arXiv subjects

Giuseppe Foffi

Publications and source records attributed to Giuseppe Foffi.

At least 19 recordsLinked to original sources

arXiv2026

Auxetic behaviour in crystals of hard polyhedra

Auxetic materials - systems that, when subjected to a compression in one direction, also compress in one or more perpendicular directions - have intrigued researchers for decades due to their counterintuitive mechanical properties. Their unique behaviour gives auxetic materials potential for a wide range of applications such as shock absorbers, and electrodes in piezoelectric sensors. Most known auxetic materials are realized by connecting rigid, anisotropic units in a hinging manner, which are systems that often are hard to realize experimentally on the microscopic scale. Here, we explore the elastic behaviour of six crystals composed of discrete space-filling hard polygons or polyhedra. We show that some of these systems show partial auxetic behaviour, emerging from the interplay of entropy and geometry alone. To demonstrate the feasibility and robustness of this phenomenon, we create two experimental realizations of square-shaped particles, spanning both colloidal particles driven by Brownian motion and granular particles driven by external vibrations, and confirm the emergence of auxeticity in both cases.

cond-mat.soft↗

arXiv2026

Self-assembly of quasicrystals under cyclic shear

We investigate the self-assembly of two-dimensional dodecagonal quasicrystals driven by cyclic shear, effectively replacing thermal fluctuations with plastic rearrangements. Using particles interacting via a smoothed square-shoulder potential, we demonstrate that cyclic shearing drives initially random configurations into ordered quasicrystalline states. The resulting non-equilibrium phase diagram qualitatively mirrors that of thermal equilibrium, exhibiting square, quasicrystalline, and hexagonal phases, as well as phase coexistence. Remarkably, the shear-stabilised quasicrystal appears even where the zero-temperature equilibrium ground state favours square-hexagonal coexistence, suggesting that mechanical driving can stabilise quasicrystalline order in a way analogous to entropic effects in thermal systems. The structural quality of the self-assembled state is maximised near the yielding transition, even though the dynamics are slowest there. Yet, the system still quickly forms monodomain quasicrystals without any complex annealing protocols, unlike at equilibrium, where thermal annealing would be required. Finite-size scaling analysis reveals that global orientational order decays slowly with system size, indicative of quasi-long-range order comparable to equilibrium hexatic phases. Overall, our results establish cyclic shear as an efficient pathway for the self-assembly of complex structures.

cond-mat.soft↗

arXiv2025

Conservation laws and slow dynamics determine the universality class of interfaces in active matter

While equilibrium interfaces display universal large-scale statistics, interfaces in phase-separated active and driven systems are predicted to belong to distinct non-equilibrium universality classes. Yet, such behavior has proven difficult to observe, with most systems exhibiting equilibrium-like fluctuations despite their strongly non-equilibrium microscopic dynamics. We introduce a hard-disk model driven by active collisions, conceived as an effective 2D description of a vibrofluidized granular system that, contrary to self-propelled models, displays clear non-equilibrium interfacial scaling. We observe for the first time, the $|\boldsymbol q|$KPZ and wet-$|\boldsymbol q|$KPZ universality classes while revealing a new, previously overlooked universality class arising in systems with slow solid-like or glassy dynamics. Conservation laws and slow dynamics select these distinct classes.

cond-mat.stat-mech↗

arXiv2025

Dynamical and structural properties of an absorbing phase transition: a case study from granular systems

We investigate the dynamical and structural properties of absorbing phase transitions (APTs) within granular systems. Specifically, we examine a model for vibrofluidized systems of spherical grains, which undergo a transition from a state of purely vertical motion to one characterized by horizontal diffusion as the density increases. Numerical simulations reveal that, depending on the specific system parameters, both continuous and discontinuous transitions can occur, each associated with markedly distinct structural properties at the transition point. We explain this using a theoretical analysis based on kinetic theory applied to an effective 2D model, which elucidates the role of a synchronization effect in determining the nature of the transition. A fluctuating hydrodynamic theory, which quantitatively describes the structural and dynamical properties of the active state such as hyperuniformity is derived from the microscopic dynamics, together with an equilibrium-like assumption concerning the noises on the hydrodynamic fields. This work expands on previous studies by providing a comprehensive examination of the APT characteristics and proposing new theoretical models to interpret the observed behaviors.

cond-mat.soft↗

arXiv2024

Transient contacts between filaments impart its elasticity to branched actin

Branched actin networks exert pushing forces in eukaryotic cells, and adapt their stiffness to their environment. The physical basis for their mechanics and adaptability is however not understood. Indeed, here we show that their high density and low connectivity place them outside the scope of standard elastic network models for actin. We combine high-precision mechanical experiments, molecular dynamics simulations and a mean-field elastic theory to show that they are instead dominated by the proliferation of interfilament contacts under compression. This places branched actin in the same category as undercoordinated, fibrous materials such as sheep's wool. When the network is grown under force, filaments entangle as if knitted together and trap contacts in their structure. Trapped contacts play a similar role as crosslinkers in rigidifying the network, and are thus key to its active adaptive mechanics.

cond-mat.soft↗

arXiv2023

Quasi-crystalline order in vibrated granular matter

Quasi-crystals are aperiodic structures that present crystallographic properties which are not compatible with that of a single unit cell. Their revolutionary discovery in a metallic alloy, less than three decades ago, has required a full reconsideration of what we defined as a crystal structure. Surprisingly, quasi-crystalline structures have been discovered also at much larger length scales in different microscopic systems for which the size of elementary building blocks ranges between the nanometric to the micrometric scale. Here, we report the first experimental observation of spontaneous quasi-crystal self-assembly at the millimetric scale. This result is obtained in a fully athermal system of macroscopic spherical grains vibrated on a substrate. Starting from a liquid-like disordered phase, the grains begin to locally arrange into three types of squared and triangular tiles that eventually align, forming 8-fold symmetric quasi-crystal that has been predicted in simulation but not yet observed experimentally in non-atomic systems. These results are not only the proof of a novel route to spontaneously assemble quasi-crystals but are of fundamental interest for the connection between equilibrium and non-equilibrium statistical physics.

cond-mat.soft↗

arXiv2023

A hard-sphere quasicrystal stabilized by configurational entropy

Due to their aperiodic nature, quasicrystals are one of the least understood phases in statistical physics. One significant complication they present in comparison to their periodic counterparts is the fact that any quasicrystal can be realized as an exponentially large number of different tilings, resulting in a significant contribution to the quasicrystal entropy. Here, we use free-energy calculations to demonstrate that it is this configurational entropy which stabilizes a dodecagonal quasicrystal in a binary mixture of hard spheres on a plane. Our calculations also allow us to quantitatively confirm that in this system all tiling realizations are essentially equally likely, with free-energy differences less than 0.0001$k_BT$ per particle -- an observation that could be the related to the observation of only random tilings in soft matter quasicrystals. Owing to the simplicity of the model and its available counterparts in colloidal experiments, we believe that this system is a excellent candidate to achieve the long-awaited quasicrystal self-assembly on the micron scale.

cond-mat.soft↗

arXiv2022

Self-assembly of dodecagonal and octagonal quasicrystals in hard spheres on a plane

Quasicrystals are fascinating structures, characterized by strong positional order but lacking the periodicity of a crystal. In colloidal systems, quasicrystals are typically predicted for particles with complex or highly specific interactions, which makes experimental realization difficult. Here, we propose an ideal colloidal model system for quasicrystal formation: binary mixtures of hard spheres sedimented onto a flat substrate. Using computer simulations, we explore both the close-packing and spontaneous self-assembly of these systems over a wide range of size ratios and compositions. Surprisingly, we find that this simple, effectively two-dimensional model systems forms not only a variety of crystal phases, but also two quasicrystal phases: one dodecagonal and one octagonal. Intriguingly, the octagonal quasicrystal consists of three different tiles, whose relative concentrations can be continuously tuned via the composition of the binary mixture. Both phases form reliably and rapidly over a significant part of parameter space, making hard spheres on a plane an ideal model system for exploring quasicrystal self-assembly on the colloidal scale.

cond-mat.soft↗

arXiv2021

Correlation between plastic rearrangements and local structure in a cyclically driven glass

The correlation between local structure and the propensity for structural rearrangements has been widely investigated in glass forming liquids and glasses. In this paper we use the excess two-body entropy $S_2$ and tetrahedrality $\n_{tet}$ as the per-particle local structural order parameters to explore such correlations in a three-dimensional model glass subjected to cyclic shear deformation. We first show that for both liquid configurations and the corresponding inherent structures, local ordering increases upon lowering temperature, signaled by a decrease in the two-body entropy and an increase in tetrahedrality. When the inherent structures, or glasses, are periodically sheared athermally, they eventually reach absorbing states for small shear amplitudes, which do not change from one cycle to the next. Large strain amplitudes result in the formation of shear bands, within which particle motion is diffusive. We show that in the steady state, there is a clear difference in the local structural environment of particles that will be part of plastic rearrangements during the next shear cycle and that of particles that are immobile. In particular, particles with higher $S_2$ and lower $n_{tet}$ are more likely to go through rearrangements irrespective of the average energies of the configurations and strain amplitude. For high shear, we find very distinctive local order outside the mobile shear band region, where almost $30\%$ of the particles are involved in icosahedral clusters, contrasting strongly with the fraction of $<5\%$ found inside the shear band.

cond-mat.stat-mech↗

arXiv2021

Stiffening attractive suspensions with solid inclusions or granularity effect in loaded colloidal gels

The elastic properties of a soft matter material can be greatly altered by the presence of solid inclusions whose microscopic properties, such as their size and interactions, can have a dramatic effect. In order to shed light on these effects we use extensive rheology computer simulations to investigate colloidal gels with solid inclusions of different size. We show that the elastic properties vary in a highly non trivial way as consequence of the interactions between the gel backbone and the inclusions. In particular, we show that the key aspects are the presence of the gel backbone and its mechanical alteration originated by the inclusions. To confirm our observations and their generality, we performed experiments on an emulsion that presents strong analogies with colloidal gels and confirms the trends observed in the simulations.

cond-mat.soft↗

arXiv2021

Yielding transition of a two dimensional glass former under athermal cyclic shear deformation

We study numerically the yielding transition of a two dimensional model glass subjected to athermal quasi-static cyclic shear deformation, with the aim of investigating the effect on the yielding behaviour of the degree of annealing, which in turn depends on the preparation protocol. We find two distinct regimes of annealing separated by a threshold energy. Poorly annealed glasses progressively evolve towards the threshold energy as the strain amplitude is increased towards the yielding value. Well annealed glasses with initial energies below the threshold energy exhibit stable behaviour, with negligible change in energy with increasing strain amplitude, till they yield. Discontinuities in energy and stress at yielding increase with the degree of annealing, consistently with recent results found in three dimensions. We observe significant structural change with strain amplitude that closely mirrors the changes in energy and stresses. We investigate groups of particles that are involved in plastic rearrangements. We analyse the distributions of avalanche sizes, of clusters of connected rearranging particles, and related quantities, employing finite size scaling analysis. We verify previously investigated relations between exponents characterising these distributions, and a newly proposed relation between exponents describing avalanche and cluster size distributions.

cond-mat.soft↗

arXiv2021

Avalanches and Structural Change in Cyclically Sheared Silica Glass

We investigate avalanches associated with plastic rearrangements and the nature of structural change in the prototypical strong glass, silica, computationally. Although qualitative aspects of yielding in silica are similar to other glasses, we find that the statistics of avalanches exhibits non-trivial behaviour. Investigating the statistics of avalanches and clusters in detail, we propose and verify a new relation between exponents characterizing the size distribution of avalanches and clusters. Across the yielding transition, anomalous structural change and densification, associated with a suppression of tetrahedral order, is observed to accompany strain localisation.

cond-mat.soft↗

arXiv2020

Hyperuniformity in cyclically driven glasses

We present a numerical investigation of the density fluctuations in a model glass under cyclic shear deformation. At low amplitude of shear, below yielding, the system reaches a steady absorbing state in which density fluctuations are suppressed revealing a clear fingerprint of hyperuniformity up to a finite length scale. The opposite scenario is observed above yielding, where the density fluctuations are strongly enhanced. We demonstrate that the transition to this state is accompanied by a spatial phase separation into two distinct hyperuniform regions, as a consequence of shear band formation above the yield amplitude.

cond-mat.dis-nn↗

arXiv2020

Infinite-pressure phase diagram of binary mixtures of (non)additive hard disks

One versatile route to the creation of two-dimensional crystal structures on the nanometer to micrometer scale is the self-assembly of colloidal particles at an interface. Here, we explore the crystal phases that can be expected from the self-assembly of mixtures of spherical particles of two different sizes, which we map to (additive or non-additive) hard-disk mixtures. We map out the infinite-pressure phase diagram for these mixtures, using Floppy Box Monte Carlo simulations to systematically sample candidate crystal structures with up to 12 disks in the unit cell. As a function of the size ratio and number ratio of the two species of particles, we find a rich variety of periodic crystal structures. Additionally, we identify random tiling regions to predict random tiling quasicrystal stability ranges. Increasing non-additivity both gives rise to additional crystal phases and broadens the stability regime for crystal structures involving a large number of large-small contacts, including random tilings. Our results provide useful guidelines for controlling the self-assembly of colloidal particles at interfaces.

cond-mat.soft↗

arXiv2020

Autonomously revealing hidden local structures in supercooled liquids

Few questions in condensed matter science have proven as difficult to unravel as the interplay between structure and dynamics in supercooled liquids and glasses. The conundrum: close to the glass transition, the dynamics slow down dramatically and become heterogeneous while the structure appears largely unperturbed. Largely unperturbed, however, is not the same as unperturbed, and many studies have attempted to identify "slow" local structures by exploiting dynamical information. Nonetheless, the question remains open: is the key to the slow dynamics imprinted in purely structural information? And if so, is there a way to determine the relevant structures without any dynamical information? Here, we use a newly developed unsupervised machine learning (UML) algorithm to identify structural heterogeneities in three archetypical glass formers. In each system, the UML approach autonomously designs an order parameter based purely on structural variation within a single snapshot. Impressively, this order parameter strongly correlates with the dynamical heterogeneity. Moreover, the structural characteristics linked to slow particles disappear as we move away from the glass transition. Our results demonstrate the power of machine learning techniques to detect structural patterns even in disordered systems, and provide a new way forward for unraveling the structural origins of the slow dynamics of glassy materials.

cond-mat.soft↗

arXiv2020

Rotational and translational dynamics in dense fluids of patchy particles

We explore the effect of directionality on rotational and translational relaxation in glassy systems of patchy particles. Using molecular dynamics simulations we analyze the impact of two distinct patch geometries, one that enhance local icosahedral structure and one which does not strongly affect local order. We find that in nearly all investigated cases, rotational relaxation takes place on a much faster time scale than translational relaxation. By comparing to a simplified dynamical Monte Carlo model, we illustrate that rotational diffusion can be qualitatively explained as purely local motion within a fixed environment, which is not coupled strongly to the cage-breaking dynamics required for translational relaxation. Nonetheless, icosahedral patch placement has a profound effect on the local structure of the system, resulting in a dramatic slowdown at low temperatures which is strongest at an intermediate "optimal" patch size.

cond-mat.soft↗

arXiv2019

Multi-component colloidal gels: interplay between structure and mechanical proprieties

We present a detailed numerical study of multi-component colloidal gels interacting sterically and obtained by arrested phase separation. Under deformation, we found that the interplay between the different intertwined networks is key. Increasing the number of component leads to softer solids that can accomodate progressively larger strain before yielding. The simulations highlight how this is the direct consequence of the purely repulsive interactions between the different components, which end up enhancing the linear response of the material. Our work {provides new insight into mechanisms at play for controlling the material properties and open the road to new design principles for} soft composite solids

cond-mat.soft↗

arXiv2019

The role of annealing in determining the yielding behavior of glasses under cyclic shear deformation

Yielding behavior in amorphous solids has been investigated in computer simulations employing uniform and cyclic shear deformation. Recent results characterise yielding as a discontinuous transition, with the degree of annealing of glasses being a significant parameter. Under uniform shear, discontinuous changes in stresses at yielding occur in the high annealing regime, separated from the poor annealing regime in which yielding is gradual. In cyclic shear simulations, relatively poorly annealed glasses become progressively better annealed as the yielding point is approached, with a relatively modest but clear discontinuous change at yielding. To understand better the role of annealing on yielding characteristics, we perform athermal quasistaic cyclic shear simulations of glasses prepared with a wide range of annealing in two qualitatively different systems -- a model of silica (a network glass), and an atomic binary mixture glass. Two strikingly different regimes of behavior emerge: Energies of poorly annealed samples evolve towards a unique threshold energy as the strain amplitude increases, before yielding takes place. Well annealed samples, in contrast, show no significant energy change with strain amplitude till they yield, accompanied by discontinuous energy changes that increase with the degree of annealing. Significantly, the threshold energy for both systems correspond to dynamical crossover temperatures associated with changes in the character of the energy landscape sampled by glass forming liquids. Uniform shear simulations support the recently discussed scenario of a random critical point separating ductile and brittle yielding, which our results now associate with dynamical crossover temperatures in the corresponding liquids.

cond-mat.dis-nn↗