A Study On Dynamic Process Of Formation And Rupture Of Liquid Bridge

Detecting interact force between the tip and the sample, Atomic Force Microscope(AFM) can provide atomic scale images, whose mechanism of formation images is different from traditional optical microscope. In an AFM measurement, the maximum image resolution can reach at 0.01 nm approximately, which is vital to study the nature of mirco-nano scale fields. However, there will be liquid bridge formed between the AFM tip and the sample in ambient air. The capillary force due to liquid bridge can affect the amplitude of AFM cantilever, which will give rise to artifacts of height image. In addition, the quality of tapping mode AFM phase image is largely dependent on the dissipation energy resulted from liquid bridge. In order to acquire the true image of the sample, here the dynamic process of the liquid bridge about formation and ruputure will be studied.Firstly, three formation mechanism of liquid bridge between the AFM tip and the sample are presented, which are capillary condensation, the squeeze and motion of thin liquid film at 65% relative humidity, the characteristic equilibrium time and volume of liquid bridge are theoretically calculated based on the three formation models. The characteristic of liquid bridge formation with different AFM working modes is discussed, respectively.Secondly, based on thermodynamical and mechanicical methods, the capillary force and dissipation energy are investigated during the process of liquid bridge extended. The results in these two methods come very close. With the relative humidity increasing, the dissipation energy becomes larger, which order is about 100nN·nm, but the maximun capillary force increases firstly and then decreases, which order is about 10nN. In a certain relative humidity environment, the capillary force decreases along with the distance increases between the AFM tip and the sample.In the end, the relative dissipation energy in AFM tapping mode is presented experimental in different relative humidity, which is consistent to the theoretical results and verifies the validity of the formation and rupture mechanism of liquid bridge.