143 research outputs found
Transport of topological defects in a biphasic mixture of active and passive nematic fluids
Collectively moving cellular systems often contain a proportion of dead cells or non-motile genotypes. When mixed, nematically aligning motile and non-motile agents are known to segregate spontaneously. However, the role that topological defects and active stresses play in shaping the distribution of the two phases remains unresolved. In this study, we investigate the behaviour of a two-dimensional binary mixture of active and passive nematic fluids to understand how topological defects are transported between the two phases and, ultimately, how this leads to the segregation of topological charges. When the activity of the motile phase is large, and the tension at the interface of motile and non-motile phases is weak, we find that the active phase tends to accumulate +1/2 defects and expel −1/2 defects so that the motile phase develops a net positive charge. Conversely, when the activity of the motile phase is comparatively small and interfacial tension is strong, the opposite occurs so that the active phase develops a net negative charge. We then use these simulations to develop a physical intuition of the underlying processes that drive the charge segregation. Lastly, we quantify the sensitivity of this process on the other model parameters, by exploring the effect that anchoring strength, orientational elasticity, friction, and volume fraction of the motile phase have on topological charge segregation. As +1/2 and −1/2 defects have very different effects on interface morphology and fluid transport, this study offers new insights into the spontaneous pattern formation that occurs when motile and non-motile cells interact
Bacteria solve the problem of crowding by moving slowly
Bacteria commonly live attached to surfaces in dense collectives containing billions of cells1. While it is known that motility allows these groups to expand en masse into new territory2,3,4,5, how bacteria collectively move across surfaces under such tightly packed conditions remains poorly understood. Here we combine experiments, cell tracking and individual-based modelling to study the pathogen Pseudomonas aeruginosa as it collectively migrates across surfaces using grappling-hook-like pili3,6,7. We show that the fast-moving cells of a hyperpilated mutant are overtaken and outcompeted by the slower-moving wild type at high cell densities. Using theory developed to study liquid crystals8,9,10,11,12,13, we demonstrate that this effect is mediated by the physics of topological defects, points where cells with different orientations meet one another. Our analyses reveal that when defects with topological charge +1/2 collide with one another, the fast-moving mutant cells rotate to point vertically and become trapped. By moving more slowly, wild-type cells avoid this trapping mechanism and generate collective behaviour that results in faster migration. In this way, the physics of liquid crystals explains how slow bacteria can outcompete faster cells in the race for new territory
Twist-induced crossover from two-dimensional to three-dimensional turbulence in active nematics
While studies of active nematics in two dimensions have shed light on various
aspects of the flow regimes and topology of active matter, three-dimensional
properties of topological defects and chaotic flows remain unexplored. By
confining a film of active nematics between two parallel plates, we use
continuum simulations and analytical arguments to demonstrate that the
crossover from quasi-2D to 3D chaotic flows is controlled by the morphology of
the disclination lines. For small plate separations, the active nematic behaves
as a quasi-2D material, with straight topological disclination lines spanning
the height of the channel and exhibiting effectively 2D active turbulence. Upon
increasing channel height, we find a crossover to 3D chaotic flows due to the
contortion of disclinations above a critical activity. We further show that
these contortions are engendered by twist perturbations producing a sharp
change in the curvature of disclinations.Comment: Accepted for PRE Rapid Communication
Evidence of robust, universal conformal invariance in living biological matter
Collective cellular movement plays a crucial role in many processes
fundamental to health, including development, reproduction, infection, wound
healing, and cancer. The emergent dynamics that arise in these systems are
typically thought to depend on how cells interact with one another and the
mechanisms used to drive motility, both of which exhibit remarkable diversity
across different biological systems. Here, we report experimental evidence of a
universal feature in the patterns of flow that spontaneously emerges in groups
of collectively moving cells. Specifically, we demonstrate that the flows
generated by collectively moving dog kidney cells, human breast cancer cells,
and by two different strains of pathogenic bacteria, all exhibit conformal
invariance. Remarkably, not only do our results show that all of these very
different systems display robust conformal invariance, but we also discovered
that the precise form of the invariance in all four systems is described by the
Schramm-Loewner Evolution (SLE), and belongs to the percolation universality
class. A continuum model of active matter can recapitulate both the observed
conformal invariance and SLE form found in experiments. The presence of
universal conformal invariance reveals that the macroscopic features of living
biological matter exhibit universal translational, rotational, and scale
symmetries that are independent of the microscopic properties of its
constituents. Our results show that the patterns of flows generated by diverse
cellular systems are highly conserved and that biological systems can
unexpectedly be used to experimentally test predictions from the theories for
conformally invariant structure
Verticalization of bacterial biofilms
Biofilms are communities of bacteria adhered to surfaces. Recently, biofilms
of rod-shaped bacteria were observed at single-cell resolution and shown to
develop from a disordered, two-dimensional layer of founder cells into a
three-dimensional structure with a vertically-aligned core. Here, we elucidate
the physical mechanism underpinning this transition using a combination of
agent-based and continuum modeling. We find that verticalization proceeds
through a series of localized mechanical instabilities on the cellular scale.
For short cells, these instabilities are primarily triggered by cell division,
whereas long cells are more likely to be peeled off the surface by nearby
vertical cells, creating an "inverse domino effect". The interplay between cell
growth and cell verticalization gives rise to an exotic mechanical state in
which the effective surface pressure becomes constant throughout the growing
core of the biofilm surface layer. This dynamical isobaricity determines the
expansion speed of a biofilm cluster and thereby governs how cells access the
third dimension. In particular, theory predicts that a longer average cell
length yields more rapidly expanding, flatter biofilms. We experimentally show
that such changes in biofilm development occur by exploiting chemicals that
modulate cell length.Comment: Main text 10 pages, 4 figures; Supplementary Information 35 pages, 15
figure
IGAPS: the merged IPHAS and UVEX optical surveys of the Northern Galactic Plane
The INT Galactic Plane Survey (IGAPS) is the merger of the optical photometric surveys, IPHAS and UVEX, based on data from the Isaac Newton Telescope (INT) obtained between 2003 and 2018. Here, we present the IGAPS point source catalogue. It contains 295.4 million rows providing photometry in the filters, i, r, narrow-band Hα, g, and U_(RGO). The IGAPS footprint fills the Galactic coordinate range, |b| 5σ confidence)
A growing bacterial colony in two dimensions as an active nematic
Rod-shaped bacteria are an example of active matter. Here the authors find that a growing bacterial colony harbours internal cellular flows affecting orientational ordering in its interior and at the boundary. Results suggest this system may belong to a new active matter universality class
IGAPS: the merged IPHAS and UVEX optical surveys of theNorthern Galactic Plane
The INT Galactic Plane Survey (IGAPS) is the merger of the optical
photometric surveys, IPHAS and UVEX, based on data from the Isaac Newton
Telescope (INT) obtained between 2003 and 2018. Here, we present the IGAPS
point source catalogue. It contains 295.4 million rows providing photometry in
the filters, i, r, narrow-band Halpha, g and U_RGO. The IGAPS footprint fills
the Galactic coordinate range, |b| < 5deg and 30deg < l < 215deg. A uniform
calibration, referred to the Pan-STARRS system, is applied to g, r and i, while
the Halpha calibration is linked to r and then is reconciled via field
overlaps. The astrometry in all 5 bands has been recalculated on the Gaia DR2
frame. Down to i ~ 20 mag (Vega system), most stars are also detected in g, r
and Halpha. As exposures in the r band were obtained within the IPHAS and UVEX
surveys a few years apart, typically, the catalogue includes two distinct r
measures, r_I and r_U. The r 10sigma limiting magnitude is ~21, with median
seeing 1.1 arcsec. Between ~13th and ~19th magnitudes in all bands, the
photometry is internally reproducible to within 0.02 magnitudes. Stars brighter
than r=19.5 have been tested for narrow-band Halpha excess signalling line
emission, and for variation exceeding |r_I-r_U| = 0.2 mag. We find and flag
8292 candidate emission line stars and over 53000 variables (both at >5sigma
confidence). The 174-column catalogue will be available via CDS Strasbourg.Comment: 28 pages, 22 figure
MicroMotility: State of the art, recent accomplishments and perspectives on the mathematical modeling of bio-motility at microscopic scales
Mathematical modeling and quantitative study of biological motility (in particular, of motility at microscopic scales) is producing new biophysical insight and is offering opportunities for new discoveries at the level of both fundamental science and technology. These range from the explanation of how complex behavior at the level of a single organism emerges from body architecture, to the understanding of collective phenomena in groups of organisms and tissues, and of how these forms of swarm intelligence can be controlled and harnessed in engineering applications, to the elucidation of processes of fundamental biological relevance at the cellular and sub-cellular level. In this paper, some of the most exciting new developments in the fields of locomotion of unicellular organisms, of soft adhesive locomotion across scales, of the study of pore translocation properties of knotted DNA, of the development of synthetic active solid sheets, of the mechanics of the unjamming transition in dense cell collectives, of the mechanics of cell sheet folding in volvocalean algae, and of the self-propulsion of topological defects in active matter are discussed. For each of these topics, we provide a brief state of the art, an example of recent achievements, and some directions for future research
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