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 R09: DSOFT 2021 PRIZE sessionLive
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Sponsoring Units: DSOFT Chair: Zvonimir Dogic, University of California, Santa Barbara |
Thursday, March 18, 2021 8:00AM - 8:36AM Live |
R09.00001: Early Career Award for Soft Matter Research (2021): How a functional distribution network builds itself Invited Speaker: Eleni Katifori The basic function of a distribution network is to broadly deliver nutrients, solutes and other load to space. However, distribution networks that are designed to be the most efficient in terms of minimizing construction and operation costs, tend to do poorly at their basic function. In fact, implementing the most efficient design in terms of cost to build and dissipation, results in a single tube running from inlet to outlet - a network that is not space-filling and that leaves large areas without any nutrients. This architecture is in stark contrast to biological vascular networks that fill space and perfuse tissues rather uniformly. To address this apparent paradox, we solve for the solute distribution in a distribution network, and add “equiperfusion”, the equal distribution of nutrients to the “mechanical” objective functions of dissipation and building cost. By minimizing these objective functions together, we derive biologically plausible adaptation rules that rely only on local measurements of the current (or shear stress), the vessel diameter, and nutrient density at each location. We show that a network remodeling under these rules will reach a state which is both cost-effective and space-filling, equally distributing nutrients to the whole domain. We consider two sets of rules, one geometry based, where the network can continuously change the location of the nodes and the vessels but not the lumen diameter, and one topology based, where the geometry remains intact, but vessels can widen or disappear. These rules once more demonstrate how it is possible for biology to build optimal, robust and adaptable structures for a variety of functions with rather limited genetic information, by utilizing self-organization principles. |
Thursday, March 18, 2021 8:36AM - 8:48AM Live |
R09.00002: Flow-driven orientational self-organization of the bronchial epithelium Simon Gsell, Etienne Loiseau, Umberto D'Ortona, Julien Favier, Annie Viallat In human lungs, the long-range oriented transport of mucus by the bronchial epithelium multi-ciliated cells, a process referred to as muco-ciliary clearance, is a key self-defense mechanism protecting the respiratory tract from inhaled particles and pathogens. Yet, the biophysical processes allowing such orientational organization remain unclear. In this work, we show that in-vitro reconstituted human bronchial epithelia exhibit active orientational self-organization driven by the mucus-cilia coupling. The ciliary orientation typically forms circular patterns associated with mucus swirls, on length scales depending on the ciliary density and mucus properties. Numerical simulations are used to investigate the two-dimensional flow driven by streamwise-oriented local forces over a substrate, as a model for ciliary-driven flows. Simulations predict multi-scale swirls and the emergence of a long-range orientational order controlled by the viscosity/friction competition (setting the hydrodynamic length scale) and the ciliary density. Model-based predictions of the effect of the ciliary density on the transport length scale show quantitative similarities with in-vitro measurements. |
Thursday, March 18, 2021 8:48AM - 9:00AM Live |
R09.00003: Wavelength selection by interrupted coarsening in reaction-diffusion systems Fridtjof Brauns, Henrik Weyer, Jacob Halatek, Junghoon Yoon, Erwin Frey Wavelength selection in reaction-diffusion systems can be understood as a coarsening process that is interrupted by counteracting processes at certain wavelengths. We first show that coarsening in mass-conserving systems is driven by self-amplifying mass transport between neighboring high-density domains. We derive a general coarsening criterion and show that coarsening is generically uninterrupted in two-component systems that conserve mass, independently of the specific form of the reaction term. This answers a critical open question, since two-component reaction-diffusion systems with mass conservation serve as important conceputal models for intracellular pattern formation. The theory is then generalized to study interrupted coarsening and anti-coarsening due to weakly-broken mass conservation, providing a general path to analyze wavelength selection in pattern formation far from equilibrium. |
Thursday, March 18, 2021 9:00AM - 9:12AM Live |
R09.00004: Curved crack patterns by drying bacteria suspensions Xiaolei Ma, Zhengyang Liu, Tianyi Lin, Yiming Qiao, Xiang Cheng Particulate suspensions undergoing drying are prone crack as the tensile stress builds up and exceeds a critical value. In contrast to conventional straight desiccation cracks in dried colloidal films, two new types of curved cracks are observed by drying sessile drops of bacteria Escherichia coli (E. coli) in water on glass slides. We use wild-type E. coli with run-and-tumbling motions and mutant E. coli of tumblers in our experiments. After deposition onto the substrate, the first drying front occurs at the drop periphery and moves continuously towards the drop center followed by a secondary drying front moving in the same direction. Spiral-like cracks are finally observed for dried E. coli tumblers, whereas dried wild-type E. coli display circular crack patterns. These results suggest the crucial role of bacterial mobility in the self-assembly of bacteria during drying, which alters the direction of the tensile stress development and consequently leads to cracks of different morphologies. The spacing and its dominant controlling parameters of the observed cracks are also discussed. Our findings shed light on the drying dynamics of bacterial suspensions and can potentially be used to design functional coatings and novel self-assembly structures of active particles. |
Thursday, March 18, 2021 9:12AM - 9:24AM Live |
R09.00005: Tradeoffs between energy efficiency and mechanical response in fluid flow networks Sean Fancher, Eleni Katifori Optimization of transport networks is a ubiquitous problem that can be found in a variety of natural and artificial systems. In the case of systems such as the animal vasculature, the transport of fluids is not only hindered by the inherent resistance to flow but also kept in a dynamic state by the pulsatile nature of the heart and elastic properties of the vessel walls. By linearizing the Navier-Stokes equation, we show that while this imparted pulsatility necessarily increases the dissipation of energy caused by the resistance, the vessel elasticity helps to reduce overall dissipation by attenuating the amplitude of the pulsatile components of the flow. However, we find that this reduction in energy loss comes at the price of increasing the time required to respond to changes in the flow boundary conditions. Dissipation and response time are found to obey a simple power law scaling relation in single vessels as well as hierarchically structured networks with relatively few loops. |
Thursday, March 18, 2021 9:24AM - 9:36AM Live |
R09.00006: Odd Transport in Active Fluids Cory Hargus, Jeffrey Epstein, Kranthi K Mandadapu Active fluids, which are composed of self-propelled particles, are known to exhibit novel transport properties. Familiar laws such as those of Fourier, Fick, and Newton -- describing the linear constitutive behavior governing heat, mass, and momentum transport, respectively -- must be revisited in this context. In particular, the breaking of time-reversal symmetry at the particle scale can dramatically affect transport at the continuum scale. One example is the emergence of so-called odd viscosity in two dimensional chiral active fluids. In this talk, we describe how the statistical mechanical origins of odd viscosity are encapsulated in a set of Green-Kubo relations, which we verify in simulations of a model fluid composed of self-rotating active dumbbells. In this same spirit, we describe consequences of time-reversal symmetry breaking for diffusive transport, identifying common features which span a range of physically-distinct diffusive active systems. |
Thursday, March 18, 2021 9:36AM - 9:48AM Live |
R09.00007: Optimal transport of an active drop Suraj Shankar, Vidya Raju, L. Mahadevan The Monge-Kantorovich problem of optimal mass transport is an old one, with deep connections to optimization theory and inviscid hydrodynamics and a range of applications to image analysis, machine learning etc. But can one use it or its variants to also construct policies to optimally transport real matter that obey complex physical dynamics? We consider the motion of a drop of an active suspension by dynamically controlling the spatial profile of its internal active stress. Within the lubrication approximation, we use optimal control theory to pose and solve the problem of transporting such a drop with minimal expenditure of mechanical work. By parametrizing the position, size and shape of the drop, we uncover a general trade-off that bounds the maximum achievable displacement of the drop by its size, along with bistability in the optimal policies, determined using Pontryagin’s Maximum Principle. Our analysis marries hydrodynamics and optimal control in a tractable and interpretable framework, paving the way forward for the spatio-temporal manipulation of active media. |
Thursday, March 18, 2021 9:48AM - 10:00AM Live |
R09.00008: Are non-dissipative hydrodynamic equations necessarily Hamiltonian? Gustavo Monteiro, Sriram Ganeshan, Alexander Abanov Isotropic fluids in two spatial dimensions can break parity symmetry and sustain transverse stresses which do not lead to dissipation. Corresponding transport coefficients include odd viscosity, odd torque and odd pressure. We consider the most general isotropic Galilean invariant fluid dynamics with momentum and particle density conservation. We limit ourselves to the terms of the second order within some counting scheme. We find exact conditions on transport coefficients which correspond to dissipationless and separately to Hamiltonian fluid dynamics. We find that the dissipationless fluids are not necessarily Hamiltonian. We discuss the consequences of this observation on the structure of effective fluid dynamics defined by reductions from more microscopically refined theories. As a consequence, we identify terms in the stress tensor that can only arise in out-of-equilibrium active matter. |
Thursday, March 18, 2021 10:00AM - 10:12AM Live |
R09.00009: Time-reversal symmetry breaking and odd viscosity in active fluids Jeffrey Epstein, Cory Hargus, Katherine Klymko, Kranthi K Mandadapu Odd viscosity is a non-dissipative transport coefficient that appears in certain two-dimensional active fluids. We argue that the standard discussion of the connection of odd viscosity to time-reversal symmetry breaking involves a mistaken application of Onsager's reciprocal relations, and provide a derivation that circumvents this issue by directly applying Onsager's regression hypothesis. In doing so, we derive a Green-Kubo equation for the odd viscosity in terms of stress fluctuations. These are verified in a system of active dumbbells, thus supporting our application of the regression hypothesis to fluctuations about nonequilibrium steady states. |
Thursday, March 18, 2021 10:12AM - 10:24AM Live |
R09.00010: Pattern formation in active model C: bands, aster networks, and foams Ivan Maryshev, Erwin Frey We study the dynamics of pattern formation in a minimal model for active mixtures made of biofilaments and molecular motors. In particular, we monitor the evolution of the conserved density of filaments and of the non-conserved nematic order parameter, focusing on the effects of an "anchoring" term that provides a direct coupling between the preferred filament direction and their density gradient. The key control parameter is the ratio between activity and elasticity. When elasticity dominates, the interplay between activity and anchoring leads to the formation of banded structures that can undergo additional bending or rotational instabilities. When activity dominates, the nature of anchoring instead gives rise to a range of active cellular solids, including aster-like networks, and disordered foams. We speculate that the introduced "active model C" with anchoring is a minimal model to describe pattern formation in a biomimetic analog of the cytoskeleton. Finally, we demonstrate how the modified active model C can be applied for the investigation of spindle-like structures occurring during the cell division. |
Thursday, March 18, 2021 10:24AM - 10:36AM Not Participating |
R09.00011: Flow-driven vascularization in soft frangible materials Aditi Chakrabarti, S Ganga Prasath, L. Mahadevan Vascularization, the formation of a branched network of vessels, enables the transport of nutrients and removal of wastes in biological organisms. In plants and animals, vascularization is self-organized in response to biochemical and biophysical cues. However, in synthetic organ systems, one of the main challenges in scaling up organoids has been proper vascularization of the tissues. Here, we use flow induced channelization in a crosslinkable gel to enable the controlled formation of synthetic vasculature in soft frangible tissue-like materials. This allows us to control and guide the formation of both divergent and convergent channel patterns, hierarchical formation of thick and thin capillaries, all the while being space-filling. We rationalize our results using a theoretical framework. |
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