Everything about Microspheres
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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.

  • Bichromal Janus Particles, Microspheres, Microbeads – Stock selection or Custom-made

    Bichromal (half-white half-black or any other color) Microspheres, Janus Particles

    Bichromal (half-white half-black or any other color) Microspheres, Janus Particles. In this picture - Paramagnetic black microspheres with partial white coating - Magnification 40x.

    Cospheric offers unique capability to manufacture Janus microspheres and microparticles with partial coatings and dual functionality. Currently half-shell or hemispherical coatings can be applied to any sphere (glass, polymer, ceramic) in sizes 45micron in diameter and higher. Coatings can be customized for any color and coverage of between 20% to 60% of the sphere. Each coating is custom formulated for color, charge, magnetic, electric, and surface properties, and solvent resistance per customers’ needs.

    Half-coated glass microspheres - Partial coating on glass particles

    Half-coated glass microspheres - Partial coating on glass particles. In this picture - Soda lime glass microspheres with partial red coating - Magnification 40x.

    Hemispherical coatings of less than 1 micron with tolerances as low as 0.25 micron have been routinely demonstrated.  Color combinations are truly unlimited. White, black, silver, blue, green, red, yellow, brown, purple as well as transparent microspheres have been made. Sphericity of greater than 90% and custom particle size ranges are offered.

    We have successfully coated solid and hollow glass microsphere, including soda-lime, borosilicate, and barium titanate glass microspheres. We have also coated on silver.

    Half-coated Microspheres

    Half-coated Microspheres

    Optically anisotropic spheres and janus particles with magnetic half-shells have been originally developed for applications in electronic displays, such as e-paper, but are now widely used in numerous applications in diagnostics, medical research, microscopy and biotechnology, as well as electronics, due to their ability to orient themselves in response to electromagnetic field and show a visual response. This is achieved by making spheres both bipolar and bichromal, with dipole precisely aligned with two differently colored hemispheres. As the spheres align themselves, the viewer will observe the color of one hemisphere, while the other hemisphere will be hidden from view, providing an obvious strong visible indication of the presence of the field or other stimuli.  In alternating electromagnetic field, these microspheres have been proven to spin at hundreds of times per second.

  • 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

  • Calculating microspheres per gram

    During scientific experiment design and analysis it is common to need to know the number of spheres per gram of dry material.  We have put together the table below to help speed up the process.

    If you have material of a density different from those listed in the table, divide the number of spheres per gram in the density ~1.0 g/cc column by the true particle density of your material to get an estimate of the number of spheres per gram.

    Product Size Polyethylene
    Density ~1.0 (g/cc)
    Soda Lime Glass
    Density ~2.5 (g/cc)
    Lower (um) Upper (um) Spheres per Gram Spheres Per Gram
    20 27 147,162,715 58,630,564
    27 32 74,393,558 29,638,868
    32 45 33,467,185 13,333,540
    45 53 16,233,536 6,467,544
    53 63 9,788,528 3,899,812
    63 75 5,813,720 2,316,223
    75 90 3,401,258 1,355,083
    90 106 2,029,192 808,443
    106 125 1,239,525 493,835
    125 150 734,672 292,698
    150 180 425,157 169,385
    180 212 253,649 101,055
    212 250 154,941 61,729
    250 300 91,834 36,587
    300 355 54,371 21,662
    355 425 32,196 12,827
    425 500 19,305 7,691
    500 600 11,479 4,573
    600 710 6,796 2,708
    710 850 4,025 1,603
    850 1000 2,413 961
    1180 1400 890 354
    1400 1700 513 204
    1700 2000 302 120
    2000 2360 184 73
    2360 2800 111 44
    2800 3350 66 26

    Note: This table assumes the mean diameter is half way between the upper and lower size.

  • Monodisperse Silica Nanospheres and Microspheres – Dry Powder

    Scientists are always pushing the limits of the technology and looking for more precise products and equipment to enable the next technology. Advances in nanotechnology is what enables this progress.  Nanoparticles are particles that have the dimensions of no less than 1 nanometer and no greater than 1000nanometers (1 micron). Nanospheres are specialized highly spherical nanoparticles. In an effort to provide our customers in the nanotechnology industry with specialty nanomaterials, Cospheric LLC has added a selection of highly precise and spherical monodisperse silica nanospheres to their broad product line and created CosphericNano – a website dedicated exclusively to nanospheres and nanoparticles.

    Silica Microspheres 3.8um DiameterSilica nanoparticles (nanospheres) and silica microspheres are now available in distributions of less than 10% CV in the sizes of 0.25 micron, 0.5 micron, 0.62 micron, 1 micron, 4 micron and 8 micron. Each product is extensively characterized under SEM (scanning electron microscope) to ensure perfect sphericity of each nanosphere and tight particle size specifications.

    Selection of Silica Nanospheres and Nanoparticles are available:

    SIO2MS 0.25um (250nm) <10% CV

    SIO2MS 0.48um (480nm) <10% CV

    SIO2MS 0.62um (620nm) <10% CV

    SIO2MS 1.15um  <10% CV

    SIO2MS 3.8um <10% CV

    SIO2MS 7.8um <10% CV

    SIO2MS 4-6um  (2-19um range)

    Dry precise monodisperse silica nanospheres are frequently used for biotechnology applications, sintering, microfluidics, electronics, optical coatings, spacers and other high tech applications.