Rheological Properties of High-Viscosity Solutions of Long Molecules

Garner, F. H.; Nissan, A. H.

doi:10.1038/158634a0

Cited by 64 publications

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“…The diagonal elements of the stress tensor exhibit anisotropy in entangled polymer melts, which can lead to novel effects such as rod climbing [15]. Under simple shear, the low-Re form of the stress tensor for an incompressible viscoelastic fluid is [16] s ij pd ij …”

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confidence: 99%

Vorticity Elongation in Polymeric Emulsions

Hobbie

Migler

1999

Phys. Rev. Lett.

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Video microscopy is used to measure the shear-induced deformation of dilute emulsions composed of viscoelastic polymer melts. In the limit of strong shear, a transition in which the droplets elongate perpendicular to the flow field is observed. A force-balance argument relates this behavior to the change in first normal stress difference across the droplet interface. [S0031-9007(99)09504-6] PACS numbers: 83.50. Ax, 47.55.Dz, 61.25.Hq, 83.70.Hq When a fluid dispersed in a second fluid is subjected to shear, the droplets deform and burst, and the use of shear flow to emulsify immiscible liquids is ubiquitous in the processing of soft materials. Although the pioneering work of Taylor [1-4] on isolated Newtonian emulsions remains at the foundation of our understanding, with the underlying intuition that the droplets elongate along the direction of flow at sufficiently high shear rates, the shear response of fluids that exhibit rheological complexity, such as semidilute entangled polymer solutions [5][6][7][8], wormlike micelles [9], and thixotropic clay gels [10], suggests that the shear can induce domains with extended correlation along the direction of vorticity, perpendicular to the flow field. In this Letter, stroboscopic video microscopy is used to measure the shear response of dilute emulsions composed of viscoelastic polymer melts. At low shear rates, the behavior is in agreement with the Taylor picture. In the limit of strong shear, however, the droplets become extended along the vorticity axis, and a simple forcebalance argument is presented that relates this unusual transition to the change in first normal stress difference across the droplet interface.The pressure-driven optical flow cell [11] is shown schematically in Fig. 1. The instrument easily achieves shear rates at which elastic stresses in the melt become much larger than the shear stress, which typically cannot be achieved in a rotating-geometry device. The shear rate ͑ ᠨ g͒ is a function of the distance from one of the walls, and the measurements presented here were taken at the surface of the bottom window [12]. The materials are polystyrene (PS) and polyethylene (PE) at T 195 ± C, with molecular weights [13] M w 195 3 10 3 ͑M n 83 3 10 3 ͒ for PS 1 , M w 342 3 10 3 ͑M n 141 3 10 3 ͒ for PS 2 , M w 85.2 3 10 3 ͑M n 15.4 3 10 3 ͒ for PE 1 , and M w 83.1 3 10 3 ͑M n 25.6 3 10 3 ͒ for PE 2 , where PE 1 is low-density with long-chain branching and PE 2 is linear high-density. The melt rheology at 195 ± C was measured independently in a rotating-plate rheometer, and the shear viscosity is shown in Fig. 2. Under small-amplitude oscillatory shear, the storage modulus, G 0 ͑v͒, exceeds the loss modulus, G 00 ͑v͒, at v c 8, 30, 700, and 800 rad͞s for PS 1 , PS 2 , PE 1 , and PE 2 , respectively. The extruded blends are 0.01 mass fraction PS 1 in PE 1 , PS 1 in PE 2 , and PS 2 in PE 1 .We observe a variety of droplet shapes as a function of ᠨ g and the viscosity ratio l ͑h 0 ͒ PS ͑͞h 0 ͒ PE , where h 0 is the low-ᠨ g viscosity. For PS 2 ͞PE 1 (l 240), ...

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confidence: 99%

Vorticity Elongation in Polymeric Emulsions

Hobbie

Migler

1999

Phys. Rev. Lett.

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“…The postulate of flow-induced diffusion has emerged out of the conviction that the diffusion of macromolecules is governed by a generalized chemical potential gradient [6,[21][22][23][24][25]. Accordingly, in inhomogeneous stress fields, spatial entr0py variations induce concentration changes as the system tends toward equalization of the free energy gradients irnplied by deformation induced entropy gradients [6,7,[21][22][23][24][25]. In capillary tube flow, for example, macromolecules will migrate away from the wall region, where they are highly extended and hence at a lower entropy level than those which are located away from the wall where the shearing stresses are lower.…”

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confidence: 99%

An analysis of apparent slip flow of polymer solutions

Cohen

Metzner

1986

Rheol Acta

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Abstract:The apparent slip flow phenomenon of polymer solutions in capillary tubes is analyzed by a thermodynamic diffusion model. An approximate solution of the developing concentration profiles shows a significant decrease in the polymer wall concentration. The approximate concentration profiles are coupled with the concentration-dependent viscosity for aqueous polyacrylamide solutions to provide a priori predictions of the magnitude of the effective slip velocity at the wall. The results are in a reasonable agreement with the available apparent slip data for 1% and 0.5% solutions of partially hydrolyzed polyacrylamide.

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“…The entropic driven force arising out of this was postulated to lead to a macromolecular migration away from the wall region. Garner and Nissan [1] suggested that such molecular migration may be responsible for the Slip phenomena in capillary tubes. An approximate analysis of macromolecular migration was reported by Schreiber et al [3].…”

Section: Theoretical Framework On Macromolecular Migrationmentioning

confidence: 98%

“…For instance, Garner et al [1] and Busse [2] speculated that the random coils of high-molecular-weight polymer will get distorted to a higher degree near the tube wall; thus, they will have reduced conformational entropies. The entropic driven force arising out of this was postulated to lead to a macromolecular migration away from the wall region.…”

Section: Theoretical Framework On Macromolecular Migrationmentioning

confidence: 98%

On the role of stress-induced migration on time-dependent terminal velocities of falling spheres

Ambeskar

Mashelkar

1990

Rheol Acta

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Abstract." There is experimental evidence to suggest that even under steady-state conditions the velocity of solid spheres or bubbles moving through viscoelastic fluids can become time dependent. One of the possible explanations offered for interpreting this phenomenon has been the generation of a polymer depleted layer in the line of passage Of the particles, which disappears due to the counterbalancing effect of molecular diffusion in the long range. We have done some careful experiments and measured these concentrations to show that no such depletion layers are formed. Alternative explanations of the phenomenon have been examined and the importance of the possible effects of microstructures generated through temporary associations has been emphasised.

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Rheological Properties of High-Viscosity Solutions of Long Molecules

Cited by 64 publications

References 0 publications

Vorticity Elongation in Polymeric Emulsions

Vorticity Elongation in Polymeric Emulsions

An analysis of apparent slip flow of polymer solutions

On the role of stress-induced migration on time-dependent terminal velocities of falling spheres

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