Tip Pressure on Semicircular Specimens in Tapping Mode Atomic Force Microscopy in Viscous Fluid Environments.

Hua-Ju Shih,Po-Jen Shih,Ching-Liang Dai

doi:10.3390/s17102182

Abstract

Tapping mode (TM) atomic force microscopy (AFM) in a liquid environment is widely used to measure the contours of biological specimens. The TM triggers the AFM probe approximately at the resonant frequencies and controls the tip such that it periodically touches the specimen along the scanning path. The AFM probe and its tip produce a hydrodynamic pressure on the probe itself and press the specimen. The tip to specimen size ratio is known to affect the measurement accuracy of AFM, however, few studies have focused on the hydrodynamic pressure caused by the effects of specimen size. Such pressure affects the contour distortion of the biological specimen. In this study, a semi-analytical method is employed for a semicircular specimen to analyze the vorticity and pressure distributions for specimens of various sizes and at various tip locations. Changes in pressure distribution, fluid spin motion, and specimen deformation are identified as the tip approaches the specimen. The results indicate the following: the specimen surface experiences the highest pressure when the specimen diameter equals the tip width; the vorticity between tip and specimen is complex when the tip is close to the specimen center line; and the specimen inflates when the tip is aligned with the specimen center line.

Highlights

To investigate nanoscale biological specimens—measuring their surface contours, for example—atomic force microscopy (AFM) is a superior technique to scanning electron microscopy (SEM) [1,2]
The authors of the present study demonstrated that the liquid pressure varies with the tip geometry; the maximum hydrodynamic pressure caused by a cone-shaped tip is Sensors 2017, 17, 2182; doi:10.3390/s17102182
The authors use a semi-analytical method to analyze the pressure and vorticity caused by a vibrating tip on a specimen surface

Summary

Introduction

To investigate nanoscale biological specimens—measuring their surface contours, for example—atomic force microscopy (AFM) is a superior technique to scanning electron microscopy (SEM) [1,2]. The two basic AFM operation modes for probing a liquid environment are the contact mode (CM) [3] and tapping mode (TM). Compared with CM, TM is capable of scanning soft surfaces with certain contact forces. The force at the tip is weak in the TM, the vibrating tip body produces fluid pressure and fluid vorticity at the edges of the tip and of the specimen, thereby distorting the measurement results. This distortion of surface morphology is different from the geometric effect between tip and specimen in air [4]

Methods

Results

Discussion

Conclusion