425 research outputs found
Casimir stresses in active nematic films
We calculate the Casimir stresses in a thin layer of active fluid with nematic order. By using a stochastic hydrodynamic approach for an active fluid layer of finite thickness L, we generalize the Casimir stress for nematic liquid crystals in thermal equilibrium to active systems. We show that the active Casimir stress differs significantly from its equilibrium counterpart. For contractile activity, the active Casimir stress, although attractive like its equilibrium counterpart, diverges logarithmically as L approaches a threshold of the spontaneous flow instability from below. In contrast, for small extensile activity, it is repulsive, has no divergence at any L and has a scaling with L different from its equilibrium counterpart
Droplet Ripening in Concentration Gradients
Living cells use phase separation and concentration gradients to organize
chemical compartments in space. Here, we present a theoretical study of droplet
dynamics in gradient systems. We derive the corresponding growth law of
droplets and find that droplets exhibit a drift velocity and position dependent
growth. As a consequence, the dissolution boundary moves through the system,
thereby segregating droplets to one end. We show that for steep enough
gradients, the ripening leads to a transient arrest of droplet growth that is
induced by an narrowing of the droplet size distribution.Comment: 12 pages, 4 figure
Extreme-value statistics of stochastic transport processes
We derive exact expressions for the finite-time statistics of extrema (maximum and minimum) of the spatial displacement and the fluctuating entropy flow of biased random walks. Our approach captures key features of extreme events in molecular motor motion along linear filaments. For one-dimensional biased random walks, we derive exact results which tighten bounds for entropy production extrema obtained with martingale theory and reveal a symmetry between the distribution of the maxima and minima of entropy production. Furthermore, we show that the relaxation spectrum of the full generating function, and hence of any moment, of the finite-time extrema distributions can be written in terms of the Marcenko-Pastur distribution of random-matrix theory. Using this result, we obtain efficient estimates for the extreme-value statistics of stochastic transport processes from the eigenvalue distributions of suitable Wishart and Laguerre random matrices. We confirm our results with numerical simulations of stochastic models of molecular motors
Determining physical properties of the cell cortex
Actin and myosin assemble into a thin layer of a highly dynamic network
underneath the membrane of eukaryotic cells. This network generates the forces
that drive cell and tissue-scale morphogenetic processes. The effective
material properties of this active network determine large-scale deformations
and other morphogenetic events. For example,the characteristic time of stress
relaxation (the Maxwell time)in the actomyosin sets the time scale of
large-scale deformation of the cortex. Similarly, the characteristic length of
stress propagation (the hydrodynamic length) sets the length scale of slow
deformations, and a large hydrodynamic length is a prerequisite for long-ranged
cortical flows. Here we introduce a method to determine physical parameters of
the actomyosin cortical layer (in vivo). For this we investigate the relaxation
dynamics of the cortex in response to laser ablation in the one-cell-stage {\it
C. elegans} embryo and in the gastrulating zebrafish embryo. These responses
can be interpreted using a coarse grained physical description of the cortex in
terms of a two dimensional thin film of an active viscoelastic gel. To
determine the Maxwell time, the hydrodynamic length and the ratio of active
stress and per-area friction, we evaluated the response to laser ablation in
two different ways: by quantifying flow and density fields as a function of
space and time, and by determining the time evolution of the shape of the
ablated region. Importantly, both methods provide best fit physical parameters
that are in close agreement with each other and that are similar to previous
estimates in the two systems. We provide an accurate and robust means for
measuring physical parameters of the actomyosin cortical layer.It can be useful
for investigations of actomyosin mechanics at the cellular-scale, but also for
providing insights in the active mechanics processes that govern tissue-scale
morphogenesis.Comment: 17 pages, 4 figure
Lipid membranes with an edge
Consider a lipid membrane with a free exposed edge. The energy describing
this membrane is quadratic in the extrinsic curvature of its geometry; that
describing the edge is proportional to its length. In this note we determine
the boundary conditions satisfied by the equilibria of the membrane on this
edge, exploiting variational principles. The derivation is free of any
assumptions on the symmetry of the membrane geometry. With respect to earlier
work for axially symmetric configurations, we discover the existence of an
additional boundary condition which is identically satisfied in that limit. By
considering the balance of the forces operating at the edge, we provide a
physical interpretation for the boundary conditions. We end with a discussion
of the effect of the addition of a Gaussian rigidity term for the membrane.Comment: 8 page
Energy and matter supply for active droplets
Chemically active droplets provide simple models for cell-like systems that
can grow and divide. Such active droplet systems are driven away from
thermodynamic equilibrium and turn over chemically, which corresponds to a
simple metabolism. We consider two scenarios of non-equilibrium driving. First,
droplets are driven via the system boundaries by external reservoirs that
supply nutrient and remove waste (boundary-driven). Second, droplets are driven
by a chemical energy provided by a fuel in the bulk (bulk-driven). For both
scenarios, we discuss the conservation of energy and matter as well as the
balance of entropy. We use conserved and non-conserved fields to analyze the
non-equilibrium steady states of active droplets. Using an effective droplet
model, we explore droplet stability and instabilities leading to droplet
division. Our work reveals that droplet division occurs quite generally in
active droplet systems. Our results suggest that life-like processes such as
metabolism and division can emerge in simple non-equilibrium systems that
combine the physics of phase separation and chemical reactions
Heat fluctuations in chemically active systems
Chemically active systems such as living cells are maintained out of thermal equilibrium due to chemical events which generate heat and lead to active fluctuations. A key question is to understand on which time and length scales active fluctuations dominate thermal fluctuations. Here, we formulate a stochastic field theory with Poisson white noise to describe the heat fluctuations which are generated by stochastic chemical events and lead to active temperature fluctuations. We find that on large length- and timescales, active fluctuations always dominate thermal fluctuations. However, at intermediate length- and timescales, multiple crossovers exist which highlight the different characteristics of active and thermal fluctuations. Our work provides a framework to characterize fluctuations in active systems and reveals that local equilibrium holds at certain length- and timescales
Salt-dependent rheology and surface tension of protein condensates using optical traps
An increasing number of proteins with intrinsically disordered domains have
been shown to phase separate in buffer to form liquid-like phases. These
protein condensates serve as simple models for the investigation of the more
complex membrane-less organelles in cells. To understand the function of such
proteins in cells, the material properties of the condensates they form are
important. However, these material properties are not well understood. Here, we
develop a novel method based on optical traps to study the frequency-dependent
rheology and the surface tension of PGL-3 condensates as a function of salt
concentration. We find that PGL-3 droplets are predominantly viscous but also
exhibit elastic properties. As the salt concentration is reduced, their elastic
modulus, viscosity and surface tension increase. Our findings show that salt
concentration has a strong influence on the rheology and dynamics of protein
condensates suggesting an important role of electrostatic interactions for
their material properties.Comment: 5 pages, 3 figures, 1 supplemen
Theory of nematic and polar active fluid surfaces
We derive a fully covariant theory of the hydrodynamics of nematic and polar active surfaces, subjected to internal and external forces and torques. We study the symmetries of polar and nematic surfaces and find that in addition to five different types of in-plane isotropic surfaces, polar and nematic surfaces can be classified into five polar, two pseudopolar, five nematic and two pseudonematic types of surfaces. We give examples of physical realisations of the different types of surfaces we have identified. We obtain expressions for the equilibrium tensions, moments, and external forces and torques acting on a passive polar or nematic surface. We calculate the entropy production rate using the framework of thermodynamics close to equilibrium and find constitutive equations for polar and nematic active surfaces with different symmetries. We study the instabilities of a confined flat planar-chiral polar active layer and of a confined deformable polar active surface with broken up-down symmetry
Chemically Active Wetting
Wetting of liquid droplets on passive surfaces is ubiquitous in our daily
lives, and the governing physical laws are well-understood. When surfaces
become active, however, the governing laws of wetting remain elusive. Here we
propose chemically active wetting as a new class of active systems where the
surface is active due to a binding process that is maintained away from
equilibrium. We derive the corresponding non-equilibrium thermodynamic theory
and show that active binding fundamentally changes the wetting behavior,
leading to steady, non-equilibrium states with droplet shapes reminiscent of a
pancake or a mushroom. The origin of such anomalous shapes can be explained by
mapping to electrostatics, where pairs of binding sinks and sources correspond
to electrostatic dipoles along the triple line. This is an example of a more
general analogy, where localized chemical activity gives rise to a multipole
field of the chemical potential. The underlying physics is relevant for cells,
where droplet-forming proteins can bind to membranes accompanied by the
turnover of biological fuels
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