Motivated by the recent exact solution of the stationary-state Kardar-Parisi-Zhang (KPZ) statistics by Imamura & Sasamoto (Phys. Rev. Lett. 108, 190603 (2012)), as well as a precursor experimental signature unearthed by Takeuchi (Phys. Rev. Lett. 110, 210604 (2013)), we establish here the universality of these phenomena, examining scaling behaviors of directed polymers in a random medium, the stochastic heat equation with multiplicative noise, and kinetically roughened KPZ growth models. We emphasize the value of cross KPZ-Class universalities, revealing crossover effects of experimental relevance. Finally, we illustrate the great utility of KPZ scaling theory by an optimized numerical analysis of the Ulam problem of random permutations.

The Type VI secretion system (T6SS) is a broadly distributed interbacterial weapon that can be used to eliminate competing bacterial populations. Although unarmed target populations are typically used to study T6SS function, bacteria most likely encounter other T6SS-armed competitors in nature. The outcome of such battles is not well understood, neither is the connection between the outcomes with the subcellular details of the T6SS. Here, we incorporated new biological data derived from natural competitors of Vibrio fischeri light organ symbionts to build a biochemical model for T6SS function at the single cell level. The model accounts for activation of structure formation, structure assembly, and deployment. By developing an integrated agent-based model (IABM) that incorporates strain-specific T6SS parameters, we replicated outcomes of biological competitions, validating our approach. We used the IABM to isolate and manipulate strain-specific physiological differences between competitors, in a way that is not possible using biological samples, to identify winning strategies for T6SS-armed populations. We found that a tipping point exists where the cost of building more T6SS weapons outweighs their protective ability. Furthermore, we found that competitions between a T6SS-armed population and a unarmed target had different outcomes dependent on the geometry of the battlefield: target cells survived at the edges of a range expansion scenario where unlimited territory could be claimed, while competitions within a confined space, much like the light organ crypts where natural V. fischeri compete, resulted in the rapid elimination of the unarmed competitor.

We present a numerical method specifically designed for simulating three-dimensional fluid–structure interaction (FSI) problems based on the reference map technique (RMT). The RMT is a fully Eulerian FSI numerical method that allows fluids and large-deformation elastic solids to be represented on a single fixed computational grid. This eliminates the need for meshing complex geometries typical in other FSI approaches and greatly simplifies the coupling between fluid and solids. We develop a three-dimensional implementation of the RMT, parallelized using the distributed memory paradigm, to simulate incompressible FSI with neo-Hookean solids. As part of our method, we develop a field extrapolation scheme that works efficiently in parallel. Through representative examples, we demonstrate the method’s suitability in investigating many-body and active systems, as well as its accuracy and convergence. The examples include settling of a mixture of heavy and buoyant soft ellipsoids, lid-driven cavity flow containing a soft sphere, and swimmers actuated via active stress.

Biological small-angle X-ray solution scattering (BioSAXS) is now widely used to gain information on biomolecules in the solution state. Often, however, it is not obvious in advance whether a particular sample will scatter strongly enough to give useful data to draw conclusions under practically achievable solution conditions. Conformational changes that appear to be large may not always produce scattering curves that are distinguishable from each other at realistic concentrations and exposure times. Emerging technologies such as time-resolved SAXS (TR-SAXS) pose additional challenges owing to small beams and short sample path lengths. Beamline optics vary in brilliance and degree of background scatter, and major upgrades and improvements to sources promise to expand the reach of these methods. Computations are developed to estimate BioSAXS sample intensity at a more detailed level than previous approaches, taking into account flux, energy, sample thickness, window material, instrumental background, detector efficiency, solution conditions and other parameters. The results are validated with calibrated experiments using standard proteins on four different beamlines with various fluxes, energies and configurations. The ability of BioSAXS to statistically distinguish a variety of conformational movements under continuous-flow time-resolved conditions is then computed on a set of matched structure pairs drawn from the Database of Macromolecular Motions (http://molmovdb.org). The feasibility of experiments is ranked according to sample consumption, a quantity that varies by over two orders of magnitude for the set of structures. In addition to photon flux, the calculations suggest that window scattering and choice of wavelength are also important factors given the short sample path lengths common in such setups.