On data benchmarking and verification of discrete granular simulations.

Since the seminal work of Cundall and Strack (1979), the Discrete Element Method (DEM) has now become accepted as a key tool amongst researchers exploring the fundamental behavior of granular materials. Along with a sustained increase in the number of publications documenting use of DEM in research, intensive development of new open-source and commercial DEM codes has taken place in the last decades. The credibility of these software packages depends on their capacity to replicate physical observations and to reproduce theoretical expressions. Researchers often calibrate DEM codes against laboratory data to gain confidence about their predictions, however, theoretical verifications at the macro and particle levels are often omitted or not explicitly documented or acknowledged. The validation of DEM codes against theoretical expressions is fundamental to guarantee reproducibility and generality of the software, and to avoid bias in more complex simulations. In this article, a dataset providing numerical simulation data along with input files is presented. The dataset relates to a series of theoretical validation approaches, previously documented in the literature, that were here applied to verify the open-source DEM code LAMMPS. The ability of LAMMPS to capture the macroscopic behaviour of granular packages is evaluated by shearing a face-center-cubic (FCC) array of monosized spheres. The calculation of particle translational/rotational motions and forces/torques is checked by considering a clump rolling down an inclined plane. Additionally, the stress-strain behavior of Toyoura sand under "drained" and "undrained" shearing is characterized by a series of LAMMPS outputs. The dataset collected from these simulations can be employed by users to benchmark new or existing DEM codes. Both the LAMMPS input scripts and the simulation results for all the cases are available in a public repository.

Substrate and cell fusion influence on slime mold network dynamics.

The acellular slime mold Physarum polycephalum provides an excellent model to study network formation, as its network is remodelled constantly in response to mass gain/loss and environmental conditions. How slime molds networks are built and fuse to allow for efficient exploration and adaptation to environmental conditions is still not fully understood. Here, we characterize the network organization of slime molds exploring homogeneous neutral, nutritive and adverse environments. We developed a fully automated image analysis method to extract the network topology and followed the slime molds before and after fusion. Our results show that: (1) slime molds build sparse networks with thin veins in a neutral environment and more compact networks with thicker veins in a nutritive or adverse environment; (2) slime molds construct long, efficient and resilient networks in neutral and adverse environments, whereas in nutritive environments, they build shorter and more centralized networks; and (3) slime molds fuse rapidly and establish multiple connections with their clone-mates in a neutral environment, whereas they display a late fusion with fewer connections in an adverse environment. Our study demonstrates that slime mold networks evolve continuously via pruning and reinforcement, adapting to different environmental conditions.

Modeling root system growth around obstacles.

State-of-the-Art models of Root System Architecture (RSA) do not allow simulating root growth around rigid obstacles. Yet, the presence of obstacles can be highly disruptive to the root system. We grew wheat seedlings in sealed petri dishes without obstacle and in custom 3D-printed rhizoboxes containing obstacles. Time-lapse photography was used to reconstruct the wheat root morphology network. We used the reconstructed wheat root network without obstacle to calibrate an RSA model implemented in the R-SWMS software. The root network with obstacles allowed calibrating the parameters of a new function that models the influence of rigid obstacles on wheat root growth. Experimental results show that the presence of a rigid obstacle does not affect the growth rate of the wheat root axes, but that it does influence the root trajectory after the main axis has passed the obstacle. The growth recovery time, i.e. the time for the main root axis to recover its geotropism-driven growth, is proportional to the time during which the main axis grows along the obstacle. Qualitative and quantitative comparisons between experimental and numerical results show that the proposed model successfully simulates wheat RSA growth around obstacles. Our results suggest that wheat roots follow patterns that could inspire the design of adaptive engineering flow networks.

Transportation networks inspired by leaf venation algorithms.

Biological systems have adapted to environmental constraints and limited resource availability. In the present study, we evaluate the algorithm underlying leaf venation (LV) deployment using graph theory. We compare the traffic balance, travel and cost efficiency of simply-connected LV networks to those of the fan tree and of the spanning tree. We use a Pareto front to show that the total length of leaf venations (LVs) is close to optimal. Then we apply the LV algorithm to design transportation networks in the city of Atlanta. Results show that leaf-inspired models can perform similarly or better than computer-intensive optimization algorithms in terms of network cost and service performance, which could facilitate the design of engineering transportation networks.

Substrate composition directs slime molds behavior.

Cells, including unicellulars, are highly sensitive to external constraints from their environment. Amoeboid cells change their cell shape during locomotion and in response to external stimuli. Physarum polycephalum is a large multinucleated amoeboid cell that extends and develops pseudopods. In this paper, changes in cell behavior and shape were measured during the exploration of homogenous and non-homogenous environments that presented neutral, and nutritive and/or adverse substances. In the first place, we developed a fully automated image analysis method to measure quantitatively changes in both migration and shape. Then we measured various metrics that describe the area covered, the exploration dynamics, the migration rate and the slime mold shape. Our results show that: (1) Not only the nature, but also the spatial distribution of chemical substances affect the exploration behavior of slime molds; (2) Nutritive and adverse substances both slow down the exploration and prevent the formation of pseudopods; and (3) Slime mold placed in an adverse environment preferentially occupies previously explored areas rather than unexplored areas using mucus secretion as a buffer. Our results also show that slime molds migrate at a rate governed by the substrate up until they get within a critical distance to chemical substances.