Measurement of Van der Waals force using oscillating optical tweezers

We employ oscillating optical tweezers as a probe to measure the surface forces between polystyrene and silica. Thus, we modulate a trapped polystyrene particle with an external sinusoidal force in close proximity (∼80 nm) of a silica surface. The particle motion is influenced by several factors which include an increased drag force according to Faxen's correction, a spurious force that comes into play due to the diffusion coefficient of the medium becoming position dependent, and finally, the London-Van der Waals (LVdW) force which becomes substantial when the particle approaches the surface. By accounting for the other forces from the analytically known results, we are able to directly quantify the LVdW force from the experimentally measured amplitude of the oscillating particle. Thereby, we determine the Hamaker constant H for the LVdW force between polystyrene and silica, and obtain a good agreement with the value reported in the literature. Our method is general in nature and can be extended toward probing other surface effects or other interaction forces using oscillating optical tweezers.