Quantitative measurement of voids formed during liquid impregnation of nonwoven multifilament glass networks using an optical visualization technique

A technique based on matching the refractive index of an invading liquid to that of a fiber mat has been used to study entrapment of air (“voids”) that occurs during forced in‐plane radial flow into nonwoven multifilament glass networks. The usefulness of this technique is demonstrated in quantifying and mapping the air pockets. Experiments with a series of fluids, with surface tensions varying from 28 × 10⁻³ to 36 × 10⁻³ N/m, viscosities from 45 × 10⁻³ to 80 × 10⁻³ Pa · s, and inlet flow rates from 0.15 × 10⁻⁶ to 0.75 × 10⁻⁶ m³/s, have shown that void content is a function of the capillary number characterizing the flow process. A critical value of capillary number Ca = 2.5 × 10⁻³ identifies a zone below which void content increases exponentially with decreasing capillary number. Above this critical value, negligible entrapment of voids is observed.