34,950 research outputs found

    Active transport in a channel: stabilisation by flow or thermodynamics

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    Recent experiments on active materials, such as dense bacterial suspensions and microtubule-kinesin motor mixtures, show a promising potential for achieving self-sustained flows. However, to develop active microfluidics it is necessary to understand the behaviour of active systems confined to channels. Therefore here we use continuum simulations to investigate the behaviour of active fluids in a two-dimensional channel. Motivated by the fact that most experimental systems show no ordering in the absence of activity, we concentrate on temperatures where there is no nematic order in the passive system, so that any nematic order is induced by the active flow. We systematically analyze the results, identify several different stable flow states, provide a phase diagram and show that the key parameters controlling the flow are the ratio of channel width to the length scale of active flow vortices, and whether the system is flow aligning or flow tumbling

    Isochronal synchrony and bidirectional communication with delay-coupled nonlinear oscillators

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    We propose a basic mechanism for isochronal synchrony and communication with mutually delay-coupled chaotic systems. We show that two Ikeda ring oscillators (IROs), mutually coupled with a propagation delay, synchronize isochronally when both are symmetrically driven by a third Ikeda oscillator. This synchronous operation, unstable in the two delay-coupled oscillators alone, facilitates simultaneous, bidirectional communication of messages with chaotic carrier waveforms. This approach to combine both bidirectional and unidirectional coupling represents an application of generalized synchronization using a mediating drive signal for a spatially distributed and internally synchronized multi-component system

    Glycine Betaine Fluxes in Lactobacillus plantarum during Osmostasis and Hyper- and Hypo-osmotic Shock

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    Bacteria respond to changes in medium osmolarity by varying the concentrations of specific solutes in order to maintain constant turgor. The primary response of Lactobacillus plantarum to an osmotic upshock involves the accumulation of compatible solutes such as glycine betaine, proline, and glutamate. We have studied the osmotic regulation of glycine betaine transport in L. plantarum by measuring the overall and unidirectional rates of glycine betaine uptake and exit at osmostasis, and under conditions of osmotic upshock and downshock. At steady state conditions, a basal flux of glycine betaine (but no net uptake or efflux) is observed that amounts to about 20% of the rate of “activated” uptake (uptake at high osmolarity). No direct exchange of 14C-labeled glycine betaine in the medium for unlabeled glycine betaine in the cytoplasm was observed in glucose metabolizing and resting cells, indicating that a separate glycine betaine efflux system is responsible for the exit of glycine betaine. Upon osmotic upshock, the uptake system for glycine betaine is rapidly activated (within seconds), whereas the basal efflux is inhibited. These two responses account for a rapid accumulation of glycine betaine until osmostasis is reached. Upon osmotic downshock, glycine betaine is rapidly released by the cells in a process that has two kinetic components, i.e. one with a half-life of less than 2 s which is unaffected by the metabolic status of the cells, the other with a half-life of 4–5 min in glucose-metabolizing cells which is dependent on internal pH or a related parameter. We speculate that the former activity corresponds to a stretch-activated channel, whereas the latter may be facilitated by a carrier protein. Glycine betaine uptake is strongly inhibited immediately after an osmotic downshock, but slowly recovers in time. These studies demonstrate that in L. plantarum osmostasis is maintained through positive and negative regulation of both glycine betaine uptake and efflux, of which activation of uptake upon osmotic upshock and activation of a “channel-like” activity upon osmotic downshock are quantitatively most important.

    Calculating potentials of mean force and diffusion coefficients from nonequilibirum processes without Jarzynski's equality

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    In general, the direct application of the Jarzynski equality (JE) to reconstruct potentials of mean force (PMFs) from a small number of nonequilibrium unidirectional steered molecular dynamics (SMD) paths is hindered by the lack of sampling of extremely rare paths with negative dissipative work. Such trajectories, that transiently violate the second law, are crucial for the validity of JE. As a solution to this daunting problem, we propose a simple and efficient method, referred to as the FR method, for calculating simultaneously both the PMF U(z) and the corresponding diffusion coefficient D(z) along a reaction coordinate z for a classical many particle system by employing a small number of fast SMD pullings in both forward (F) and time reverse (R) directions, without invoking JE. By employing Crook's transient fluctuation theorem (that is more general than JE) and the stiff spring approximation, we show that: (i) the mean dissipative work W_d in the F and R pullings are equal, (ii) both U(z) and W_d can be expressed in terms of the easily calculable mean work of the F and R processes, and (iii) D(z) can be expressed in terms of the slope of W_d. To test its viability, the FR method is applied to determine U(z) and D(z) of single-file water molecules in single-walled carbon nanotubes (SWNTs). The obtained U(z) is found to be in very good agreement with the results from other PMF calculation methods, e.g., umbrella sampling. Finally, U(z) and D(z) are used as input in a stochastic model, based on the Fokker-Planck equation, for describing water transport through SWNTs on a mesoscopic time scale that in general is inaccessible to MD simulations.Comment: ReVTeX4, 13 pages, 6 EPS figures, Submitted to Journal of Chemical Physic

    Airborne microbial monitoring in an operational cleanroom using an instantaneous detection system and high efficiency microbial samplers

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    The airborne microbial contamination in a non-unidirectional airflow cleanroom, occupied by personnel wearing either full cleanroom attire or only cleanroom undergarments was simultaneously determined using an instantaneous microbial detection (IMD) system and efficient microbial air samplers that detected both aerobic and anaerobic microbes. Depending on the type of cleanroom clothing, the IMD system recorded between 7 to 94 times more ‘biological’ particles than microbe carrying particles (MCPs) recovered by the air samplers. Change in the airborne concentration of ‘biological’ particles due to the different clothing was not consistent with the change in the concentration of MCPs. The median size of the ‘biological’ particles was smaller than the MCPs and the associated particle size distributions were considerably different. A number of sterile materials in the cleanroom were shown to disperse substantial quantities of ‘biological’ particles and it was concluded that the number of particles of microbiological origin, and the relationship between the counts of ‘biological’ particles to MCPs, were masked by non-microbial fluorescent particles dispersed from these materials. Consequently, adequate monitoring of this type of cleanroom operation to confirm appropriate airborne microbiological contamination control, using only an IMD system of the type used for this programme of work, is considered to be unfeasible. However, if the IMD system could be improved to more accurately differentiate between micro-organisms and non-microbial fluorescent particles, or if the dispersion of fluorescent particles from nonmicrobiological cleanroom materials could be reduced, then this system should provide an effective cleanroom airborne monitoring method

    Lattice-Boltzmann Method for Non-Newtonian Fluid Flows

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    We study an ad hoc extension of the Lattice-Boltzmann method that allows the simulation of non-Newtonian fluids described by generalized Newtonian models. We extensively test the accuracy of the method for the case of shear-thinning and shear-thickening truncated power-law fluids in the parallel plate geometry, and show that the relative error compared to analytical solutions decays approximately linear with the lattice resolution. Finally, we also tested the method in the reentrant-flow geometry, in which the shear-rate is no-longer a scalar and the presence of two singular points requires high accuracy in order to obtain satisfactory resolution in the local stress near these points. In this geometry, we also found excellent agreement with the solutions obtained by standard finite-element methods, and the agreement improves with higher lattice resolution
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