Direct Measurement of Infinitesimal Depletion Force in a Colloid-Polymer Mixture by Laser Radiation Pressure

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Abstract
It is widely believed that depletion force causes formation of colloidal crystal or flocculation in the stable colloidal suspension mixed with nonadsorbing polymers. This force works between a colloidal particle and plane, which can be regarded as a sphere with infinite radius. We report a novel technique that enables us to measure the depletion force working between a colloidal particle and a glass surface using laser radiation pressure. We found that the observed force almost coincides with the predicted one.