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  • 1
    Electronic Resource
    Electronic Resource
    FILAMENT-STRETCHING RHEOMETRY OF COMPLEX FLUIDS (2002)
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Fluid Mechanics 34 (2002), S. 375-415 
    Details
    ISSN: 0066-4189
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Filament-stretching rheometers are devices for measuring the extensional viscosity of moderately viscous non-Newtonian fluids such as polymer solutions. In these devices, a cylindrical liquid bridge is initially formed between two circular end-plates. The plates are then moved apart in a prescribed manner such that the fluid sample is subjected to a strong extensional deformation. Asymptotic analysis and numerical computation show that the resulting kinematics closely approximate those of an ideal homogeneous uniaxial elongation. The evolution in the tensile stress (measured mechanically) and the molecular conformation (measured optically) can be followed as functions of the rate of stretching and the total strain imposed. The resulting rheological measurements are a sensitive discriminant of molecularly based constitutive equations proposed for complex fluids. The dynamical response of the elongating filament is also coupled to the extensional rheology of the polymeric test fluid, and this can lead to complex viscoelastic-flow instabilities such as filament necking and rupture or elastic peeling from the rigid end-plates.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1146/annurev.fluid.34.083001.125207
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  • 2
    Electronic Resource
    Electronic Resource
    Non-isothermal modification of purely elastic flow instabilities in torsional flows of polymeric fluids (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 13 (2001), S. 382-396 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Previous experimental measurements and linear stability analyses of curvilinear shearing flows of viscoelastic fluids have shown that the combination of streamwise curvature and elastic normal stresses can lead to flow destabilization. Torsional shear flows of highly elastic fluids with closed streamlines can also accumulate heat from viscous dissipation resulting in nonuniformity in the temperature profile within the flow and nonlinearity in the viscometric properties of the fluid. Recently, it has been shown by Al-Mubaiyedh et al. [Phys. Fluids 11, 3217 (1999)] that the inclusion of energetics in the linear stability analysis of viscoelastic Taylor–Couette flow can change the dominant mode of the purely elastic instability from a nonaxisymmetric and time-dependent secondary flow to an axisymmetric stationary Taylor-type toroidal vortex that more closely agrees with the stability characteristics observed experimentally. In this work, we present a detailed experimental study of the effect of viscous heating on the torsional steady shearing of elastic fluids between a rotating cone and plate and between two rotating coaxial parallel plates. Elastic effects in the flow are characterized by the Deborah number, De, while the magnitude of the viscous heating is characterized by the Nahme–Griffith number, Na. We show that the relative importance of these two competing effects can be quantified by a new dimensionless thermoelastic parameter, aitch-theta=Na1/2/De, which is a material property of a given viscoelastic fluid independent of the rate of deformation. By utilizing this thermoelastic number, experimental observations of viscoelastic flow stability in three different fluids and two different geometries over a range of temperatures can be rationalized and the critical conditions unified into a single flow stability diagram. The thermoelastic number is a function of the molecular weight of the polymer, the flow geometry, and the temperature of the test fluid. The experiments presented here were performed using test fluids consisting of three different high molecular weight monodisperse polystyrene solutions in various flow geometries and over a large range of temperatures. By systematically varying the temperature of the test fluid or the configuration of the test geometry, the thermoelastic number can be adjusted appreciably. When the characteristic time scale for viscous heating is much longer than the relaxation time of the test fluid (aitch-theta(very-much-less-than)1) the critical conditions for the onset of the elastic instability are in good agreement with the predictions of isothermal linear stability analyses. As the thermoelastic number approaches a critical value, the strong temperature gradients induced by viscous heating reduce the elasticity of the test fluid and delay the onset of the instability. At even larger values of the thermoelastic parameter, viscous heating stabilizes the flow completely. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1338540
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  • 3
    Electronic Resource
    Electronic Resource
    Cavity flows of elastic liquids: Two-dimensional flows (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 9 (1997), S. 3123-3140 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: There is a wealth of experimental and computational results available for the motion of Newtonian fluids in the lid-driven cavity geometry, however little is known about the corresponding motion of viscoelastic fluids. We use laser Doppler velocimetry (LDV) and digital particle image velocimetry (DPIV) to probe the dynamics of viscoelastic fluid motion in the classic "lid-driven cavity" problem for a range of industrially important aspect ratios (0.25≤Λ≤4) using an ideal elastic fluid as the test material. The magnitude of non-Newtonian effects in the cavity are characterized by the dimensionless Deborah number and the experiments span the range 0≤De≤0.35. Elastic effects break the symmetry observed in the velocity field of cavity flows of viscous Newtonian fluids at zero Reynolds number. At low De, the flow remains two-dimensional but increasing the imposed velocity causes the center of the primary recirculating vortex in the cavity to shift progressively upstream. At larger Deborah numbers, the fluid motion becomes unstable and a three-dimensional flow develops. Upon cessation of the forcing boundary motion, a pronounced elastic recoil is observed which leads to a rapid reversal in the direction of the recirculating vortex. This transient motion subsequently decays through viscous dissipative effects on the elastic time scale of the fluid. The kinematics of the localized corner flow near the downstream corner are studied in detail and the distinguishing features of the viscoelastic corner flow with respect to the classic knife-edge problem of Taylor are reported. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869430
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  • 4
    Electronic Resource
    Electronic Resource
    Cavity flows of elastic liquids: Purely elastic instabilities (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 10 (1998), S. 1058-1070 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experimental observations of a purely elastic flow instability occurring in the lid-driven cavity flow of two semi-dilute polymer solutions are reported and the effect of cavity aspect ratio on the dynamical structure of the unstable flow is quantitatively investigated. The spatial and temporal characteristics of the secondary flow are measured using flow visualization, laser Doppler velocimetry, and digital particle image velocimetry. At the onset conditions the disturbances appear in the form of spatially periodic flow cells which propagate along the neutral direction of the cavity. The secondary flow structure is analogous to the Taylor–Görtler vortices observed in inertially driven hydrodynamic instabilities. The critical onset conditions for two elastic test fluids and five different aspect ratios correlate with a recently proposed dimensionless stability criterion which incorporates measures of the local streamline radius of curvature and the non-Newtonian normal stresses in the flow domain. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869631
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  • 5
    Electronic Resource
    Electronic Resource
    Structural limitation to the material strength of electrorheological fluids (1997)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 71 (1997), S. 333-335 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The variation of the dynamic modulus of a model electrorheological fluid with strain amplitude is shown to closely resemble that of traditional physical gels. Comparison of the in-phase stress component in each system indicates that the material strength of electrorheological fluids in shear is limited by the small strain amplitude to structural failure. An anisotropic network model is proposed for electrorheological fluids, in which the primary structure consists of chains of particles spanning the electrode gap along the field direction, while many-body interactions form a secondary structure of short chains tilted with respect to the field direction and interconnecting the primary chains. A geometrical argument shows that the tensile strain in the secondary structure can be an order of magnitude larger than that in the primary chains. This nonuniform strain distribution poses an inherent structural limitation on the shear material strength of electrorheological fluids. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.119530
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  • 6
    Electronic Resource
    Electronic Resource
    Using filament stretching rheometry to predict strand formation and “processability” in adhesives and other non-Newtonian fluids (2000)
    Springer
    Rheologica acta 39 (2000), S. 321-337 
    Details
    ISSN: 1435-1528
    Keywords: Key words Capillary thinning ; Filament stretching ; Giesekus model ; Micro-filament rheometer ; Pressure sensitive adhesives ; Processability ; Spinnability ; Stringiness
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract The spinning of polymeric fibers, the processing of numerous foodstuffs and the peel and tack characteristics of adhesives are all associated with the formation, stability and, ultimately, the longevity of thin fluid `strands'. This tendency to form strands is usually described in terms of the tackiness of the fluid or by heuristic concepts such as `stringiness' (Lakrout et al. J Adhesion 1999). The dynamics of such processes are complicated due to spatially and temporally non-homogeneous growth of extensional stresses, the action of capillary forces and the evaporation of volatile solvents. We describe the development and application of a simple instrument referred to as a microfilament rheometer (MFR) that can be used to readily differentiate between the dynamical response of different pressure-sensitive adhesive fluid formulations. The device relies on a quantitative observation of the rate of extensional thinning or `necking' of a thin viscous or viscoelastic fluid filament in which the solvent is free to evaporate across the free surface. This high-resolution measurement of the radial profile provides a direct indication of the ultimate time to break up of the fluid filament. This critical time is a sensitive function of the rheological properties of the fluid and the mass transfer characteristics of the solvent, and can be conveniently reported in terms of a new dimensionless quantity we refer to as a processability parameter P. We demonstrate the usefulness of this technique by presenting our results in the form of a case study in which we measure the visco-elasto-capillary thinning of slender liquid filaments for a number of different commercial polymer/solvent formulations and relate this to the reported processing performance of the materials. We also compare the MFR observations with the prediction of a simple 1D theory derived from the governing equations that model the capillary thinning of an adhesive filament.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003970000072
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  • 7
    Electronic Resource
    Electronic Resource
    Digital particle imaging velocimetry of viscoelastic fluids (1997)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 43 (1997), S. 289-302 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: We investigate the utility of digital particle imaging velocimetry (DPIV) for performing kinematic measurements in non-Newtonian flows. With the advances in numerical techniques for simulation of viscoelastic flows, acquisition of spatially dense 2-D kinematic data in steady and time-dependent deformations can be useful in verifying predictions of the corresponding computational studies. Furthermore, kinematic measurements of the velocity field and rate of deformation in prototypical industrial processes can significantly enhance the rational design and optimization of polymer processing unit operations. Application of a high seeding density DPIV technique in viscoelastic media is discussed, and quantitative data are obtained in a number of industrially relevant flow geometries. The issues of velocity-position assignment and the effects of a velocity gradient across DPIV correlation regions are discussed. A simple yet effective averaging technique preserves the order of accuracy and assigns the velocity vectors to their appropriate positions using an overlapping discretization scheme. The examples studied experimentally include steady flow in circular pipes, flow past obstructions, flow in a lid-driven cavity, and time-dependent free-surface extensional flows in a liquid filament. With the exception of the first example, these flow geometries constitute an important collection of configurations in which quantitative experimental data for non-Newtonian fluids are scarce or nonexistent.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690430202
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  • 8
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    Spontaneous wettability patterning via creasing instability (2016)
    National Academy of Sciences
    Details
    Publication Date: 2016-07-05
    Description: Surfaces with patterned wettability contrast are important in industrial applications such as heat transfer, water collection, and particle separation. Traditional methods of fabricating such surfaces rely on microfabrication technologies, which are only applicable to certain substrates and are difficult to scale up and implement on curved surfaces. By taking advantage of a mechanical instability on a polyurethane elastomer film, we show that wettability patterns on both flat and curved surfaces can be generated spontaneously via a simple dip coating process. Variations in dipping time, sample prestress, and chemical treatment enable independent control of domain size (from about 100 to 500 μm), morphology, and wettability contrast, respectively. We characterize the wettability contrast using local surface energy measurements via the sessile droplet technique and tensiometry.
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
    Published by National Academy of Sciences
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  • 9
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    Viscoelastic fishbones (2019)
    American Physical Society
    Details
    Publication Date: 2019-10-24
    Electronic ISSN: 2469-990X
    Topics: Physics
    Published by American Physical Society
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  • 10
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    Hydrodynamics control shear-induced pattern formation in attractive suspensions (2019)
    National Academy of Sciences
    Details
    Publication Date: 2019-06-04
    Description: Dilute suspensions of repulsive particles exhibit a Newtonian response to flow that can be accurately predicted by the particle volume fraction and the viscosity of the suspending fluid. However, such a description fails when the particles are weakly attractive. In a simple shear flow, suspensions of attractive particles exhibit complex, anisotropic microstructures and flow instabilities that are poorly understood and plague industrial processes. One such phenomenon, the formation of log-rolling flocs, which is ubiquitously observed in suspensions of attractive particles that are sheared while confined between parallel plates, is an exemplar of this phenomenology. Combining experiments and discrete element simulations, we demonstrate that this shear-induced structuring is driven by hydrodynamic coupling between the flocs and the confining boundaries. Clusters of particles trigger the formation of viscous eddies that are spaced periodically and whose centers act as stable regions where particles aggregate to form flocs spanning the vorticity direction. Simulation results for the wavelength of the periodic pattern of stripes formed by the logs and for the log diameter are in quantitative agreement with experimental observations on both colloidal and noncolloidal suspensions. Numerical and experimental results are successfully combined by means of rescaling in terms of a Mason number that describes the strength of the shear flow relative to the rupture force between contacting particles in the flocs. The introduction of this dimensionless group leads to a universal stability diagram for the log-rolling structures and allows for application of shear-induced structuring as a tool for assembling and patterning suspensions of attractive particles.
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
    Published by National Academy of Sciences
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