Enhanced bacterial swimming speeds in macromolecular polymer solutions
Nature Physics (2019)
Abstract:
© 2019, The Author(s), under exclusive licence to Springer Nature Limited. The locomotion of swimming bacteria in simple Newtonian fluids can successfully be described within the framework of low-Reynolds-number hydrodynamics 1 . The presence of polymers in biofluids generally increases the viscosity, which is expected to lead to slower swimming for a constant bacterial motor torque. Surprisingly, however, experiments have shown that bacterial speeds can increase in polymeric fluids 2–5 . Whereas, for example, artificial helical microswimmers in shear-thinning fluids 6 or swimming Caenorhabditis elegans worms in wet granular media 7,8 increase their speeds substantially, swimming Escherichia coli bacteria in polymeric fluids show just a small increase in speed at low polymer concentrations, followed by a decrease at higher concentrations 2,4 . The mechanisms behind this behaviour are currently unclear, and therefore we perform extensive coarse-grained simulations of a bacterium swimming in explicitly modelled solutions of macromolecular polymers of different lengths and densities. We observe an increase of up to 60% in swimming speed with polymer density and demonstrate that this is due to a non-uniform distribution of polymers in the vicinity of the bacterium, leading to an apparent slip. However, this in itself cannot predict the large increase in swimming velocity: coupling to the chirality of the bacterial flagellum is also necessary.Reconfigurable Flows and Defect Landscape of Confined Active Nematics
(2019)
Emergence of Active Nematic Behavior in Monolayers of Isotropic Cells.
Physical review letters 122:4 (2019) 048004-048004
Abstract:
There is now growing evidence of the emergence and biological functionality of liquid crystal features, including nematic order and topological defects, in cellular tissues. However, how such features that intrinsically rely on particle elongation emerge in monolayers of cells with isotropic shapes is an outstanding question. In this Letter, we present a minimal model of cellular monolayers based on cell deformation and force transmission at the cell-cell interface that explains the formation of topological defects and captures the flow-field and stress patterns around them. By including mechanical properties at the individual cell level, we further show that the instability that drives the formation of topological defects, and leads to active turbulence, emerges from a feedback between shape deformation and active driving. The model allows us to suggest new explanations for experimental observations in tissue mechanics, and to propose designs for future experiments.Topological states in chiral active matter: Dynamic blue phases and active half-skyrmions.
The Journal of chemical physics 150:6 (2019) 064909-064909
Abstract:
We numerically study the dynamics of two-dimensional blue phases in active chiral liquid crystals. We show that introducing contractile activity results in stabilised blue phases, while small extensile activity generates ordered but dynamic blue phases characterised by coherently moving half-skyrmions and disclinations. Increasing extensile activity above a threshold leads to the dissociation of the half-skyrmions and active turbulence. We further analyse isolated active half-skyrmions in an isotropic background and compare the activity-induced velocity fields in simulations to an analytical prediction of the flow. Finally, we show that confining an active blue phase can give rise to a system-wide circulation, in which half-skyrmions and disclinations rotate together.Active transport in a channel: stabilisation by flow or thermodynamics.
Soft matter (2019)