Rheology and Pinching Dynamics of Associative Polysaccharide Solutions

Enhanced oil recovery via dual cross-linked polysaccharide network for superior rheology and flow efficiency

Welan gum has been widely used in harsh reservoir development, such as good thickening ability, temperature and salt resistance. However, viscoelasticity is crucial to polymer injectability and microscopic displacement efficiency, Welan gum have rigid helical conformation and show poor rheological properties., therefore, we proposed a double network strategy to improve the molecular conformation and rheological properties of Welan gum. Specifically, we synthesized β-cyclodextrin functionalized polyacrylamide (HPAM-CD) and adamantane modified Weland gum (WL-AD), and polyacrylamide and Welan gum through the supramolecular interaction to obtain double network crosslinking system (WL-AD&HPAM-CD). Rheology show that the double-network system driven by multiple non-covalent interaction has excellent rheological properties. Circular dichroism, atomic force microscopy and scanning electron microscope show that WL-AD&HPAM-CD forms a special molecular conformation, significantly improve the molecule elasticity of biopolymers, it has good conformational stability under harsh conditions. Therefore, oil recovery rate of WL-AD&HPAM-CD after water flooding in 200 mD cores is 4.99 % and 11.5 % higher than that of the same concentration of Welan gum and grafted Welan gum. In addition, WL-AD&HPAM-CD has excellent fluidity in 10-50 mD low permeability reservoirs, and can increase oil recovery by 12.17-17.16 % after water flooding. This study provides a research ideas for the development and application of biopolymers.

Host-guest strategy improves rheological properties, conformational stability and oil displacement efficiency of xanthan gum

The low cost and environmental advantages of Xanthan gum make its production and application scale exceed that of other polysaccharides. However, the temperature resistance of Xanthan gum limits its application. In this study, polysaccharide supramolecular Xanthan gum network (XG-β-CD/AD) based on β-cyclodextrin and adamantane was prepared for enhanced oil recovery. The structure of Xanthan gum was characterized by Fourier infrared spectroscopy, nuclear magnetic resonance spectroscopy and thermogravimetric analysis. The rheological properties of the modified polysaccharide network in aqueous solution were systematically studied. The results showed that physical cross-linking of host-guest interacion enhanced the thickening ability of the polymer. Shear rheology, extensional rheology and dynamic modulus test proved that XG-β-CD/AD had excellent rheological properties. The micromorphology, dynamic light scattering and circular dichroism clarified the molecular conformation, the host-guest interaction can improve conformational transition temperature (Tm) and inorganic salt tolerance of Xanthan gum. Under harsh environment (90 °C, 30000 mg/L brine), the oil recovery of XG-β-CD/AD is 6 %-11 % higher than that of XG at the same conditions, showing a better ability to improve the recovery rate. This study provides a research idea for the selection, development and application of biomacromolecular materials.

The fluid dynamics of a viscoelastic fluid dripping onto a substrate

Extensional flows of complex fluids play an important role in many industrial applications, such as spraying and atomisation, as well as microfluidic-based drop deposition. The dripping-on-substrate (DoS) technique is a conceptually-simple, but dynamically-complex, probe of the extensional rheology of low-viscosity, non-Newtonian fluids. It incorporates the capillary-driven thinning of a liquid bridge, produced by a single drop as it is slowly dispensed from a syringe pump onto a solid partially-wetting substrate. By following the filament thinning and pinch-off process the extensional viscosity and relaxation time of the sample can be determined. Importantly, DoS allows experimentalists to measure the extensional properties of lower viscosity solutions than is possible with commercially available capillary break-up extensional rheometers. Understanding the fluid mechanics behind the operation of DoS will enable users to optimise and extend the performance of this protocol. To achieve this understanding, we employ a computational rheology approach, using adaptively-refined time-dependent axisymmetric numerical simulations with the open-source Eulerian code, Basilisk. The volume-of-fluid technique is used to capture the moving interface, and the log-conformation transformation enables a stable and accurate solution of the viscoelastic constitutive equation. Here, we focus on understanding the roles of surface tension, elasticity and finite chain extensibility in controlling the elasto-capillary (EC) regime, as well as the perturbative effects that gravity and substrate wettability play in setting the evolution of the self-similar thinning and pinch-off dynamics. To illustrate the interplay of these different forces, we construct a simple one-dimensional model that captures the initial rate of thinning when the dynamics are dominated by a balance between inertia and capillarity. This model also captures the structure of the transition region to the nonlinear EC regime in which the rapidly growing elastic stresses in the thread balance the capillary pressure as the filament thins towards breakup. Finally, we propose a fitting methodology based on the analytical solution for FENE-P fluids to improve the accuracy in determining the effective relaxation time of an unknown fluid.

Pinching dynamics, extensional rheology, and stringiness of saliva substitutes

Saliva substitutes are human-made formulations extensively used in medicine, food, and pharmaceutical research to emulate human saliva's biochemical, tribological, and rheological properties. Even though extensional flows involving saliva are commonly encountered in situations such as swallowing, coughing, sneezing, licking, drooling, gleeking, and blowing spit bubbles, rheological evaluations of saliva and its substitutes in most studies rely on measured values of shear viscosity. Natural saliva possesses stringiness or spinnbarkeit, governed by extensional rheology response, which cannot be evaluated or anticipated from the knowledge of shear rheology response. In this contribution, we comprehensively examine the rheology of twelve commercially available saliva substitutes using torsional rheometry for rate-dependent shear viscosity and dripping-onto-substrate (DoS) protocols for extensional rheology characterization. Even though most formulations are marketed as having suitable rheology, only three displayed measurable viscoelasticity and strain-hardening. Still, these too, failed to emulate the viscosity reduction with the shear rate observed for saliva or match perceived stringiness. Finally, we explore the challenges in creating saliva-like formulations for dysphagia patients and opportunities for using DoS rheometry for diagnostics and designing biomimetic fluids.

Rheology and dispensing of real and vegan mayo: the chickpea or egg problem

The rheology, stability, texture, and taste of mayonnaise, a dense oil-in-water (O/W) emulsion, are determined by interfacially active egg lipids and proteins. Often mayonnaise is presented as a challenging example of an egg-based food material that is hard to emulate using plant-based or vegan ingredients. In this contribution, we characterize the flow behavior of animal-based and plant-based mayo emulsions, seeking to decipher the signatures that make the real mayonnaise into such an appetizing complex fluid. We find that commercially available vegan mayos can emulate the apparent yield stress and shear thinning of yolk-based mayonnaise by the combined influence of plant-based proteins (like those extracted from chickpeas) and polysaccharide thickeners. However, we show that the dispensing and dipping behavior of egg-based and vegan mayos display striking differences in neck shape, sharpness, and length. The ratio of apparent extensional to shear yield stress value is found to be larger than the theoretically predicted square root of three for all mayo emulsions. The analysis of neck radius evolution of these extension thinning yield stress fluids reveals that even when the power law exponent governing the intermediate pinching dynamics is similar to the exponent obtained from the shear flow curve, the terminal pinching dynamics show strong local effects, possibly influenced by interstitial fluid properties, finite drop size and deformations, and capillarity.

Temperature-controlled dripping-onto-substrate (DoS) extensional rheometry of polymer micelle solutions

Capillary-driven thinning of a liquid bridge is commonly used to measure the extensional rheology of macromolecular solutions for assessment of material sprayability, printability, and jettability. Methods like dripping-onto-substrate (DoS) rheometry are often preferred to methods like capillary breakup extensional rheometry (CaBER) due to low required sample volume and ability to measure low-viscosity fluids; however, DoS measurements to-date have been limited to ambient temperatures. Here, an environmental control chamber is developed to enable temperature-controlled DoS (TC-DoS) measurements, and the temperature-dependent extensional rheology of a model system of poloxamer 234 (P234) in NaF brine is examined. Spherical P234 micelles at ambient conditions exhibit inertiocapillary (IC) thinning; above the sphere-to-rod transition temperature, the liquid bridge evolves towards viscocapillary (VC) thinning as micelles lengthen and shear viscosity increases. Above 37 °C, wormlike micelle (WLM) formation results in pronounced elastocapillary (EC) thinning, and further WLM growth and entanglement results in three elasticity-dominated flow regimes: EC thinning, beads-on-a-string (BOAS) instability formation, and BOAS thinning. Despite having a substantially larger amphiphile molecular weight and micelle cross-sectional radius than surfactant WLMs, entangled P234 WLMs exhibit similar extensional behavior and achieve comparable maximum Trouton ratios. Comparing DoS measurements of P234 WLMs with prior studies on surfactant WLMs reveals that the maximum Trouton ratio depends on the ratio of shear and extensional relaxation times, a trend undetectable via CaBER due to pre-deformation during the initial step stretch. These findings reveal rich temperature-dependent flow behaviors in polymer micelles and highlight the importance of using a minimally-disruptive method such as TC-DoS when measuring the extensional rheology of microstructured and thermosensitive fluids.