Suction is a focus of unsaturated soil research and investigation on suction is challenging. Many physical phenomena in nature can be explained by suction

and the research challenge is that the quantitative measurement technology of unsaturated soil suction is immature. Unsaturated soil

as a three-phase composite material widely exists on the earth surface

has far more complicated mechanical properties than saturated soil. The main reason is that a curved liquid surface contraction film is formed at the water-air interface in the unsaturated soil

which causes the soil pores to produce suction. The strength and deformation characteristics of unsaturated soil are the macroscopic manifestations of its internal suction

and the microstructure serves as a bridge to connect macroscopic physical and mechanical properties with suction. It is an important way to improve soil mechanics to grasp the microstructure of unsaturated soil as well as the theory and experiment of suction. It is pointed out that the research on the dynamic structure evolution of unsaturated soil-liquid bridge and the distribution characteristics of suction force chain is an important basis to fully understand the mechanical properties of unsaturated soil

and it is also an important means to grasp and quantify the suction of unsaturated soil.