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We present a method for measuring the van der Waals force between two microspheres based on photonic force microscopy. We trapped a microsphere as probe by optical tweezers. The restricted Brownian motion of Gaussian distribution could be found in this system. The vibration center of the probe was affected by the van der Waals force when a target microsphere was closer to the probe. We measured the vibration center of the probe at different separation between the pair of microspheres. Based on this, the measurement of the van der Waals force between the two microspheres was realized with a high precision. Our method can realize the direct measurement of van der Waals force without using the variation rules of it. This method results in a simple structure, would not damage the sample, and can be suitable for the surface of any shape. It is general and has a wide range of applications in other fields of micro-force measurement.

We presented an optical system that could measure the viscosity coefficient of liquid in a micro-area. The orbital rotation of a polystyrene microsphere was realized by a dual-beam fiber-optic trap with a transverse offset. The rotation rate increased with the viscosity coefficient of the environmental medium. On this basis, the viscosity coefficients of ethanol solutions with different concentrations were measured successfully. The volume of solution samples was less than 1 μL. This provides a basis for the viscosity measurement of rare liquid or enchylema, which is of great significance for biological applications such as cell characteristics and reaction dynamics.