Dynamic capillary impact on longitudinal micro-flow in vacuum assisted impregnation and the unsaturated permeability of inner fiber tows

This paper addresses issues of the synergetic dynamic effect of capillary force on the longitudinal impregnation driven by external pressures, especially under vacuum assistance. An apparatus was designed to detect the axial infiltration along unidirectional fiber bundles which were all aligned closely to give a representation of micro-flow channel of inner fiber tows. The external driving pressures were controlled sufficiently low, 20–60 kPa, on the order of capillary pressures. Based on the analysis of infiltration velocities under different external pressures, dynamic capillary pressures can be determined experimentally. The results showed that capillary pressures, the most important force of microscopic flow through inner fiber yarns, acted as a drag force on the infiltration flow for vacuum assisted penetration into unidirectional fiber bundles. This unique drag effect is very different from traditional unsaturated infiltration, different from the compressed air driving permeation and the theoretical calculated data in this paper. Moreover, values and even signs of the dynamic capillary pressures varied with the fiber fraction of the assemblies as well as the fluid types. Further analysis demonstrated that the function of capillary pressure was closely related to the capillary number (Ca), acting as drag force when Ca larger than a critical value, and as a promotive force with smaller Ca. Consequently, unsaturated permeabilities of the unidirectional fiber bundles were estimated by taking consideration of both dynamic and quasi-static capillary pressures.