Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session A11: Mechanics of Cells and Tissues IFocus Session Live
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Sponsoring Units: DBIO Chair: Moumita Das, Rochester Institute of Technology; Joshua Shaevitz, Princeton University |
Monday, March 15, 2021 8:00AM - 8:36AM Live |
A11.00001: Cilia driven flows: Linking micro- with macroscopic dynamics Invited Speaker: Guillermina Ramirez-San Juan In living organisms arrays of thousands of micrometer-scale motile cilia must coordinate their activity over centimeters to generate flows. These cilia driven flows have diverse properties that are tailored to execute a wide variety of biological functions, ranging from feeding and swimming in single-cell protists to mucus clearance in humans. |
Monday, March 15, 2021 8:36AM - 8:48AM Live |
A11.00002: Planarian Asexual Reproduction via Mechanical Tearing - One Problem, Multiple Solutions Tapan Goel, Danielle Ireland, Vir Shetty, Patrick Henry Diamond, Eva-Maria S Collins Asexual freshwater planarians reproduce by tearing themselves into a head and tail piece (“fission”) using only their musculature and substrate traction. What sets the division plane and how they divide has been a longstanding open question. We have previously explained where and how Dugesia japonica planarians divide using a linear elastic model [1]. Surprisingly, we found that other planarian species - Schmidtea mediterranea and Girardia tigrina, found different solutions to the same problem. Since the 3 species have distinct reproductive strategies, optimizing different parameters (resource allocation to offspring, reproductive waiting time, number of offspring)[2], we hypothesized a link between the fission mechanism on the organismal level and the population level strategy. We quantified the fission dynamics and offspring sizes for all 3 species and found that substrate traction and differences in body shape changes during fission constrain the location of the division plane and thus determine relative resource allocation to offspring. |
Monday, March 15, 2021 8:48AM - 9:00AM Live |
A11.00003: Peristaltic pumping with valves in low Reynolds numbers Aaron Winn, Eleni Katifori Frequently in the vascular systems of living organisms, flow has to propagate against negative pressure gradients. For example, in humans, the pressure provided by the pumping heart alone is not enough to combat gravity and viscous forces and return the blood from the lower limbs back to the heart. Inspired by similar challenges in the lymphatic system, we employ a one-dimensional fluid model to study a pumping mechanism consisting of a viscous fluid in a contracting tube with valves. A simple model for valves is proposed to understand the nonlinear fluid-structure interaction. The valves open and close according to the sign of the pressure drop across the valve, in such a way that promotes unidirectional flow. We begin by demonstrating that oscillations in the pressure or cross-sectional area can lead to positive time-averaged flow even with an overall negative pressure gradient. Peristaltic pumping is modeled by imposing an oscillatory function that represents a contraction wave governing the cross-sectional area of the tube. It is demonstrated that, in some cases, it does not matter which direction the contraction wave propagates when calculating the average flow in the presence of valves. Furthermore, the effect of valve placement on the flow is investigated. |
Monday, March 15, 2021 9:00AM - 9:12AM Live |
A11.00004: Fluid dynamics of flow networks with fluid-storage function Yongtian Luo, Che-Ling Ho, Brent Helliker, Eleni Katifori We study the dynamics of flow networks that are able to store fluid, by theoretically modeling networks with local fluid-storage capacitance. We develop a spatially explicit capacitive model which is able to capture the local changes of flow rate and fluid status (such as water potential and fluid content). This electrical-circuit analogue model is useful for the study of plant leaf hydraulics, in which water-storage capacitance is critical to the resilience and survival of plants in arid conditions such as a drought. Traditionally, lumped-element models of large-scale plant tissues are used for this hydraulic research. We take a novel approach for our modeling, and implement spatially varying capacitors and resistances through fluid pathways (corresponding to leaf xylem, stomatal pores and water-storage cells) to investigate their effects on the local status of the flow networks. We focus on the theoretical findings of our modeling, which reveal the importance of collaboration between capacitance and stomatal control to maintain leaf water status, and we discuss the applicability of the model to grass leaf hydraulics. |
Monday, March 15, 2021 9:12AM - 9:24AM Live |
A11.00005: Vertex Modelling of Cephalic Furrow Formation Redowan Ahmed Niloy, Jeffrey thomas, Jerzy Blawzdziewicz The notion of mechanical cues regulating tissue formation, although, is well known, how the cells orchestrate the information among themselves to generate specific movements is still not fully understood. In the current research, this question has been addressed to discover critical mechanical cues for cephalic furrow formation in Drosophila melanogaster. The 2D Vertex modelling approach has been used to model the relevant segment of the epithelial layer tissue. We have found inverting nature of dynamics in terms of varying stresses along the cellular membrane before and after the invagination occurs. Also the accumulation of increasing stress along the membranes of invaginating cells is another mechanical cue, that we have found critical, to generate the cephalic furrow. |
Monday, March 15, 2021 9:24AM - 9:36AM Live |
A11.00006: Measuring mechanical stress in Myxococcus xanthus monolayers with traction force microscopy Endao Han, Katherine Copenhagen, Joshua Shaevitz During development, a population of Myxococcus xanthus cells transitions from a two-dimensional monolayer to three-dimensional fruiting bodies. The initial motion of cells into the third dimension occurs at special points in the monolayer where there is a topological defect in the cellular alignment field. To understand how these defects are related to three-dimensional motion, we need to observe both how cells move and the relevant mechanical forces. Using traction force microscopy, we map out the spatial distribution of forces exerted by M. xanthus cells on a soft hydrogel substrate and link the position of defects to the distribution of stresses. |
Monday, March 15, 2021 9:36AM - 9:48AM Live |
A11.00007: Wings deployment in Drosophila melanogaster Simon Hadjaje, Ignacio Andrade-Silva, Raphael Clement, Marie-Julie Dalbe, Pierre-Thomas Brun, Joel Marthelot During its final transformation to morph into its adult shape, an insect deploys its wings over just a couple of minutes. The wings rapidly unfold from a wrinkled compact structure to a plane which subsequently solidifies to generate rigidity. We study the wing expansion in Drosophila melanogaster. The expansion is regulated by an increase of the internal pressure and by the injection of a viscous liquid (hemolymph) into a network of deformable veins under hormonal control (Bursicon). We first characterize the kinematic of the deployment through macroscopic observation and quantify the fluid pressure, the fluid flow and the elastic properties of the wing structure during expansion. We then image sections of Drosophila wings using scanning electron microscopy to study the morphological evolution of the cross-section of the wings at different expansion stages. Combining scaling analysis and numerical simulations of the fluid-structure interaction between the viscous loading and the elastic deformation of the structure, we build a fundamental understanding of the dynamic of the wing expansion. |
Monday, March 15, 2021 9:48AM - 10:00AM Live |
A11.00008: Viscoelasticity of Myxococcus xanthus Fruiting Bodies Matthew Black, Joshua Shaevitz When starved, the social bacteria Myxoccocus xanthus mounts a population-level, collective response through the formation of multicellular fruiting bodies. Beginning with the aggregation of motile cells into mounds and culminating in the constituent cells' sporulation, the process of fruiting body formation both provides the environment within which cells sporulate and creates a structure that then buffers the spore population against environmental stresses. We use atomic force microscope-based microrheology to characterize the mechanical state of these structures throughout their development. Doing so reveals that the process of fruiting body formation is mechanically akin to a gelation process wherein nascent mounds of cells show a rheodictic creep response while mature fruiting bodies exhibit more elastic, arrheodictic behavior. We further show that these changes are endogenously driven by energy expenditure of the structure's constituent cells. Finally, we demonstrate that changes in how these cells expend energy—whether that be towards motility at early times or sporulation at later times—governs the transition to a solid, mechanically resilient structure. |
Monday, March 15, 2021 10:00AM - 10:12AM Live |
A11.00009: Fruit Morphogenesis in Arabidopsis thaliana Kurien Parel, Andre Gomez-Felipe, Daniel Kierzkowski, Frederick P. Gosselin, Anne-Lise Routier-Kierzkowska The gynoecium is the female reproductive organ of a plant, that eventually becomes a fruit. While the development of this organ is well studied from a genetic perspective, little is know of the mechanical factors controlling gynoecium growth and form. Here, we investigate how mechanics affect both local deformation and the final organ shape in a growing gynoecium. We propose that organ shape emerges from the differences in specified growth between tissues. The growth incompatibilities between tissues also result in residual stresses, which determine local expansion patterns on the organ surface. The evolution of this organ’s shape is modelled with finite elements, using thermal expansion as a proxy for material growth. We compare simulation results with biological growth values obtained from live imaging of the gynoecium of the model plant Arabidopsis thaliana.. Our work provides some insight into the bio-mechanical factors controlling plant organ shape during development. |
Monday, March 15, 2021 10:12AM - 10:48AM Live |
A11.00010: Award for Outstanding Doctoral Thesis Research in Biological Physics (2020): Vortex arrays and chaotic mixing by swimming invertebrate larvae Invited Speaker: William Gilpin Many marine invertebrates have larval stages covered in linear arrays of beating cilia, which propel the animal while simultaneously entraining planktonic prey. These ciliary bands are strongly conserved, and they are responsible for the unusual morphologies of many invertebrates. However, few studies have investigated their underlying hydrodynamics. Using starfish larvae as a model system, we study the fluid dynamics of ciliary bands, and discover that they create a beautiful pattern of slowly-evolving vortices around swimming invertebrate larvae. Closer inspection of the bands reveals unusual ciliary "tangles" analogous to topological defects that break-up and re-form as the animals adjust their swimming strokes. Quantitative experiments and modeling demonstrate that these vortices create a physical tradeoff between feeding and swimming in heterogenous environments, which manifests as distinct flow patterns representing each behavior. We find that this low-dimensional behavior effectively functions as a stirring protocol, creating intricate fluid dynamical patterns reminiscent of mixing patterns found in chaotic dynamical systems. We use three-dimensional Stokesian dynamics simulations to place these findings in the context of the broader morphospace of invertebrate ciliary bands. We find a quantitative interplay between larval form and hydrodynamic function that generalizes to other invertebrates, providing an example of how physical forces shaped anatomical adaptation by early animals. |
Monday, March 15, 2021 10:48AM - 11:00AM Live |
A11.00011: Environmental Influence on the Ventilation Mechanism of Termite Mounds: A Computational Study Saurabh Saxena, Neda Yaghoobian Termites are famously known for building massive soil-based porous mound structures with intricate internal architectures. The purpose of these mounds is believed to be providing controlled microclimates and effective ventilations in the mound for the termite colony through harnessing wind, solar energy, and the colony’s metabolic heat. This study investigates the underlying physics of the ventilation mechanism within the complex structures of termite mounds. Here, we develop and employ an energy balance-based model that simulates the spatiotemporally variable surface temperatures in high resolutions and dynamically couple it with a computational fluid dynamic (CFD) model to investigate the effect of the diurnally variable surface temperatures on the mound internal flow characteristics. The Navier–Stokes equations are modified in a Direct Numerical Simulation (DNS) using the Darcy-Brinkman-Forchheimer model to represent porosity. The results indicate new insights into the mound’s ventilation process and its internal convective flow features as a result of the diurnal variations in solar heating. |
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