Physical, mechanical, chemical, and biological processes for various problems span length and time scales of many orders of magnitude [1][2][3][4][5][6]. As a typical dynamic process, moving contact line (MCL) problem or droplet spreading consists of 7-8 length and time scales from the atomistic to the continuum [2], as shown in Figure 1. A grand challenge in MCL problems is to link these vastly different length and time scales.All these length and time scales origin from the competition among governing forces [7], which indicates that they can be bridged by specific dimensionless numbers, as listed in Table 1. The dimensionless numbers provide clear physical interpretation of MCL problems under study and also produce valuable scale estimates. Thus, we can choose the proper governing equations. As the first example, MCL consists of three regions as shown in Figure 2. In the macroscale region, when the droplet size is much less than the capillary length, the gravity forces is neglected compared with surface tension, which remarkably simplify the governing equations. In the mesoscale region, i.e. L≈10 -6 m or smaller, which is on the same order of magnitude of the viscous length, MCL is dissipated by the viscous resistance. It has been shown that in such region, the viscous dissipation results in significantly different droplet Left: in the macroscale region, the contact angle of a water droplet with a base diameter of~10 -3 m can be measured using a droplet contour analyzer. Middle: in the mesoscale region, the edge shape of the droplet can be analysed using an optical microscope according to the interference in the liquid film. Right: in the microscale region, the most anterior part of the MCL was imaged by an atomic-force microscopy (AFM), when the liquid molecules or solvents have a distinguishable size compared with the roughness of the substrate.