A Shadow-Based Nano Scale Precision Force Sensor

The meniscus surrounding a hydrophobic circular plate floating on a transparent liquid surface could refract light to form a shadow with a bright edge at the bottom of the vessel. This paper demonstrated the feasibility of a simple method to visually measure the force applied on the top surface of this circular plate by using its shadow image. The shadow area represented the floating force according to the updated Archimedes' principle. The viscous damping of liquid could reduce the high-frequency noises from the environmental vibration, which improved the stability and sensitivity of the measuring system. The water depth, initial weight of the plate, and the size of the circular plate effects on the shadows and the edge quality have been evaluated. With an ordinary camera to monitor the shadow, a simple circular plate device could realize a force resolution of up to 10 nN. The force measuring range and sensitivity can be easily extended by changing the radius of the circular plate. The shadow method could be developed into a novel nano scale precision force measurement apparatus.