  As a typical multiphase fluid flow process, drainage in porous media is of
fundamental interest in nature and industrial applications. During drainage
processes in unsaturated soils and porous media in general, saturated clusters,
in which a liquid phase fully occupies the pore space between solid grains,
affect the relative permeability and effective stress of the system. In this
study, we experimentally studied drainage processes in unsaturated granular
media as a model porous system. The distribution of saturated clusters is
analysed by an optical imaging method under different drainage conditions, in
which pore-scale information from Voronoi and Delaunay tessellation was used to
characterise the topology of saturated cluster distributions. By employing
statistical analyses, the observed spatial and temporal information of
multiphase flow and fluid entrapment in porous media are described. The results
indicate that the distributions of both the crystallised cell size and pore
size are positively correlated to the spatial and temporal distribution of
saturated cluster sizes. The saturated cluster size is found to follow a
lognormal distribution, in which the generalised Bond number correlates
negatively to the scale parameter and positively to the shape parameter. These
findings can be used to connect pore-scale behaviour with overall
hydro-mechanical characteristics in partially saturated porous media, using
both the degree of saturation and generalised Bond number.
