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  • Particle Image Velocimetry – Particle Size and Distribution

    Particle size:

    Particle size is connected to many of the other parameters of seed particles in a PIV system. With size affecting visibility, flow conformity, and being integral in relation to pixel size. A rough number for ideal particle size is 1-100um though sizes in the nm and mm’s have been used for certain PIV applications. With smaller sizes being necessary for micro-PIV methods and larger sizes being a requirement for large scale flow visualization. The importance of size is related to how truly the tracer will follow the flow with particle diameter having the largest effect on stokes number, which is a representation of flow tracer fidelity. Though when particle size becomes too small it can be difficult to confirm that the tracer is not being affected by minor currents or other factors within the fluid. Also as size decreases visualizing the spheres can become quite challenging. However, the stokes number can provide a decent representation of how well particles follow the flow. Though, the stokes number is an approximation based on assumptions and therefore can only provide a useful representation rather than a confirmation of tracer fidelity.

    Particle size distribution:

    Fluorescent Red Polyethylene Tracers

    A parameter that should be considered in conjunction with particle size is distribution. As particles in the sizes used for PIV are so small that no meaningful quantity of tracers can be produced in a specific size and rather size ranges need to be considered. With tighter size distributions, there will be less error attributable to differences in visibility of particles and a better approximation of how well each particle being used will conform to the flow. For example fluorescent red polyethylene has multiple size ranges available (10-22um, 10-45um, 10-90um, and 10-150um). With tighter size distributions being more difficult to obtain and as such being more expensive. Raising the question of what the trade off between price and size distribution is. Wide distributions can be used within PIV, however they may necessitate further image processing and may reduce accuracy of measurements. Therefore, there is no perfect size distribution choice. Though, with the understanding of what is available the choice of a correct size and size distribution can be determined.

  • Particle Image Velocimetry – Intro to Tracer Particle Parameters

    PIV is a vast field with varying techniques and differing areas of research. Techniques vary from 2D PIV, only viewing velocity in a plane of the fluid system, to high speed TOMO PIV which views a 3D area of fluid and can be time resolved allowing for acceleration data to also be obtained. Another difference is that the size of liquid PIV set-ups can range from micron sized micro channels to multi thousand-gallon tanks. While the area being imaged may not vary as much as the

    Barium Sulfate Tracer for X-ray imaging

    systems themselves, it can still differ from units of micro meters to potentially meters. With viewing windows growing as new advancements in science and technology progress, the need for seed particles to match them will grow. One example of this is the rise of helium filled soap bubble seeders that provide an easily visualized 300um bubble for air systems allowing for large areas to be seeded and visualized. Or barium sulfate polyethylene microspheres which are useful due to being a radio contrast agent allowing for visualization via x-ray imaging.

    Therefore, a one solution fits all approach is not feasible when it comes to seed particle selection. As each experiment will have differing size, density, light intensity/visibility, particle material, and seeding concentration needs based on desired results.

  • PIV Seeding – Microparticle Recommendations

    Flow Visualization can best be accomplished with colored or fluorescent tracing particles of the same density as the fluid being studied.Particle image velocimetry (PIV) is the term used for imaging the fluid flow using colored tracer particles.  Through high speed particle imaging the velocity of the particles can be obtained and mapped.

    Photo Courtesy of Wiki-CommonsFor PIV experiments in water there are a wide variety of 1g/ml microspheres available as seeding particles, Cospheric offers polyethylene seeding particles in a variety of colors (fluorescent and non-fluorescent) and sizes from 10-20um, on up to 1mm.

    Particle size selection is important to ensure that observations can be accurately made.  For imaging systems without any magnification or unaided human observation it is recommended to use micro-spheres that are larger than 200um.   Human eyes can resolve features down to about 75um, at 200um+ the particles will be large enough to be discernible.

    Correctly seeding the system is also important.  For most applications loadings of 0.1-0.3% are sufficient.  This is the equivalent of about 1 gram of particles per gallon of fluid in systems of 10-50 gallons (40-200 liters) .  Exact loading levels will depend on the individual experiment requirements.

    For aqueous systems, polyethylene micro-particles will require a surfactant to wet properly.  For general flow studies pre-wetting the PE microspheres with Simple Green concentrated cleaner works well.  For biological systems a bio-compatable surfactant such as Tween 20 or Tween 80 is recommended.

    Seeding particle color selection is important to ensure that sufficient contrast is achieved during testing.  Very bright fluorescent micro-particles are available in densities of 1g/cc  these offer not only bright testing under daylight conditions, but also the option of illumination with lasers or uv lights for increased system contrast.

    The recommended PIV seeding particles are available from Cospheric.

    For green lasers (530nm)  we recommend UVPMS-BR
    For dark backgrounds: UVPMS-BY, UVPMS-BG, BLPMS, ORGPMS, BSPMS, or WPMS
    For light backgrounds: GRYPMS, VIOPMS, UVPMS-BR, UVPMS-BO

  • Microparticles for simulating fish egg dispersion and recruitment

    Understanding survival and mortality of fish in the early life stages has been a fundamental issue in biology and a central problem in fisheries oceanographic study for more than a century. It has been argued that most marine fishes begin life as an egg that floats in the sea, and, during their evolutionary history, the early life of fishes has surely been shaped to ensure the “continued existence of species” by the sheer pressure of natural selection, and stated that a fish to survive must deal with and exploit its physical and biological environments. However, although we are now in the 21st Century, there still remains a lot to be made scientifically clear in the early life of fishes. In the present lecture, I will talk about fundamental issues in the isolated floating eggs of marine fish, which many pelagic species spawn in thousands, millions, or sometimes almost billions during a life of an individual female. The topics contain description and discussion on the egg size, buoyancy and rising speed. Measurements on the eggs naturally spawned in aquaculture systems are firstly introduced. Several examples of egg vertical distribution, accumulation and dispersion observed through field surveys will be shown to consider how the egg size and buoyancy are adaptive to survive in the pelagic environment.1

    UVPMS-BG 180-212um - 40x MagnificationScientists who study fish require artificial micro-particles to simulate fish eggs and their dispersion behavior in water. In order to accurately simulate the dispersion of fish eggs it is important to use particles of the proper size and buoyancy/density. Particles with accurate size ranges and densities are now available from Cospheric LLC. Densities from 1.00g/cc up to 1.12 g/cc are available in size ranges from 10-27um on up to 0.85-1.0mm. Sea water particles of 1.025g/cc (UVPMS-BG-1.025), and fresh water beads of 1.00g/cc are in stock and available for quick delivery.

    Most fish eggs are in the size of 0.5-5mm1 with the typical size of 1mm being the most common.   Salt water fish eggs tend to be slightly less dense than medium saltwater at a density of about 1.020g/cc1.

    Continue reading “Microparticles for simulating fish egg dispersion and recruitment” »

  • Neutrally Buoyant in Water – Microspheres with Specific Gravity of 1g/cc

    Cospheric offers polymer microspheres with specific gravity of 1g/cc are specifically designed to match the density of water for optimal suspension of particles. Suspension of microspheres in water enables the visualization and characterization of fluid flow and testing the capability of devices to withstand particulate matter in the fluid stream, ensuring that microspheres do not settle and do not float on the surface.  Most of these polymer microspheres are at least moderately opaque and clearly visible in water, clear or translucent liquids.

    Many Colors are available in sizes from 10um up to 1180um,  of polyethylene microspheres are available with specific density of 1g/cc, designed for optimum suspension in water for fluid flow visualization.  Colors include Violet, Orange, Yellow, Fluorescent, Grey, Pink, Blue, and many fluorescent colors.

    600-710um violet PE microspheres250-300um ORANGE PE Microspheresblue microspheres 355-425um Yellow microspheresGrey Microspheres PE