular to the helical axis. In the case of shear perpendicular to the helical axis, the nematic twist-bend phase is stable and flows without disordering. Conversely, shear along the helical axis disrupts order and leads to the emergence of fractionally charged Skyrmion defects, i.e. merons. These defects act as topological machines, locally rotating rods into a re-ordered and stable orientation of the $N_{\rm TB}$ phase. These findings reveal a new mechanism to create and control merons and highlight the potential application of bent-core liquid crystals in designing functional material with specific optical and computational properties.

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arXiv2026

Bonded-particle model for magneto-elastic rods

We develop a bonded-particle model for magneto-elastic rods that unifies large deformations, contact, and long-range magnetic interactions within a single discrete-element framework. The rod is discretized into orientable particles connected by co-rotational bonds that capture stretching, shearing, twisting, and bending through a symmetric decomposition of relative displacement and rotation. Magnetic coupling is introduced at the particle level: each particle carries a dipole moment that rotates with it, enabling both external-field actuation and long-range dipole--dipole interactions without modifying the structural formulation. We implement the model in LAMMPS to take advantage of its parallel efficiency, long-range electrostatic solvers, and multiphysics capabilities. We validate the model on three problems spanning writhing instabilities, non-uniform magnetic actuation, and dipole-induced mechanical hysteresis. To demonstrate multiphysics capability, we couple the model with a lattice Boltzmann fluid solver via the immersed boundary method and simulate filaments in oscillatory channel flow and fluid pumping by magnetically actuated cilia arrays. Across all examples, the model shows good agreement with experimental, analytical, and numerical reference results.

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arXiv2026

Shear and crystallization in deformable granular packings: why don't auxetics order?

Shear of three-dimensional, highly compressed granular packings is simulated using a bonded particle approach that explicitly resolves elastic deformation. Varying Poisson's ratio $ν$ produces significant changes in rheology, packing structure, and grain morphology. During flow, conventional systems ($ν> 0$) readily crystallize while auxetics ($ν< 0$) resist ordering. This duality reflects the fact that conventional grains develop polyhedral-like facets but conserve volume while auxetics behave oppositely, demonstrating an unexpected interaction between elasticity, geometry, and crystallization.

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arXiv2026

Reverse segregation and self-organization in inclined chute flows of bidisperse granular mixtures

In the usual segregation scenario for stable inclined chute flows of bidisperse mixtures of fine and coarse spherical particles, coarse particles rise toward the free surface, forming a coarse-rich region atop the flowing pile. Beyond a threshold coarse-to-fine diameter ratio of approximately 4, conversely, the weight of the coarse particles exceeds the segregation driving forces, causing individual coarse particles to sink within the pile and producing a reversed segregation state. However, an understanding of the collective evolution of the pile structure is still lacking when the particle diameter ratio exceeds 4 {\textit{and}} the coarse particle mass fraction is appreciable. To explore this broadly bidisperse limit, we perform discrete element method simulations considering mean particle diameter ratios of up to 8 and coarse particle mass fractions spanning 0.1 to 0.9. The steady-state flow profiles reveal several intriguing behaviors that depend on the diameter ratio and mass fraction. These include a previously identified transition from usual to reverse segregation and a newfound tendency to self-organize into alternating coarse- and fine-rich particle layers stacked along the shear gradient direction, with layer thickness dictated by the coarse particle diameter. A fuller understanding of segregation at this scale could pave the way for enhanced mixing or demixing techniques at the commercial scale.

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arXiv2026

Shape Elasticity in Colloidal Bent-Core Liquid Crystals

Curved particles have been shown to stabilize a range of states with unique order in dense suspensions of colloidal bent core liquid crystals. The shape of the colloidal rods encourages the formation of curved director fields. However, states of constant bend cannot uniformly fill either two or three dimensional Euclidean space and are therefore geometrically frustrated. As a result, curved rods are forced to couple their preference for bend with additional twist and splay deformations, giving rise to twist-bend and splay-bend states of nematic and smectic order. In this article, we study the effect of rod curvature on these diverse states of liquid crystalline order using molecular dynamics simulations of a bonded particle model of curved rods with tunable shape elasticity. Focusing on the case of intermediately curved rods, we find that curved rods go through a sequence of isotropic, nematic twist-bend and smectic splay-bend ordering as the density is increased from the dilute limit, in agreement with previous studies of rigid rods. As the rods become more elastic, the critical concentration separating these phases is shifted to higher density. Lastly, we find that flexibility weakens the first-order phase transition separating the isotropic and nematic twist-bend phases.

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arXiv2024

Onset and impact of plastic deformation in granular compaction

The role of plastic deformation in the high-pressure compaction of granular material is investigated using bonded particle model simulations. Grains are discretized into a set of computational particles connected by pairwise bonds. Bonds are harmonic up to a plastic onset strain $ε_p$ above which they yield, capping out at a maximum force and producing an elastic-perfectly-plastic-like mechanical response in grains. Packings containing over one thousand monodisperse spherical grains are isotropically compacted to different packing fractions to quantify how decreasing $ε_p$ softens the rise in pressure and impacts effective elastic properties of the confined system. By isolating the relative decrease in pressure due to plasticity, we find data can be collapsed across a wide range of values of $ε_p$ suggesting that relatively simple mathematical descriptions may capture plasticity's effect in granular compaction. Lastly, we study the microscopic statistics of local strains in grains and connect their evolution to the observed macroscopic behavior.

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arXiv2023

A soft departure from jamming: the compaction of deformable granular matter under high pressures

The high-pressure compaction of three dimensional granular packings is simulated using a bonded particle model (BPM) to capture linear elastic deformation. In the model, grains are represented by a collection of point particles connected by bonds. A simple multibody interaction is introduced to control Poisson's ratio and the arrangement of particles on the surface of a grain is varied to model both high- and low-frictional grains. At low pressures, the growth in packing fraction and coordination number follow the expected behavior near jamming and exhibit friction dependence. As the pressure increases, deviations from the low-pressure power-law scaling emerge after the packing fraction grows by approximately 0.1 and results from simulations with different friction coefficients converge. These results are compared to predictions from traditional discrete element method simulations which, depending on the definition of packing fraction and coordination number, may only differ by a factor of two. As grains deform under compaction, the average volumetric strain and asphericity, a measure of the change in the shape of grains, are found to grow as power laws and depend heavily on the Poisson's ratio of the constituent solid. Larger Poisson's ratios are associated with less volumetric strain and more asphericity and the apparent power-law exponent of the asphericity may vary. The elastic properties of the packed grains are also calculated as a function of packing fraction. In particular, we find the Poisson's ratio near jamming is 1/2 but decreases to 1/4 before rising again as systems densify.

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arXiv2023

Universal behavior in fragmenting brittle, isotropic solids across material properties

A bonded particle model is used to explore how variations in the material properties of brittle, isotropic solids affect critical behavior in fragmentation. To control material properties, a new model is proposed which includes breakable two- and three-body particle interactions to calibrate elastic moduli and mode I and II fracture toughnesses. In the quasistatic limit, fragmentation leads to a power-law distribution of grain sizes which is truncated at a maximum grain mass that grows as a non-trivial power of system size. In the high-rate limit, truncation occurs at a mass that decreases as a power of increasing rate. A scaling description is used to characterize this behavior by collapsing the mean squared grain mass across rates and system sizes. Consistent scaling persists across all material properties studied although there are differences in the evolution of grain size distributions with strain as the initial number of grains at fracture and their subsequent rate of production depend on Poisson's ratio. This evolving granular structure is found to induce a unique rheology where the ratio of the shear stress to pressure, an internal friction coefficient, decays approximately as the logarithm of increasing strain rate. The stress ratio also decreases at all rates with increasing strain as fragmentation progresses.

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arXiv2022

Large-scale frictionless jamming with power-law particle size distributions

Due to significant computational expense, discrete element method simulations of jammed packings of size-dispersed spheres with size ratios greater than 1:10 have remained elusive, limiting the correspondence between simulations and real-world granular materials with large size dispersity. Invoking a recently developed neighbor binning algorithm, we generate mechanically-stable jammed packings of frictionless spheres with power-law size distributions containing up to nearly four million particles with size ratios up to 1:100. By systematically varying the width and exponent of the underlying power laws, we analyze the role of particle size distributions on the structure of jammed packings. The densest packings are obtained for size distributions that balance the relative abundance of large-large/intermediate and small-small particle contacts. Although the proportion of rattler particles and mean coordination number strongly depend on the size distribution, the mean coordination of non-rattler particles attains the frictionless isostatic value of six in all cases. The size distribution of non-rattler particles that participate in the load-bearing network exhibits no dependence on the width of the total particle size distribution beyond a critical particle size for low-magnitude exponent power laws. This signifies that only particles with sizes greater than the critical particle size contribute to the mechanical stability. However, for high-magnitude exponent power laws, all particle sizes participate in the mechanical stability of the packing.

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arXiv2022

Shear Is Not Always Simple: Rate-Dependent Effects of Flow Type on Granular Rheology

Despite there being an infinite variety of types of flow, most rheological studies focus on a single type such as simple shear. Using discrete element simulations, we explore bulk granular systems in a wide range of flow types at large strains and characterize invariants of the stress tensor for different inertial numbers and interparticle friction coefficients. We identify a strong dependence on the type of flow which grows with increasing inertial number or friction. Standard models of yielding, repurposed to describe the dependence of the stress on flow type in steady-state flow and at finite rates, are compared with data.

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arXiv2021

Jamming of Bidisperse Frictional Spheres

By generalizing a geometric argument for frictionless spheres, a model is proposed for the jamming density $ϕ_J$ of mechanically stable packings of bidisperse, frictional spheres. The monodisperse, $μ_s$-dependent jamming density $ϕ_J^{\mathrm{mono}}(μ_s)$ is the only input required in the model, where $μ_s$ is the coefficient of friction. The predictions of the model are validated by robust estimates of $ϕ_J$ obtained from computer simulations of up to $10^7$ particles for a wide range of $μ_s$, and size ratios up to 40:1. Although $ϕ_J$ varies nonmonotonically with the volume fraction of small spheres $f^s$ for all $μ_s$, its maximum value $ϕ_{J,\mathrm{max}}$ at an optimal $f^{s}_{\mathrm{max}}$ are both $μ_s$-dependent. The optimal $f^{s}_{\mathrm{max}}$ is characterized by a sharp transition in the fraction of small rattler particles.

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arXiv2021

Criticality in sheared, disordered solids. I. Rate effects in stress and diffusion

Rate-effects in sheared disordered solids are studied using molecular dynamics simulations of binary Lennard-Jones glasses in two and three dimensions. In the quasistatic (QS) regime, systems exhibit critical behavior: the magnitudes of avalanches are power-law distributed with a maximum cutoff that diverges with increasing system size $L$. With increasing rate, systems move away from the critical yielding point and the average flow stress rises as a power of the strain rate with exponent $1/β$, the Herschel-Bulkley exponent. Finite-size scaling collapses of the stress are used to measure $β$ as well as the exponent $ν$ which characterizes the divergence of the correlation length. The stress and kinetic energy per particle experience fluctuations with strain that scale as $L^{-d/2}$. As the largest avalanche in a system scales as $L^α$, this implies $α< d/2$. The diffusion rate of particles diverges as a power of decreasing rate before saturating in the QS regime. A scaling theory for the diffusion is derived using the QS avalanche rate distribution and generalized to the finite strain rate regime. This theory is used to collapse curves for different system sizes and confirm $β/ν$.

cond-mat.soft↗

arXiv2021

Criticality in sheared, disordered solids. II. Correlations in avalanche dynamics

Disordered solids respond to quasistatic shear with intermittent avalanches of plastic activity, an example of the crackling noise observed in many nonequilibrium critical systems. The temporal power spectrum of activity within disordered solids consists of three distinct domains: a novel power-law rise with frequency at low frequencies indicating anticorrelation, white-noise at intermediate frequencies, and a power-law decay at high frequencies. As the strain rate increases, the white-noise regime shrinks and ultimately disappears as the finite strain rate restricts the maximum size of an avalanche. A new strain-rate- and system-size-dependent scaling theory is derived for power spectra in both the quasistatic and finite-strain-rate regimes. This theory is validated using data from overdamped two- and three-dimensional molecular dynamics simulations. We identify important exponents in the yielding transition including the dynamic exponent $z$ which relates the size of an avalanche to its duration, the fractal dimension of avalanches, and the exponent characterizing the divergence in correlations with strain rate. Results are related to temporal correlations within a single avalanche and between multiple avalanches.

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arXiv2019

Anisotropic Avalanches and Critical Depinning of Three-Dimensional Magnetic Domain Walls

Simulations with more than $10^{12}$ spins are used to study the motion of a domain wall driven through a three-dimensional random-field Ising magnet (RFIM) by an external field $H$. The interface advances in a series of avalanches whose size diverges at a critical external field $H_c$. Finite-size scaling is applied to determine critical exponents and test scaling relations. Growth is intrinsically anisotropic with the height of an avalanche normal to the interface $\ell_\perp$ scaling as the width along the interface $\ell_\|$ to a power $χ=0.85 \pm 0.01$. The total interface roughness is consistent with self-affine scaling with a roughness exponent $ζ\approx χ$ that is much larger than values found previously for the RFIM and related models that explicitly break orientational symmetry by requiring the interface to be single-valued. Because the RFIM maintains orientational symmetry, the interface develops overhangs that may surround unfavorable regions to create uninvaded bubbles. Overhangs complicate measures of the roughness exponent but decrease in importance with increasing system size.

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