@article {669, title = {Fiber plucking by molecular motors yields large emergent contractility in stiff biopolymer networks}, journal = {Soft Matter}, volume = {15}, year = {2019}, month = {Feb-21-2020}, pages = {1481 - 1487}, abstract = {

The mechanical properties of the cell depend crucially on the tension of its cytoskeleton, a biopolymer network that is put under stress by active motor proteins. While the fibrous nature of the network is known to strongly affect the transmission of these forces to the cellular scale, our understanding of this process remains incomplete. Here we investigate the transmission of forces through the network at the individual filament level, and show that active forces can be geometrically amplified as a transverse motor-generated force \"plucks\&$\#$39;\&$\#$39; the fiber and induces a nonlinear tension. In stiff and densely connected networks, this tension results in large network-wide tensile stresses that far exceed the expectation drawn from a linear elastic theory. This amplification mechanism competes with a recently characterized network-level amplification due to fiber buckling, suggesting that that fiber networks provide several distinct pathways for living systems to amplify their molecular forces.

}, issn = {1744-683X}, doi = {10.1039/C8SM00979A}, url = {http://xlink.rsc.org/?DOI=C8SM00979Ahttp://pubs.rsc.org/en/content/articlepdf/2019/SM/C8SM00979A}, author = {Ronceray, Pierre and Broedersz, Chase P. and Lenz, Martin} } @article {668, title = {Spontaneous rotation can stabilise ordered chiral active fluids}, journal = {Nature Communications}, volume = {10}, year = {2019}, month = {Feb-22-2019}, pages = { Article Number: 920 }, abstract = {

Active hydrodynamic theories are a powerful tool to study the emergent ordered phases of internally driven particles such as bird flocks, bacterial suspension and their artificial analogues. While theories of orientationally ordered phases are by now well established, the effect of chirality on these phases is much less studied. In this paper, we present a complete dynamical theory of orientationally ordered chiral particles in two-dimensional incompressible systems. We show that phase-coherent states of rotating chiral particles are remarkably stable in both momentum-conserved and non-conserved systems in contrast to their non-rotating counterparts. Furthermore, defect separation-which drives chaotic flows in non-rotating active fluids-is suppressed by intrinsic rotation of chiral active particles. We thus establish chirality as a source of dramatic stabilisation in active systems, which could be key in interpreting the collective behaviors of some biological tissues, cytoskeletal systems and collections of bacteria.

}, doi = {10.1038/s41467-019-08914-7}, url = {http://www.nature.com/articles/s41467-019-08914-7}, author = {Maitra, Ananyo and Lenz, Martin} } @article {655, title = {Stress-dependent amplification of active forces in nonlinear elastic media}, journal = {Soft Matter}, volume = {15}, year = {2019}, month = {Jan-14-2019}, pages = {331 - 338}, abstract = {

The production of mechanical stresses in living organisms largely relies on localized, force-generating active units embedded in filamentous matrices. Numerical simulations of discrete fiber networks with fixed boundaries have shown that buckling in the matrix dramatically amplifies the resulting active stresses. Here we extend this result to a continuum elastic medium prone to buckling subjected to an arbitrary external stress, and derive analytical expressions for the active, nonlinear constitutive relations characterizing the full active medium. Inserting these relations into popular active gel descriptions of living tissues and the cytoskeleton will enable investigations into nonlinear regimes previously inaccessible due to the phenomenological nature of these theories.

}, issn = {1744-683X}, doi = {10.1039/C8SM00949J}, url = {http://xlink.rsc.org/?DOI=C8SM00949J}, author = {Ronceray, Pierre and Broedersz, Chase P. and Lenz, Martin} } @article {631, title = {A nonequilibrium force can stabilize 2D active nematics}, journal = {Proceedings of the National Academy of Sciences}, volume = {115}, year = {2018}, month = {Jul-03-2018}, pages = {6934-6939}, abstract = {

Suspensions of actively driven anisotropic objects exhibit distinctively nonequilibrium behaviors, and current theories predict that they are incapable of sustaining orientational order at high activity. By contrast, here we show that nematic suspensions on a substrate can display order at arbitrarily high activity due to a previously unreported, potentially stabilizing active force. This force moreover emerges inevitably in theories of active orientable fluids under geometric confinement. The resulting nonequilibrium ordered phase displays robust giant number fluctuations that cannot be suppressed even by an incompressible solvent. Our results apply to virtually all experimental assays used to investigate the active nematic ordering of self-propelled colloids, bacterial suspensions, and the cytoskeleton and have testable implications in interpreting their nonequilibrium behaviors.

}, issn = {0027-8424}, doi = {10.1073/pnas.1720607115}, url = {http://www.pnas.org/lookup/doi/10.1073/pnas.1720607115}, author = {Maitra, Ananyo and Srivastava, Pragya and Marchetti, M. Cristina and Lintuvuori, Juho S. and Ramaswamy, Sriram and Lenz, Martin} }