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Capillary-driven ink infiltration through a porous powder bed in three-dimensional (3D) binder jet printing (inkjet printing onto a powder bed) controls the printing resolution and as-printed “green” strength of the resulting object. However, a full understanding of the factors controlling the kinetics of the infiltration remains incomplete. Here, high-resolution in situ synchrotron radiography provides time-resolved imaging of the penetration of an aqueous solution of eythylene glycol through a porous alumina powder bed, used as a model system. A static drop-on-demand inkjet printer was used to dispense liquid droplets onto a powder surface. The subsequent migration of the liquid front and its interactions with powder particles were tracked using fast synchrotron X-radiography in the Diamond Synchrotron, with phase-contrast imaging at a frame rate of 500 Hz. Image processing and analysis reveal that both the time-dependent increment in the wetting area and the propagation of the “interface leading edge” exhibit heterogeneous behavior in both temporal and spatial domains. However, mean infiltration kinetics are shown to be consistent with existing infiltration models based on the Washburn equation modified to account for the spreading of the liquid drop on the powder surface and using a modified term for the bed porosity.