Drop formation and breakup of low viscosity elastic fluids: Effects of molecular weight and concentration

Tirtaatmadja, V., McKinley, G. H. and Cooper-White, J. J. (2006) Drop formation and breakup of low viscosity elastic fluids: Effects of molecular weight and concentration. Physics of Fluids, 18 4: Article number 043101. doi:10.1063/1.2190469

Author Tirtaatmadja, V.
McKinley, G. H.
Cooper-White, J. J.
Title Drop formation and breakup of low viscosity elastic fluids: Effects of molecular weight and concentration
Journal name Physics of Fluids   Check publisher's open access policy
ISSN 1070-6631
Publication date 2006-04
Sub-type Article (original research)
DOI 10.1063/1.2190469
Volume 18
Issue 4
Start page Article number 043101
Total pages 18
Editor John Kim
L. Gary Leal
Place of publication College Park, MD, United States
Publisher American Institute of Physics
Collection year 2006
Language eng
Subject 290600 Chemical Engineering
670705 Plastic products (incl. construction materials)
Abstract The dynamics of drop formation and pinch-off have been investigated for a series of low viscosity elastic fluids possessing similar shear viscosities, but differing substantially in elastic properties. On initial approach to the pinch region, the viscoelastic fluids all exhibit the same global necking behavior that is observed for a Newtonian fluid of equivalent shear viscosity. For these low viscosity dilute polymer solutions, inertial and capillary forces form the dominant balance in this potential flow regime, with the viscous force being negligible. The approach to the pinch point, which corresponds to the point of rupture for a Newtonian fluid, is extremely rapid in such solutions, with the sudden increase in curvature producing very large extension rates at this location. In this region the polymer molecules are significantly extended, causing a localized increase in the elastic stresses, which grow to balance the capillary pressure. This prevents the necked fluid from breaking off, as would occur in the equivalent Newtonian fluid. Alternatively, a cylindrical filament forms in which elastic stresses and capillary pressure balance, and the radius decreases exponentially with time. A (0+1)-dimensional finitely extensible nonlinear elastic dumbbell theory incorporating inertial, capillary, and elastic stresses is able to capture the basic features of the experimental observations. Before the critical "pinch time" t(p), an inertial-capillary balance leads to the expected 2/3-power scaling of the minimum radius with time: R-min similar to(t(p)-t)(2/3). However, the diverging deformation rate results in large molecular deformations and rapid crossover to an elastocapillary balance for times t>t(p). In this region, the filament radius decreases exponentially with time R-min similar to exp[(t(p)-t)/lambda(1)], where lambda(1) is the characteristic time constant of the polymer molecules. Measurements of the relaxation times of polyethylene oxide solutions of varying concentrations and molecular weights obtained from high speed imaging of the rate of change of filament radius are significantly higher than the relaxation times estimated from Rouse-Zimm theory, even though the solutions are within the dilute concentration region as determined using intrinsic viscosity measurements. The effective relaxation times exhibit the expected scaling with molecular weight but with an additional dependence on the concentration of the polymer in solution. This is consistent with the expectation that the polymer molecules are in fact highly extended during the approach to the pinch region (i.e., prior to the elastocapillary filament thinning regime) and subsequently as the filament is formed they are further extended by filament stretching at a constant rate until full extension of the polymer coil is achieved. In this highly extended state, intermolecular interactions become significant, producing relaxation times far above theoretical predictions for dilute polymer solutions under equilibrium conditions. (C) 2006 American Institute of Physics
Keyword Mechanics
Physics, Fluids & Plasmas
Elongational Rheometer
Capillary Instability
Viscoelastic Jets
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Non-UQ

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Created: Wed, 15 Aug 2007, 10:39:23 EST