Bioink Rheology Regulates Stability of Bioprinted Strands

Extrusion three-dimensional (3D) bioprinting typically requires an ad-hoc trial-and-error optimization of the bioink composition towards enhanced resolution. The bioink solutions are solidified after leaving cone-shaped or cylindrical nozzles. The presence of bioink instability not only hampers the extrusion resolution but also affects the behavior of embedded cellular components. This is a key factor in selecting bioinks and bioprinting design parameters for well-established desktop and handheld bioprinters. In this work, we developed an analytical solution for the process of bioink deposition and compared its predictions against numerical simulations of the deposition. We estimated the onset of bioink instability as a function of bioink rheological properties and nozzle geometry. Both analytical and simulation results demonstrated that enhancing shear-thinning behavior of the bioink stabilizes the printing process whereas bioink shear-thickening behavior induces an opposite effect through extending the toe region of the deposition. The present study serves as a benchmark for detailed simulations of the extrusion process for optimal bioprinting.

Microstructure of the fluid particles around the rigid body at the shear-thickening state toward understanding of the fluid mechanics

We studied the shear-thickening behavior of systems containing rigid spherical bodies immersed in smaller particles using non-equilibrium molecular dynamics simulations. We generated shear-thickening states through particle mass modulation of the systems. From the microstructures, i.e., two-dimensional pair distribution functions, we found anisotropic structures resulting from shear thickening, that are explained by the difference between the velocities of rigid bodies and fluid particles. The increasing viscosity in our system originated from collisions between fluid particles and rigid bodies. The lubrication forces defined in macroscale physics are then briefly discussed.

The Effect of Rheology of Viscoelastic Polymer on Pressure Transient Response in Near-Wellbore Regions

Viscoelastic polymer solution shows shear thinning behavior at low shear rates and shear thickening behavior at high shear rates in reservoirs. However, models that ignored shear thickening behavior were commonly employed to interpret transient pressure data derived from tested wells in viscoelastic polymer flooding systems; although, viscoelastic polymer solutions show shear thickening behavior in the near-wellbore region due to high shear rate. To better characterize the oilfield with pressure transient analysis in viscoelastic polymer flooding systems, we developed a numerical model that takes into account both shear thinning behavior and shear thickening behavior. A finite volume method was employed to discretize partially differential flow equations in a hybrid grid system including PEBI mesh and Cartesian grid, and the Newton-Raphson method was used to solve the fully implicit nonlinear system. To illustrate the significance of our model, we compared our model with a model that ignores the shear thickening behavior by graphing their solutions on log-log plots. In the flow regime of near-wellbore damage, the pressure derivative computed by our model is distinctly larger than that computed by the model ignoring shear thickening behavior. Furthermore, the effect of shear thickening behavior on pressure derivative differs from that of near-wellbore damage. We then investigated the influence of shear thickening behavior on pressure derivative with different polymer injection rates, injection rates, and permeabilities. The results can provide a benchmark to better estimate near-wellbore damage in viscoelastic polymer flooding systems. Besides, we demonstrated the applicability and accuracy of our model by interpreting transient pressure data from a field case in an oilfield with viscoelastic polymer flooding treatments.

The Impact of Rheology on Viscous Oil Displacement by Polymers Analyzed by Pore-Scale Network Modelling

Several experimental studies have shown significant improvement in heavy oil recovery with polymers displaying different types of rheology, and the effect of rheology has been shown to be important. These experimental studies have been designed to investigate why this is so by applying a constant flow rate and the same polymer effective viscosity at this injection rate. The types of rheology studied vary from Newtonian and shear thinning behavior to complex rheology involving shear thinning and thickening behavior. The core flood experiments show a significantly higher oil recovery with polyacrylamide (HPAM), which exhibits shear thinning/thickening behavior compared to biopolymers like Xanthan, which is purely shear thinning. Various reasons for these observed oil recovery results have been conjectured, but, to date, a clear explanation has not been conclusively established. In this paper, we have investigated the theoretical rationale for these results by using a dynamic pore scale network model (DPNM), which can model imbibition processes (water injection) in porous media and also polymer injection. In the DPNM, the polymer rheology can be shear thinning, shear thinning/thickening, or Newtonian (constant viscosity). Thus, the local effective viscosity in a pore within the DPNM depends on the local shear rate in that pore. The predicted results using this DPNM show that the polymer causes changes in the local flow velocity field, which, as might be expected, are different for different rheological models, and the changes in the velocity profile led to local diversion of flow. This, in turn, led to different oil recovery levels in imbibition. However, the critical result is that the DPNM modelling shows exactly the same trend as was observed in the experiments, viz. that the shear thinning/thickening polymer gave the highest oil recovery, followed by the Newtonian Case and the purely shear thinning polymer gave the lowest recover, but this latter case was still above the waterflood result. The DPNM simulations showed that the shear-thinning/thickening polymer show a stabilized frontal velocity and increased oil mobilization, as observed in the experiments. Simulations for the shear-thinning polymer show that, in high-rate bonds, the average viscosity is greatly reduced, and this causes enhanced water fingering compared to the Newtonian polymer case. No other a priori model of the two-phase fluid physics of imbibition, coupled with the polymer rheology, has achieved this degree of predictive explanation, of these experimental observations, to our knowledge.

Hydrogels in Hand Sanitizers

Hand hygiene can be considered a strategic key useful in the containment of infections such as COVID-19 both at home and in communities because it can dramatically reduce the widespread outbreak of infections. In case of the unavailability of soap and water, “instant” hand sanitizers are recommended because their application can be considered easy, versatile, quick and often less aggressive for the skin. For these reasons, alcoholic and alcohol-free hand rub gels can be considered the best performing formulations on the market. Together with disinfectants and antiseptic agents, hydrogels play a fundamental role in obtaining stable formulations and are easy to disperse, with a pleasant skin feel and an overall good performance. Several compounds commonly used in the pharmaceutical, cosmetic and food industry are available for this purpose, in particular, cellulose derivatives and synthetic polymers derivatives. Each of them is available in several grades, presenting different thickening behavior, rheological properties and compatibility with other ingredients, alcohols in particular. For all these reasons, it is important to explore hydrogel properties and behaviors in different contexts (i.e., hydroalcoholic and aqueous media) in order to develop new and performing hand rub gels, always taking into account the different international legal frameworks regarding disinfectant and sanitizing formulations.