Study of adhesivity of surfaces using rotational optical tweezers

Optical tweezers are powerful tools for high resolution study of surface properties. Such experiments are traditionally performed by studying the active or the Brownian fluctuation of trapped particles in the X, Y and Z directions. Here we find that employing the fourth dimension, rotation, allows for sensitive and fast probing of the surface, and happen when birefringent microparticles are applied with circularly polarized light, thus called the rotational optical tweezers. When the trapped birefringent microparticle is far away from the surface, the rotation rate is dependent only on the laser power. 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 specific type of substrates. We suspect this to be due to the interaction between the substrate and the birefringent particle, keeping in mind that the Faxen correction for this mode of rotation cannot increase beyond 1.2 times. We use this to probe some surfaces and find that there is no binding for hydrophobic ones but hydrophilic ones particularly tend to show a laser power threshold to start rotating. We calculate that the threshold energy of the tweezers is consistent with the Van der Waals potential energy, when the mode of interaction with the surface is purely physical. We also find that for chitosan, the mode of interaction is possibly different from Van der Waals. Further, we place the particle on the threshold and observe 'stick-slip' kind of rotational behaviour.