35 research outputs found
Decomposition of nitric oxide in a hot nitrogen stream to synthesize air for hypersonic wind tunnel combustion testing
A clean source of high enthalpy air was obtained from the exothermic decomposition of nitric oxide in the presence of strongly heated nitrogen. A nitric oxide jet was introduced into a confined coaxial nitrogen stream. Measurements were made of the extent of mixing and reaction. Experimental results are compared with one- and two-dimensional chemical kinetics computations. Both analyses predict much lower reactivity than was observed experimentally. Inlet nitrogen temperatures above 2400 K were sufficient to produce experimentally a completely reacted gas stream of synthetic air
Synchronization by Reactive Coupling and Nonlinear Frequency Pulling
We present a detailed analysis of a model for the synchronization of
nonlinear oscillators due to reactive coupling and nonlinear frequency pulling.
We study the model for the mean field case of all-to-all coupling, deriving
results for the initial onset of synchronization as the coupling or
nonlinearity increase, and conditions for the existence of the completely
synchronized state when all the oscillators evolve with the same frequency.
Explicit results are derived for Lorentzian, triangular, and top-hat
distributions of oscillator frequencies. Numerical simulations are used to
construct complete phase diagrams for these distributions
Synchronization by Nonlinear Frequency Pulling
We analyze a model for the synchronization of nonlinear oscillators due to
reactive coupling and nonlinear frequency pulling motivated by the physics of
arrays of nanoscale oscillators. We study the model for the mean field case of
all-to-all coupling, deriving results for the onset of synchronization as the
coupling or nonlinearity increase, and the fully locked state when all the
oscillators evolve with the same frequency
Stress Generation and Filament Turnover during Actin Ring Constriction
We present a physical analysis of the dynamics and mechanics of contractile actin rings. In particular, we analyze the dynamics of ring contraction during cytokinesis in the Caenorhabditis elegans embryo. We present a general analysis of force balances and material exchange and estimate the relevant parameter values. We show that on a microscopic level contractile stresses can result from both the action of motor proteins, which cross-link filaments, and from the polymerization and depolymerization of filaments in the presence of end-tracking cross-linkers
Sarcomeric Pattern Formation by Actin Cluster Coalescence
Contractile function of striated muscle cells depends crucially on the almost crystalline order of actin and myosin filaments in myofibrils, but the physical mechanisms that lead to myofibril assembly remains ill-defined. Passive diffusive sorting of actin filaments into sarcomeric order is kinetically impossible, suggesting a pivotal role of active processes in sarcomeric pattern formation. Using a one-dimensional computational model of an initially unstriated actin bundle, we show that actin filament treadmilling in the presence of processive plus-end crosslinking provides a simple and robust mechanism for the polarity sorting of actin filaments as well as for the correct localization of myosin filaments. We propose that the coalescence of crosslinked actin clusters could be key for sarcomeric pattern formation. In our simulations, sarcomere spacing is set by filament length prompting tight length control already at early stages of pattern formation. The proposed mechanism could be generic and apply both to premyofibrils and nascent myofibrils in developing muscle cells as well as possibly to striated stress-fibers in non-muscle cells
Continuum theory of contractile fibres
The generation of contractile forces by living cells often
involves linear arrangements of actively interacting polar
filaments. We develop a physical description of the dynamics of
active fibers based on a general expression for the tension in
terms of the filament density and the bundle polarisation. We
discuss the long-time behaviour of oriented and of nonpolar
fibres, discuss effects of polymerization and depolymerization,
and relate this continuum theory to nonlocal descriptions of
filament-motor systems. We show that a nonpolar arrangement of
filaments suppresses oscillatory instabilities which could be
relevant for muscle fibers