Far-field Dynamics Research Articles

Hydrodynamic interactions between squirmers near walls: far-field dynamics and near-field cluster stability

Confinement increases contacts between microswimmers in dilute suspensions and affects their interactions. In particular, boundaries have been shown experimentally to lead to the formation of clusters that would not occur in bulk fluids. To what extent does hydrodynamics govern these boundary-driven encounters between microswimmers? We consider theoretically the symmetric boundary-mediated encounters of model microswimmers under gravity through far-field interaction of a pair of weak squirmers, as well as the lubrication interactions occurring after contact between two or more squirmers. In the far field, the orientation of microswimmers is controlled by the wall and the squirming parameter. The presence of a second swimmer influences the orientation of the original squirmer, but for weak squirmers, most of the interaction occurs after contact. We thus analyse next the near-field reorientation of circular groups of squirmers. We show that a large number of swimmers and the presence of gravity can stabilize clusters of pullers, while the opposite is true for pushers; to be stable, clusters of pushers thus need to be governed by other interactions (e.g. phoretic). This simplified approach to the phenomenon of active clustering enables us to highlight the hydrodynamic contribution, which can be hard to isolate in experimental realizations.

Calcite twinning strain variations across the Proterozoic Grenville orogen and Keweenaw-Kapuskasing inverted foreland, USA and Canada

We report the calcite twinning strain results of a traverse across the Grenville orogen from Parry Sound, Ontario (NW) to Ft. Ann, New York (SE), including the younger, adjacent Ordovician Taconic allochthon. Fifty four carbonates (marbles, calcite veins, Ordovician limestone) were collected resulting in 68 strain analyses on mechanically twinned calcite (n = 2337 grains) across the Central Gneiss Belt (CGB; 3 samples), the Central Metasedimentary Belt (CMB; 27 samples), the Central Granulite Terrane (CGT; Adirondack's; 13 samples) and the Ottawan Orogenic Lid (OOL; 11 samples). Twinning strains in the greenschist-grade OOL marbles preserve N–S shortening and U-Pb titanite ages (∼1150 Ma; n = 4) document these marbles formed during the Shawinigan (1190–1140 Ma) part of the Grenville orogen. From northwest to southeast, the Ottawan (1095–1020 Ma) twinning strain is dominantly a layer-parallel shortening fabric oriented N–S (Parry Sound), then becomes parallel to the Grenville thrust direction (NW–SE) across the CMB to the Adirondack Highlands where the sub-horizontal shortening strain becomes margin-parallel (SW–NE). Within the regional sample suite there are two areas studied in detail, the Bancroft shear zone (n = 11) and a roadcut on the southeast side of the Adirondack Mountains (Ft. Ann, NY; n = 8). Marbles from the Bancroft shear zone contain calcite grains with 2 sets of twin lamellae (e1 and e2). The better-developed e1 sets (n = 406) record a horizontal fabric oriented NW–SE whereas the younger e2 lamellae (n = 146) preserve a margin-parallel (SW–NE) horizontal fabric. Both the e1 and e2 strains record an overprint vertical shortening strain (NEV), perhaps related to extensional orogenic collapse. We also report an Ottawan orogen-aged granoblastic mylonite (1093 Ma, U-Pb zircon; 1102 Ma Ar-Ar biotite) in the Keweenaw thrust hanging wall 500 km inboard of the Grenville front and interpret the relations of Grenville-Keweenaw far-field dynamics.

Immiscible surfactant droplets on thin liquid films: Spreading dynamics, subphase expulsion and oscillatory instabilities

After deposition of immiscible, surface-active liquids on thin liquid films of higher surface tension, Marangoni stresses thin the liquid film around the surfactant droplet and induce a radially outward flow. We observed an oscillatory instability, caused by temporary trapping and subsequent release of subphase liquid from underneath the surfactant droplet. Height profiles of the thin liquid films were monitored using optical interferometry and fluorescence microscopy, both in the vicinity of the deposited surfactant droplet and at larger distances. Numerical calculations based on the lubrication approximation are compared to the experimental results. Good agreement between the experimental and calculated far-field dynamics and values of the spreading exponents was found.

Characteristics of a Shallow River Plume: Observations from the Saco River Coastal Observing System

Interest in the coastal dynamics of river plumes has mainly focused on large rivers, but plumes from the more numerous smaller rivers have important local consequences and may, in aggregate, be significant contributors to coastal circulation. We studied the dynamics of the plume from the Saco River in Saco Bay, Gulf of Maine, over a 3-year period. The transport and salinity in the region are governed by river discharge, tides, winds, and interaction with the Western Maine Coastal Current. The dynamics of the flow field vary with location within the plume and discharge. The far-field dynamics of the Saco River plume are dominated by inertial processes (hence qualifying it as a small-scale river plume), during times of low discharge, with low salinity water present both up and downstream of the river mouth, but are dominated by rotational processes during times of high discharge (thus qualifying it as a large-scale river plume), with buoyant water primarily advected downshelf. Near-field dynamics are governed by weak, subcritical flow during low discharge but strongly inertial, supercritical flow during high discharge. Offshore movement of the plume is not governed by Ekman dynamics but is instead a result of discharge, wind-induced vertical mixing, and the geography of the coastline and adjacent islands.

Far-field dynamics of tidal energy extraction in channel networks

Tidal hydrokinetic power generation involves the conversion of the kinetic power in swiftly moving tidal currents to renewable electricity. Resource assessment is critical to understand the tidal hydrokinetic potential, but is complicated by a number of factors, including far-field effects. These are changes to the tidal regime caused by the increased resistance to flow as power is extracted from a channel network. This study addresses far-field effects in four prototypical channel networks: multiply-connected flow around an island, a branching network in which the flow bifurcates but does not converge downstream, and a network with multiple constrictions in series. These networks are modelled as one-dimensional channels with hydrokinetic power extraction in high current constrictions. Changes to tides, transport, frictional power dissipation, and kinetic power density are quantified for a range of extraction options. Depending on the type of network, the tidal regime may be either locally augmented or reduced by kinetic power extraction. The changes to kinetic power density throughout the network have important implications for resource assessment, particularly for networks with multiple extraction sites. Results suggest that existing analytical methods tend to over- or under-estimate the hydrokinetic resource because they do not allow for changes to the tidal forcing as a consequence of extraction. In general, site-specific numerical modelling is required to quantitatively predict far-field extraction effects and assess the hydrokinetic resource.

Near-field and far-field dynamics of (Al,In)GaN laser diodes

Near- and far-field dynamics of edge-emitting (Al,In)GaN laser diodes are measured simultaneously with a 100 nm spatial and a 5 ns temporal resolution using a scanning near-field microscope. We reconstruct the phase distribution at the laser diode facet. Beam steering and near-field mode dynamics are interpreted in terms of thermal and carrier induced change of refractive index in the waveguide.

Subrelativistic Jets from Black Hole Accretion Vortices. II. The Broad Absorption Lines in Quasars

This article is an analysis of the broad absorption line properties of a theory of quasars described in a companion paper (Paper I) that differentiates radio-loud quasars from radio-quiet quasars by the magnetic field structure, or lack thereof, in the vortex of an accretion flow around a supermassive rapidly rotating black hole. Nonmagnetic vortices (the radio-quiet case) can initiate radiation pressure-driven jets, as shown in Paper I and these bipolar outflows are candidate broad absorption line regions. The primary emphases of this article are the far-field dynamics and the observational implications of the radiation-driven jets from accretion vortices conjectured to exist in radio-quiet quasars. The well-known confinement problem of the broad line absorbing gas is solved by these hypersonic underexpanded jets as they are collimated kinematically with an opening angle of the order of the inverse of the Mach number. The asymptotic jet velocities are found by a detailed analysis of the force of gravity, continuum radiation pressure, and line-driving forces on the jet plasma. The dynamics are explored through time stationary solutions. However, the general solution is considered to be a superposition of episodic quasistationary funnel ejecta as the accretion flow is most likely dynamically unsteady. The final state outflow is a series of geometrically thin clouds with a distribution of velocities ranging from 0 to ? 35,000 km s-1. The most significant implication of this model is that radio-loud quasars (magnetic flux filled accretion vortices) do not have broad absorption line outflows.