Characterization of voids formed during liquid impregnation of non-woven multifilament glass networks as related to composite processing

Anant D. Mahale, Robert K. Prud'homme, Ludwig Rebenfeld

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13 Scopus citations


A technique based on matching the refractive index of an invading liquid to that of a fibre mat has been used to study entrapment of air ('voids') that occurs during forced in-plane radial flow into non-woven multifilament glass networks. This technique is useful for 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-1, viscosities from 45 × 10-3 to 290 × 10-3 Pa.s, and inlet flow rates from 0.15 × 10-6 to 0.75 × 10-6 m3 s-1, 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. Similar experiments carried out on surface treated non-woven mats spanning a range of equilibrium contact angles from 20 to 78°, have shown that there is a critical contact angle above which negligible entrapment is observed. Below this value, there is no apparent effect of contact angle on the void fraction-capillary number relationship described earlier. Studies on the effect of filament wettability, and fluid velocity and viscosity on the size of the entrapment (voids) have also been carried out. These indicate that larger sized voids which envelop more than one pore are favoured at a low capillary number in comparison to smaller, pore level bubbles. Experiments were carried out on deformed mats: imposing high permeability spots at regular intervals on a background of low permeability. The effect of these spatial fluctuations in heterogeneity of the mat on entrapment is currently being studied.

Original languageEnglish (US)
Pages (from-to)199-207
Number of pages9
JournalComposites Manufacturing
Issue number4
StatePublished - Dec 1993

All Science Journal Classification (ASJC) codes

  • General Engineering


  • capillary number
  • fibre mats
  • liquid impregnation
  • permeability
  • refractive index matching
  • void formation


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