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This article presents the results of an experimental and analytical investigation of the fluid flow in a pocketed thrust bearing. An experimental test rig was designed, developed, and used to visualize fluid flow in pocketed thrust bearings. Microparticle image velocimetry (μPIV) was used to measure fluid flow inside the pocket of a thrust bearing. The thrust bearings were constructed by gluing precision shim stocks to a flat BK7 glass disk in contact with a polished steel disk. The precision shim stock provides the desired pocket depth for the bearing. A polished steel disk in contact with the thrust bearing was driven by a motor in order to induce fluid flow within the pockets. μPIV was then employed to measure the shear-driven cavity flow and generate the quiver plots of the flow field. Three different lubricants were used at various speeds and a constant load to measure the effects of speed and viscosity on the flow out of the pocketed thrust bearing. In order to achieve the analytical aspect of this research, a model was developed to predict the film thickness, cavitation area, and pressure distribution generated within the bearing. The cavitation areas obtained from the model were compared with the experimental results. The results corroborate well. The calculated pressure and film thickness were then used to determine the 3D velocity profiles within the pocketed thrust bearing. The measured velocities obtained from the experimental images were compared to the analytical velocity fields. Comparing the measured velocities with the analytical model, the depth of the microparticles in the bearing pocket was determined. Using this approach, the μPIV-measured 2D velocity field was converted into a 3D velocity field, which illustrates the fluid motion inside a pocketed thrust bearing at various speeds and viscosities. [ABSTRACT FROM AUTHOR]

Formation of wear particles within total hip replacement is one of the main causes of its failure. In addition to improving the lubrication and wear resistance of materials used as bearing surfaces, understanding of wear particle distribution patterns within lubricants inside an implant gap could be used to improve design parameters and implants’ lifespan. In this study, the behaviours of biolubricants (with compositions similar to human joint synovial fluid) and suspended particles were investigated by micro-particle image velocimetry in curved mini channels under oscillatory Couette flow conditions. The studied biolubricants had shear-thinning viscoelastic characteristics. The authors found that increasing shear-thinning, elasticity or motion frequency levels did not affect the trend behaviours of biolubricant flows due to the low strain values of the experimental conditions applied. However, suspended particles formed strings along flow directions and exhibited cross-stream migration to channel walls. Motion frequency, fluid shear thinning and elasticity characteristics and channel dimensions strongly affected particle behaviours.

In this study, the flow of lubricating greases in a labyrinth seal geometry is studied using microparticle image velocimetry (µPIV). The aim is to evaluate the grease velocity distribution inside the gap of a labyrinth seal and to find a relationship between the grease consistency and the transferred speed from the rotating ring in order to choose the correct grease as a sealing medium. In addition, the grease flow characteristics are important for the understanding of fracture due to grease layer displacement. For these purposes, four greases with different rheological properties were used in µPIV experiments. It was found that the grease consistency plays a crucial role in speed development as well as the grease composition and presence of a slip effect at the grease–rotating wall interface. [ABSTRACT FROM AUTHOR]

Microbioreactors are gaining increased interest in biopharmaceutical research. Due to their decreasing size, the parallelization of multiple reactors allows for simultaneous experiments. This enables the generation of high amounts of valuable data with minimal consumption of precious pharmaceutical substances. However, in bioreactors of all scales, fast mixing represents a crucial condition. Efficient transportation of nutrients to the cells ensures good growing conditions, homogeneous environmental conditions for all cultivated cells, and therefore reproducible and valid data. For these reasons, a new type of batch microbioreactor was developed in which any moving mixer component is rendered obsolete through the utilization of capillary surface waves for homogenization. The bioreactor was fabricated in photosensitive glass and its fluid volume of up to 8 µL was provided within a bowl-shaped volume. External mechanical actuators excited capillary surface waves and stereo microparticle image velocimetry (µPIV) was used to analyze resulting convection at different excitation conditions in varied reactor geometries. Typical vortex patterns were observed at certain resonance frequencies where best mixing conditions occurred. Based on the results, a simplified 1D model which predicts resonance frequencies was evaluated. Cultivation of Escherichia coli BL21 under various mixing conditions showed that mixing in resonance increased the biomass growth rate, led to high biomass concentrations, and provided favorable growth conditions. Since glass slides containing multiple bowl reactors can be excited as a whole, massive parallelization is foreseen.

Induced-charge electro-osmosis around multiple gold-coated stainless steel rods under various AC electric fields is investigated using the techniques of microparticle image velocimetry and numerical simulation. In this study, the results of interactions between induced electric double layers of two identical conductive cylinders on surrounding fluid are presented. The induced-charge electro-osmosis flow around multiple rods in touch and with one cylinder diameter gap reveals quadrupolar flow structures with four vortices. The induced-charge electro-osmotic flow structure and velocity magnitude also depend on the cylinder geometry and orientation. It is seen that four small vortices develop in the close region of cylinder surface for multiple rods with gap, while the other four large vortices are surrounding them. The distributions of vorticity patterns also strongly depend on cylinder orientation in the close region of cylinder surface. [ABSTRACT FROM AUTHOR]

We describe the development of an optical flow visualization method for resolving the flow velocity vector field in lymphatic vessels in vitro. The aim is to develop an experimental protocol for accurately estimating flow parameters, such as flow rate and shear stresses, with high spatial and temporal resolution. Previous studies in situ have relied on lymphocytes as tracers, but their low density resulted in a reduced spatial resolution whereas the assumption that the flow was fully developed in order to determine the flow parameters of interest may not be valid, especially in the vicinity of the valves, where the flow is undoubtedly more complex. To overcome these issues, we have applied the time-resolved microparticle image velocimetry (μ-PIV) technique, a wellestablished method that can provide increased spatial and temporal resolution that this transient flow demands. To that end, we have developed a custom light source, utilizing high-power light-emitting diodes, and associated control and image processing software. This paper reports the performance of the system and the results of a series of preliminary experiments performed on vessels isolated from rat mesenteries, demonstrating, for the first time, the successful application of the μ-PIV technique in these vessels. [ABSTRACT FROM AUTHOR]

The internal flow field during droplet formation stage accompanied with mass transfer is heuristically investigated in a T-junction micro-channel. De-ionized water/phosphate acid/butanol is used as the working system. Micro-PIV measurement is conducted to capture the velocity distribution inside the droplet. Experimental results illustrate that as the mass transfer is involved in this process, the dispersion size of droplet is decreased. There exists an internal circulation inside the droplet during formation stage in the T-shape micro-channel. The velocity of vortex inside the droplet can be reduced when the mass transfer process is enhanced. A parameter of swirling strength is applied to demonstrate the convection effect inside the droplet. At the initial stage of the formation process, swirling strength of the vortices is increased as the phosphate acid concentration increases. But, as the generation of droplet, the swirling strength of the condition with lower phosphate acid concentration increases much faster than that with higher phosphate acid concentration. The experiment results can help to explore the mass transfer mechanism in micro-devices. [ABSTRACT FROM AUTHOR]

Fluid flows in microchannels with microcavities at low Reynolds number are increasingly used in microfluidic applications such as trapping and sorting of cells and particles. For optimizing the microcavity configuration and better controlling the microenvironment in the microcavities, it is important to thoroughly understand the flow behaviors in the microcavities. Hence, using microparticle image velocimetry (μPIV), we investigated quantitatively the flow characteristics of rectangular microcavities with a wide range of aspect ratio ( λ = 0.25-3) and Reynolds numbers ( Re = 0-100). Depending on the control parameters ( Re and λ), a flow regime map in microcavities has been constructed, including three different flow patterns: attached flow, separated flow, and transitional flow. The critical parameters for the transform of flow patterns were determined. Only a single central microvortex appears in the microcavities, and the evolution and characteristics of the microvortex were investigated in detail. The results revealing the flow mechanism of different flow patterns in the rectangular microcavities can provide useful design guidelines of microfluidic-based devices, as well as a map to help microfluidic users in their design of application-driven microcavities. [ABSTRACT FROM AUTHOR]

Grease is extensively used to lubricate various machine elements such as rolling bearings, seals, and gears. Understanding the flow dynamics of grease is relevant for the prediction of grease distribution for optimum lubrication and for the migration of wear and contaminant particles. In this study, grease flow is visualized using microparticle image velocimetry (μPIV). The experimental setup includes a concentric cylinder configuration with a rotating shaft to simulate the grease flow in a double restriction seal geometry with two different grease pocket sizes. It is shown that the grease is partially yielded in the large grease pocket geometry and fully yielded in the small grease pocket. For the small grease pocket, it is shown that three distinct grease flow layers are present: a high shear rate region close to the stationary wall, a bulk flow layer, and a high shear rate boundary region near the rotating shaft. The grease shear thinning behavior and its wall slip effects have been identified. The μPIV experimental results have been compared with a numerical model for both the large and small gap size. It is shown that the flow is close to one-dimensional in the center of the small pocket. A one-dimensional analytical model based on the Herschel-Bulkley rheology model has been developed, showing good agreement with the measured velocity profiles in the small grease pocket. Furthermore, wall slip effects and shear banding are observed, where the latter imply that using the assumption of uniform shear in conventional concentric cylinder rheometers may result in erroneous rheological results. [ABSTRACT FROM PUBLISHER]

This paper reports the visualization of droplet formation in co-flowing microfluidic devices using food-grade aqueous biopolymer-surfactant solutions as the dispersed droplet phase and sunflower oil as the continuous phase. Microparticle image velocimetry and streak imaging techniques are utilized to simultaneously recover the velocity profiles both within and around the dispersed phase during droplet formation and detachment. Different breakup mechanisms are found for Newtonian-Newtonian and non-Newtonian-Newtonian model water-in-oil emulsions, emphasizing the influence of process and material parameters such as the flow rates of both phases, interfacial tension, and the elastic properties of the non-Newtonian droplet phase on the droplet formation detachment dynamics. [ABSTRACT FROM AUTHOR]

Microparticle image velocimetry (μPIV) is used to measure the grease velocity profile in small seal-like geometries and the radial migration of contaminant particles is predicted. In the first part, the influence of shaft speed, grease type, and temperatures on the flow of lubricating greases in a narrow double restriction sealing pocket is evaluated. Such geometries can be found in, for example, labyrinth-type seals. In a wide pocket the velocity profile is one-dimensional and the Herschel-Bulkley model is used. In a narrow pocket, it is shown by the experimental results that the side walls have a significant influence on the grease flow, implying that the grease velocity profile is two-dimensional. In this area, a single empirical grease parameter for the rheology is sufficient to describe the velocity profile. In the second part, the radial migration of contaminant particles through the grease is evaluated. Centrifugal forces acting on a solid spherical particle are calculated from the grease velocity profile. Consequently, particles migrate to a larger radius and finally settle when the grease viscosity becomes large due to the low shear rate. This behavior is important for the sealing function of the grease in the pocket and relubrication. [ABSTRACT FROM AUTHOR]

This article presents the results of experimental investigations of lubricant flow out of a micropocket in a conformal contact due to surface shear. A test rig was designed and developed to perform a micropocket flow under boundary and starved lubrication conditions. The test rig consists of a laser-machined circular micropocket on a flat specimen operating against a rotating glass disk under an applied load. Silicone oil was used as the test fluid. Optical microvideography was used to investigate lubricant extraction from micropockets. A high-speed camera was used to observe the lubricant extraction phenomena from micropockets under various operating conditions. Microparticle image velocimetry was also implemented to quantify and analyze lubricant flow from a micropocket. The following results were obtained: (1) the role of a micropocket by comparing the lubrication mechanism of a contact with and without a micropocket; (2) the effects of load, speed, and micropocket geometry on lubricant extraction; and (3) the detail of the micropocket flow during the extraction using microparticle image velocimetry (μ-PIV). [ABSTRACT FROM AUTHOR]

Through confocal-like microparticle image velocimetry experiments, we reconstruct, for the first time, the three-dimensional flow field structure of the azimuthal fluid recirculation in a sessile drop induced by asymmetric surface acoustic wave radiation, which, in previous two-dimensional planar studies, has been shown to be a powerful mechanism for driving inertial microcentrifugation for micromixing and particle concentration. Supported through finite element simulations, these insights into the three-dimensional flow field provide valuable information on the mechanisms by which particles suspended in the flow collect in a stack at a central position on the substrate at the bottom of the drop once they are convected by the fluid to the bottom region via a helical spiral-like trajectory around the drop periphery. Once close to the substrate, the inward radial velocity then forces the particles into this central stagnation point where they are trapped by sedimentary forces, provided the convective force is insufficient to redisperse them along with the fluid up a central column and into the bulk of the drop. [ABSTRACT FROM AUTHOR]

This article presents the results of an experimental and corresponding analytical investigation of fluid flow out of a shallow microcavity (dimple) due to surface shear. An experimental test rig was designed, developed, and used to demonstrate shear-driven fluid flow out of a microcavity. The experimental test rig consists of a flat steel belt driven against a microcavity filled with a fluid. Microparticle image velocimetry (µPIV) was employed to visualize the shear-driven cavity flow and generate the streamline contours of the flow field. Two different sets of experiments were conducted. The first set examines the important factors that affect the flow field, and these include the cavity depth-to-width ratio (aspect ratio) and the speed of the flat belt. The second set of experiments was designed to observe the important fluid flow phenomena including the extraction and stagnation of the fluid above and below a critical cavity depth. A model was also developed using the computational fluid dynamic software COMSOL Multiphysics (1) to corroborate the experimental and analytical results. The results are in good agreement. [ABSTRACT FROM AUTHOR]