Measurement of near-wall microparticle motion induced by evanescent-field radiation pressure

This study confirmed through experimental and theoretical investigations that considering the influences of the neighboring wall on the particle motion due to intermolecular, electrostatic, and hydrodynamic interactions is crucial when studying microparticles’ motion under evanescent-field radiation pressure. The velocity of polystyrene microparticles parallel to the interface was measured in the evanescent field generated at a glass–solution interface. The measured velocity and the estimated radiation force increased with the laser power, decreased rapidly as the incident angle increased from the critical angle and almost overlapped for p- and s-polarizations as reported in previous studies, and scaled with the integrated evanescent intensity over the particle surface. The theoretical estimation of the equilibrium wall–particle separation distance deq revealed that the increase in the NaCl concentration from 0 to 60 mM caused a decrease in deq from 63 to 10 nm by reducing the repulsive electric double-layer force between the wall and particles through electrostatic screening. This reduction in deq increased both the evanescent-field radiation force and hydrodynamic drag force exerted on the particle and significantly affected the resultant near-wall particle velocity. The measured velocity was constant at 20 < deq < 70 nm and rapidly decreased for deq < 20 nm, which was consistent with the theoretical model based on the balance between the radiation force and the modified Stokes drag force. These findings suggest that the motion of near-wall microparticles can be accurately predicted or controlled by appropriately incorporating the wall effect.