Some aspects of the interfacial phenomena that occur during liquid infiltration of porous media are discussed. Experimental data are presented on the critical speed at which air bubbles are trapped when a horizontal liquid surface passes (from below) across an isolated cylindrical rod or filament whose axis is horizontal. The results are correlated over a wide range of liquid viscosities and surface tensions. We observe that bubble entrapment occurs when the Capillary number exceeds a value defined by $ where γ is the so-called Property number, defined by $ The coefficientsAandmdepend upon the wetting characteristics of the liquid with respect to the solid (filament). A simple physical model is proposed which is in qualitative agreement with the observations.