Distinct impacts of polar and nematic self-propulsion on active unjamming.

Though jamming transitions are long studied in condensed matter physics and granular systems, much less is known about active jamming (or unjamming), which commonly takes place in living materials. In this paper, we explore, by molecular dynamic simulations, the jamming-unjamming transition in a dense system of active semiflexible filaments. In particular, we characterize the distinct impact of polar vs nematic driving for different filament rigidities and at varying densities. Our results show that high densities of dynamic active filaments can be achieved by only changing the nature of the active force, nematic or polar. Interestingly, while polar driving is more effective at unjamming the system at high densities below confluency, we find that at even higher densities, nematic driving enhances unjamming compared to its polar counterpart. The effect of varying the rigidity of filaments is also significantly different in the two cases: While for nematic driving, lowering the bending rigidity unjams the system, we find an intriguing reentrant jamming-unjamming-jamming transition for polar driving as the filament rigidity is lowered. While the first transition (unjamming) is driven by softening due to reduced rigidity, the second transition (jamming) is a cooperative effect of ordering and coincides with the emergence of nematic order in the system. Together, through a generic model of self-propelled flexible filaments, our results demonstrate how tuning the nature of self-propulsion and flexibility can be employed by active materials to achieve high densities without getting jammed.

Frictional Granular Matter: Protocol Dependence of Mechanical Properties.

Theoretical treatments of frictional granular matter often assume that it is legitimate to invoke classical elastic theory to describe its coarse-grained mechanical properties. Here, we show, based on experiments and numerical simulations, that this is generically not the case since stress autocorrelation functions decay more slowly than what is expected from elasticity theory. It was theoretically shown that standard elastic decay demands pressure and torque density fluctuations to be normal, with possibly one of them being hyperuniform. However, generic compressed frictional assemblies exhibit abnormal pressure fluctuations, failing to conform with the central limit theorem. The physics of this failure is linked to correlations built in the material during compression from a dilute configuration prior to jamming. By changing the protocol of compression, one can observe different pressure fluctuations, and stress autocorrelations decay at large scales.

Universal Low-Frequency Vibrational Modes in Silica Glasses.

It was recently shown that different simple models of glass formers with binary interactions define a universality class in terms of the density of states of their quasilocalized low-frequency modes. Explicitly, once the hybridization with standard Debye (extended) modes is avoided, a number of such models exhibit a universal density of states, depending on the mode frequencies as D(ω)∼ω^{4}. It is unknown, however, how wide this universality class is, and whether it also pertains to more realistic models of glass formers. To address this issue we present analysis of the quasilocalized modes in silica, a network glass that has both binary and ternary interactions. We conclude that in three dimensions silica exhibits the very same frequency dependence at low frequencies, suggesting that this universal form is a generic consequence of amorphous glassiness.

The sandpile revisited: computer assisted determination of constitutive relations and the breaking of scaling.

We revisit the problem of the stress distribution in a frictional sandpile with both normal and tangential (frictional) inter-granular forces, under gravity, equipped with a new numerical method of generating such assemblies. Numerical simulations allow a determination of the spatial dependence of all the components of the stress field, principle stress axis, angle of repose, as a function of systems size, the coefficient of static friction and the frictional interaction with the bottom surface. We compare these results with the predictions of a theory based on continuum equilibrium mechanics. Basic to the theory of sandpiles are assumptions about the form of scaling solutions and constitutive relations for cohesive-less hard grains for which no typical scale is available. We find that these constitutive relations must be modified; moreover for smaller friction coefficients and smaller piles these scaling assumptions break down in the bulk of the sandpile due to the presence of length scales that must be carefully identified. Fortunately, for larger friction coefficient and for larger piles the breaking of scaling is weak in the bulk, allowing an approximate analytic theory which agrees well with the observations. After identifying the crucial scale, triggering the breaking of scaling, we provide a predictive theory to when scaling solutions are expected to break down. At the bottom of the pile the scaling assumption breaks always, due to the different interactions with the bottom surface. The consequences for measurable quantities like the pressure distribution and shear stress at the bottom of the pile are discussed. For example one can have a transition from no dip in the base-pressure to a dip at the center of the pile as friction increases.

Excess vibrational density of states and the brittle to ductile transition in crystalline and amorphous solids.

The conditions which determine whether a material behaves in a brittle or ductile fashion on mechanical loading are still elusive and comprise a topic of active research among materials physicists and engineers. In this study, we present the results of in silico mechanical deformation experiments from two very different model solids in two and three dimensions. The first consists of particles interacting with isotropic potentials and the other has strongly direction dependent interactions. We show that in both cases, the excess vibrational density of states is one of the fundamental quantities which characterizes the ductility of the material. Our results can be checked using careful experiments on colloidal solids.

Thermodynamic anomalies of a network former in a periodic field: Network former in a periodic field.

We study the effect of an external one-dimensional periodic field on thermodynamic anomalies associated with a two-dimensional model liquid with anisotropic interactions. The model system, a 50 : 50 binary mixture of two species of particles interacting with an angle-dependent Lennard-Jones potential, has a rich phase diagram and shows many features of network-forming liquids like water and silica such as a prominent minimum in the pressure-temperature isochore. Confining the system by a commensurate one-dimensional periodic field shifts the temperature of minimum pressure to higher temperatures. A mean-field theory of a lattice-gas in an external field which couples to internal orientational states reproduces these results.

Dipole screening in pure shear strain protocols of amorphous solids.

When amorphous solids are subjected to simple or pure strain, they exhibit elastic increase in stress, punctuated by plastic events that become denser (in strain) upon increasing the system size. It is customary to assume in theoretical models that the stress released in each plastic event is redistributed according to the linear Eshelby kernel, causing avalanches of additional stress release. Here we demonstrate that, contrary to the uniform affine strain resulting from simple or pure strain, each plastic event is associated with a nonuniform strain that gives rise to a displacement field that contains quadrupolar and dipolar charges that typically screen the linear elastic phenomenology and introduce anomalous length scales and influence the form of the stress redistribution. An important question that opens up is how to take this into account in elastoplastic models of shear induced phenomena like shear banding.

Anomalous elasticity and plastic screening in amorphous solids.

Amorphous solids appear to react elastically to small external strains, but in contrast to ideal elastic media, plastic responses abound immediately at any value of the strain. Such plastic responses are quasilocalized in nature, with the "cheapest" one being a quadrupolar source. The existence of such plastic responses results in screened elasticity in which strains and stresses can either quantitatively or qualitatively differ from the unscreened theory, depending on the specific screening mechanism. Here we offer a theory of such screening effects by plastic quadrupoles, dipoles, and monopoles, explain their natural appearance, and point out the analogy to electrostatic screening by electric charges and dipoles. For low density of quadrupoles the effect is to normalize the elastic moduli without a qualitative change compared to pure elasticity theory; for higher density of quadrupoles the screening effects result in qualitative changes. Predictions for the spatial dependence of displacement fields caused by local sources of strains are provided and compared to numerical simulations. We find that anomalous elasticity is richer than electrostatics in having a screening mode that does not appear in the electrostatic analog.

Elementary plastic events in amorphous silica.

Plastic instabilities in amorphous materials are often studied using idealized models of binary mixtures that do not capture accurately molecular interactions and bonding present in real glasses. Here we study atomic-scale plastic instabilities in a three-dimensional molecular dynamics model of silica glass under quasistatic shear. We identify two distinct types of elementary plastic events, one is a standard quasilocalized atomic rearrangement while the second is a bond-breaking event that is absent in simplified models of fragile glass formers. Our results show that both plastic events can be predicted by a drop of the lowest nonzero eigenvalue of the Hessian matrix that vanishes at a critical strain. Remarkably, we find very high correlation between the associated eigenvectors and the nonaffine displacement fields accompanying the bond-breaking event, predicting the locus of structural failure. Both eigenvectors and nonaffine displacement fields display an Eshelby-like quadrupolar structure for both failure modes, rearrangement, and bond breaking. Our results thus clarify the nature of atomic-scale plastic instabilities in silica glasses, providing useful information for the development of mesoscale models of amorphous plasticity.

Glass-Like Slow Dynamics in a Colloidal Solid with Multiple Ground States.

We study the phase-ordering dynamics of a 2D model colloidal solid using molecular dynamics simulations. The colloid particles interact with each other with a Hamaker potential modified by the presence of equatorial "patches" of attractive and repulsive regions. The total interaction potential between two such colloids is, therefore, strongly directional and has a 3-fold symmetry. Working in the canonical ensemble, we determine the phase diagram in the density-temperature plane. We obtain three distinct crystalline ground states, viz., a low density honeycomb solid, a rectangular solid at intermediate density, and finally a high-density triangular structure. We show that when cooled rapidly from the liquid phase along iso-chores, the system undergoes a transition to a "strong glass", while slow cooling gives rise to crystalline phases. We claim that geometrical frustration arising from the presence of many competing crystalline ground states causes glassy ordering and dynamics in this solid. Our results may be easily confirmed by suitable experiments on patchy colloids.

Origin of chains of Au-PbS nano-dumbbells in space.

Self-assembled, one-dimensional (1D) nanomaterials are amenable building blocks for bottom-up nanofabrication processes. A current shortcoming in the self-assembly of 1D nanomaterials in solution phase is the need for specific linkers or templates under very precise conditions to achieve a handful of systems. Here we report on the origin of a novel self-assembly of 1D dumbbells consisting of Au tipped PbS nanorods into stable chains in solution without any linkers or templates. A realistic multi-particle model suggests that the mesophase comprises 1D dumbbells arrayed in chains formed by anisotropic van der Waals type interactions. We demonstrate an alternative recognition mechanism for directing the assembly of the 1D dumbbells, based on effective interaction between the neighboring dumbbells consisting of Au tips with complementary crystallographic facets that guides the entire assembly in space.

Single-file diffusion and kinetics of template-assisted assembly of colloids.

We report computer simulation studies of the kinetics of ordering of a two-dimensional system of particles on a template with a one-dimensional periodic pattern. In equilibrium, one obtains a reentrant liquid-solid-liquid phase transition as the strength of the substrate potential is varied. We show that domains of crystalline order grow as ~t(1/z), with z~4, with a possible crossover to z~2 at late times. We argue that the t(1/4) law originates from single-file motion and annihilation of defect pairs of opposite topological charge along channels created by the template.

Polymorphism, thermodynamic anomalies, and network formation in an atomistic model with two internal states.

Using molecular dynamics simulations we study, in two dimensions, the temperature-density phase diagram of a simple model with two internal states labeled 1 and -1. The particles interact with a modified Lennard-Jones potential, which depends on relative positions of the particles as well as on their state. Working in an ensemble where the system reduces to a 50:50 binary mixture of two species of particles, we obtain a rich phase diagram showing many features of common network-forming liquids such as water and silica, including polymorphism and thermodynamic anomalies. We believe our model may be useful for studies concerning generic features of such complex liquids.