has been devoted to these areas due to the appearance of divergent phenomena at different scales, such as macroscopic fluid dynamics, mesoscopic hydrogen bond network, and nanoscopic hydration layer. Especially, as the basis of numerous important but complex physicochemical processes, such as dissolution, crystallization, corrosion, catalysis, protein folding, and so on, [10][11][12][13][14][15] the research of the underlying mechanism and atomic dynamics at the solid-liquid interface call for characterization technology of both high spatial and temporal resolution in liquid. Furthermore, forces between solid objects in liquid are very different from those in vacuum or air due to the emergence of the solid-liquid interface, and the measurement of these forces could provide a unique pathway to understanding the system dynamics in liquid. As a whole, an in-depth understanding of a solid-liquid system calls for efficient characterization techniques of high spatial and temporal resolution, as well as force measurement of high sensitivity. For various widely adopted characterization methods including Fourier transform infrared spectroscopy, [16] sum-frequency vibrational spectroscopy, [17] X-ray photoelectron spectroscopy, [18] environment scanning electron microscope, [19] surface force apparatus, [20] and so on, only one or two of these goals could be achieved. By contrast, the fast-developing atomic force microscopy (AFM) provides a feasible strategy for the investigation of solid-liquid interfaces.AFM was invented in 1986 for obtaining the high-resolution surface topography of materials [21] regardless of their electrical conductivity, which utilizes a cantilever as the force sensor to raster scan the interaction between a sample and a tip at the cantilever's free end and obtains the sample topography through a feedback control system. Three years later, Hansma's group [22] applied AFM in the liquid environment for the first time and obtained atomic-scale images of a mica surface, which unveiled the prelude to the application of LE-AFM. However, the static mode used by early LE-AFM was invasive to the sample during raster scanning. When LE-AFM was applied to susceptible samples such as biological materials, the static scanning mode is no longer adequate. To overcome this issue, various dynamic modes were proposed, including amplitude modulation (AM) mode, frequency modulation (FM) mode, and so on. In 1994, Putman et al. [23,24] applied AM mode LE-AFM to image monkey Liquid environment is essential for the occurrence of various vital processes in fields of chemistry, biology, mechanics, and so on, underscoring the importance of understanding the solid-liquid interfaces. In the past decades, liquid-environment atomic force microscopy (LE-AFM) has played a nonsubstitutable role in studying solid-liquid interfaces because of its sub-nanometer spatial resolution, video-level imaging rates as well as piconewton-level force measuring capabilities. Various state-of-the-art developments and exciting applications are made r...

No abstract