EXPLORE WETTING DYNAMICS AT MICRO AND NANO SCALES: APPLICATIONS AND PROGRESS OF LONG-NEEDLE ATOMIC FORCE MICROSCOPE

“How can we measure interface phenomena on the microscopic level” is a key scientific question in micro- and nanoscale fluid mechanics and is also listed in recent 125 questions (Sanders S, Science, 2021). Traditional optical methods face limitations due to the optical diffraction limit, making it difficult to directly measure flow and interfacial phenomena at micro- or nanoscale. However, atomic force microscopy (AFM) offers a solution by enabling precise manipulation and force measurements. The AFM-based microrheometer, which is assembled with a long-needle probe, can directly measure the capillary force acting at the gas-liquid-solid three-phase contact line, as well as the dynamic variations of forces during the vertical motion of the probe. This enables the precise characterization of the dynamic behaviors of fluid interfaces and the mechanical properties of various materials in liquid environments at micro- and nanoscales. In this paper, the experimental principles and methods of long-needle AFM will be reviewed, along with its latest progress in the study of wetting dynamics at the micro- and nanoscale non-ideal interfaces, including asymmetric and speed-dependent capillary force hysteresis on the surfaces with low energy barrier, avalanches and extreme value statistics of a moving contact line on disordered rough surfaces, state- and rate-dependent contact line dynamics over an aging soft surface, and manipulation of contact angle hysteresis at electrified ionic liquid-solid interfaces. This experimental method provides reliable data for testing various theoretical models and numerical simulations. The application of this technology in emerging fields may inspire us to explore the physical nature of complex phenomena at interfaces.