We investigate the collision behaviour of a shear thinning non-Newtonian fluid xanthan, by binary droplet collision experiments. Droplet collisions of non-Newtonian fluids are more complex than their Newtonian counterpart as the viscosity no longer remains constant during the collision process. Despite the complex collision dynamics, we are able to present a complete regime map based on non-dimensional Weber (We) number and impact parameter (B). We compare the collision outcomes of xanthan, glycerol and a milk concentrate at similar impact conditions. These experiments reveal very rich and complex collision morphologies for shear thinning xanthan solution, strikingly different from Newtonian droplet collisions. Unlike glycerol and milk, xanthan collisions show no reflexive separation even at very high We number. Instead of breakup, we observe disc-like shapes with an oscillating behaviour of the colliding droplets. A detailed analysis reveals that this outcome is related to increased viscous energy dissipation and extensional effects.

@en

Information on the viscoelastic response to extensional deformation of polymeric solutions and other complex fluids is crucial for understanding their behavior in a wide variety of industrial applications. While the characterization of complex fluids using simple shear experiments is well established, a complete characterization of their extensional rheological properties is still challenging. Currently, methods such as CaBER, ROJER, and microfluidics are used for this. The downside of these techniques is that fluids with low viscosity and/or short relaxation times are still hard to characterize. Here we propose an alternative approach for measuring extensional rheological properties by using a microscopic air bubble as an extensional rheometer. We pursue this idea experimentally by submerging a single bubble in a pool of liquid and studying the dynamics of the bubble under influence of a change in hydrostatic pressure. This situation is analogous to a stress-controlled shear rheology experiment, where a time-dependent shear stress is applied to a sample and the resultant strain is measured. We test our approach on a range of polymeric fluids with different relaxation times and find that the bubble microrheology experiments provide results and trends that are indeed consistent with theoretical expectations and with previous data acquired using established methods. The concept of bubble microrheology is thus very promising. While the experimental technique is straightforward, practical improvements can still be made on the experimental setup and the data analysis, which could extend the range of investigable materials and the amount of information that can be extracted from the measurements. A chief advantage of our methodology is that it ensures the measurement of only extensional properties and that fluids with very short relaxation times can be measured. Finally, by applying a sinusoidal pressure variation instead of a pressure jump, our method could potentially be extended to oscillatory measurements.@en

Polymer liquids are used in the oil industry to improve the volumetric sweep efficiency and displacement efficiency of the oil from a reservoir. Surprisingly, it is not only the viscosity but also the elastic properties of the displacing fluid that determine the displacement efficiency. This may be caused by the ability of a viscoelastic fluid to pull oil out of dead-ends. The aim of our work is to obtain a fundamental understanding of the effect of fluid elasticity, by developing an advanced computer simulation methodology for the flow of non-Newtonian fluids through porous media.We simulate a 3D unsteady viscoelastic flow through a model porous medium using computational fluid dynamics. The primitive variables velocity, pressure and stresses are used in the formulation. The physical and rheological properties of actual polymer solutions used in polymer flooding have been incorporated, where the viscoelastic stress part is formulated using a FENE-P type of constitutive equation. The simulations are performed using a finite volume methodology with a staggered grid. The solid-fluid interfaces of the porous structure are modeled with a second ordered immersed boundary method.The porous medium is generated by placing stationary spherical particles of equal size in random positions using a Monte Carlo method. By means of 3D periodic boundary conditions we model the flow behavior for Newtonian and viscoelastic fluids through such a porous structure. The effect of porosity and different Deborah numbers (De) is studied in detail. The simulations provide insight on how flow structure and viscoelastic stresses change with increasing De number. To our surprise we observe completely different flow structures at high De through various pore configurations. The simulations provide a detailed understanding of the strong interplay between fluid rheology and flow topology in a random porous medium. This work has a significant importance for applications in oil recovery, polymer and food processing, and other industries.@en

Controlled particle migration in non-Newtonian fluids is very important in many biological, environmental and industrial applications like hydraulic fracturing. The phenomenon of particle transport in non-Newtonian fluid is generally governed by complex interplay of different effects like shear-thinning or thickening behaviour of fluid, viscoelasticity and secondary flow. The present literature on the particle migration in a channel shows that particles move towards the centreline in highly viscoelastic fluids at very low Reynolds number (negligible inertial effects) and move away from the centreline in Newtonian fluids. However, very little is known about how flow and particle dynamics is affected in a rock fracture network or porous medium. To obtain insights into flow dynamics in such a complex geometries, we use microchannels with the specific arrangement of pillars in order to mimic the hydraulic fracture network and porous media. In this study, we perform experiments on the migration of non-Brownian particles in such pillared microchannel of different porosities with different types of fluids - Newtonian, shear-thinning and viscoelastic. The comparison of the particle behaviour in these fluids reveals the influence of the viscous and elastic forces on particle migration. In this work, standard micro-PIV technique is used to obtain the fluid velocity and PTV is used to track the particle velocities. This work provides fundamental insight on effect of viscoelasticity on particle migration in a model fracture network/porous media. @en

Polymer liquids are used in the oil industry to improve the volumetric sweep efficiency and displacement efficiency of the oil from a reservoir. Surprisingly, it is not only the viscosity but also the elastic properties of the displacing fluid that determine the displacement efficiency. To understand the effects of viscoelasticity on enhanced oil recovery, both single and multiphase experiments are performed in pillared microchannels of different pitches. Different shear thinning viscoelastic fluids are used to obtain insights into flow structures in these pillared micro channels. Particle image velocimetry (PIV) technique is applied to characterise the complex flow structures at very low Reynolds number (< 0.01). The pressure drop across the channel for a range of Deborah numbers is measured using a pressure sensor. We observe an onset of flow asymmetry for the viscoelastic fluid after a critical Deborah number depending on the channel configuration. This flow asymmetry increases with an increase in Deborah number and shows characteristics of elastic turbulence. This can be characterised by an increase in velocity and pressure fluctuations which has a temporal and spatial dependency. The sudden increase in the velocity fluctuations is also visible from the micro-PIV experiments. Though the fluid is shear thinning in nature, we observe an increase in the apparent viscosity after the critical De number is reached, which represents a thickening behaviour. So strong shear and extensional effects of the viscoelastic fluid are responsible for the asymmetric flow structures. Next we performed multiphase flow experiments, by saturating the microchannel with oil, and then displacing it by different non-Newtonian fluids. Surprisingly we found the enhanced recovery corresponds to the critical De number where the onset of instability initiates for the single phase experiments. The elastic instabilities and the relation to enhanced displacement provides fundamental insights in the mechanism of polymer flooding.@en