Priscilla Ribeiro Varges

Spherical forcing by means of propellers has proven to be a convenient method for generating homogeneous isotropic turbulence with a reduced mean flow. Being a relatively recent approach, it has been exclusively used with Newtonian fluids. The present work consists of a first experimental characterization of the flow properties of aqueous solutions of Carbopol, a well-known test fluid possessing shear thinning and yield stress properties, under spherical forcing. Experimental conditions generated from two Carbopol concentrations, together with pure water, and five propeller speeds are evaluated by means of stereo particle image velocimetry. We assess the suitability of this forcing method for fundamental studies by quantifying the degree of homogeneity, isotropy, and mean flow magnitude and by obtaining fundamental flow properties, namely integral length and velocity scales, Taylor microscales, and dissipation rates. In addition, to assess the influence of viscosity variability on the flow dynamics, we combine rheometric measurements with velocimetry data to estimate viscosity statistics. For the Carbopol solutions, the velocity fields present levels of homogeneity and isotropy comparable to water flows, as well as excellent homogeneity for viscosity statistics. Although the mean flow magnitude increases in the non-Newtonian cases, it remains within acceptable limits. Among the modifications to turbulent properties introduced by Carbopol, viscosity fluctuations generate new mechanisms of energy dissipation, which present a significant impact. These findings provide valuable insights into the flow response of Carbopol solutions to mechanical forcing and highlight the critical importance of a proper understanding of the impact of shear-dependent viscosity on flow dynamics.

Cryo-scanning electron microscopy (cryo-SEM) has become a powerful tool for characterizing complex fluids, bridging microscopic structure and macroscopic rheology. In this work, we provide practical guidelines for optimizing for the application of cryo-microscopy, emphasizing the importance of tailoring experimental parameters to the specific chemical and physical properties of the fluid. Key considerations include freezing methods, fracturing precision, controlled sublimation, and optimized coating thickness. Using Carbopol, xanthan gum, and Laponite as model fluids, we demonstrate that the observed microstructures reflect intrinsic properties rather than artifacts of sample preparation. However, inadequate freezing or over-sublimation, as well as improper handling such as rapid sublimation, air bubble entrapment, or excessive coating, can compromise the results, whereas optimized conditions effectively preserve the native network structures. The microstructural changes revealed by cryo-SEM correlated directly with rheological properties, such as yield stress increase with Carbopol concentration and reversible thixotropic recovery in Laponite. We highlight both the potential and the limitations of cryo-SEM, underscoring its role as a complementary methodology to rheological analysis in soft matter research and industrial applications.

Flow assurance in the oil industry aims to prevent flow interruptions in production caused by phenomena such as wax crystallization, emulsion formation, and the development of complex microstructures that alter viscosity and yield stress. The present study implements a standardized methodology to evaluate five crude oils and their corresponding 50/50 (v/v) emulsions with synthetic brine, under controlled cooling rates of 0.8 and 0.08 °C/min. The protocol combines stress ramp and creep tests to obtain the static yield stress, when applicable, and the flow curves to obtain the viscosity function for the other cases. The results revealed Newtonian, shear-thinning, and viscoplastic behaviors, demonstrating the diversity and complexity of the samples analyzed. Differences between crude oils and emulsions indicate that the coexistence of wax crystallization and dispersed droplets can influence the macroscopic rheological response. The discussion explores how these interactions may contribute to the observed behaviors. The proposed approach enables a robust evaluation of flow performance at low temperatures and provides essential data for designing efficient pipeline, pumping, and separation systems, with broad applicability to other complex fluids encountered in flow assurance scenarios.

The demand for minimally invasive body contouring procedures, particularly for gluteal augmentation, has grown significantly. This study evaluates the morphologic and rheological properties of four commercially available hyaluronic acid (HA) fillers used for body contouring and explores their clinical implications. Critical parameters such as storage modulus (G′), loss modulus (G″), complex modulus (G*), and damping factor (tan δ) were measured using oscillatory rheological tests to assess each filler’s elasticity, viscosity, and viscoelastic profile. Scanning Electron Microscopy (SEM) was performed to analyze the microstructure of the fillers, providing insights into their microscopic architecture. The results showed differences in mechanical properties and viscoelastic behavior among the fillers. These variations suggest that the choice of filler may need to be tailored to specific body contouring requirements. Understanding these differences is crucial for achieving the best clinical results and patient satisfaction, helping professionals select the most suitable filler for each case.

Summary: The success of an oilwell drilling operation is directly associated with the correct formulation of drilling fluids and their rheological measurements. The goal of this study is to investigate the usage of a Fann 35A viscometer and the methodology for rheological characterization of drilling fluids by comparison with the use of a rotational rheometer. Flow curves and gel strength tests were performed considering classic measurement artifacts such as apparent wall slip, secondary flows, steady-state (SS) regime, and inertial effects, among others. In addition, a study of the relationship between pressure drop and flow rate in a tube and in an annular space was carried out to investigate the influence of the viscosity function and of the rheological properties on the design of pipelines and the correct sizing of pumps. Use of American Petroleum Institute (API) equations and curve fitting were explored as potential choices for viscosity functions. The results indicate that the use of API equation predictions can compromise the effectiveness of the drilling process, while the choice of an adequate viscosity function is essential for the correct sizing of pumps. The gel strength was evaluated in the viscometer and presented divergent results from those obtained in the rheometer. Furthermore, a grooved geometry was developed for the viscometer to avoid the effects of apparent slip at low shear rates. Some recommendations are made based on the results obtained, which lead to better accuracy in the rheological results of drilling fluids and, consequently, better performance of some functions assigned to it. The proposed improvements and methodologies proved to be promising, although in some cases the cost-benefit remained unchanged.

The cement plug placement operations are represented by buoyancy-driven exchange flows between two yield stress materials. It is an unstable situation since cement slurries are usually denser than drilling fluids and consequently their positions tend to invert. To better understand this phenomenon, experimental and numerical analysis were performed considering a denser liquid

We present an experimental study of the flow of yield stress materials through annular abrupt expansions–contractions, to evaluate the flow invasion into the cavity formed in the larger cross section region. Steady inertialess flows of Carbopol® aqueous dispersions were investigated. The flow pattern reveals yielded and unyielded regions, which were visualized using tracer particles, laser sheets, and a digital camera. The yield surfaces were identified in the experiments by choosing large enough exposure times that allow sufficient particle displacement in the yielded region. To estimate the amount of fluid that remains stagnant in the cavity, we defined the invasion ratio, a quantity that was determined through image processing for different combinations of the governing parameters. The influence of the cavity diameter and axial length, eccentricity, and inlet velocity on the invasion ratio was investigated. Fore-aft asymmetric yield surfaces were observed for all tests, probably due to elastic effects.

We experimentally studied the viscous fingering instability considering a Newtonian oil displacing viscoelastic shear-thinning liquids and vice versa. The non-Newtonian liquids are aqueous solutions of polyacrylamide and xanthan gum, i.e., flexible and rigid polymers, respectively. A rectangular Hele-Shaw cell, connected by two plenum chambers, was developed to evaluate the displacement of a fixed volume. The experiment consists of analyzing the interface time evolution through a digital camera as a function of the geometric, dynamic, and rheological parameters. The displacement efficiency was determined through image processing in order to identify the formation of fingers or plugs. Unlike the Newtonian case, the transition does not occur when the viscosity ratio is roughly equal to one, but nevertheless, it was observed that the stability of the interface depends on the viscosity ratio. Specifically, more branches are observed at low viscosity ratios. Furthermore, a higher stability is observed when the Newtonian liquid displaces the shear thinning liquid, especially when the polymer is more rigid. When the Newtonian liquid is being displaced, elastic effects favor the displacing efficiency.

In this work we perform an experimental study of the buoyancy-driven exchange flow of the cement plug placement in oil wells, most commonly for abandonment and sidetracking. In this process, a denser fluid is placed above a less dense one, generating a highly unstable situation that can lead to unsuccessful operations when their positions invert before the cement curing. To analyze the exchange flow we perform visualization experiments with real drilling fluids positioned above a lower-density liquid in a Plexiglas vertical tube. The use of a drilling fluid, instead of a real cement plug at the top, was chosen in order to eliminate the curing influence on the preliminary tests. Thus, the same density difference between cement and other well fluid can be evaluated. A sliding gate ensures an initially flat interface that evolves with time immediately after the gate opening. The interface shape and front speed is recorded with the aid of a digital camera, and measured through image processing and analysis. Since in the real situations the well diameters are larger than the ones used in the laboratory, the interfacial tension effects are naturally negligible. Hence, surfactants are used to reduce the influence of interfacial tension on the experiments. The effects of the governing parameters on the inversion velocity and flow regime are analyzed for different combinations of fluids. Drilling fluids and cement plugs have non-Newtonian viscoplastic behavior, and careful rheological tests are performed to obtain the rheological parameters. Therefore, yield stress, viscosity levels, geometric parameters and density ratios between the fluids can lead to different flow patterns that will contribute to a successful or unsuccessful operation. The analysis of the experimental results will allow one to identify, for a given pair of fluids, the operational window in the governing parameter space within which the inversion velocity is sufficiently low (or zero) to ensure the heavier fluid placement success.