Pico-Newton Friction Force Measurements Using a Laser-trapped Microsphere

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Abstract
We have developed a system for measuring the force of friction by using the technique of laser-trapping. A polystyrene microsphere of a 3 µm diameter is trapped in a water layer using a focused near infrared laser. While the sample is scanned, all of the viscous, friction, and radiation forces are induced in the microsphere. The friction force acting on the microsphere is estimated by the viscous force obtained from the scanning speed of the stage using Stokes law, and the radiation force is obtained by the displacement of the microsphere. We demonstrate experimentally the existence of a linear relationship between the force of radiation and the displacement. We found that the value of the spring constant of the radiation force is 7.2 µN/m. Furthermore, we measured both the friction force acting on the probe on a flat quartz substrate surface and that at the edge of small steps. The magnitude of these forces are 0.88 pN and 1.23 pN, respectively.