Study On The Influence Of Relative Humidity On Micro/Nano Adhesion Forces Based On Atomic Force Microscopes

With the development of nanotechnology,the adhesion between two solid surfaces plays an important role in MEMS.At the micro and nano scales,adhesion is one of the main causes of failure.In order to reduce adhesion,it is necessary to carry out a lot of experiments and theories to further study the adhesion mechanism.In this paper,the influence of relative humidity on adhesion force was studied.Specific research contents are as follows:1)Evolution of adhesion between silica and HOPG interfaces from RH-independent to RH-dependentThe adhesion forces between atomic force microscopy(AFM)silica cantilevers and highly oriented pyrolytic graphite(HOPG)were measured to study the influence.The results show that the adhesion force behavior can evolve from RH-independent to RH-dependent with repeated contacts,and the RH dependence is related to contact history.In the beginning,the adhesion force is relatively small and RH-independent.However,it becomes RH-dependent after some time at a high RH and never changes again.The RH-dependent adhesion force usually changes slightly at low RHs(<40%),increases gradually with RH,and then remains the same at high RHs.With a suddenly changed RH,the RH-dependent force usually increases sharply,although sometimes there is a lag.With a continually changed RH,the RH-dependent force can keep pace with RH initially,but shows a certain lag and advance after some time.With a constant high RH,the RH-dependent force can either increase gradually with repeated contacts or remain the same or decrease to some extent and then remain the same.2)Influence of RH on adhesion between silica and mica interfacesThe adhesion force between silica micro cantilever and mica was measured using AFM,and the evolution of adhesion force with RH was investigated.There are three methods to change the humidity:(1)increasing the relative humidity in a step wise manner;(2)continuously changing the humidity in the way of increasing-constant-decreasing;and(3)abruptly changing from low humidity to a high humidity and maintain stability.The results revealed various evolution of adhesion force with relative humidity depending on the contact history,as follows:(1)In the initial stage of contact,the adhesion force showed weak RH-dependent on relative humidity.In the entire range of humidity or at low to moderate humidity levels,the adhesion force did not vary with increasing humidity.However,in the middle to later stages of contact,the adhesion force became sensitive to humidity changes.Generally,the adhesion force tended to change with the relative humidity,particularly in the latest stages of contact where it exhibited a strong RH-dependent.(2)When abruptly transitioning from low humidity to slightly higher humidity(still within the low humidity range),the adhesion force generally increased slightly,followed by a slight decrease with increasing number of repeated contacts.(3)When transitioning from low humidity to moderate humidity,the adhesion force might suddenly increase.However,if a large liquid bridge could not be formed(only small liquid bridges were present),the adhesion force would continuously decrease with repeated contacts until the small liquid bridge disappeared.(4)When transitioning from low humidity to high humidity,the adhesion force generally experienced a sharp increase initially,followed by fluctuations of increase and decrease.In other words,when maintained at high humidity,the adhesion force might even decrease.This phenomenon is attributed to the change in liquid bridge morphology,leading to a decrease in Laplace pressure and ultimately a decrease in capillary force.Additionally,at high humidity,a sudden drop in adhesion force to a minimum value could be observed,which is attributed to the rupture of large liquid bridges.(5)The adhesion force could also exhibit behaviors that do not synchronize with relative humidity changes,and even exhibit opposite behaviors:(1)when maintained at high humidity,the adhesion force decreases to a minimum value,and then continues to increase and remain constant;(2)when reduced to low humidity,the adhesion force remains at a relatively high value even after several tens of hours of probe-sample separation;and(3)when increasing the humidity,the adhesion force suddenly decreases to a minimum value.3)Difference in RH-dependent of adhesion at silica-silica and silica-carbon film interfaces The adhesion force between silica micro cantilever and silica,diamond-like(DLC)films and graphene surfaces were measured using AFM.The influence of relative humidity on the three surfaces was investigated and the differences in adhesion behavior were compared.The results show that DLC film can reduce the amplitude of adhesive force and the sensitivity of adhesive force to relative humidity.The adhesion of the silica-graphene interface shows RH-independent at the initial contact stages,and the adhesion is small.However,the adhesion of the silica-graphene interface shows a strong RH-dependent at the latest stage of contact.(1)In the initial contact stage,the adhesion force at the silica-silica interface was insensitive to humidity.When abruptly transitioning from low humidity to high humidity,the adhesion force usually exhibited only small variations or remained unchanged.In the middle to later stage of contact,the adhesion force at the silica-silica interface showed strong RH-dependent,with a sharp increase upon sudden humidity rise.At high humidity,the large adhesion force could remain constant or decrease with repeated contacts.(2)When transitioning from low humidity to high humidity,the adhesion force at the silica-DLC film interface typically exhibited only small variations,indicating low sensitivity to humidity.Even in the latest stage of contact,the adhesion force at the silica-DLC film interface only showed a slight increase with increasing relative humidity,and decreased when maintained at high humidity.However,the adhesion force at the silica-DLC film interface could exhibit strong RH-dependent due to probe wear and contamination.(3)With increasing relative humidity,the adhesion force at the silica-graphene interface remained unchanged and had a small magnitude in the initial contact stage.However,with an increasing number of contacts,it also displayed strong RH-dependent.When abruptly transitioning from low humidity to high humidity,the adhesion force typically increased gradually with a certain delay.The maximum adhesion force could be reached after a period of maintaining high humidity.This is in stark contrast to the rapid increase observed at the silica-silica interface.