We introduce OpenRAND, a C++17 library aimed at facilitating reproducible scientific research by generating statistically robust yet replicable random numbers in as little as two lines of code, overcoming some of the unnecessary complexities of existing RNG libraries. OpenRAND accommodates single and multi-threaded applications on CPUs and GPUs and offers a simplified, user-friendly API that complies with the C++ standard’s random number engine interface. It is lightweight; provided as a portable, header-only library. It is statistically robust: a suite of built-in tests ensures no pattern exists within single or multiple streams. Despite its simplicity and portability, it remains performant—matching and sometimes outperforming native libraries. Our tests, including a Brownian walk simulation, affirm its reproducibility and ease-of-use while highlight its computational efficiency, outperforming CUDA’s cuRAND by up to 1.8 times.

Microorganisms living in microfluidic environments often form multispecies swarms, where they can leverage collective motions to achieve enhanced transport and spreading. Nevertheless, there is a general lack of physical understandings of the origins of the multi-scale unstable dynamics observed within these systems. In this study, we build a theoretical model to study the hydrodynamic instabilities arising in a dilute mixture of microswimmers that have different propulsion mechanisms and populations. Especially, we consider the scenario of an "activity-balanced" binary suspension that produces zero mean extra stress. We construct a continuum kinetic model that describes the transient dynamics as the system deviates from uniform isotropy. We perform linear stability analyses to show that such binary active suspensions may exhibit rich instability behavior that are far more complex than the single-species cases (e.g., pure pusher or puller suspensions) that have been well studied.

Dense assemblies of self-propelling rods (SPRs) may exhibit fascinating collective behaviors and anomalous physical properties that are far away from equilibrium. Using large-scale Brownian dynamics simulations, we investigate the dynamics of disclination defects in 2D fluidized swarming motions of dense dry SPRs (i.e., without hydrodynamic effects) that form notable local positional topological structures that are reminiscent of smectic order. We find the deformations of smectic-like rod layers can create unique polar structures that lead to slow translations and rotations of ±1/2-order defects, which are fundamentally different from the fast streaming defect motions observed in wet active matter. We measure and characterize the statistical properties of topological defects and reveal their connections with the coherent structures. Furthermore, we construct a bottom-up active-liquid-crystal model to analyze the instability of polar lanes, which effectively leads to defect formation between interlocked polar lanes and serves as the origin of the large-scale swarming motions.

Multispecies swarms are found for microorganisms living in microfluidic environments where they can take advantage of collective motions during transport and spreading. Nevertheless, there is a general lack of physical understandings of the origins of the multiscale unstable dynamics. Here we build a computation model to study the binary suspensions of rear- and front-actuated microswimmers, or respectively the so-called pusher and puller particles, that have different populations and swimming speeds. We perform direct particle simulations to reveal that even in the scenarios of stress-balanced mixtures which produce approximately zero net extra stresses, the longtime dynamics can exhibit non-trivial density fluctuations and spatially-correlated motions. We then construct a continuum kinetic model and perform linear stability analysis to reveal the underlying mechanisms of hydrodynamic instabilities. Our theoretical predictions qualitatively agree with numerical results and explain the onsets of the observed collective motions.

Microorganisms living in microfluidic environments often form multi-species swarms, where they can leverage collective motions to achieve enhanced transport and spreading. Nevertheless, there is a general lack of physical understandings of the origins of the multiscale unstable dynamics observed within these systems. Here, we build a computational model to study binary suspensions of rear- and front-actuated microswimmers, or respectively the so-called "pusher" and "puller" particles, that have different populations and swimming speeds. We perform direct particle simulations to reveal that collective system dynamics are possible even in the scenario of an "activity-balanced" mixture, which produces near zero mean extra stress. We first construct a continuum kinetic model to describe the initial transient period when the system is near uniform isotropy and then perform linear stability analysis to reveal the system's finite-wavelength hydrodynamic instabilities, in contrast with the long-wavelength instabilities of pure pusher/puller suspensions. Then, we carry out slender-body discrete particle simulations to resolve both the short time instabilities and the the longtime dynamics, which feature non-trivial density fluctuations and spatially-correlated motions, distinct from those of single-species.