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Author index ►Caroline E. Wagner, Alexander C. Barbati, Jan Engmann, Adam S. Burbidge, Gareth H. McKinley

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Apparent shear thickening at low shear rates in polymer solutions can be an artifact of non-equilibration

Appl. Rheol.26:5 (2016) 54091 (5 pages) ►

While shear thickening is a well-known feature of some polymer solutions, its observation in the absence of a clear specific chemical or structural mechanism and at very low rates of shear (shear rate ≤ 0.1/s) in our own data, as well as in several polymer systems in the literature, has prompted further investigation. Using the rheologically-reproducible and commercially available dysphagia product ResourceÒ Thicken Up Clear (produced by Nestle Health Science) as a canonical demonstration fluid, we show that the observation of a local maximum in the steady shear viscosity at very low shear rates can be completely attributed to the sample not having reached steady state conditions, and correspondingly, to the measurement not having been performed in steady simple shear flow. We propose two criteria to ensure equilibration during steady state flow rheological measurements: a substantial increase in the measurement time allotted for each point such that the total material strain accumulated in the sample is allowed to reach shear strain ≤ 5 and/or a stricter convergence criterion of 10 consecutive readings within a tolerance of 1%.► Cite this publication as follows:

Wagner CE, Barbati AC, Engmann J, Burbidge AS, McKinley GH: Apparent shear thickening at low shear rates in polymer solutions can be an artifact of non-equilibration, Appl. Rheol. 26 (2016) 54091.

An enhanced version of the flexure-based microgap rheometer (FMR) is described which enables rheological measurements in steady state shearing flows of bulk fluid samples of PDMS with an absolute gap separation between the shearing surfaces of 100 nm - 100 μmm. Alignment of the shearing surfaces to a parallelism better then 10_{-7}rad allows us to reliably measure shear stresses at shear rates up to 104 s_{-1}. At low rates and for shearing gaps < 5 mm the stress response is dominated by sliding friction between the surfaces that is independent of the viscosity of the fluid and only determined by the residual particulate phase (dust particles) in the fluid.This behaviour is similar to the boundary lubrication regime in tribology.The absolute gap control of the FMR allows us to systematically investigate the flow behaviour at low degrees of confinement (gap separations 100 nm - 2 μm) that cannot be accessed with conventional (controlled normal load) tribological test protocols.► Cite this publication as follows:

Clasen C, Kavehpour HP, McKinley GH: Bridging Tribology and Microrheology of Thin Films, Appl. Rheol. 20 (2010) 45049.

We provide an overview of transient extensional rheometry techniques for wheat flour doughs in which the deformation and material response is well defined. The behavior of a range of model doughs was explored with a Filament Stretching Extensional Rheometer (FISER). The measurements were also compared to data obtained with a new windup extensional rheometer; the SER universal testing platform. A simple empirical constitutive equation, which allows characterization of the experimental results with a small number of parameters, is presented to describe the resulting measurements. To characterize the relaxation modulus of the doughs, small amplitude shear tests were performed on samples that have been shear-mixed in a mixograph for varying lengths of time. The linear viscoelastic properties were found to exhibit a broad power-law dependence on the imposed oscillatory frequency that is very reminiscent of that exhibited by a critical gel. The critical gel model of Winter and Chambon was used as the basis for constructing a non-linear constitutive equation for the material stress by combining the relaxation modulus for the critical gel with a Lodge rubber-like liquid form for the kinematics. Transient uniaxial extensional data recorded from the FISER and SER instruments were then compared to the predictions of the constitutive equation. The model captures the initial powerlaw response and subsequent strain-hardening; however additional physics is required to describe the rheological phenomena at very large Hencky strains, including finite extensibility effects and filament rupture in extensional flows.► Cite this publication as follows:

Ng TS, McKinley GH, Padmanabhan M: Linear to Non-linear Rheology of Wheat Flour Dough, Appl. Rheol. 16 (2006) 265.

We investigate the dynamics of the capillary thinning and break-up process for low viscosity elastic fluids such as dilute polymer solutions. Standard measurements of the evolution of the midpoint diameter of the necking fluid filament are augmented by high speed digital video images of the break up dynamics. We show that the successful operation of a capillary thinning device is governed by three important time scales (which characterize the relative importance of inertial, viscous and elastic processes), and also by two important length scales (which specify the initial sample size and the total stretch imposed on the sample). By optimizing the ranges of these geometric parameters, we are able to measure characteristic time scales for tensile stress growth as small as 1 millisecond for a number of model dilute and semi-dilute solutions of polyethylene oxide (PEO) in water and glycerol. If the final aspect ratio of the sample is too small, or the total axial stretch is too great, measurements are limited, respectively, by inertial oscillations of the liquid bridge or by the development of the well-known beads-on-a-string morphology which disrupt the formation of a uniform necking filament. By considering the magnitudes of the natural time scales associated with viscous flow, elastic stress growth and inertial oscillations it is possible to construct an .operability diagram. characterizing successful operation of a capillary breakup extensional rheometer. For Newtonian fluids, viscosities greater than approximately 70 mPas are required; however for dilute solutions of high molecular weight polymer, the minimum viscosity is substantially lower due to the additional elastic stresses arising from molecular extension. For PEO of molecular weight 2 · 106 g/mol, it is possible to measure relaxation times of order 1 ms in dilute polymer solutions with zero-shear-rate viscosities on the order of 2 . 10 mPas.► Cite this publication as follows:

Rodd LE, Scott TP, Cooper-White JJ, McKinley GH: Capillary Break-up Rheometry of Low-Viscosity Elastic Fluids, Appl. Rheol. 15 (2005) 12.

► Cite this publication as follows:

McKinley GH: 72nd annual meeting of the society of rheology (Hilton Head Island, South Carolina/U.S.A) , Appl. Rheol. 11 (2001) 153.

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Braithwaite GJC, McKinley GH: Microrheometry for Studying the Rheology and Dynamics of Polymers near, Appl. Rheol. 9 (1999) 27.

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