X-ray micro-tomography was used to comprehensively study the pore-scale drying process and characterize the liquid film region and the invasion front. Our finding reveals that the decrease in saturation is not uniform across the pore size. The capillary rearrangement, which refers to the restructuring of the liquid phase due to the capillary pressure difference, drives the liquid to flow from large pores to small pores. As a result, the decrease in liquid saturation occurs preferentially within the larger pores. A sharp decrease in liquid saturation occurred at the invasion front, where saturation ranged from 1 to around 0.15. The obtained invasion width that characterized the extent of roughness and disorder of the invasion front obeyed the scaling law of the percolation model. Therefore, this region is significantly limited by the interaction between capillary and gravity. However, with more multi-pore connected clusters observed within the porous medium, the critical exponent of the cumulative cluster size distribution obtained from the cluster region deviates from the universal power-law behavior. This result indicates the presence of liquid film within the cluster region, where the viscous force becomes comparable to the capillary force. We also found that the total gas-liquid-specific interfacial area has no effect on the drying rate. This result provides new evidence that the liquid film region is vapor saturated.
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