Tracer trajectories and displacement due to a micro-swimmer near a surface

2015 ◽  
Vol 773 ◽  
pp. 498-519 ◽  
Author(s):  
A. J. T. M. Mathijssen ◽  
D. O. Pushkin ◽  
J. M. Yeomans
Keyword(s):  
Particle Transport ◽  
Numerical Study ◽  
Tracer Particle ◽  
Persistence Length ◽  
Theoretical Work ◽  
Solid Boundary ◽  
Tracer Transport ◽  
Tracer Particles ◽  
Recent Theoretical Work ◽  
Run And Tumble

We study tracer particle transport due to flows created by a self-propelled micro-swimmer, such as a swimming bacterium, alga or a microscopic artificial swimmer. Recent theoretical work has shown that as a swimmer moves in the fluid bulk along an infinite straight path, tracer particles far from its path perform closed loops, whereas those close to the swimmer are entrained by its motion. However, in biologically and technologically important cases tracer transport is significantly altered for swimmers that move in a run-and-tumble fashion with a finite persistence length, and/or in the presence of a free surface or a solid boundary. Here we present a systematic analytical and numerical study exploring the resultant regimes and their crossovers. Our focus is on describing qualitative features of the tracer particle transport and developing quantitative tools for its analysis. Our work is a step towards understanding the ecological effects of flows created by swimming organisms, such as enhanced fluid mixing and biofilm formation.

scholarly journals Tracer particle momentum effects in vortex flows

2013 ◽  
Vol 723 ◽  
pp. 665-691 ◽  
Author(s):  
David M. Birch ◽  
Nicholas Martin
Keyword(s):  
Particle Transport ◽  
Tracer Particle ◽  
Classical Model ◽  
Tracking Error ◽  
Selection Criterion ◽  
Tangential Velocity ◽  
Stokes Number ◽  
Radial Component ◽  
Vortex Flows ◽  
Tracer Particles

AbstractThe measurement of vortex flows with particle-image velocimetry (PIV) is particularly susceptible to error arising from the finite mass of the tracer particles, owing to the high velocities and accelerations typically experienced. A classical model of Stokes-flow particle transport is adopted, and an approximate solution for the case of particle transport within an axisymmetric, quasi-two-dimensional Batchelor $q$-vortex is presented. A generalized expression for the maximum particle tracking error is proposed for each of the velocity components, and the importance of finite particle size distributions is discussed. The results indicate that the tangential velocity component is significantly less sensitive to tracking error than the radial component, and that the conventional particle selection criterion (based on the particle Stokes number) may result in either over- or under-sized particles for a specified allowable error bound. Results were demonstrated by means of PIV measurements carried out in air and water using particles with very different properties.

Dispersion resulting from flow through spatically periodic porous media II. Surface and intraparticle transport

1982 ◽  
Vol 307 (1498) ◽  
pp. 149-200 ◽  
Keyword(s):  
Periodic Structures ◽  
Brownian Particle ◽  
Tracer Particle ◽  
Continuous Phase ◽  
Tracer Transport ◽  
Tracer Particles ◽  
Spatially Periodic ◽  
Discontinuous Phase ◽  
External Forces ◽  
Transfer Problems

A rigorous theory of Brownian particle flow and dispersion phenomena in spatially periodic structures is presented within the context of generalized Taylor dispersion theory. The analysis expands upon a prior work, which was limited to transport within the continuous phase, to include convective and diffusive transport of the tracer particle within the interior of the discontinuous phase, as well as surface adsorption and transport along the phase boundary separating the discontinuous and continuous phases. Incorporated within the generalization are considerations of tracer particles of non-zero size, and situations wherein external forces act upon the tracer, the novel effect of each being to cause the tracer to move with a different velocity from that of the fluid in which it is suspended. Applications to various chromatographic separation phenomena are cited. Extensions of the analysis to heat-transfer problems and to situations involving homogeneous, first-order chemical reactions are also made. Both Eulerian and Lagrangian interpretations of the tracer transport phenomena are given.

The light-scattering Mueller matrices for Rayleigh and Rayleigh-Gans-Debye approximation

1990 ◽  
Vol 55 (12) ◽  
pp. 2889-2897
Author(s):  
Jaroslav Holoubek
Keyword(s):  
Light Scattering ◽  
Theoretical Work ◽  
Matrix Elements ◽  
Elastic Light Scattering ◽  
Yield Information ◽  
Recent Theoretical Work ◽  
Particle Parameters ◽  
Complete Set ◽  
Single Matrix ◽  
Scattering Matrix Elements

Recent theoretical work has shown that the complete set of polarized elastic light-scattering studies should yield information about scatterer structure that has so far hardly been utilized. We present here calculations of angular dependences of light-scattering matrix elements for spheres near the Rayleigh and Rayleigh-Gans-Debye limits. The significance of single matrix elements is documented on examples that show how different matrix elements respond to changes in particle parameters. It appears that in the small-particle limit (Rg/λ < 0.1) we do not loose much information by ignoring "large particle" observables.

scholarly journals Magnetospheric Structure of Rotation-Powered Neutron Stars

1992 ◽  
Vol 128 ◽  
pp. 56-77 ◽  
Author(s):  
Jonathan Arons
Keyword(s):  
Radio Emission ◽  
Gamma Radiation ◽  
Heavy Ions ◽  
Theoretical Work ◽  
Future Research ◽  
Observational Constraints ◽  
Polar Caps ◽  
Recent Theoretical Work ◽  
Density Flow ◽  
Promising Avenue

AbstractI survey recent theoretical work on the structure of the magnetospheres of rotation-powered pulsars, within the observational constraints set by their observed spindown, their ability to power synchrotron nebulae and their ability to produce beamed collective radio emission, while putting only a small fraction of their energy into incoherent X- and gamma radiation. I find no single theory has yet given a consistent description of the magnetosphere, but I conclude that models based on a dense outflow of pairs from the polar caps, permeated by a lower density flow of heavy ions, are the most promising avenue for future research.

TRACER TIME DISTRIBUTION OF PARTICLES IN POROUS MEDIA

Fractals ◽  
1993 ◽  
Vol 01 (04) ◽  
pp. 1075-1079
Author(s):  
MARIELA ARAUJO
Keyword(s):  
Porous Medium ◽  
Transit Time ◽  
Power Law ◽  
Time Distribution ◽  
Tracer Particle ◽  
Power Law Distribution ◽  
Low Velocity ◽  
Constant Flow Rate ◽  
Variable Permeability ◽  
Tracer Particles

We study the transit time distributions of tracer particles in a porous medium through which a constant flow rate is established. Our model assumes that non-Gaussian dispersion is due to the presence of low velocity zones or channels in parallel with a faster flow path. Each channel is represented as a trap and simulates the existence of variable permeability blocks inside the porous medium. The time the tracer particle spends inside each channel is related to the heterogeneity of the sample, and is assumed here to have a power-law distribution. We compare the transit time distribution of these particles for the case in which the traps are Poisson distributed with the one in which the trap distribution is a power-law function.

scholarly journals THE EXOTIC BARIUM BISMUTHATES

1996 ◽  
Vol 10 (08) ◽  
pp. 863-955 ◽  
Author(s):  
A. TARAPHDER ◽  
RAHUL PANDIT ◽  
H. R. KRISHNAMURTHY ◽  
T. V. RAMAKRISHNAN
Keyword(s):  
Bound States ◽  
Random Phase ◽  
Density Wave ◽  
Mean Field ◽  
Experimental Tests ◽  
Theoretical Work ◽  
Recent Theoretical Work ◽  
Effects Of Disorder ◽  
Exotic Physics ◽  
Transport Gaps

We review the remarkable properties, including superconductivity, charge-density-wave ordering and metal–insulator transitions, of lead- and potassium-doped barium bismuthate. We will discuss some of the early theoretical studies of these systems. Our recent theoretical work, on the negative-U, extended-Hubbard model for these systems, will also be described. Both the large- and intermediate-U regimes of this model were examined, using mean-field and random-phase approximations, particularly with a view to fitting various experimental properties of these bismuthates. On the basis of our studies, we point out possibilities for exotic physics in these systems. We also emphasize the different consequences of electronic and phonon-mediated mechanisms for the negative U. We show that, for an electronic mechanism, the semiconducting phases of these bismuthates must be unique, with their transport properties dominated by charge±2eCooperon bound states. This can explain the observed difference between the optical and transport gaps. We propose other experimental tests for this novel mechanism of charge transport and comment on the effects of disorder.

scholarly journals Landauer in the age of synthetic biology: energy consumption and information processing in biochemical networks

2015 ◽  
Author(s):  
Pankaj Mehta ◽  
Alex H Lang ◽  
David J Schwab
Keyword(s):  
Information Processing ◽  
Energy Consumption ◽  
Synthetic Biology ◽  
Protein Modification ◽  
Theoretical Work ◽  
Biochemical Networks ◽  
Fundamental Physics ◽  
Recent Theoretical Work ◽  
Cellular Circuits ◽  
Molecular Components

A central goal of synthetic biology is to design sophisticated synthetic cellular circuits that can perform complex computations and information processing tasks in response to specific inputs. The tremendous advances in our ability to understand and manipulate cellular information processing networks raises several fundamental physics questions: How do the molecular components of cellu- lar circuits exploit energy consumption to improve information processing? Can one utilize ideas from thermodynamics to improve the design of synthetic cellular circuits and modules? Here, we summarize recent theoretical work addressing these questions. Energy consumption in cellular cir- cuits serves five basic purposes: (1) increasing specificity, (2) manipulating dynamics, (3) reducing variability, (4) amplifying signal, and (5) erasing memory. We demonstrate these ideas using several simple examples and discuss the implications of these theoretical ideas for the emerging field of synthetic biology. We conclude by discussing how it may be possible to overcome these limitations using “post-translational” synthetic biology that exploits reversible protein modification.

scholarly journals Mixed-polarity random-dot stereograms alter GABA and Glx concentration in the early visual cortex

2019 ◽  
Author(s):  
Reuben Rideaux ◽  
Nuno Goncalves ◽  
Andrew E Welchman
Keyword(s):  
Visual Cortex ◽  
Three Dimensional ◽  
Theoretical Work ◽  
Dimensional Structure ◽  
Resonance Spectroscopy ◽  
Gaba Concentration ◽  
Recent Theoretical Work ◽  
Random Dot Stereograms ◽  
Mixed Polarity ◽  
Early Visual Cortex

ABSTRACTThe offset between images projected onto the left and right retinae (binocular disparity) provides a powerful cue to the three-dimensional structure of the environment. It was previously shown that depth judgements are better when images comprise both light and dark features, rather than only dark or only light elements. Since Harris and Parker (1995) discovered the “mixed-polarity benefit”, there has been limited evidence supporting their hypothesis that the benefit is due to separate bright and dark channels. Goncalves and Welchman (2017) observed that single- and mixed-polarity stereograms evoke different levels of positive and negative activity in a deep neural network trained on natural images to make depth judgements, which also showed the mixed-polarity benefit. Motivated by this discovery, here we seek to test the potential for changes in the balance of excitation and inhibition that are produced by viewing these stimuli. In particular, we use magnetic resonance spectroscopy to measure Glx and GABA concentration in the early visual cortex of adult humans while viewing single- and mixed-polarity random-dot stereograms (RDS). We find that observers’ Glx concentration is significantly higher while GABA concentration is significantly lower when viewing mixed-polarity RDS than when viewing single-polarity RDS. These results indicate that excitation and inhibition facilitate processing of single- and mixed-polarity stereograms in the early visual cortex to different extents, consistent with recent theoretical work (Goncalves & Welchman, 2017).

scholarly journals Imaging Mueller matrix determination of transparent, unpolarizing samples using a classically entangled polarization state

2018 ◽  
Vol 64 (4) ◽  
pp. 407
Author(s):  
Jacqueline Isamar Muro-Ríos ◽  
R. Espinosa-Luna
Keyword(s):  
Open Space ◽  
Polarization State ◽  
Theoretical Work ◽  
Mueller Matrix ◽  
Experimental Setup ◽  
Wave Plate ◽  
Recent Theoretical Work ◽  
Study Results ◽  
Theoretical Proposal

Inspired in a recent theoretical work for the determination of the Mueller matrix, using as incidence a single classically entangled polarization state (F. Töppel et al., New J. Phys. 16 (2014) 073019), an experimental setup is proposed and tested.  The open space and two wave plate retarders are used as the transparent, nondepolarizing samples under study. Results show some experimental improvements are necessary in order to implement accurately the theoretical proposal in which this work is based.

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