arXiv subjects

Robert L. Jack

Publications and source records attributed to Robert L. Jack.

At least 37 records · Page 2Linked to original sources

arXiv2024

Yielding in colloidal gels: from local structure to meso-scale strand breakage and macroscopic failure

We study creep flow and yielding of particulate depletion gels under constant shear stress, combining data on different length and time scales. We characterise the breakage of meso-scale strands in the gel. Breakage events are distributed homogeneously in space, corresponding to macroscopically ductile flow. At the microscale, a spatio-temporal analysis of structural and mechanical metrics connects properties of strands before and after they fail, indicating that strand breakage is statistically predictable. Using results from different scales, we discuss the interplay between creeping and aging dynamics, and we demonstrate a viscosity bifurcation.

cond-mat.soft↗

arXiv2024

Thermodynamically consistent flocking: From discontinuous to continuous transitions

We introduce a family of lattice-gas models of flocking, whose thermodynamically consistent dynamics admits a proper equilibrium limit at vanishing self-propulsion. These models are amenable to an exact coarse-graining which allows us to study their hydrodynamic behavior analytically. We show that the equilibrium limit here belongs to the universality class of Model C, and that it generically exhibits tricritical behavior. Self-propulsion has a non-perturbative effect on the phase diagram, yielding novel phase behaviors depending on the type of aligning interactions. For aligning interactions that increase monotonically with the density, the tricritical point diverges to infinite density reproducing the standard scenario of a discontinuous flocking transition accompanied by traveling bands. In contrast, for models where the aligning interaction is non-monotonic in density, the system can exhibit either (the nonequilibrium counterpart of) an azeotropic point, associated with a continuous flocking transition, or a state with counterpropagating bands.

cond-mat.stat-mech↗

arXiv2024

Effects of phase separation on extinction times in population models

We study the effect of phase separating diffusive dynamics on the mean time to extinction in several reaction-diffusion models with slow reactions. We consider a continuum theory similar to model AB, and a simple model where individual particles on two sites undergo on-site reactions and hopping between the sites. In the slow-reaction limit, we project the models' dynamics onto suitable one-dimensional reaction coordinates, which allows derivation of quasi-equilibrium effective free energies. For weak noise, this enables characterisation of the mean time to extinction. This time can be enhanced or suppressed by the addition of phase separation, compared with homogeneous reference cases. We also discuss how Allee effects can be affected by phase separation.

cond-mat.stat-mech↗

arXiv2024

Unravelling Metastable Markovian Open Quantum Systems

We analyse the dynamics of metastable Markovian open quantum systems by unravelling their average dynamics into stochastic trajectories. We use quantum reset processes as examples to illustrate metastable phenomenology, including a simple three-state model whose metastability is of classical type, and a two-qubit model that features a metastable decoherence free subspace. In the three-state model, the trajectories exhibit classical metastable phenomenology: fast relaxation into distinct phases and slow transitions between them. This extends the existing correspondence between classical and quantum metastability. It enables the computation of committors for the quantum phases, and the mechanisms of rare transitions between them. For the two-qubit model, the decoherence-free subspace appears in the unravelled trajectories as a slow manifold on which the quantum state has a continuous slow evolution. This provides a classical (non-metastable) analogue of this quantum effect. We discuss the general implications of these results, and we highlight the useful role of quantum reset processes for analysis of quantum trajectories in metastable systems.

quant-ph↗

arXiv2024

Competition between lanes and transient jammed clusters in driven binary mixtures

We consider mixtures of oppositely driven particles, showing that their non-equilibrium steady states form lanes parallel to the drive, which coexist with transient jammed clusters where particles are temporarily immobilised. We analyse the interplay between these two types of non-equilibrium pattern formation, including their implications for macroscopic demixing perpendicular to the drive. Finite-size scaling analysis indicates that there is no critical driving force associated with demixing, which appears as a crossover in finite systems. We attribute this effect to the disruption of long-ranged order by the transient jammed clusters.

cond-mat.stat-mech↗

arXiv2023

Heterogeneous nucleation in the random field Ising model

We investigate the nucleation dynamics of the three-dimensional random field Ising model (RFIM) under an external field. We use umbrella sampling to compute the free-energy cost of a critical nucleus, and use forward flux sampling for the direct estimation of nucleation rates. For moderate to strong disorder, our results indicate that the size of the nucleating cluster is not a good reaction coordinate, contrary to the pure Ising model. We rectify this problem by introducing a coordinate that also accounts for the location of the nucleus. Using the free energy barrier to predict the nucleation rate, we find reasonable agreement, although deviations become stronger as disorder increases. We attribute this effect to cluster shape fluctuations. We also discuss finite-size effects on the nucleation rate.

cond-mat.stat-mech↗

arXiv2023

Partial and complete wetting of droplets of active Brownian particles

We study wetting droplets formed of active Brownian particles in contact with a repulsive potential barrier, in a wedge geometry. Our numerical results demonstrate a transition between partially wet and completely wet states, as a function of the barrier height, analogous to the corresponding surface phase transition in passive fluids. We analyse partially wet configurations characterised by a nonzero contact angle $θ$ between the droplet surface and the barrier, including the average density profile and its fluctuations. These findings are compared with two equilibrium systems: a Lennard-Jones fluid and a simple contour model for a liquid-vapour interface. We locate the wetting transition where $\cos(θ)=1$, and the neutral state where $\cos(θ)=0$. We discuss the implications of these results for possible definitions of surface tensions in active fluids.

cond-mat.stat-mech↗

arXiv2023

Active-passive mixtures with bulk loading: a minimal active engine in one dimension

We study a one-dimensional mixture of active (run-and-tumble) particles and passive (Brownian) particles, with single-file constraint, in a sawtooth potential. The active particles experience a ratchet effect: this generates a current, which can push passive particles against an applied load. The resulting system operates as an active engine. Using numerical simulations, we analyse the efficiency of this engine, and we discuss how it can be optimised. Efficient operation occurs when the active particles self-organise into teams, which can push the passive ones against large loads by leveraging collective behaviour. We discuss how the particle arrangement, conserved under the single-file constraint, affects the engine efficiency. We also show that relaxing this constraint still allows the engine to operate effectively.

cond-mat.stat-mech↗

arXiv2023

How do Cicadas Emerge Together? Thermophysical Aspects of Their Collective Decision-Making

Certain periodical cicadas exhibit life cycles with durations of 13 or 17 years, and it is now generally accepted that such large prime numbers arise evolutionarily to avoid synchrony with predators. Less well explored is the question of how, in the face of intrinsic biological and environmental noise, insects within a brood emerge together in large successive swarms from underground during springtime warming. Here we consider the decision-making process of underground cicada nymphs experiencing random but spatially-correlated thermal microclimates like those in nature. Introducing short-range communication between insects leads to a model of consensus building that maps on to the statistical physics of an Ising model with a quenched, spatially correlated random magnetic field and annealed site dilution, which displays the kinds of collective swarms seen in nature.

cond-mat.soft↗

arXiv2023

Emerging mesoscale flows and chaotic advection in dense active matter

We study two models of overdamped self-propelled disks in two dimensions, with and without aligning interactions. Active mesoscale flows leading to chaotic advection emerge in both models in the homogeneous dense fluid away from dynamical arrest, forming streams and vortices reminiscent of multiscale flow patterns in turbulence. We show that the characteristics of these flows do not depend on the specific details of the active fluids, and result from the competition between crowding effects and persistent propulsions. Our results suggest that dense active fluids present a type of `active turbulence' distinct from collective flows reported in other types of active systems.

cond-mat.soft↗

arXiv2023

Intermittent relaxation and avalanches in extremely persistent active matter

We use numerical simulations to study the dynamics of dense assemblies of self-propelled particles in the limit of extremely large, but finite, persistence times. In this limit, the system evolves intermittently between mechanical equilibria where active forces balance interparticle interactions. We develop an efficient numerical strategy allowing us to resolve the statistical properties of elastic and plastic relaxation events caused by activity-driven fluctuations. The system relaxes via a succession of scale-free elastic events and broadly distributed plastic events that both depend on the system size. Correlations between plastic events lead to emergent dynamic facilitation and heterogeneous relaxation dynamics. Our results show that dynamical behaviour in extremely persistent active systems is qualitatively similar to that of sheared amorphous solids, yet with some important differences.

cond-mat.soft↗

arXiv2023

Tricritical behavior in dynamical phase transitions

We identify a new scenario for dynamical phase transitions associated with time-integrated observables occurring in diffusive systems described by the macroscopic fluctuation theory. It is characterized by the pairwise meeting of first- and second-order bias-induced phase transition curves at two tricritical points. We formulate a simple, general criterion for its appearance and derive an exact Landau theory for the tricritical behavior. The scenario is demonstrated in three examples: the simple symmetric exclusion process biased by an activity-related structural observable; the Katz-Lebowitz-Spohn lattice gas model biased by its current; and in an active lattice gas biased by its entropy production.

cond-mat.stat-mech↗

arXiv2023

Necking and failure of a colloidal gel arm: signatures of yielding on different length scales

Colloidal gels consist of percolating networks of interconnected arms. Their mechanical properties depend on the individual arms, and on their collective behaviour. We use numerical simulations to pull on a single arm, built from a model colloidal gel-former with short-ranged attractive interactions. Under elongation, the arm breaks by a necking instability. We analyse this behaviour at three different length scales: a rheological continuum model of the whole arm; a microscopic analysis of the particle structure and dynamics; and the local stress tensor. Combining these different measurements gives a coherent picture of the necking and failure: the neck is characterised by plastic flow that occurs for stresses close to the arm's yield stress. The arm has an amorphous local structure and has large residual stresses from its initialisation. We find that neck formation is associated with increased plastic flow, a reduction in the stability of the local structure, and a reduction in the residual stresses; this indicates that {the} system loses its solid character and starts to behave more like a viscous fluid. We discuss the implications of these results for the modelling of gel dynamics.

cond-mat.soft↗

arXiv2022

Dimensionality reduction of local structure in glassy binary mixtures

We consider unsupervised learning methods for characterizing the disordered microscopic structure of supercooled liquids and glasses. Specifically, we perform dimensionality reduction of smooth structural descriptors that describe radial and bond-orientational correlations, and assess the ability of the method to grasp the essential structural features of glassy binary mixtures. In several cases, a few collective variables account for the bulk of the structural fluctuations within the first coordination shell and also display a clear connection with the fluctuations of particle mobility. Fine-grained descriptors that characterize the radial dependence of bond-orientational order better capture the structural fluctuations relevant for particle mobility, but are also more difficult to parametrize and to interpret. We also find that principal component analysis of bond-orientational order parameters provides identical results to neural network autoencoders, while having the advantage of being easily interpretable. Overall, our results indicate that glassy binary mixtures have a broad spectrum of structural features. In the temperature range we investigate, some mixtures display well-defined locally favored structures, which are reflected in bimodal distributions of the structural variables identified by dimensionality reduction.

cond-mat.stat-mech↗

arXiv2022

Role of initial conditions in $1D$ diffusive systems: compressibility, hyperuniformity and long-term memory

We analyse the long-lasting effects of initial conditions on fluctuations in one-dimensional diffusive systems. We consider both the fluctuations of current for non-interacting diffusive particles starting from a step-like initial density profile, and the mean-square displacement of tracers in homogeneous systems with single-file diffusion. For these two cases, we show analytically (via the propagator and Macroscopic Fluctuation Theory, respectively) that the long-term memory of initial conditions is mediated by a single static quantity: a generalized compressibility that quantifies the density fluctuations of the initial state. We thereby identify a universality class of hyperuniform initial states whose dynamical variances coincide with the `quenched' cases studied previously; we also describe a continuous family of other classes among which equilibrated (or `annealed') initial conditions are but one family member. We verify our predictions through extensive Monte Carlo simulations.

cond-mat.stat-mech↗

arXiv2022

Thermal vestiges of avalanches in the driven random field Ising model

We investigate the non-equilibrium behavior of the $3d$ random field Ising model at finite temperature, as an external field is increased through its coercive field. We show by numerical simulations that the phenomenology of avalanches -- which are sharply defined only at zero temperature -- also persists over a significant range of finite temperatures. We analyse the main differences between the thermal and zero-temperature systems, including an excess of small avalanches in the thermal case, whose behaviour is consistent with activated dynamical scaling. We also investigate the extent to which individual avalanches at finite temperature can be traced back to parent avalanches in the athermal system.

cond-mat.stat-mech↗

arXiv2022

Entropy production and its large deviations in an active lattice gas

Active systems are characterized by a continuous production of entropy at steady state. We study the statistics of entropy production within a lattice-based model of interacting active particles that is capable of motility-induced phase separation. Exploiting a recent formulation of the exact fluctuating hydrodynamics for this model, we provide analytical results for its entropy production statistics in both typical and atypical (biased) regimes. This complements previous studies of the large deviation statistics of entropy production in off-lattice active particle models that could only be addressed numerically. Our analysis uncovers an unexpectedly intricate phase diagram, with five different phases arising (under bias) within the parameter regime where the unbiased system is in its homogeneous state. Notably, we find the concurrence of first order and second order nonequilibrium phase transition curves at a bias-induced tricritical point, a feature not yet reported in previous studies of active systems.

cond-mat.stat-mech↗

arXiv2022

Mechanical analysis of a dynamical phase transition for particles in a channel

We analyse biased ensembles of trajectories for a two-dimensional system of particles, evolving by Langevin dynamics in a channel geometry. This bias controls the degree of particle clustering. On biasing to large clustering, we observe a dynamical phase transition where the particles break symmetry and accumulate at one of the walls. We analyse the mechanical properties of this symmetry-broken state using the Irving-Kirkwood stress tensor. The biased ensemble is characterised by body forces which originate in random thermal noises, but have finite averages in the presence of the bias. We discuss the connection of these forces to Doob's transform and optimal control theory.

cond-mat.stat-mech↗