Active Jamming: Self-propelled soft particles at high density

We study numerically the phases and dynamics of a dense collection of self-propelled particles with soft repulsive interactions in two dimensions. The model is motivated by recent in vitro experiments on confluent monolayers of migratory epithelial and endothelial cells. The phase diagram exhibits a liquid phase with giant number fluctuations at low packing fraction and high self-propulsion speed and a jammed phase at high packing fraction and low self-propulsion speed. The dynamics of the jammed phase is controlled by the low frequency modes of the jammed packing.Comment: 4 pages, 4 figure

Driven flux-line lattices in the presence of weak random columnar disorder: Finite-temperature behavior and dynamical melting of moving Bose glass

We use 3D numerical simulations to explore the phase diagram of driven flux line lattices in presence of weak random columnar disorder at finite temperature and high driving force. We show that the moving Bose glass phase exists in a large range of temperature, up to its melting into a moving vortex liquid. It is also remarkably stable upon increasing velocity : the dynamical transition to the correlated moving glass expected at a critical velocity is not found at any velocity accessible to our simulations. Furthermore, we show the existence of an effective static tin roof pinning potential in the direction transverse to motion, which originates from both the transverse periodicity of the moving lattice and the localization effect due to correlated disorder. Using a simple model of a single elastic line in such a periodic potential, we obtain a good description of the transverse field penetration at surfaces as a function of thickness in the moving Bose glass phase.Comment: 5 pages, 4 figures, New title and minor changes in text and figures. Accepted for publication in Physical Review

Athermal Phase Separation of Self-Propelled Particles with no Alignment

We study numerically and analytically a model of self-propelled polar disks on a substrate in two dimensions. The particles interact via isotropic repulsive forces and are subject to rotational noise, but there is no aligning interaction. As a result, the system does not exhibit an ordered state. The isotropic fluid phase separates well below close packing and exhibits the large number fluctuations and clustering found ubiquitously in active systems. Our work shows that this behavior is a generic property of systems that are driven out of equilibrium locally, as for instance by self propulsion.Comment: 5 pages, 4 figure

Critical behavior of plastic depinning of vortex lattices in two dimensions: Molecular dynamics simulations

Using molecular dynamics simulations, we report a study of the dynamics of two-dimensional vortex lattices driven over a disordered medium. In strong disorder, when topological order is lost, we show that the depinning transition is analogous to a second order critical transition: the velocity-force response at the onset of motion is continuous and characterized by critical exponents. Combining studies at zero and nonzero temperature and using a scaling analysis, two critical expo- nents are evaluated. We find v\sim (F-F_c)^\beta with \beta=1.3\pm0.1 at T=0 and F>F_c, and v\sim T^{1/\delta} with \delta^{-1}=0.75\pm0.1 at F=F_c, where F_c is the critical driving force at which the lattice goes from a pinned state to a sliding one. Both critical exponents and the scaling function are found to exhibit universality with regard to the pinning strength and different disorder realizations. Furthermore, the dynamics is shown to be chaotic in the whole critical region.Comment: 8 pages, 6 figure

Mechanical pressure and momentum conservation in dry active matter

We relate the breakdown of equations of states for the mechanical pressure of generic dry active systems to the lack of momentum conservation in such systems. We show how sources and sinks of momentum arise generically close to confining walls. These typically depend on the interactions of the container with the particles, which makes the mechanical pressure a container-dependent quantity. We show that an equation of state is recovered if the dynamics of the orientation of active particles are decoupled from other degrees of freedom and lead to an apolar bulk steady-state. This is related to the fact that the mean steady-state active force density is the divergence of the flux of "active impulse", an observable which measures the mean momentum particles will receive from the substrate in the future

Structure and mechanics of active colloids

11 pages Acknowledgments MCM thanks Xingbo Yang and Lisa Manning for their contribution to some aspects of the work reviewed here and for fruitful discussions. MCM was supported by NSF-DMR-305184. MCM and AP acknowledge support by the NSF IGERT program through award NSF-DGE-1068780. MCM, AP and DY were additionally supported by the Soft Matter Program at Syracuse University. AP acknowledges use of the Syracuse University HTC Campus Grid which is supported by NSF award ACI-1341006. YF was supported by NSF grant DMR-1149266 and the Brandeis Center for Bioinspired Soft Materials, an NSF MRSEC, DMR-1420382.Peer reviewedPreprin

Behaviour of concrete incorporating ground glass and alkali-reactive aggregates

L'objectif de cette étude doctorale était de mieux comprendre le comportement d'un ajout cimentaire alternatif à haute teneur en alcalis comme le verre broyé (VB) face au phénomène de la réaction alcalis-silice (RAS). Ainsi, des systèmes cimentaires binaires et ternaires composés de ciments de différentes teneurs en alcalis (0.63 à 1.25% Na₂Oeq), de VB (10 - 30%) et d'un des ajouts cimentaires suivants : fumée de silice (FS) (5 - 10%), métakaolin (MK) (5 - 15%), cendre volante (CV) (15 - 30%) ou laitier de hauts fourneaux (LHF) (20 - 40%), ont été utilisés pour la fabrication de prismes de béton et de pâtes cimentaires. Les prismes de béton incorporant un calcaire hautement réactif (Spratt) ont été conditionnés à 38°C avec une H.R.>95% et leur changement dimensionnel suivi pour une période de deux ans. Les échantillons de pâtes cimentaires ont été, quant à eux, conservés dans des contenants hermétiques à 38°C et leur solution interstitielle extraite sous pression et analysée après 28 et 182 jours de conditionnement pour en déterminer la concentration en Na⁺ et K⁺. Contrairement à ce qui est attendu en fonction des connaissances actuelles, les résultats des analyses chimiques indiquent que le sodium du VB n'est pas relâché dans la solution interstitielle en proportion de son contenu original (>13% Na₂O), du moins pour les systèmes ternaires étudiés. L'ajout de VB réduit la [K⁺] de manière plus importante que ne le ferait une simple dilution du ciment, alors que la [OH⁻] est modestement influencée par l'ajout de VB. L'expansion des éprouvettes fabriquées à partir des mélanges ternaires étudiés n'est pas fonction du pourcentage de VB dans le système; d'ailleurs, de nombreux mélanges se trouvent de part et d'autre de la limite de 0.040% de deux ans préconisée par la Pratique Normalisée CSA A23.2-28A. La nature et la proportion de FS/MK/CV/LHF a un impact plus important que la quantité de VB. L'ajout de NaOH au mélange, tel que recommandé pour fins d'accélération lors des essais réalisés en laboratoire, semble réduire l'expansion de nombreux mélanges ternaires incorporant du VB.The aim of this Ph.D. project was to deepen the understanding of the behaviour of a high-alkali Supplementary Cementitious Material (SCM) like Ground Glass (GG) on Alkali-Silica Reaction (ASR). Thus, binary and ternary cementitious systems made of 1) cements of different alkali contents (0.63 to 1.25% Na₂Oeq) 2); GG (10 - 30%) and 3) one of the following supplementary cementitious materials: Silica Fume (SF) (5 - 10%), Metakaolin (MK) (5 - 15%), Fly Ash (FA) (15 - 30%) or Blast Furnace Slag (BFS) (20 - 40%), were used to manufacture concrete prisms and cement pastes. Concrete prisms incorporating a highly reactive limestone (Spratt) were stored at 38°C with a R.H.>95% and their dimensional change monitored for a period of two years. Cement paste samples were stored at 38°C in sealed containers and their pore solution was extracted under pressure and analyzed for their concentration of Na⁺ and K⁺ (28 and 182 days). To the contrary of the expectations based on actual knowledge, the results of the chemical analysis suggest that the sodium of the GG is not released into the pore solution proportionally to its original content (13% Na₂O), at least for the cementitious systems studied. The addition of GG reduces the [K⁺] more significantly than would do a simple cement dilution, while the [OH⁻] is modestly influenced by the addition of GG in the system. The expansion of the prisms cast from the ternary mixtures studied is not a function of the percentage of GG in the system and many expansion results of mixtures are found on either side of the 0.040% two-year limit proposed by CSA Standard Practice A23.2-28A. The nature and proportion of FS/MK/CV/LHF in the system is more significant that the GG content. The addition of NaOH to the mixture, as recommended for acceleration purposes in laboratory tests, appears to reduce the expansion of many ternary mixtures incorporating GG