Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session FV: Minisymposium: Polymeric Fluids in Simple Shear: From Interfacial Slip to Constitutive Discontinuity |
Hide Abstracts |
Chair: Shi-Qing Wang, University of Akron Room: Hilton Chicago Marquette |
Monday, November 21, 2005 8:00AM - 8:26AM |
FV.00001: Stick or Slip? Slippery Questions of Boundary Question in Fluid Dynamics Steve Granick Fluid dynamics within small channels draws great interest due to its fundamental interest on the one hand, and the development of microfluidic devices on the practical side, yet detailed knowledge about flow immediately at a solid surface remains too vague. Some experiments were previously performed in which hydrodynamic flow was compared to predictions using the classical stick boundary condition point to a certain amount of slip, but those measurements (from our own laboratory as well as others) suffered from the deficiency that the slip is inferred indirectly. Seeking to remedy this deficiency, this laboratory has undertaken experiments to measure the near-surface flow rate from direct measurement. Previous attempts to measure surface flow rate were limited to a resolution of the optical wavelength. Here, using fluorescence resonance energy transfer (FRET) and fluorescence quenching approaches, we improve the resolution by 1-2 orders of magnitude. Two different flow systems, hydrodynamic flow and electrokinetic flow, were investigated using this novel technique. [Preview Abstract] |
Monday, November 21, 2005 8:26AM - 8:52AM |
FV.00002: Flow boundary conditions for fluid mixtures at solid walls and moving contact lines Mark Robbins Molecular simulations of slip at solid surfaces have focused on single component systems, but polymers are frequently blended to optimize performance. This talk will examine counterintuitive behavior that can arise when binary fluid mixtures flow past stationary solid walls in simple shear and at moving contact lines. In general the velocities of the two species do not go to zero at the walls. In addition to the slip found for single fluids, there may be velocity discontinuities due to diffusive fluxes and to interfacial forces when there is a concentration gradient.$^1$ Cases where the fluid velocity is largest near the wall and where the apparent slip length diverges will be shown, and a general boundary condition for multi-phase flow presented. The no-slip boundary condition leads to singular dissipation when the contact line between a fluid interface and solid moves, but it was suggested that a diffusive flux could remove this singularity.$^2$ The flow and stress near moving contact lines are analyzed for a range of interfacial widths, velocities and interactions. A significant diffusive flux is only observed in the layer closest to the solid and is not sufficient to remove the singularity. Instead, the finite molecular size and non-Newtonian effects cutoff the singularity.\\ 1. C. Denniston and M. O. Robbins, Phys. Rev. Lett. {\bf 87}, 178302 (2001).\\ 2. H.-Y. Chen and D. Jasnow and J. Vinals, Phys. Rev. Lett. {\bf 85}, 1686 (2000). [Preview Abstract] |
Monday, November 21, 2005 8:52AM - 9:18AM |
FV.00003: Influence of Surface Conditions on the Slip Behavior at Liquid/Solid Interfaces: Comparison Between Molecular-Based Models and Continuum Predictions Sandra Troian Advanced design of micro- and nanofluidic devices requires better understanding of the surface conditions affecting small scale transport. The smaller the device, the more critical are the boundary conditions and resulting frictional losses in degrading performance. Viscous drag can be reduced by enhancing slip at liquid/solid (L/S) interfaces. Recent experiments indicate measureable slip in flow against silanized and topologically textured substrates and in systems conducive to nanobubble nucleation at the L/S interface. Entangled polymer melts tend to generate even larger slip lengths, defined as the extrapolated distance within the solid phase where the tangential flow speed vanishes. While hydrodynamic analyses are useful in providing a continuum description of fluidic response, molecular dynamics (MD) simulations offer detailed resolution of the molecular behavior near chemically or topologically modified surfaces. In this talk we will focus on the slip length of simple and polymeric liquids subject to planar shear at vanishing Reynolds number and investigate the influence of chain length, surface roughness, chemical patterning and shear rate. Direct comparison between hydrodynamic predictions, molecular dynamics (MD) simulations and a molecular based friction model reveals the geometric and molecular parameters influencing slip at different length scales. Excellent agreement between continuum and MD simulations is obtained when the substrate feature size is about an order of magnitude larger than the fluid diameter. Below this limit, we describe how the substrate features cause either an enhancement or reduction in the continuum estimate. For molecular-scale features, a Green-Kubo analysis of the friction coefficient successfully reproduces the MD results for periodic surface roughness. In combination, these continuum and molecular-scale investigations provide a detailed picture of slip spanning multiple length scales. (This work, performed in collaboration with N. V. Priezjev and A. A. Darhuber, is funded by the NSF, the NASA Microgravity Fluid Physics Program and the Princeton Institute for the Science and Technology of Materials.) [Preview Abstract] |
Monday, November 21, 2005 9:18AM - 9:44AM |
FV.00004: Nonlinear rheology of entangled systems: The case of living polymers (micelles) Y. Thomas Hu We have studied shear banding in Couette flow using a combination of particle tracking velocimetry (PTV), small angle light scattering, microscopic visualization, and flow birefringence. Time-resolved local shear rate characterization by PTV has enabled a first direct study of the kinetics of shear banding. A first stage, which precedes banding, is tilting of shear rate during which local shear rate increases towards the inner and decreases towards the outer gap surface. A shear banding stage then proceeds with a low shear band growing away from the outer gap surface. Shear rate tilting is found to be due to a coupling of local shear thinning with the non-zero stress gradient of the flow geometry. The low shear band starts when local shear rate at the outer surface ``touches down'' to a critical re-entanglement shear rate. The effective lifetime of the shear bands is the same as the chain re-entanglement time. These factors lead us to suggest that both the progression from tilt to shear banding and the interface between the low and high shear bands are subject to a common local entanglement / disentanglement criterion. For solutions with surfactant concentration above a critical vlaue, constitutive curves constructed from local shear rates show that there is not a unique stress -- shear rate relation in the shear band coexistence regime, suggesting constitutive instability. The critical entanglement density at which consitutive instability occurs is estimated. The similarities and differences between the rheology of living polymers and that of the conventional polymers are discussed. [Preview Abstract] |
Monday, November 21, 2005 9:44AM - 10:10AM |
FV.00005: From interfacial slip to bulk flow: surprises in non-Newtonian flow of polymers? Shi-Qing Wang All types of flow involve interfacial contact between the confining solid boundary and the fluid. Thus, understanding the nature of the hydrodynamic boundary condition (HBC) is crucial to a realistic description of the fluid mechanics of both simple and structured liquids including polymeric liquids. Polymeric liquids are uniquely capable of violating no-slip HBC on large length scales because of a high level of chain entanglement and not because ``the molecular scale over which slip might occur is large enough to result in macroscopic effects'' [1]. This dynamic structure of chain entanglement appears to make polymeric liquids behave similarly to other yield-stress fluids such as foams, gels, dense suspensions and glassy colloids. In other words, the ``structure'' is breakable by external stress to display yield-like flow. This unanticipated feature of polymeric liquids has begun to produce several big surprises. In this presentation, we will describe these surprising results revealed by a combination of mechanical and optical measurements. Specifically, we show, using a newly developed particle tracking velocimetry, that not only the well-known protocol of controlled-rate seems problematic as a reliable way to delineate the nature of polymer flow, but also questionable is the popular apparatus of cone-plate shear cell in terms of its ability to generate uniform simple shear. Guided by these experimental results we are pursuing the fundamental questions of (a) how to revise the prevailing molecularly based theoretical understanding of entangled polymers, (b) whether the widely used cone-plate flow cell is suitable for exploring flow behavior of various non-Newtonian fluids, (c) what options and choices of experimental apparatus we should equip in any lab working on non-Newtonian flow. \newline \newline [1] M.M. Denn, abstract of APS 2005 March Meeting. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700