Van Der Waals Force Measurement Technology Based On Photonic Force Microscope

Optical tweezers use the mechanical effect of light to capture a microsphere,and have good application prospects in precision measurement and other fields.Photonic force microscope(PFM)is a technology based on optical tweezers.PFM captures a microsphere as the probe and performs the measurement of tiny force by measuring the variation of the motion of the probe.The London-Van der Waals(LVd W)force is a kind of intermolecular force,which affects the precision and stability of precision instruments in daily life.In this paper,we presented a method of van der Waals force measurement by using Photonic force microscope.We trapped a microsphere as probe by optical tweezers.The restricted Brownian motion of Gaussian distribution could be found in this system.This 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.The research work of this paper mainly includes the following contents:1.We deduced the expressions of the van der Waals force expressions of the microsphere-microsphere system and the microsphere-plane system based on the LannaJones potential model.Besides,we presented the relationship between the van der Waals force and the separation between the two surfaces.And we also deduced the motion state of the trapped microspheres in the optical trap and the influence of van der Waals force on the motion of the microspheres based on the Langevin equation.The effect of LVd W forces was obvious when the separation was less than 300 nm and it was pronounced in the motion of the probe particle when it was moved toward the surface of the target particle,resulting in obvious displacement.Based on this,we could calculate the LVd W force by the displacement of vibration center of the probe.2.Based on photon force microscopy technology,the LVd W force between the probe microsphere and the target microsphere was measured in a liquid environment.The measured LVd W was curved with the separation between the two surfaces and was consistent with the simulation results.The error was less than 10 f N.The LVd W force between two microspheres decreased with the increase of the separation between them and the results were consistent with the simulation.And The LVd W between the two silicon microspheres was stronger than between the silicon microsphere and the PS microsphere.The LVd W force between the silica microspheres with carboxyl and sulfhydryl groups and the silica microspheres without groups was stronger than the van der Waals force between the two microspheres without groups.3.Accurate measurement of van der Waals forces in a microsphere-plane system in air was achieved based on the PFM.The van der Waals force at different positions of the target plane was measured.The scanning function of PFM was realized,which highlighted the advantages of non-contact measurement without damaging samples.