Abstract

Rotational optical tweezers uses circularly polarized light to rotate birefringent microparticles. Normally, if the particle is trapped far away from a surface, the rotation rate only goes to zero when the tweezers laser power is turned to zero. However, we find that if one traps close to a surface, the rotation rate goes to zero even at finite tweezers laser powers for some type of substrates. We suspect this to be due to binding between the substrate and the birefringent particle, keeping in mind that the hydrodynamic drag for this mode of rotation cannot increase beyond 1.2 times the drag away from the surface. We use this to probe some surfaces and find that there is no binding for hydrophilic ones but hydrophilic ones particularly tend to show a power threshold beyond which the birefringent particle starts rotating. We also place the particle on the threshold and observe ”stick-slip” kind of rotational behaviour.

© 2019 The Author(s)

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