Study On The Effect Of Repeated Contact At The Same Position On Micro-nano Adhesion Based On Atomic Force Microscopy

The interaction between two surfaces is one of the basic problems in surface science.In the microcosmic field,microelectromechanical systems are affected by surface effects due to abnormally large surface-body ratios.Adhesion is one of the main reasons leading to the failure of micro-nano system.In order to further reduce and control adhesion,a large number of experimental and theoretical studies are needed to explore the mechanism of adhesion.The main tool for measuring adhesion is the atomic force microscope(AFM).Among the many factors affecting adhesion,the influence of repeated contact on adhesion cannot be ignored.Earlier studies suggested that repeated contact had little effect on adhesion,but it did not.At present,the effect of repeated contact at the same position on adhesion is not very clear.Therefore,in different environments(dry,wet,different humidity,continuous humidity change),this paper studied the influence of repeated contact at the same position on adhesion.The main research work is as follows:(1)The effect of repeated contact between polystyrene(PS)microspheres and polymethyl methacrylate(PMMA)samples on the adhesion was studied by AFM in a dry nitrogen environment.The results show that the adhesion at the same position depends largely on the surface topography,the number and distribution of roughness peaks,the nonlinearity of the instrument and the contact history.The adhesion behavior is different at different positions,and the adhesion is usually divided into several layers and jumps between them,especially to adjacent layers.The difference of adhesion between adjacent layers is almost the same at the same position.It is also observed that the adhesion increases,decreases and stabilizes with the increase of contact times at different levels.(2)In a humid laboratory environment,AFM was used to measure the adhesion between a needle-free silicon microcantilever and different hydrophilic surfaces,and the evolution behavior of the adhesion with repeated contact was investigated.The results show that the adhesive force does not remain constant,but exhibits various evolution behaviors with the contact number:(1)it keeps increasing;(2)Continuous decrease;(3)Maintain basic stability;(4)Suddenly jump to a large value;(5)Suddenly jumps to a small value.At the same time,the adhesion sometimes exhibits a behavior of "layering" and jumps back and forth between layers depending on the number of contacts.Even for different positions on the surface of the same sample,the law of adhesion evolution is varied.These evolution behaviors are attributed to the evolution of interfacial liquid Bridges.(3)At different relative humidity(10%?90%),AFM was used to measure the adhesion between the needle-free silicon microcantilever and silica and graphene surface(substrate is silica),and the evolution of adhesion with repeated contact at the same position was investigated.The results show that the adhesion on the surface of silica exhibits different evolution behaviors according to relative humidity:(1)at low humidity(10%-40%),the adhesion decreases sharply at first,and then remains unchanged or increases;(2)At medium humidity(40%-60%),the adhesive force basically remains unchanged(with slight fluctuations);(3)At high humidity(70%-90%),the adhesive force increases first and then decreases,or decreases sharply first and then remains stable.When it is stable,the adhesive force also tends to increase or decrease slowly,or shows the phenomenon of stratification and jump between layers.These evolution behaviors are attributed to the size and shape evolution of the interface liquid bridge with repeated contact.By contrast,the evolution behavior of adhesion is different after monolayer graphene is attached.At 10%?70% relative humidity,the adhesion becomes smaller(10?40 n N,about1/2?1/8 of that of silica),and usually changes sharply in the first dozens of contacts,and the phenomenon of stratification and jump between layers appears with repeated contacts.This stratification is attributed to the change in the number of small liquid Bridges at the interface.At each level,adhesion may increase or decrease or remain roughly constant with the number of contacts.However,at 80% and 90% humidity,the adhesion is still very large(up to ?480n N),and increases with repeated contact until saturation or decreases.These behaviors have also been attributed to the evolution of liquid Bridges with repeated contact.(4)The adhesion forces on some materials with different hydrophilicity were measured with atomic force microscopy(AFM)cantilevers to investigate the evolution behavior with RH.With continually changed RH(increasing,stable,and decreasing),the behavior in the forcevolume mode is usually inverted U-shaped with two “hat brims”(stable-increasing-stabledecreasing-stable).However,the behavior on some samples is deformed inverted U-shaped due to the decrease in magnitude at high RHs.The increment on partially hydrophobic and hydrophobic surfaces are much larger that on a hydrophilic surface.The behavior with RH at a location can also be inverted U-shaped with two “hat brims”.The behaviors of both gradual and sudden change were observed at a location with changed RH.At high stable RHs,the adhesion force shows complex behaviors.Some other abnormal behaviors were observed,indicating the particularity at a location.Therefore,the measurement at a location is not suitable to study the RH dependence.These behaviors were attributed to the evolution of a water bridge,largely owing to thin-film flow.