Particle–wall interactions are important in biology, micromachining, coagulation studies, and many other areas of science. As a contactless tool, optical tweezers are ideal for measuring these kind of interactions. Here we will present a new method for calculating the non-optical forces acting on a trapped particle using simultaneous position and force detection. Analysis of the particle's Brownian motion when trapped gives a measure of all the forces experienced by the particle. In contrast, measuring only the light's momentum change directly gives the solely optical force. This is achieved measuring the changes in the scattered light. The difference between the forces recorded by the two techniques reveals the external forces acting on the trapped particle. Therefore, by trapping the particle close to a wall, one can study the particle-wall interaction force in details. The simulation were done using the optical tweezer toolbox  to find the optical force acting on a particle. The net force was calculated from a Brownian motion’s statistics of a trapped particle in the presence of the exponential external force. By using the proposed method, we were able to successfully reconstruct the external force. The experiment was done on a trapped spherical PMMA particle (d=2.2um) close to the 3D-printed wall. For the particle-wall distance ~0.7um the non-optical force is ~100fN . The experiment and simulation results confirm the efficiency of the proposed method for an external force measurements.  Nieminen et al., J. Opt. A 9, S196-S203 (2007).
Anatolii V. Kashchuk, Ann A. M. Bui, Alexander B. Stilgoe, David M. Carberry, Timo A. Nieminen, and Halina Rubinsztein-Dunlop, "Measurements of particle-wall interaction forces using simultaneous position and force detection
(Conference Presentation)," Proc. SPIE 9922, Optical Trapping and Optical Micromanipulation XIII, 99221C (Presented at SPIE Nanoscience + Engineering: August 31, 2016; Published: 10 November 2016); https://doi.org/10.1117/12.2236419.5161498045001.
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