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Microspheres are often used for studies that involve long term
suspension of microspheres in an aqueous solution. This is particularly important
for closed-system flow visualization studies, where researchers do not
have the opportunity to agitate the liquid and re-suspend the
microspheres. For long term suspension of particles, neutrally-buoyant
microspheres are required.
The density of water changes with temperature and pressure. It also changes when other materials are dissolved in it (for example, salt and sugar). When long-term suspension of microspheres in an aqueous solution is required, it is critical that the density of microspheres is matched as close as possible to the density of water at the correct temperature. If there is a mismatch in density, the microspheres will either sink or float. The rate of separation (or settling) is determined primarily by the difference in densities of the microspheres and the liquid, as well as particle size. The smaller the microspheres, the less sensitive they are to the density mismatch. For larger microspheres (such as close to 1mm in diameter) the density needs to match at least to 0.001g/cc in order for the particles to stay suspended for some time. Particles below 200micron might be able to tolerate density mismatch of 0.005g/cc. Particles below 50micron can potentially handle a difference in density of 0.01g/cc.
Every Cospheric microspheres product has been characterized with a NIST-traceable density measurement with the accuracy of +/-0.002g/cc, using the most accurate density measurement tools available on the market today.
The tables of Density of Water below are supplied to help the researcher select the right density of the microsphere for their application by matching it to the density of water at the correct temperature.
compiled by Niels Ramsing and Jens Gundersen