It is well known that a binary system of nonactive disks that experience driving in opposite directions exhibits jammed, phase separated, disordered, and laning states. In active matter systems, such as a crowd of pedestrians, driving in opposite directions is common and relevant, especially in conditions which are characterized by high pedestrian density and emergency. In such cases, the transition from laning to disordered states may be associated with the onset of a panic state. We simulate a laning system containing active disks that obey run-and-tumble dynamics, and we measure the drift mobility and structure as a function of run length, disk density, and drift force. The activity of each disk can be quantified based on the correlation timescale of the velocity vector. We find that in some cases, increasing the activity can increase the system mobility by breaking up jammed configurations; however, an activity level that is too high can reduce the mobility by increasing the probability of disk-disk collisions. In the laning state, the increase of activity induces a sharp transition to a disordered strongly fluctuating state with reduced mobility. We identify a novel drive-induced clustered laning state that remains stable even at densities below the activity-induced clustering transition of the undriven system. We map out the dynamic phase diagrams highlighting transitions between the different phases as a function of activity, drive, and density.

Crack initiation emerges due to a combination of elasticity, plasticity, and disorder, and it is heavily dependent on the material's microstructural details. In this paper, we investigate brittle metals with coarse-grained, microstructural disorder that could originate in a material's manufacturing process, such as alloying. As an investigational tool, we consider crack initiation from a surface, ellipsoidal notch: As the radius of curvature at the notch increases, there is a dynamic transition from notch-induced crack initiation to bulkdisorder crack nucleation. We perform extensive and realistic simulations using a phase-field approach coupled to crystal plasticity. Furthermore, the microstructural disorder and notch width are varied in order to study the transition. We identify this transition for various disorder strengths in terms of the damage evolution. Above the transition, we identify detectable precursors to crack initiation that we quantify in terms of the expected stress drops during mode I fracture loading. We discuss ways to observe and analyze this brittle to quasi-brittle transition in experiments.

Effects of disorder in models of active and inactive systems Joshua Thibault Disordered systems can be present in diverse contexts, such as material science, biology and social science. The statistical effects of disorder can vary according to whether the system is active or inactive. In general terms, the activity labels the capacity of local degrees of freedom to generate energy. The focus of this thesis is the demonstration of similarities and differences of disorder effects between active and inactive systems, by focusing on two basic examples. Disorder in inactive systems is understood by focusing on the statistical behavior of a brittle heterogeneous alloy with disorder at the micrometer-scale. In this system, we study how microstructural disorder impacts crack formation and growth characteristics. The Weierstrass-Mandelbrot fractal function is used to generate a fluctuational variation in the critical strain energy release rate, a parameter used to determine damage in the phase field. An ellipsoidal notch is placed at the lateral edge of the sample and the sample is subjected to mode I loading via a defined strain rate. This is an investigational tool to study the dynamic transition from brittle, notch-based crack initiation to quasi-brittle, disorder-driven crack nucleation in the material bulk. This transition is identified for various disorder strengths in terms of the damage evolution and a discussion of the observations and analyses of this brittle to quasi-brittle transition in experiments is presented. Disorder in active systems is explored by focusing on a basic model for bacteria populations, crowds of pedestrians or flocks of birds. A single drive force imparted on all the particles in a system can be approximated as what motivates the collective group's overall motion. The motion that accounts for the individualistic, random behavior of particles in a system is modeled by a runand-tumble, local velocity dependent, force. Run-and-tumble dynamics are considered a sufficient approximation of a particle's individual motion. For this system, a study is conducted in a channeltype flow with bi-directional traffic, analogous to humans in a tunnel-like space or airport terminal. A molecular dynamics model is used to describe the possible collective behaviors that these systems can form through self-organization from a quenched disordered orientation. A dynamic transition in the order of the particles within the simulation is studied. These simulations appear to have two competing effects: the individual motion of the particles due to the run-and-tumble effect and the overall drive force on the group of particles. The four identified phases of the disks as a function of increasing drive force are: a jammed state, a phase separated state, a mixed state, and a laning state. These transitions in phases are corroborated by visible drops in the average velocity of one type of particle and the six-fold disk ordering as a function of the drive force.

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