223 research outputs found
Fluctuating, Lorentz-force-like coupling of Langevin equations and heat flux rectification
In a description of physical systems with Langevin equations, interacting
degrees of freedom are usually coupled through symmetric parameter matrices.
This coupling symmetry is a consequence of time-reversal symmetry of the
involved conservative forces. If coupling parameters fluctuate randomly, the
resulting noise is called multiplicative. For example, mechanical oscillators
can be coupled through a fluctuating, symmetric matrix of spring "constants".
Such systems exhibit well-studied instabilities. In this note, we study the
complementary case of antisymmetric, time-reversal symmetry breaking coupling
that can be realized with Lorentz forces or various gyrators. We consider the
case that these antisymmetric couplings fluctuate. This type of multiplicative
noise does not lead to instabilities in the stationary state but renormalizes
the effective non-equilibrium friction. Fluctuating Lorentz-force-like
couplings also allow to control and rectify heat transfer. A noteworthy
property of this mechanism of producing asymmetric heat flux is that the
controlling couplings do not exchange energy with the system.
Fluctuating, Lorentz-force-like coupling of Langevin equations and heat flux rectification
In a description of physical systems with Langevin equations, interacting
degrees of freedom are usually coupled through symmetric parameter matrices.
This coupling symmetry is a consequence of time-reversal symmetry of the
involved conservative forces. If coupling parameters fluctuate randomly, the
resulting noise is called multiplicative. For example, mechanical oscillators
can be coupled through a fluctuating, symmetric matrix of spring "constants".
Such systems exhibit well-studied instabilities. In this note, we study the
complementary case of antisymmetric, time-reversal symmetry breaking coupling
that can be realized with Lorentz forces or various gyrators. We consider the
case that these antisymmetric couplings fluctuate. This type of multiplicative
noise does not lead to instabilities in the stationary state but renormalizes
the effective non-equilibrium friction. Fluctuating Lorentz-force-like
couplings also allow to control and rectify heat transfer. A noteworthy
property of this mechanism of producing asymmetric heat flux is that the
controlling couplings do not exchange energy with the system.
Role of the membrane for mechanosensing by tethered channels
Biologically important membrane channels are gated by force at attached
tethers. Here, we generically characterize the non-trivial interplay of force,
membrane tension, and channel deformations that can affect gating. A central
finding is that minute conical channel deformation under force leads to
significant energy release during opening. We also calculate channel-channel
interactions and show that they can amplify force sensitivity of tethered
channels
Rapport fait au nom de la Commission du Marché Commun sur la coopération de la Haute Autorité et des Gouvernements des Etats membres d'après le quatrième Rapport général sur l'activité de la Communauté (1 avril 1955-8 avril 1956) Exercise 1955-1956 Session ordinaire (Seconde partie). = Report on behalf of the Committee on the Common Market on the cooperation of the High Authority and the Member States governments after the Fourth General Report on the Activities of the Community (11 April 1955-8 April 1956). Common Assembly: ordinary session (Second part) Document No 20 1955-1956, June 1956
Efficiency of surface-driven motion: nano-swimmers beat micro-swimmers
Surface interactions provide a class of mechanisms which can be employed for
propulsion of micro- and nanometer sized particles. We investigate the related
efficiency of externally and self-propelled swimmers. A general scaling
relation is derived showing that only swimmers whose size is comparable to, or
smaller than, the interaction range can have appreciable efficiency. An upper
bound for efficiency at maximum power is 1/2. Numerical calculations for the
case of diffusiophoresis are found to be in good agreement with analytical
expressions for the efficiency
Collective force generation by groups of migrating bacteria
From biofilm and colony formation in bacteria to wound healing and embryonic
development in multicellular organisms, groups of living cells must often move
collectively. While considerable study has probed the biophysical mechanisms of
how eukaryotic cells generate forces during migration, little such study has
been devoted to bacteria, in particular with regard to the question of how
bacteria generate and coordinate forces during collective motion. This question
is addressed here for the first time using traction force microscopy. We study
two distinct motility mechanisms of Myxococcus xanthus, namely twitching and
gliding. For twitching, powered by type-IV pilus retraction, we find that
individual cells exert local traction in small hotspots with forces on the
order of 50 pN. Twitching of bacterial groups also produces traction hotspots,
however with amplified forces around 100 pN. Although twitching groups migrate
slowly as a whole, traction fluctuates rapidly on timescales <1.5 min. Gliding,
the second motility mechanism, is driven by lateral transport of substrate
adhesions. When cells are isolated, gliding produces low average traction on
the order of 1 Pa. However, traction is amplified in groups by a factor of ~5.
Since advancing protrusions of gliding cells push on average in the direction
of motion, we infer a long-range compressive load sharing among sub-leading
cells. Together, these results show that the forces generated during twitching
and gliding have complementary characters and both forces are collectively
amplified in groups
Sensory adaptation in a continuum model of bacterial chemotaxis—working range, cost-accuracy relation, and coupled systems
Sensory adaptation enables organisms to adjust their perception in a changing environment. A paradigm is bacterial chemotaxis, where the output activity of chemoreceptors is adapted to different baseline concentrations via receptor methylation. The range of internal receptor states limits the stimulus magnitude to which these systems can adapt. Here, we employ a highly idealized, Langevin-equation based model to study how the finite range of state variables affects the adaptation accuracy and the energy dissipation in individual and coupled systems. Maintaining an adaptive state requires constant energy dissipation. We show that the steady-state dissipation rate increases approximately linearly with the adaptation accuracy for varying stimulus magnitudes in the so-called perfect adaptation limit. This result complements the well-known logarithmic cost-accuracy relationship for varying chemical driving. Next, we study linearly coupled pairs of sensory units. We find that the interaction reduces the dissipation rate per unit and affects the overall cost-accuracy relationship. A coupling of the slow methylation variables results in a better accuracy than a coupling of activities. Overall, the findings highlight the significance of both the working range and collective operation mode as crucial design factors that impact the accuracy and energy expenditure of molecular adaptation networks
Dynamics and efficiency of a self-propelled, diffusiophoretic swimmer
Active diffusiophoresis - swimming through interaction with a self-generated,
neutral, solute gradient - is a paradigm for autonomous motion at the
micrometer scale. We study this propulsion mechanism within a linear response
theory. Firstly, we consider several aspects relating to the dynamics of the
swimming particle. We extend established analytical formulae to describe small
swimmers, which interact with their environment on a finite lengthscale. Solute
convection is also taken into account. Modeling of the chemical reaction
reveals a coupling between the angular distribution of reactivity on the
swimmer and the concentration field. This effect, which we term "reaction
induced concentration distortion", strongly influences the particle speed.
Building on these insights, we employ irreversible, linear thermodynamics to
formulate an energy balance. This approach highlights the importance of solute
convection for a consistent treatment of the energetics. The efficiency of
swimming is calculated numerically and approximated analytically. Finally, we
define an efficiency of transport for swimmers which are moving in random
directions. It is shown that this efficiency scales as the inverse of the
macroscopic distance over which transport is to occur.Comment: 16 pages, 11 figure
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