Active Particles Knead Three-Dimensional Gels into Porous Structures.

Colloidal gels are prime examples of functional materials exhibiting disordered, amorphous, yet metastable forms. They maintain stability through short-range attractive forces and their material properties are tunable by external forces. Combining persistent homology analyses and simulations of three-dimensional colloidal gels doped with active particles, we reveal novel dynamically evolving structures of colloidal gels.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.