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Sep 2009

Volume 21, Issue 9, Articles (09xxxx)

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Phys. Fluids 21, 091103 (2009); http://dx.doi.org/10.1063/1.3200935 (1 page)

Thomas Cubaud
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Introduction: 26th Annual Gallery of Fluid Motion (San Antonio, Texas, 2008)

Adonios N. Karpetis and Efstathios E. Michaelides

Phys. Fluids 21, 091101 (2009); http://dx.doi.org/10.1063/1.3200912 (1 page)

Online Publication Date: 11 September 2009

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01.10.Cr Announcements, news, and awards
47.00.00 Fluid dynamics
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Cavitation within a droplet

Lammert Heijnen, Pedro Antonio Quinto-Su, Xue Zhao, and Claus Dieter Ohl

Phys. Fluids 21, 091102 (2009); http://dx.doi.org/10.1063/1.3200931 (1 page) | Cited 2 times

Online Publication Date: 11 September 2009

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47.55.dd Bubble dynamics
47.80.Jk Flow visualization and imaging
47.32.Ff Separated flows
47.55.dp Cavitation and boiling
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Wetting and lubricating film instabilities in microchannels

Thomas Cubaud

Phys. Fluids 21, 091103 (2009); http://dx.doi.org/10.1063/1.3200935 (1 page) | Cited 1 time

Online Publication Date: 11 September 2009

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68.15.+e Liquid thin films
68.08.Bc Wetting
47.20.Gv Viscous and viscoelastic instabilities
47.60.Dx Flows in ducts and channels
47.55.D- Drops and bubbles
66.20.-d Viscosity of liquids; diffusive momentum transport
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Optically induced electrokinetic patterning and manipulation of particles

Stuart J. Williams, Aloke Kumar, and Steven T. Wereley

Phys. Fluids 21, 091104 (2009); http://dx.doi.org/10.1063/1.3200938 (1 page) | Cited 2 times

Online Publication Date: 11 September 2009

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Abstract Unavailable
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47.57.jd Electrokinetic effects
47.57.eb Diffusion and aggregation
82.70.Dd Colloids
82.70.Kj Emulsions and suspensions
47.85.Np Fluidics
47.55.-t Multiphase and stratified flows
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Tornadoes in a microchannel

C. L. Perez and J. D. Posner

Phys. Fluids 21, 091105 (2009); http://dx.doi.org/10.1063/1.3205101 (1 page)

Online Publication Date: 11 September 2009

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47.27.-i Turbulent flows
47.20.-k Flow instabilities
47.65.-d Magnetohydrodynamics and electrohydrodynamics
47.60.Dx Flows in ducts and channels
47.57.J- Colloidal systems
47.57.E- Suspensions
47.32.C- Vortex dynamics
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Forest of hairpins in a low-Reynolds-number zero-pressure-gradient flat-plate boundary layer

Xiaohua Wu and Parviz Moin

Phys. Fluids 21, 091106 (2009); http://dx.doi.org/10.1063/1.3205471 (1 page) | Cited 5 times

Online Publication Date: 11 September 2009

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47.27.W- Boundary-free shear flow turbulence
47.32.-y Vortex dynamics; rotating fluids
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Flagellar dynamics in viscous fluids

M. S. Sakar, C. Lee, and P. E. Arratia

Phys. Fluids 21, 091107 (2009); http://dx.doi.org/10.1063/1.3205479 (1 page) | Cited 1 time

Online Publication Date: 11 September 2009

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47.15.-x Laminar flows
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Evaporating cocktails

Sam Dehaeck, Christophe Wylock, and Pierre Colinet

Phys. Fluids 21, 091108 (2009); http://dx.doi.org/10.1063/1.3205483 (1 page) | Cited 2 times

Online Publication Date: 11 September 2009

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Abstract Unavailable
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47.20.Ma Interfacial instabilities (e.g., Rayleigh-Taylor)
47.80.Jk Flow visualization and imaging
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Why don’t mackerels swim like eels? The role of form and kinematics on the hydrodynamics of undulatory swimming

Iman Borazjani and Fotis Sotiropoulos

Phys. Fluids 21, 091109 (2009); http://dx.doi.org/10.1063/1.3205869 (1 page) | Cited 2 times

Online Publication Date: 11 September 2009

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47.11.-j Computational methods in fluid dynamics
02.60.-x Numerical approximation and analysis
47.15.Tr Laminar wakes
66.20.-d Viscosity of liquids; diffusive momentum transport
47.85.Dh Hydrodynamics, hydraulics, hydrostatics
47.63.-b Biological fluid dynamics
47.32.-y Vortex dynamics; rotating fluids
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Capillary origami in nature

Sunghwan Jung, Pedro M. Reis, Jillian James, Christophe Clanet, and John W. M. Bush

Phys. Fluids 21, 091110 (2009); http://dx.doi.org/10.1063/1.3205918 (1 page) | Cited 4 times

Online Publication Date: 11 September 2009

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68.03.Cd Surface tension and related phenomena
47.55.nb Capillary and thermocapillary flows
62.10.+s Mechanical properties of liquids
47.55.D- Drops and bubbles
47.20.-k Flow instabilities
47.35.Bb Gravity waves
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Bursting bubbles

Henri Lhuissier and Emmanuel Villermaux

Phys. Fluids 21, 091111 (2009); http://dx.doi.org/10.1063/1.3200933 (1 page) | Cited 2 times

Online Publication Date: 11 September 2009

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47.55.df Breakup and coalescence
47.20.-k Flow instabilities
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Bubble surface texture and pulsation due to balloon bursting in different liquids

Enrique Soto and Andrew Belmonte

Phys. Fluids 21, 091112 (2009); http://dx.doi.org/10.1063/1.3207864 (1 page)

Online Publication Date: 11 September 2009

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47.55.dd Bubble dynamics
47.35.-i Hydrodynamic waves
47.20.Ma Interfacial instabilities (e.g., Rayleigh-Taylor)
47.80.Jk Flow visualization and imaging
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Non-Poisson statistics of settling spheres

Laurence Bergougnoux and Élisabeth Guazzelli

Phys. Fluids 21, 091701 (2009); http://dx.doi.org/10.1063/1.3231828 (4 pages) | Cited 5 times

Online Publication Date: 22 September 2009

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Direct tracking of the particle positions in a sedimenting suspension indicates that the particles are not simply randomly distributed. The initial mixing of the suspension leads to a microstructure which consists of regions devoid of particles surrounded by regions where particles have an excess of close neighbors and which is maintained during sedimentation.
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47.57.ef Sedimentation and migration
82.70.Kj Emulsions and suspensions
47.51.+a Mixing

Multiple-timescale asymptotic analysis of transient coating flows

C. M. Groh and M. A. Kelmanson

Phys. Fluids 21, 091702 (2009); http://dx.doi.org/10.1063/1.3231847 (4 pages) | Cited 2 times

Online Publication Date: 22 September 2009

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New stability results for the widely studied paradigm “rotating cylinder coating flow” problem are found using a novel multiple-timescale asymptotic approach that is not only fully automated within an algebraic-manipulator platform, but also more widely applicable to diverse evolution equations, particularly those arising in thin-film flow on spatially periodic topographies. Hitherto undiscovered contributions to the capillary decay and gravitational drift in the Fourier modes comprising the coating-film thickness on the cylinder are found, the main discovery being the formal derivation of the functional form of a time-dependent decay rate that has previously been speculated only partially and heuristically. The new asymptotic approach admits analysis of the solution on a geometric progression of increasingly slow timescales, the slowest timescale being a priori dictated in the automated procedure. Theoretical results are in excellent agreement with those obtained from spectrally accurate numerical integrations of the evolution equation for the film thickness. The extent to which the predictions of prior related asymptotic studies are improved upon is quantified.
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47.15.gm Thin film flows
47.15.Fe Stability of laminar flows
47.10.A- Mathematical formulations
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back to top Micro- and Nanofluid Mechanics

Peculiar phenomenon of micro-free-jet flow

Chie Gau, C. H. Shen, and Z. B. Wang

Phys. Fluids 21, 092001 (2009); http://dx.doi.org/10.1063/1.3224012 (13 pages) | Cited 2 times

Online Publication Date: 3 September 2009

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This is the first time that free microjet flow dynamics is explored experimentally in the open literature. The microjet is issued from a microslot nozzle made by microelectromechanical system techniques. Three different sizes of slot nozzles that have the widths of 50, 100, and 200 μm are fabricated. Careful flow visualization and instantaneous velocity measurements at different locations for these jet flows are made at different Reynolds numbers. The results indicate that the vortex formation and merging processes that occurred in the large-scale macrojet are completely absent in the microjet, and the microjet flow phenomenon is drastically different from that of the macrojet. A microjet can penetrate much deeper than a macrojet. A critical Reynolds number for the breakdown of the microjet is determined. Detailed discussion on the microjet dynamics will be presented in this paper.
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47.60.Kz Flows and jets through nozzles
47.80.Jk Flow visualization and imaging
47.32.-y Vortex dynamics; rotating fluids

Transition zone dynamics in combined isotachophoretic and electro-osmotic transport

Friedhelm Schönfeld, Gabriele Goet, Tobias Baier, and Steffen Hardt

Phys. Fluids 21, 092002 (2009); http://dx.doi.org/10.1063/1.3222866 (11 pages) | Cited 6 times

Online Publication Date: 8 September 2009

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The present study focuses on the interplay of isotachophoresis (ITP) and electro-osmotic flow (EOF). While EOF is commonly suppressed in ITP applications, we investigate scenarios of the combination of both EOF and ITP. Experimental results of ITP/EOF experiments within cross-patterned polymer chips show characteristic deformations of fluorescent sample zones sandwiched between leading and trailing electrolytes. A changing curvature of the deformation is observed during ITP/EOF runs, but overall a well defined sample segment is maintained after a transport over a few centimeters. By means of numerical modeling we study the deformation attributed to the mismatch of EOF between leading and trailing electrolytes. The model results are found to qualitatively agree with our experimental findings. We introduce the ratio of the EOF velocities in the leading and trailing electrolyte, expressed via the respective mobilities, as a dimensionless parameter γ and show that in the case where electro-osmotically induced convection dominates over electromigration the deformation width scales as 1−γ. In particular, we find that the EOF-induced dispersion virtually vanishes for the case γ = 1. Hence, in this particular case isotachophoretic self-sharpening and electro-osmotic pumping can be combined without any detrimental effects on sample transport even for large EOF velocities.
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47.65.-d Magnetohydrodynamics and electrohydrodynamics
47.61.-k Micro- and nano- scale flow phenomena
47.85.Np Fluidics
82.45.Gj Electrolytes

History force on coated microbubbles propelled by ultrasound

Valeria Garbin, Benjamin Dollet, Marlies Overvelde, Dan Cojoc, Enzo Di Fabrizio, Leen van Wijngaarden, Andrea Prosperetti, Nico de Jong, Detlef Lohse, and Michel Versluis

Phys. Fluids 21, 092003 (2009); http://dx.doi.org/10.1063/1.3227903 (7 pages) | Cited 5 times

Online Publication Date: 21 September 2009

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In this paper the unsteady translation of coated microbubbles propelled by acoustic radiation force is studied experimentally. A system of two pulsating microbubbles of the type used as contrast agent in ultrasound medical imaging is considered, which attract each other as a result of the secondary Bjerknes force. Optical tweezers are used to isolate the bubble pair from neighboring boundaries so that it can be regarded as if in an unbounded fluid and the hydrodynamic forces acting on the system can be identified unambiguously. The radial and translational dynamics, excited by a 2.25 MHz ultrasound wave, is recorded with an ultrahigh speed camera at 15×106 frames/s. The time-resolved measurements reveal a quasisteady component of the translational velocity, at an average translational Reynolds number 〈Ret〉 ≈ 0.5, and an oscillatory component at the same frequency as the radial pulsations, as predicted by existing models. Since the coating enforces a no-slip boundary condition, an increased viscous dissipation is expected due to the oscillatory component, similar to the case of an oscillating rigid sphere that was first described by Stokes [“On the effect of the internal friction of fluids on the motion of pendulums,” Trans. Cambridge Philos. Soc. 9, 8 (1851) ]. A history force term is therefore included in the force balance, in the form originally proposed by Basset and extended to the case of time-dependent radius by Takemura and Magnaudet [“The history force on a rapidly shrinking bubble rising at finite Reynolds number,” Phys. Fluids 16, 3247 (2004) ]. The instantaneous values of the hydrodynamic forces extracted from the experimental data confirm that the history force accounts for the largest part of the viscous force. The trajectories of the bubbles predicted by numerically solving the equations of motion are in very good agreement with the experiment.
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47.55.dd Bubble dynamics
43.35.Bf Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in liquids, liquid crystals, suspensions, and emulsions
43.25.Qp Radiation pressure
43.25.Nm Acoustic streaming
back to top Interfacial Flows

Damping of linear oscillations in axisymmetric liquid bridges

E. J. Vega and J. M. Montanero

Phys. Fluids 21, 092101 (2009); http://dx.doi.org/10.1063/1.3216566 (8 pages) | Cited 5 times

Online Publication Date: 3 September 2009

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We analyzed experimentally the damping of both axial and lateral free oscillations of small amplitude in axisymmetric liquid bridges. We excited the first oscillation mode in nearly inviscid and in moderately viscous liquid bridges, and measured the parameters which characterize that mode. The axial spatial dependence of those parameters was determined, and the influence of the equilibrium shape on the oscillation frequency and damping rate was analyzed by considering liquid bridges with very different volumes. The experimental results were compared with the solution of the Navier–Stokes equations in the limit of zero viscous Capillary number and of two one-dimensional models. These theoretical approaches predicted accurately the axial spatial dependence of the parameters characterizing the oscillation mode. Comparison with the experimental data showed remarkable agreement for the oscillation frequency, while significant discrepancies were found for the damping rate.
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47.10.A- Mathematical formulations

Two-dimensional droplet spreading over topographical substrates

Nikos Savva and Serafim Kalliadasis

Phys. Fluids 21, 092102 (2009); http://dx.doi.org/10.1063/1.3223628 (16 pages) | Cited 12 times

Online Publication Date: 9 September 2009

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Contact line motion over a topographical substrate is considered using the spreading of a two-dimensional droplet as a model system. The spreading dynamics is modeled under the assumption of small contact angles where the long-wave expansion in the Stokes-flow regime can be employed to derive a single equation for the evolution of the droplet thickness. The contact line singularity is removed through the Navier slip condition, while the contact angle at the contact points is assumed to remain always equal to its static value. Through a singular perturbation approach, the flow in the vicinity of the contact line is matched asymptotically with the flow in the bulk of the droplet to yield a set of two coupled integrodifferential equations for the location of the two droplet fronts. Our matching procedure is verified through direct comparisons with numerical solutions to the full problem. Analysis of the equations obtained by asymptotic matching reveals a number of intriguing features that are not present when the substrate is flat. In particular, we demonstrate the existence of multiple equilibrium states which allows for a hysteresislike effect on the apparent contact line. Further, we demonstrate a stick-slip-type behavior of the contact line as it moves along the local variations of the substrate shape and the interesting possibility of a relatively brief recession of one of the contact lines.
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47.55.dr Interactions with surfaces
68.08.Bc Wetting
68.03.Cd Surface tension and related phenomena

Stretching and slipping of liquid bridges near plates and cavities

Shawn Dodds, Marcio da Silveira Carvalho, and Satish Kumar

Phys. Fluids 21, 092103 (2009); http://dx.doi.org/10.1063/1.3212963 (15 pages) | Cited 9 times

Online Publication Date: 10 September 2009

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The dynamics of liquid bridges are relevant to a wide variety of applications including high-speed printing, extensional rheometry, and floating-zone crystallization. Although many studies assume that the contact lines of a bridge are pinned, this is not the case for printing processes such as gravure, lithography, and microcontacting. To address this issue, we use the Galerkin/finite element method to study the stretching of a finite volume of Newtonian liquid confined between two flat plates, one of which is stationary and the other moving. The steady Stokes equations are solved, with time dependence entering the problem through the kinematic boundary condition. The contact lines are allowed to slip, and we evaluate the effect of the capillary number and contact angle on the amount of liquid transferred to the moving plate. At fixed capillary number, liquid transfer to the moving plate is found to increase as the contact angle on the stationary plate increases relative to that on the moving plate. When the contact angle is fixed and the capillary number is increased, the liquid transfer improves if the stationary plate is wetting, but worsens if it is nonwetting. The presence of a cavity on the stationary plate significantly affects the contact line motion, often causing pinning along the cavity wall. In these cases, liquid transfer is controlled primarily by the cavity shape, suggesting that the effects of surface topography dominate over those of surface wettability. At low capillary numbers, bridge breakup can be understood in terms of the Rayleigh–Plateau stability limit, regardless of the combination of contact angles or the plate geometry. At higher capillary numbers, the bridge is able to stretch beyond this limit although the deviation from this limit appears to depend on contact line pinning, and not directly on the combination of contact angles or the plate geometry.
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47.45.Gx Slip flows and accommodation
02.70.Dh Finite-element and Galerkin methods
47.11.Fg Finite element methods
47.15.Rq Laminar flows in cavities, channels, ducts, and conduits

Relative permeabilities and coupling effects in steady-state gas-liquid flow in porous media: A lattice Boltzmann study

Haibo Huang and Xi-yun Lu

Phys. Fluids 21, 092104 (2009); http://dx.doi.org/10.1063/1.3225144 (10 pages) | Cited 3 times

Online Publication Date: 10 September 2009

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In this paper, the viscous coupling effects for immiscible two-phase (gas-liquid) flow in porous media were studied using the Shan–Chen-type single-component multiphase lattice Boltzmann model. Using the model, the two-phase flows in porous media with density ratio as high as 56 could be simulated and the contact angle of the gas-liquid interface at a solid wall is adjustable. To investigate viscous coupling effects, the co- and countercurrent steady-state two-phase flow patterns and relative permeabilities as a function of wetting saturation were obtained for different capillary numbers, wettabilities, and viscosity ratios. The cocurrent relative permeabilities seem usually larger than the countercurrent ones. The opposing drag-force effect and different pore-level saturation distributions in co- and countercurrent flows may contribute to this difference. It is found that for both co- and countercurrent flows, for strongly wet cases and viscosity ratio M>1, knw increase with the driving force and the viscosity ratio. However, for neutrally wet cases, the variations of knw and kw are more complex. It is also observed that different initial pore-level saturation distributions may affect final steady-state distribution, and hence the relative permeabilities. Using the cocurrent and countercurrent steady flow experiments to determine the generalized relative permeabilities seems not correct.
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47.55.-t Multiphase and stratified flows
47.56.+r Flows through porous media
68.08.Bc Wetting
51.20.+d Viscosity, diffusion, and thermal conductivity
66.20.-d Viscosity of liquids; diffusive momentum transport

An integral boundary layer equation for film flow over inclined wavy bottoms

T. Häcker and H. Uecker

Phys. Fluids 21, 092105 (2009); http://dx.doi.org/10.1063/1.3224858 (15 pages) | Cited 10 times

Online Publication Date: 11 September 2009

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We study the flow of an incompressible liquid film down a wavy incline. Applying a Galerkin method with only one ansatz function to the Navier–Stokes equations, we derive a second-order weighted residual integral boundary layer equation, which, in particular, may be used to describe eddies in the troughs of the wavy bottom. We present numerical results which show that our model is qualitatively and quantitatively accurate in wide ranges of parameters, and we use the model to study some new phenomena, for instance, the occurrence of a short wave instability (at least in a phenomenological sense) for laminar flows which does not exist over a flat bottom.
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47.15.Cb Laminar boundary layers
47.20.Ib Instability of boundary layers; separation
47.10.A- Mathematical formulations

Internal instability of thin liquid sheets

V. Nagabhushana Rao and K. Ramamurthi

Phys. Fluids 21, 092106 (2009); http://dx.doi.org/10.1063/1.3234190 (10 pages) | Cited 1 time

Online Publication Date: 21 September 2009

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Linear stability analysis of an inviscid liquid sheet with different velocity profiles across its thickness is reported. The velocity profiles for which there is a progressive increase or decrease in velocities between the two interfaces are demonstrated to be inherently unstable even in the absence of the destabilizing aerodynamic shear at the liquid-gas interfaces. Compared to a flat velocity profile, a linear or a parabolic profile, symmetric at the center line of the sheet reduced both the maximum growth rate and the wavelength range over which the waves grow. The convective acceleration from the velocity gradient is found to stabilize longer waves while the growth of shorter waves is hampered by the combined effect of the surface tension and a decrease in the interface velocity between gas and liquid media. The wave forms are dominantly sinuous for symmetric velocity profiles; however, with larger velocity gradients the dilatational modes are observed. The inherent instability of liquid sheets with a progressive change in velocities between the interfaces is seen to arise from the differential convective acceleration at the two interfaces in the plane of reference of the liquid sheets.
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47.20.Ma Interfacial instabilities (e.g., Rayleigh-Taylor)
68.15.+e Liquid thin films
68.03.Cd Surface tension and related phenomena
47.35.-i Hydrodynamic waves
47.20.Dr Surface-tension-driven instability

Vortex sheet motion in incompressible Richtmyer–Meshkov and Rayleigh–Taylor instabilities with surface tension

Chihiro Matsuoka

Phys. Fluids 21, 092107 (2009); http://dx.doi.org/10.1063/1.3231837 (15 pages) | Cited 2 times

Online Publication Date: 25 September 2009

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Motion of a planar interface in incompressible Richtmyer–Meshkov (RM) and Rayleigh–Taylor (RT) instabilities with surface tension is investigated numerically by using the boundary integral method. It is shown that when the Atwood number is small, an interface rolls up without regularization of the interfacial velocity. A phenomenon known as “pinching” in the physics of drops is observed in the final stage of calculations at various Atwood numbers and surface tension coefficients, and it is shown that this phenomenon is caused by a vortex dipole induced on the interface. It is also shown that when the surface tension coefficient is large, finite amplitude standing wave solutions exist for the RM instability. This standing wave solution is investigated in detail by nonlinear stability analysis. When gravity is taken into account (RT instability), linearly stable but nonlinearly unstable motion can occur under a critical condition that the frequency of the linear dispersion relation in the system is equal to zero. Further, it is shown that the growth rate of bubbles and spikes under this critical motion is neither of the exponential type nor of the power law type at both the linear stage and the asymptotic stage.
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47.20.Ma Interfacial instabilities (e.g., Rayleigh-Taylor)
47.32.-y Vortex dynamics; rotating fluids
47.55.dd Bubble dynamics
68.03.Cd Surface tension and related phenomena
back to top Particulate, Multiphase, and Granular Flows

Nuclear magnetic resonance measurement of shear-induced particle migration in Brownian suspensions

Jennifer R. Brown, Einar O. Fridjonsson, Joseph D. Seymour, and Sarah L. Codd

Phys. Fluids 21, 093301 (2009); http://dx.doi.org/10.1063/1.3230498 (9 pages) | Cited 4 times

Online Publication Date: 8 September 2009

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The shear-induced migration of colloidal particles in capillary flow has been investigated using nuclear magnetic resonance. Nuclear magnetic resonance methods have the ability to measure spatially resolved velocity and probability distributions of displacement within a multiphase colloidal system. For a suspension of ∼ 2.49 μm Brownian model hard spheres under shear flow in a 1 mm diameter glass capillary, particle migration inward to the capillary center was found using spectrally resolved pulsed gradient spin echo techniques for a range of volume fractions. Particle migration was detected even in the dilute regime, down to ϕ<0.04. While particle migration has been measured and is expected in concentrated and noncolloidal suspensions, it has only recently been unequivocally detected in dilute Brownian suspensions.
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47.57.ef Sedimentation and migration
05.40.Jc Brownian motion
76.60.Lz Spin echoes
82.70.Dd Colloids
82.70.Kj Emulsions and suspensions
47.55.nb Capillary and thermocapillary flows
47.57.J- Colloidal systems
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