Test Fluids Research Articles

Shear resistance and composition of polyethylene protuberances from hip-simulating pin-on-disc wear tests

Protuberances on the surface of ultra-high-molecular-weight polyethylene (UHMWPE) pins were chemically and mechanically investigated in order to better understand the tribology of UHMWPE vs. CoCrMo, which is a typical material pairing for joint replacements.Pin-on-disc wear tests were performed using pins made of UHMWPE articulating against discs made of a CoCrMo alloy. Wear tests were performed using two different test fluids: a standard test liquid used for hip-simulator tests and a synthetic synovial fluid containing hyaluronic acid, albumin, immunoglobulin G, the phospholipid lecithin and additionally sodium azide to fight bacterial growth.After the wear tests, the pin surfaces exhibited scratches as well as protuberances with a pitting-like appearance. These protuberances, i.e. elevations protruding from the articulating surface, were 6 ± 3 μm high on the pins lubricated with the standard test liquid and 20 ± 5 μm high under the lubrication with the synthetic synovial fluid. Investigating the protuberances using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) showed that these were composed mainly of UHMWPE, together with amine groups from proteins. To our knowledge, the mechanical properties and namely shear resistance of these protuberances were investigated the first time. The hardness and the elastic modulus of the protuberances were similar to the bulk material, as revealed by nanoindentation. The shear resistance of the protuberances as measured by a nanoscratch test method was comparable or even higher than that of the bulk material.

A new design of soft texture analyzer tribometer (STAT) for in vitro oral lubrication study

The relative movement between food particles as well as between food particles and oral surface is considered as critically important in influencing food oral processing and sensory perception. Precise characterization of oral lubrication/friction has recently attracted a lot of attention among food researchers for insight information about the controlling mechanisms of oral sensation and sensory perception. This study is a continuation of our effort in trying to establish a feasible in vitro soft tribology technique for food oral processing and sensory studies. Specific purpose of this study is to further improve the reliability and feasibility of a TA-tribometer developed previously by the group. A lighter and soft probe was produced to make it more consistent with that of oral conditions and a new connecting mechanism between the probe and the load cell was introduced with the help of 3D printing technique. A set of near-Newtonian fluids (water, syrup solutions and glycerol) with different shear viscosity were used as testing samples to assess the reliability at a wide range of sliding speeds and load pressures. Friction coefficients obtained at various experimental conditions were integrated and plotted as Stribeck curves. Viscosity/load ratio was introduced to understand and predicate the lubrication behavior of test fluids. The calculated Viscosity/load ratio not only influenced friction coefficient magnitude but the establishment of tribological regimes. A boundary regime can be clearly seen at a low range of viscosity/load ratio (0.00569–0.0327). For viscosity/load ratios between 0.211 and 5.097, lubrication would be most likely in mixed regime. Once viscosity/load ratio goes beyond 6.627, a hydrodynamic regime becomes dominant. Results verify the feasibility of the new modification and the improved device suggests a promising application for assessment the lubrication behavior of various near-Newtonian fluids. Future experiments will involve the use of beverages and semi-solids, explore the application potential for in vitro oral lubrication studies.

Heat and mass transfer characteristics of binary droplets in acoustic levitation

The complex relationships between the flow field and heat transfer phenomena of acoustically levitated droplets under evaporation were investigated. To explain these correlations, binary droplets of ethanol and water were used as test fluids. Immediately after droplet levitation, the droplet external flow field direction was toward the droplet, with a circulating vortex forming near the droplet surface. As evaporation progressed, the external flow transitioned toward the opposite direction, while the circulation vortex expanded. To better understand the transition process of the droplet thermal boundary layer, the heat transfer coefficient time series changes were calculated by assuming that the transitions of the ethanol and water binary droplets occurred in three stages: (1) preferential evaporation of ethanol, (2) transition (evaporation of ethanol and condensation of water), and (3) evaporation and condensation of water. Finally, by comparing the flow field and thermal boundary transitions, the transition mechanism for flow structures and heat transport phenomena of acoustically levitated droplets with evaporation was considered. Our experimental and analytical results provide deeper physical insights into noncontact fluid manipulation and suggest potential future applications, such as in acoustic tweezers and microreactors.

Trends and Variation in Care and Outcomes for Children Hospitalized With Acute Gastroenteritis.

Assess trends in inpatient acute gastroenteritis (AGE) management across children's hospitals and identify elements of AGE management associated with resource use. We examined inpatient stays for children 6 months to 18 years hospitalized with AGE from 2009 to 2018 using the Pediatric Health Information System database. We characterized demographics, hospital-level resource use (ie, medications, laboratories, and imaging), and outcomes (ie, cost per case, 14-day revisit rates, and length of stay [LOS]). We compared demographic characteristics and resource use between 2009 to 2013 and 2014 to 2018 using χ2 and Wilcoxon rank-sum tests. We grouped hospitals on the basis of 2009 use of each resource and trended use over time using logistic regression. Annual change in mean cost and LOS were estimated by using models of log-transformed data. Across 32 354 hospitalizations at 38 hospitals, there was a high use of electrolyte testing (85.4%) and intravenous fluids (84.1%) without substantial changes over time. There were significant reductions in the majority of laboratory, medication, and imaging resources across hospitals over the study period. The most notable reductions were for rotavirus and stool testing. Many hospitals saw a decrease in LOS, with only 3 noting an increased revisit rate. Reductions in cost per case over time were most associated with decreases in imaging, laboratory testing, and LOS. Significant variation in resource use for children hospitalized with AGE coupled with high use of resources discouraged in AGE guidelines highlights potential opportunities to improve resource use that may be addressed in future AGE guidelines and quality improvement initiatives.

An imaging approach for in-situ measurement of refractive index of a porous medium

Refractive index matching (RIM) is often used for flow measurement in porous media studies that use optical techniques. However, the refractive index of transparent porous media, often constructed of beads, is not always readily available or measurable with commonly used measuring devices such as refractometers. Therefore, an alternative technique is needed for determining the refractive index of the beads forming the porous media. The available methods in the literature involve a rigorous procedure and use a laser to illuminate the medium. Their setups are often complex and are sensitive to small changes in the temperature or wavelength and sometimes needs continuous stirring of the medium to have a homogeneous porous media. A new approach for measuring and matching the refractive index of beads that minimizes the limits of the available techniques is presented here. Unlike common RIM measurement approaches, the proposed technique involves no laser light and therefore is relatively simpler than similar approaches found in the literature. This approach uses shadow imaging to capture images of the beads forming the porous medium while the RIM fluid flows through it. A comparison approach using a reference image and a structural similarity metric to quantify the level of the refraction index matching is used to identify the refractive index of the dispersed phase. The approach is verified experimentally using borosilicate glass beads as the dispersed phase. Two different RIM fluids, Potassium thiocyanate (KSCN) solutions and a mixture of Drakeol 7, as silicone oil and soybean oil are used as the test fluids to compare the independency performance of the approach. The approach not only identifies the general refractive index of the dispersed phase but can also be used to identify local spatial variations in the porous media. In addition, using the imaging approach minimizes the effort for determining the refractive index of the solid for flow experiments and reduces the need for equipment or changes to the setup.

Thermal performance of a mini-channel heat exchanger (MCHE) working with CNT/GNP-based non-Newtonian nanofluids

In this work, the heat transfer of the non-Newtonian nanofluids inside a wavy microchannels heat exchanger (WMCHE) in cross-flow configuration has been experimentally and numerically studied. The Reynolds number (Re) does not have unique definition for non-Newtonian fluids. The non-Newtonian carboxyl methyl cellulose (CMC) aqueous solution containing 0.2 mass% CMC is used as base fluid (BF). The single-walled carbon nanotubes (CNT) and graphene nanoparticles (GNP) were added to the BF in 0.1 mass% as two other test fluids. Here, an equation based on the assumption of laminar flow is used in order to evaluate Re as a function of experimentally measured pressure drop. However, this assumption needs a similar form of friction factor relation to that of Newtonian fluids that is verified based on numerical simulation. In the presented work, finite element method (FEM) was utilized to perform the numerical modeling through Comsol Multiphysics software. Results show that as flowrate and relative waviness(2A/2L) increase, the convective heat transfer coefficient could be intensified. In terms of pressure drop, it was seen that with increasing the flowrate and relative waviness(2A/2L) of nanofluids, pressure drop was intensified. The results are compared to the experimental data and showed good agreement. The proposed performance index implies that GNP/BF nanofluid is the best one and both of two kinds of wavy configurations enhance the heat transfer efficiency, although the wavy two configurations are better.

Planar LIF/MIE ratio droplet sizing using structured laser sheet imaging at elevated ambient pressures

LIF/Mie ratio-metric imaging was used to characterize sprays produced by a simple hollow-cone pressure atomizer, operating under elevated ambient pressures up to 10 atm. A structured laser sheet was used as the source of illumination to suppress the multiple scattered light, generating images that are free of the artefacts typically found in conventional laser sheet images. The resulting LIF/Mie ratio-metric images were calibrated using Phase Doppler anemometry to generate axial planar maps of the spray’s Sauter-mean diameter (SMD). This calibration methodology was applied over a range of ambient pressures and liquid flows to assess the robustness of the structured LIF/Mie ratio-metric imaging as a droplet sizing technique. The test fluids consisted of conventional and alternative jet fuels as well as nozzle calibration fluid. Results presented in the paper indicate both the effectiveness and certain limitations of the technique.

Comparative numerical and experimental investigation of process viscometry for flows in an agitator with a flat blade turbine impeller

This paper presents a method for measuring the viscosity of generalized Newtonian fluid directly in flows generated by flat-blade turbine impellers, which are commonly used for moderate mixing and dispersion. A flat-blade turbine with four blades is defined as a model system and analyzed through numerical simulations with experimental verification. Carbopol 940 solution, a high viscosity non-Newtonian fluid with a yield stress, and a bentonite based drilling mud solution were selected as test fluids. Numerical simulation techniques for flow in agitators with a yield stress was established using the rotating coordinate system and flow solutions were validated with experiments by comparing the torque on the impeller shaft. The Metzner-Otto constant and the energy dissipation rate constant were predicted by numerical simulations using the Metzner-Otto correlation and validated via experiments. The effective viscosity that reproduces total energy dissipation rate identical to that of a Newtonian fluid was obtained from both numerical and experimental methods at different impeller speeds, from which the material viscosity curve was established as a function of the shear rate. The accuracy of viscosity prediction was compared with a rheological measurement and the average relative error was below 12% and 7% in the experiment and simulation, respectively. This method has the advantage of being able to measure the in-situ viscosity, where a drilling mud needs to transport more and heavier cuttings and careful preparation of the mud is key issue to a successful drilling process.

Mechanistic study on silica nanoparticles-assisted guar gum polymer flooding for enhanced oil recovery in sandstone reservoirs

Nanoparticles-assisted polymer flooding has attracted the attention of researchers for its enhanced rheological properties and stability of the chemical slugs. The present work deals with the rheological properties of guar gum with and without silica nanoparticles in the temperature range from 25 °C to 75 °C. It is found that the viscosity of guar gum solution increases significantly with increasing concentration of silica nanoparticles. Imbibition experiments were performed with waxy crude oil-saturated sandstone cores using 2000 ppm brine, 4000 ppm guar gum solution, 0.2 wt% silica nanoparticles in distilled water, and 4000 ppm guar gum containing 0.2 wt% silica nanoparticles as test fluids for understanding the mechanism of wettability alteration in the presence of silica nanoparticles and guar gum. Mixture of 0.2 wt% silica nanoparticles and 4000 ppm guar gum shows higher oil recovery (∼36 % original oil-in-place) compared to other test solutions in imbibition experiments. Contact angles between crude oil and sandstone rock were measured in presence of 2000 ppm brine, 0.025 wt% silica nanoparticles in water, and guar gum (2000 ppm)-silica nanoparticles (0.025 wt%) mixture. Guar gum containing silica nanoparticles are effective in changing the contact angle towards water-wet state (Contact angle ∼115°) from oil-wet state (contact angle ∼72°). No significant phase change is observed during the phase behavior experiment except for light emulsion formation. Finally, core flooding experiments were conducted with guar gum, silica nanoparticles solution, and mixture of silica nanoparticles and guar gum for additional oil recovery to verify the effectiveness of the chemical slugs. Results showed that the addition of silica nanoparticles to guar gum solution improves the effectiveness of guar gum polymer by recovering 44.3 % of original oil-in-place. A comparative study was also conducted to show the performance of silica nanoparticles-assisted guar gum solution for additional oil recovery. Most of the research works have focused on the application of silica nanoparticles-induced polyacrylamide and xanthan gum chemical slugs in enhanced oil recovery. However, laboratory study based on the application of silica nanoparticles-induced guar gum solution in additional oil recovery adds some interesting findings to the literature compared to the existing information.

Accretion on high derivative asymptotically safe black holes * * Supported in part by Hebei Provincial Natural Science Foundation of China (A2014201068)

Asymptotically safe gravity is an effective approach to quantum gravity. It is important to differentiate modified gravity, which is inspired by asymptotically safe gravity. In this study, we examine particle dynamics near the improved version of a Schwarzschild black hole. We assume that in the context of an asymptotically safe gravity scenario, the ambient matter surrounding the black hole is of isothermal nature, and we investigate the spherical accretion of matter by deriving solutions at critical points. The analysis of various values of the state parameter for isothermal test fluids, viz., k = 1, 1/2, 1/3, 1/4 show the possibility of accretion onto an asymptotically safe black hole. We formulate the accretion problem as Hamiltonian dynamical system and explain its phase flow in detail, which reveals interesting results in the asymptotically safe gravity theory.

SYNERGISTIC EFFECT OF POLYMER-POLYMER MIXTURES AS DRAG REDUCING AGENTS ON OIL-WATER FLOWS

The synergistic effect of natural and synthetic polymers as drag reducing agents in water phase during horizontal oil-water flows was studied in 12 mm internal diameter (ID) unplasticised polyvinylchloride (uPVC) pipe. Partially hydrolyzed polyacrylamide (HPAM; trade name as magnafloc 1011 or Separan), polyethylene oxide (PEO; trade name as Polyox WSR 310), Aloe Vera mucilage (AVM; as Aloe barbadensis miller), mixture of partially hydrolyzed polyacrylamide and Aloe Vera mucilage and polyethylene oxide and Aloe Vera mucilage (HPAM-AVM and PEO-AVM) were used. Mixture Reynolds number of 62923, master solution of 2000 ppm and 20000 ppm, and total concentration (TC) of 30 ppm and 400 ppm were investigated using diesel oil (ρ = 832 kg/m3, µ = 1.66 cP) and water (ρ = 1000 kg/m3, µ = 0.89 cP) as test fluids. The results show that the drag reduction (DR) of 65.39 and 69.23%, and 69.23 and 70.77% were obtained for HPAM-AVM and PEO-AVM mixtures respectively, at mixing ratios of 3:1 and 1:19, for 25% oil input fraction and water phase Reynolds number (Rew) of 47192. These values were found to be higher than the DR obtained by individual polymer alone at the same conditions. Drag reduction decreased with increase in the oil input fraction due to the decrease in the water phase Reynolds number. The result implies drag reduction efficiency can be enhanced by combining natural and synthetic polymers.

Washing Effect of Microbubble Mixture on the Soiled Model Attached on a Metal Surface

We washed a soiled stainless-steel (SUS304) surface with attached starch paste as the soiled model because of investigating washing effect of microbubble mixtures generated by a swirling method (S-MB), a pressurized method (P-MB), and a pressurized method with a pressurized tank (NP-MB). Compared with the resultant washing rate of tap water alone, relatively high washing effect of microbubble mixtures was obtained. Furthermore, the washing effect of P-MB approximately agreed with those of S-MB. For understanding the experimental results, particle diameter change against elapsed time and static contact angle of interface between solid surface and droplet of test fluids was discussed. Particle diameters of all microbubble mixtures were changed as elapsed time. Elapsed time until collapse of P-MB higher than that of S-MB. Moreover, NP-MB exhibited highest elapsed time until collapse. Moreover, static contact angle of microbubble mixtures was lower than that of tap water alone. Correlation between washing effect and contact angle was obtained. The obtained knowledges were beneficial for many applications using fine bubble mixtures.

Influence of laser texturing on the wettability of PTFE

Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer showing excellent thermal and electrical insulation properties and a low coefficient of friction. Due to its large stability, and hydrophobic nature, the wettability of PTFE surfaces can be reduced to transform them into superhydrophobic. In this regard, laser texturing is a fast, simple and versatile method to produce superhydrophobic PTFE surfaces in one-step, and over large areas. In this work, we used a CO2 laser to modify the surface of PTFE samples. We studied the effect of the processing parameters (laser power or irradiance, scanning speed, and spacing -overlapping- between scan lines) on the wettability of textured surfaces using water, mineral oil and ethanol/water solutions as test fluids. Laser-treated surfaces showed a hierarchical micro- and nanotopography with a cotton-like appearance. The higher roughness and large quantity of air pockets make these laser-treated surfaces superhydrophobic, and highly oleophobic. Furthermore, they remain unaltered after being in contact with strong alkali and acid solutions or after slight friction. The self-cleaning performance of these surfaces was also demonstrated. The present findings suggest that CO2 laser texturing of PTFE is suitable for the large-scale preparation of surfaces with low-wettability to different liquids.

Preparation of Plasmonic Au-TiO2 Thin Films on a Transparent Polymer Substrate

In this work, plasmonic thin films composed of Au nanoparticles embedded in a TiO2 matrix were prepared in a transparent polymer substrate of poly(dimethylsiloxane) (PDMS). The thin films were deposited by reactive DC magnetron sputtering, and then subjected to heat treatment up to 150 °C in order to promote the growth of the Au nanoparticles throughout the TiO2 matrix. The transmittance spectrum of the thin films was monitored in situ during the heat treatment, and the minimum time required to have a defined localized surface plasmon resonance (LSPR) band was about 10 min. The average size of Au nanoparticles was estimated to be about 21 nm—the majority of them are sized in the range 10–40 nm, but also extend to larger sizes, with irregular shapes. The refractive index sensitivity of the films was estimated by using two test fluids (H2O and DMSO), and the average value reached in the assays was 37.3 ± 1.5 nm/RIU, resulting from an average shift of 5.4 ± 0.2 nm. The results show that it is possible to produce sensitive plasmonic Au-TiO2 thin films in transparent polymer substrates such as PDMS, the base material to develop microfluidic channels to be incorporated in LSPR sensing systems.

Experimental Investigation of the Effects of Fluid’s Physicochemical Characteristics on Piping Erosion of a Sandy Soil under Turbulent Flow

Abstract This article presents an experimental study of the relative and interactive effects of three physicochemical characteristics of permeating fluids (viscosity, pH, and ionic strength) on piping erosion of a sandy soil under turbulent flow. Full-factorial experimental design was used to produce eight types of fluids of various combinations of the three fluid characteristics. Hole erosion tests were conducted on identically prepared sandy specimens. The erosion rate index was used to evaluate the effects of fluid properties on piping. The erosion rate index was quantified for two repeat trials for each of the eight test fluids. Regression analysis was conducted on the results to generate a statistical model to describe the relative effects of the three factors and their interactions. The main findings include the following: (1) Viscosity, pH, and ionic strength were all determined to be significant factors on the rate of erosion. The two-way interactions between viscosity and pH and between viscosity and ionic strength were also determined to be significant interaction factors, while the interaction between pH and ionic strength did not prove to be statistically significant. (2) Higher pH of the test fluid causes higher erosive capacity; higher ionic strength causes lower erosive capacity of fluid. (3) At low viscosity, ionic strength does not affect the erosion, but when the viscosity is higher (or the fluid temperature is colder), higher ionic strength causes much less erosion. There is almost no interactive effect between pH and ionic strength.

Guar Gum as Flow Improver in Single-Phase Water and Liquid–Liquid Flows

In this study, the effectiveness of guar gum as a flow improver in single-phase water as well as in oil–water flows was investigated. Guar gum is one of the hydrocolloids mostly used as a flow improver because of its unique rheological behaviour. It forms highly viscous shear thinning solutions at very low concentrations. In the oil industry, pressure drop is experienced in the transportation of fluid due to skin friction in the pipeline which translates to high energy cost. The addition of small amount of heavy molecular weight synthetic polymer has been found to enhance flow in pipes. This phenomenon is called drag reduction (DR). However, very few reports exist on the potentials of natural polymers which are more environmentally friendly and cheaper as a DR agent. The experimental rig consists of a horizontal 12- and 20-mm-ID uPVC pipes with oil (ρ = 832 kg/m3; µ = 3.5 cP at 25 °C) and tap water (ρ = 998 kg/m3; µ = 1.0 cP at 25 °C) as test fluids. The molecular weight of guar gum was determined to be 994,035 ± 1.9% g/mol. Results showed that maximum drag reduction of 45% was achieved by the addition of 200 ppm of guar gum to single-phase water flow at Re of 69,000 in the 12-mm-ID pipe. In oil–water flows, maximum DR of 23% was achieved for 0.25 oil fraction and 200 ppm of guar gum. In addition, drag reduction was found to increase with pipe diameter and decrease with increasing oil fraction. DR with guar gum compares better with synthetic polymers of equivalent molecular weight with promise for wider industrial applications.

Experimental study on the oscillation characteristics of thin liquid film in a microchannel

Characterization of thin liquid film is important for comprehensive understanding the phase-change heat transfer mechanisms in an oscillating slug flow. In this study, the thin film thickness of oscillating slug flow in a square tube was experimentally investigated using a nano-particle tracking velocimetry technique. Square tubes with inner dimension of 3.0 × 3.0, 1.6 × 1.6, 0.5 × 0.5, and 0.3 × 0.3 mm2 were used for test tubes, water was used as test fluids. The results show asymmetric velocity distribution in the thin liquid film. An obvious decrease of response time was observed for the thin liquid film when the oscillation frequency increases. The measured thin film thickness fits well with Taylor’s law when the capillary number is small. Deviation from Taylor’s law was found for larger capillary number, due to the prominent effect of bubble deceleration on the thin film thickness when the oscillation frequency increases. The relationship between acceleration process and deceleration process in different oscillation frequency is also studied.

Asymmetric flow of polymer solutions around microfluidic cylinders: Interaction between shear-thinning and viscoelasticity

Viscoelastic flow around a cylinder is a model problem representing a wide range of relevant industrial processing and biological applications. Reducing the cylinder to microscopic dimensions conveniently enables the problem to be examined in the absence of inertia. Recently, we have developed glass microfluidic geometries containing long and slender, yet rigidly fixed, microfluidic cylinders, which present a low blockage ratio β=2r/W=0.1, where r=20μm is the cylinder radius and W=400μm is the channel width. Using a shear-banding viscoelastic wormlike micellar (WLM) solution, we showed how the flow around such a cylinder could destabilize beyond a critical Weissenberg number (Wi=λU/r, where λ is a characteristic time of the fluid and U is the average flow velocity), resulting in the asymmetric division of the fluid around either side of the cylinder [Haward et al, Soft Matter15:1927]. In the present work we investigate this flow instability in greater detail using a range of polymer solutions formulated from hydrolyzed poly(acrylamide) (HPAA) dissolved at different concentrations in deionized water. The test fluids present a range of shear-thinning responses under steady shear, and also a wide variety of characteristic times. At low HPAA concentrations, the flow around the cylinder remains essentially symmetric for all Wi, but as the concentration increases, so does the maximum degree of the flow asymmetry observed with increasing Wi. Interestingly, at intermediate concentrations, the flow can resymmetrize at very high Wi. We understand these effects by considering simultaneously both the degree of shear-thinning of the fluid and the imposed Wi, and our analysis shows that both strong shear-thinning and high elasticity are required for the formation of strongly asymmetric flows. Our results represent the first report of this highly asymmetric flow state in polymer solutions, showing that it is a general phenomenon and not only specific to WLM. Our analysis provides a clear insight into the origins of this unusual flow state and may also be relevant to understanding other instances of asymmetries arising in shear-thinning viscoelastic flows.

Automated extraction of in situ contact angles from micro-computed tomography images of porous media

We present a simple and robust algorithm for the automated measurement of in situ contact angles from segmented micro-computed tomography images of immiscible fluid pairs in porous media and 2D slices of them. The algorithm comprises three steps: identification of contact points, an initial coarse estimate of contact angle, and its refinement. To obtain the coarse estimate we identify the vectors which point into the wetting phase and are also normal to the fluid/fluid and fluid/solid surfaces within the vicinity of each contact point. The coarse estimate is subsequently refined by fitting planes across fluid/fluid and fluid/solid surfaces to obtain the final estimate of contact angle. The algorithm was applied to a packed bed of glass spheres using air/water as the fluid pairs. A wide distribution of contact angles spanning the full range of possible values was measured for both the 2D and 3D cases. The distributions are skewed towards their respective means and are longer-tailed compared to a Gaussian distribution. The mean contact angles were found to be approximately 65±21°, which is significantly larger than bulk contact angles of 35±3° measured on a flat glass substrate using identical test fluids. The disagreement suggests that bulk contact angles can dramatically underestimate in situ contact angles even in the simplest porous media.

Estimation of shear rate change in vertically oscillating non-Newtonian fluids: Predictions on particle settling

This study investigates the effect of oscillatory motion on velocity and shear rate change within different non-Newtonian, slightly viscoelastic fluids oscillated in a vertical U-shaped circular pipe. An estimation to quantify the influence on particle settling in an oscillatory environment is also presented. Flow visualization using particle image velocimetry (PIV) technique was deployed to compare the two-dimensional velocity field in the vertical plane of the U-tube axis and the resulting shear rate change in three different non-Newtonian fluids. The experiments were performed in a 1.2 m high, 50 mm diameter transparent test section, at room temperature (21 ± 0.5 °C) and atmospheric pressure.A piston was driven at harmonic motion via a gas buffer, to provide the driving force for the test fluids at four different low frequencies ranging from 0.1 – 0.75 Hz and at three different piston oscillation amplitude ratios of A = a/D = 0.3, 0.4 and 0.5, where a is the displacement amplitude of the piston and D is the pipe diameter. Oscillatory Reynolds number (Reδ) based on Stokes layer thickness was used as the criteria for determining the specific flow regime. 36 different experimental cases were tested within the range of 2 < Reδ < 34, and all the experimental cases exhibited the laminar flow regime.The study reveals that the axial velocity amplitude along the pipe centerline increases with the increasing frequency and with increasing oscillation amplitude irrespective of the non-Newtonian fluid type. The thickness of the shear region decreases with the increase of frequency. The change of shear rate is maximum near the wall region of the pipe and that is achieved at the maximum position of the phase cycle, where the axial velocity also possesses its highest magnitude. The most viscous and the least elastic fluid have reported a maximum reduction of viscosity as an effect of the oscillatory motion and the viscosity reduction becomes insignificant when the non-Newtonian fluids become less viscous.