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Linear and nonlinear viscoelastic properties of hydrogels significantly contribute to functionality, long-term performance, and stability of the hydrogels. With respect to the nonlinear viscoelastic response of chemically crosslinked hydrogels, the vast majority of publications have reported the type III response (weak strain overshoot). Herein, to measure the true mechanical response of hydrogels subjected to large shear deformations, we developed a technique by chemically bonding and sandwiching two surfaces of a hydrogel to treated glass slides attached to the oscillating rheometer's metal plates. Employing this method, for the first time, we were able to completely alleviate errors attributed to the wall slip in the rheological measurements of soft materials, enabling the accurate evaluation of nonlinear behavior of hydrogels. The results show that these hydrogels follow a type II (strain hardening) response. It is argued that the observed type III response of hydrogels, widely reported in the literature, originates from the wall-slip condition, rather than the inherent viscoelasticity of the hydrogels. This insight has important implications for the future development of hydrogel-based or other soft materials. [ABSTRACT FROM AUTHOR]

This study sheds light on the effect of nitrogen (N) doping of carbon nanotubes (CNTs) on medium-amplitude oscillatory shear (MAOS) response of CNT/polyvinylidene fluoride (PVDF) nanocomposites within a rheologically percolated concentration regime. Custom-synthesized CNTs without and with nitrogen heteroatom (at a nitrogen atomic percent of 3.85 at. %) were incorporated into a PVDF matrix using a miniature melt-mixer at different concentrations. In both cases, as confirmed by TEM investigations, a nanoscopic state of dispersion in the PVDF matrix was achievable using the applied melt mixing procedure. Our results indicated that N-doped nanocomposites, well below their electrical percolation, form a hybrid, load-bearing network structure where network interconnectivity is driven by N-doped CNT domains and near-surface regions of the PVDF phase. This hybrid network formation behavior combined with N-doped CNTs inferior aspect ratio and their higher susceptibility to breakage and length loss during the melt mixing process have led to delayed electrical percolation. In contrast, localized clustering and contact aggregation in a submicrometer scale was the dominant mode of network formation in the undoped CNT/PVDF nanocomposites. These microstructural inferences were further validated in the frame of molecular simulations and optical microscopy investigations. [ABSTRACT FROM AUTHOR]

Polymer flooding is one of the most effective enhanced oil recovery (EOR) methods. High temperature and a high salt content in oil reservoirs significantly decrease the performance of polymer flooding. In this work, the viscoelastic properties of a partially hydrolyzed polyacrylamide (HPAM) solution with and without salt (NaCl) and at two different temperatures (35 °C and 70 °C) were evaluated using rheological approaches. Two fumed silica nanoparticles (NPs) featuring different surface chemistries were used, and their ability to prevent destruction of the polymer network structure against salt addition and temperature increase was investigated. Linear rheological tests (frequency sweep, creep, and creep recovery) and nonlinear rheological tests (large amplitude oscillatory shear) were employed to evaluate the network structure of these systems. The results showed that either adding salt or increasing the temperature destroyed the mechanical integrity of the HPAM 3-dimensional elastic network. However, the introduction of both types of NPs at a sufficient concentration maintained the network structure of HPAM solutions in the small deformation region. In the large deformation region, it was shown that the extent of intra-cycle shear-thickening behavior in the HPAM solution (T = 35 °C and without any salt) decreased by incorporating salt or by increasing the temperature. Moreover, upon incorporating either of the NPs to the HPAM solution, the nonlinear viscoelastic behavior dramatically changed, and the critical strain (linear to nonlinear transition) decreased to a much lower strain amplitude. The outcomes of this study will help petroleum scientists to design more efficient EOR methods. [ABSTRACT FROM AUTHOR]

Partially hydrolyzed polyacrylamide (HPAM) is one of the most widely used polymers for enhanced oil recovery operations. However, high temperature and high salinity in oil reservoirs restrict its functionality and performance. To alleviate this, incorporating fumed silica nanoparticles (NPs) in HPAM solutions was found to be very effective in harsh oil reservoir conditions to improve the efficiency of polymer flooding. Studying the flow behavior of hybrid polymer and fumed silica NP solutions under real reservoir conditions can be very challenging and hard to achieve due to continuously converging and diverging flow through porous structures. In this regard, rheological analysis of such systems under well-controlled flow histories within the capability of rotational rheometers can be of great importance to fully understand the mechanical response of these hybrid solution systems. In this study, two types of fumed silica NPs with different surface chemistries and two types of HPAM polymers with different molecular weights were dispersed/dissolved in deionized water. Linear viscoelastic properties of the hybrid solution systems were studied based on their step-stress (creep) and small amplitude oscillatory shear responses. As deformation in porous media can be rapid and large, consideration of nonlinear viscoelastic properties can be very crucial. The stress decomposition method and Lissajous–Bowditch curves were used to describe the intercycle and intracycle shear-thickening and strain-stiffening ratios quantitatively and qualitatively. In brief, linear and nonlinear rheology conjugated with thermogravimetric analysis and cryo-scanning electron microscopy imaging enabled us to characterize viscoelastic properties of the hybrid systems and link our observations to microstructural features. Through polymer bridging, the slightly hydrophobic fumed silica NPs (AEROSIL R816) had a unique ability to form interconnected, predominately elastic network structures in contrast to large agglomerated structures formed by highly hydrophilic AEROSIL 300. This has led to observing very different rheological behaviors, regardless of the HPAM polymer molecular weight, below and above a critical fumed silica NPs concentration. [ABSTRACT FROM AUTHOR]

We studied the breakup of viscoelastic drops in Newtonian liquid matrices undergoing simple shear flow. The coexistence of different breakup modes was observed. The drop size determines, to a great extent, the type of drop breakup mechanism and the critical point when the mechanism changes. Small drops break along the vorticity direction, which is the direction perpendicular to the flow direction and the velocity gradient direction, whereas large drops break in the flow direction. The change in mechanism abruptly occurs as the drop size reduces. There is a big jump in the critical shear rate when the mechanism changes from breakup in the flow direction to breakup in the vorticity direction. [ABSTRACT FROM AUTHOR]

Carbon nanotube (CNT)/polymer nanocomposites exhibit excellent electrical properties by forming a percolated network. Adding a secondary filler can significantly affect the CNTs’ network, resulting in changing the electrical properties. In this work, we investigated the effect of adding manganese dioxide nanowires (MnO2NWs) as a secondary nanofiller on the CNTs’ network structure inside a poly(vinylidene fluoride) (PVDF) matrix. Incorporating MnO2NWs to PVDF/CNT samples produced a better state of dispersion of CNTs, as corroborated by light microscopy and transmission electron microscopy. The steady shear and oscillatory shear flows were employed to obtain a better insight into the nanofiller structure and viscoelastic behavior of the nanocomposites. The transient response under steady shear flow revealed that the stress overshoot of hybrid nanocomposites (two-fillers), PVDF/CNT/MnO2NWs, increased dramatically in comparison to binary nanocomposites (single-filler), PVDF/CNT and PVDF/MnO2NWs. This can be attributed to microstructural changes. Large amplitude oscillatory shear characterization was also performed to further investigate the effect of the secondary nanofiller on the nonlinear viscoelastic behavior of the samples. The nonlinear rheological observations were explained using quantitative nonlinear parameters [strain-stiffening ratio (S) and shear-thickening ratio (T)] and Lissajous-Bowditch plots. Results indicated that a more rigid nanofiller network was formed for the hybrid nanocomposites due to the better dispersion state of CNTs and this led to a more nonlinear viscoelastic behavior. [ABSTRACT FROM AUTHOR]