Band 28, Heft 2

A novel method is introduced to describe quantitatively hysteresis seen in flow curves of microfibrillated cellulose suspensions. Also, a data normalisation procedure is presented that allows a direct comparison of data from suspensions of different solids contents. The discussion of the flow curve hysteresis of an MFC suspension is proposed to provide a lot of information on the suspension morphology under flow. Such information is not only useful for process design, but also may serve as a quality control tool. Hysteresis data as a function of the suspension solids content are provided, and considered with reference to an overview made of peer work in the field. Two discrete hysteresis loop areas were found in the flow curves presented in this work, each associated with a distinct shear rate region, one where the viscosity of the flow curve during shear rate increase is higher than that of the shear rate flow curve at decreasing shear rate (named positive hysteresis) and another where it is the opposite (named negative hysteresis). This behavior seems to have been rarely reported, and where reported we offer an explanation, based on morphological models and rheometer measurement set up, as to why other researchers may find a variety of hysteresis forms. It is hypothesised that the negative normalised hysteresis is mainly depending on the excessive flocculation/structuration induced at intermediate shear rates during the shear rate increase, and that it is necessarily less with increasing solids content because of the reduced availability of free water. The positive normalised hysteresis, however, is considered to originate from the different morphologies at lower shear rates, i.e. the initial, homogeneous structure vs. the structure that was previously induced by the intermediate shear during shear rate decrease. The positive normalised hysteresis appears not to depend on the solids content, indicating a self-similarity or scaling behavior of the structuring with respect to the underlying network structure.

In petroleum drilling, aqueous Polyanionic Cellulose solutions (PAC) are often used as a drilling fluid model system in experimental laboratory studies to investigate cuttings transport. Cuttings transport refers to the transportation of drilled-off solids out of the wellbore. In these studies, PAC solutions are typically assumed to behave purely viscous, i.e. they do not show time-dependent/thixotropic and/or viscoelastic properties. In this study, a rheological characterization of PAC has been performed in combination with an evaluation of time scales characterizing the fluid to verify the conventional assumption of a purely-viscous fluid. It is found that PAC solutions are generally not purely viscous: They feature viscoelastic behavior on time scales of the order of 0.01 to 1 s, such as normal stress differences, as well as thixotropic behavior on larger time scales of the order of 10 to 1000 s because of their polymeric microstructure. If simplified to a purely viscous fluid, the degree of uncertainty in representing the measured apparent shear viscosity may increase by an order of ≈ 75 to 90 % depending on the relevant time scale. When obtaining flow curves, a sufficiently long measurement point duration (sampling time for a particular torque reading) is required to ensure that the liquid microstructure has reached its dynamic equilibrium at the desired shear rate. Due to their polymeric nature, PAC solutions feature Newtonian viscosity plateaus at both low and high shear rates. For modeling purposes, the application of a Cross/Carreau material function is recommended because it both best describes the flow curve data and minimizes extrapolation errors compared to the conventionally used Power Law material function.

A triple pressure-stage capillary rheometer was individually developed for providing an insight of pressure effect on polymeric melts viscosity during steady and continuous flow. Three capillary dies with identical/varied diameters and aspect ratio were assembled in series along the flow direction, relying on which the flow was divided into three zones with varied pressures under the same flow rate. Several polymeric melts, such as low density polyethylene (LDPE), polystyrene (PS), polypropylene (PP) as well as its nanocomposites of PP/CaCO 3 , PP/Mg(OH) 2 , and PP/halloysite nanotubes (PP/HNTs) were taken as the experimental samples. The principles for calculating the pressure sensitivity of shear viscosity in capillary flow were discussed, including methods based on constant shear rate (CSR), constant shear stress (CSS), and curve superposition (CSP). For the several polymer melts adopted in this work, a sequence of pressure dependence of viscosity was revealed as PS > PP > LDPE, which is typically acknowledged.

In this paper, we present a novel method we have developed for measuring the drag reduction in a dilute polymer solution, based on the triboelectricity phenomenon. The presence of a small quantity of polymer with high molecular density in a liquid decreases the friction of the liquid on solid walls. This property defines drag reduction. The friction itself produces electricity in the liquid known as triboelectricity. In this work, we show that drag reduction can be measured by measuring the triboelectric voltage in the solvent and in the polymer solution. The method was tested on well characterized dilute solution of polyethylene oxide (PEO) and the results obtained agree qualitatively well with those available in the literature, notably showing that for given flow rate, drag reduction by PEO increases with polymer concentration until reaching a plateau. Also, for given concentration, drag reduction increases with flow rate in the range of concentration and flow rate tested. More generally, a similar behavior is expected for any polymer solution obeying the power-law rheological model.