Study On Air Damping Dissipation In Tapping Mode Atomic Force Microscope

Phase image in tapping mode atomic force microscope(TM-AFM)can more accurately reflect the changes of the physical and chemical properties of the sample surface than the topography image.However,the reasons affecting the phase or phase contrast are very complex.Currently,the interpretation of phase image is still a hot issue in theory and experiment,and there is a great challenge.Even for very skilled operators,how to select scanning probes,determine scanning parameters,control the experimental environment,identify artifacts,eliminate artifacts,and improve image quality in the experiment are still in the stage of perceptual knowledge and non-systematic rational knowledge.It is shown that the phase image is related to the energy dissipation of the TM-AFM system.There are various forms and properties of energy dissipation when scanning in the laboratory atmosphere,and the phase image of TM-AFM is the embodiment of the energy dissipation of the whole system.Air damping is one of the most important energy dissipation mechanisms of TM-AFM.The quality factor of microcantilever can reach tens of thousands in vacuum but only a few hundred in air and even down to single digit in liquid environment.In this paper,the air damping effect in TM-AFM is studied by means of theory,simulation and experiment.Firstly,the dynamic model of micro-cantilever based on displacement excitation is established,and the quality factor expression of continuum beam model is given.On this basis,the expressions of equivalent mass,equivalent stiffness and equivalent damping are given,and the single-degree-of-freedom harmonic oscillator model is established.The similarities and differences of lumped mass and spring terms in the motion differential equations and in the boundary conditions are discussed.Secondly,the air rarefaction effect and small Reynolds number effect in micro-flow are discussed.Based on the assumption of continuous fluid,the air damping expression of micro-cantilever while vibrated in air freely is given,and the corresponding quality factor is obtained.ANSYS-FLUENT carries out bidirectional fluid-solid coupling calculation,and the pressure distribution is obtained.Based on Reynolds equation,one-dimensional and two-dimensional models of squeeze film damping of micro-cantilever were established,and the changes of quality factors were compared by changing related parameters such as tip-sample distance and aspect ratio.The nonlinear Reynolds equation was solved by MATLAB finite element method to obtain the pressure distribution.The bidirectional fluid-solid coupling calculation was carried out by ANSYSFLUENT to verify the results.The air squeeze film damping model of inclined tipless cantilever,microprobe and ordinary probe are established,and the squeeze film damping effect in TM-AFM is given quantitatively or qualitatively.The results show that the theoretical models of inclined tipless cantilever and micro-probe are in good agreement with the experimental results.For the ordinary probe,due to the complexity of the slender tip,the variation trend of quality factor with needlesample distance is qualitatively explained.When the tip-sample distance is far,the slender tip can be neglected,but when the tip-sample is far,it cannot be neglected.This study is very important for the development of energy dissipation theory and AFM imaging technology.