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Oct 2009

Volume 21, Issue 10, Articles (10xxxx)

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Phys. Fluids 21, 104102 (2009); http://dx.doi.org/10.1063/1.3243976 (13 pages)

Denis Martinand, Eric Serre, and Richard M. Lueptow
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back to top Viscous and Non-Newtonian Flows

Axisymmetric instabilities in electrospinning of highly conducting, viscoelastic polymer solutions

Colman P. Carroll and Yong Lak Joo

Phys. Fluids 21, 103101 (2009); http://dx.doi.org/10.1063/1.3246024 (10 pages) | Cited 9 times

Online Publication Date: 12 October 2009

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In this paper the axisymmetric instabilities observed during the electrospinning of highly electrically conducting, viscoelastic poly(ethylene oxide) (PEO)/water solutions are investigated. In our theoretical study, a linear stability analysis is coupled with a model for the stable electrospun jet. The combined model is used to calculate the expected bead growth rate and wave number for given electrospinning conditions. In the experimental section of the study, PEO/water solutions are electrospun and the formation of axisymmetric beads is captured using high-speed photography. Experimental values for the bead growth rate and wave number are extracted and compared with the model predictions. An energy analysis is then carried out on the stability results to investigate the mechanism of instability via the coupling between base flow and perturbation. The analysis reveals that the unstable axisymmetric mode for electrically driven, highly conducting jets is not a capillary mode, but is mainly driven by electrical forces due to the interaction of charges on the jet. We note that this axisymmetric, conducting mode often exhibits a growth rate too small to be observed during electrospinning. However, both our experiments and stability analysis demonstrate that the axisymmetric instability with a high growth rate can be seen in practice when the electrical force is effectively coupled with viscoelastic forces.
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47.57.Ng Polymers and polymer solutions
47.55.nb Capillary and thermocapillary flows
81.20.-n Methods of materials synthesis and materials processing
47.60.Kz Flows and jets through nozzles

Coating flow of viscous Newtonian liquids on a rotating vertical disk

Nilesh H. Parmar, Mahesh S. Tirumkudulu, and E. J. Hinch

Phys. Fluids 21, 103102 (2009); http://dx.doi.org/10.1063/1.3250858 (8 pages) | Cited 1 time

Online Publication Date: 22 October 2009

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We study a Newtonian viscous liquid coating a vertical rotating disk in the creeping flow regime. Experiments were performed at varying disk rotation speeds and liquid volumes, and the thickness profile at steady state was measured. While the maximum liquid supported by the rotating disk varied with rotation rate and liquid viscosity, the numerical value of a dimensionless number signifying the ratio of gravity to viscous forces was the same in all the cases, γ = 0.30. A lubrication analysis for the time evolution of the film thickness that accounted for gravity, surface tension, and viscous forces was solved numerically to steady state. The predicted thickness profiles are in quantitative agreement with those obtained experimentally. The lubrication equation at steady state was solved analytically in the absence of surface tension to obtain constant height contours that were circular and symmetric about the horizontal axis. However to obtain a complete solution, knowledge of the height variation across the contours is required, and this is controlled by the surface tension. On including this effect, we derived an asymptotic solution to predict thickness profiles that agree well with measurements for large values of viscosity or rotation rates.
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47.15.gm Thin film flows
47.85.mf Lubrication flows
47.32.Ef Rotating and swirling flows
68.03.Cd Surface tension and related phenomena
68.15.+e Liquid thin films

Thermal convection of a viscoplastic liquid with high Rayleigh and Bingham numbers

A. Vikhansky

Phys. Fluids 21, 103103 (2009); http://dx.doi.org/10.1063/1.3256166 (7 pages) | Cited 17 times

Online Publication Date: 28 October 2009

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We consider the effect of yield stress on the Rayleigh–Bénard convection of a viscoplastic material. First we consider the model problem of convection in a differentially heated loop, which is described by the (modified) Lorenz equations. The presence of the yield stress significantly alters the dynamics of the system. In particular, the chaotic motion can stop suddenly (sometimes, after a period of chaotic oscillations). Guided by the model equations we performed direct numerical simulations of convection of the Bingham liquid in a square cavity heated from bellow. Our interest has been concentrated on the situation when both buoyancy and plastic forces are large. The obtained results are in a reasonable agreement with the predictions by the Lorenz equations.
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47.50.-d Non-Newtonian fluid flows
47.52.+j Chaos in fluid dynamics
47.55.pb Thermal convection
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