<p>The purpose of this study is to experimentally investigate the infiltration characteristics of bentonite-based stabilizing fluid at the microscale to interpret the mud film formation mechanism. An infiltration testing apparatus combined with X-ray micro-CT imaging was developed to visualize the interior of the specimen before and after infiltration. Cluster labeling was conducted on the CT images to quantify the distribution and volume of bentonite. Flow velocity distributions in soil pores were calculated using an image analysis to examine their correlation with bentonite distribution. The results showed that increasing infiltration pressure and stabilizing fluid concentration led to larger volumes of bentonite being distributed within the pores as well as the formation of a uniform mud film, ceasing permeability. Bentonite clusters with larger volume tended to develop particularly in regions with high flow velocity within pores, and this tendency was more significant as the stabilizing fluid concentration increased. Based on the findings, the mud film formation mechanism can be explained as follows. When the fluid comes into contact with the specimen, the flow velocity varies significantly from low to high, and bentonite adsorbs to the soil particle surface. The adsorbed bentonite further narrows the pore space, leading to cumulation of bentonite and development of large bentonite clusters, resulting in the mud film formation. Although the mud film formation is not sufficient, the stabilizing fluid infiltrates the interior of the specimen and the relatively large bentonite clusters are distributed in pore spaces, leading to a decrease in macroscopic permeability.</p>