arXiv subjects

Michael C. Holcomb

Publications and source records attributed to Michael C. Holcomb.

4 recordsLinked to original sources

arXiv2026

Digging through 2D densely-packed coarse granular media as a critical phenomenon

We create a mechanical digger able to move within 2D densely-packed granular media, in a manner intrinsically different from the existing biomimic diggers. The characteristics of our design include that the average grain size is about one tenth as large as the digger, and the area packing density ϕ for testing is between about 0.610 and 0.762. Unlike conventional autonomous diggers that move by fluidizing the surrounding granular media with a much finer grain size, our digger uses a different mobility mechanism as coarse grains with interparticle friction are hard to be fluidized. To cope with high interparticle friction, the digger has a circular shape and singly captures granular particles near the front entrance of the recess formed by the center unit running across its body and then ejects the captured particles backwards. We validate this moving strategy in both experiments using a manual digger and the corresponding numerical simulations, and demonstrate that the moving efficiency can be enhanced by the judgement of the human operator but the effectiveness is not significantly influenced by the shape of granular particles. In addition, localized vibration is needed to degrade friction between interlocked irregular-shaped particles. Our numerical results show that the distribution of both moving distances and time intervals between consecutive ejections follow a power-law. Reducing ϕ shrinks the spatial and temporal spans over which the power laws hold. Further, we experimentally verify this finding by demonstrating that similar power-laws are observed from an automated digger which periodically randomizes its digging direction. Finding these spatio-temporal power laws indicates that digging within a densely-packed granular environment could be a critical phenomenon.

cond-mat.soft↗

arXiv2021

Enhanced flow rate by the concentration mechanism of Tetris particles when discharged from a hopper with an obstacle

We apply a holistic 2D Tetris-like model, where particles move based on prescribed rules, to investigate the flow rate enhancement from a hopper. This phenomenon was originally reported in the literature as a feature of placing an obstacle at an optimal location near the exit of a hopper discharging athermal granular particles under gravity. We find that this phenomenon is limited to a system of sufficiently many particles. In addition to the waiting room effect, another mechanism able to explain and create the flow rate enhancement is the concentration mechanism of particles on their way to reaching the hopper exit after passing the obstacle. We elucidate the concentration mechanism by decomposing the flow rate into its constituent variables: the local area packing fraction $ϕ_l^E$ and the averaged particle velocity $v_y^E$ at the hopper exit. In comparison to the case without an obstacle, our results show that an optimally placed obstacle can create a net flow rate enhancement of relatively weakly driven particles, caused by the exit-bottleneck coupling if $ϕ_l^E > ϕ_o^c$, where $ϕ_o^c$ is a characteristic area packing fraction marking a transition from fast to slow flow regimes of Tetris particles. Utilizing the concentration mechanism by artificially guiding particles into the central sparse space under the obstacle or narrowing the hopper exit angle under the obstacle, we can create a man-made flow rate peak of relatively strongly-driven particles that initially exhibit no flow rate peak. Additionally, the enhanced flow rate can be maximized by an optimal obstacle shape, particle acceleration rate towards the hopper exit, or exit geometry of the hopper.

cond-mat.soft↗

arXiv2018

Understanding the Local Flow Rate Peak of a Hopper Discharging Discs through an Obstacle Using a Tetris-like Model

Placing a round obstacle above the orifice of a flat hopper discharging uniform frictional discs has been experimentally and numerically shown in the literature to create a local peak in the gravity-driven hopper flow rate. Using frictionless molecular dynamics (MD) simulations, we show that the local peak is unrelated to the interparticle friction, the particle dispersity, and the obstacle geometry. We then construct a probabilistic Tetris-like model, where particles update their positions according to prescribed rules rather than in response to forces, and show that Newtonian dynamics are also not responsible for the local peak. Finally, we propose that the local peak is caused by an interplay between the flow rate around the obstacle, greater than the maximum when the hopper contains no obstacle, and a slow response time, allowing the overflowing particles to converge well upon reaching the hopper orifice.

cond-mat.soft↗

arXiv2016

Embryo as an active granular fluid: stress-coordinated cellular constriction chains

Mechanical stress plays an intricate role in gene expression in individual cells and sculpting of developing tissues. However, systematic methods of studying how mechanical stress and feedback help to harmonize cellular activities within a tissue have yet to be developed. Motivated by our observation of the cellular constriction chains (CCCs) during the initial phase of ventral furrow formation in the Drosophila melanogaster embryo, we propose an active granular fluid (AGF) model that provides valuable insights into cellular coordination in the apical constriction process. In our model, cells are treated as circular particles connected by a predefined force network, and they undergo a random constriction process in which the particle constriction probability P is a function of the stress exerted on the particle by its neighbors. We find that when P favors tensile stress, constricted particles tend to form chain-like structures. In contrast, constricted particles tend to form compact clusters when P favors compression. A remarkable similarity of constricted-particle chains and CCCs observed in vivo provides indirect evidence that tensile-stress feedback coordinates the apical constriction activity. We expect that our particle-based AGF model will be useful in analyzing mechanical feedback effects in a wide variety of morphogenesis and organogenesis phenomena.

cond-mat.soft↗