Capillarity in fibrous filter media: Relationship to filter properties

This work examines the wetting of a range of low packing density (high porosity), porous, fibrous media by mineral oils. The fibrous media were suspended above a vessel of oil, and the rate of imbibation of oil was measured by means of a balance under the oil and a load cell above the media. It was found that the height of the oil column within the media over time generally resembled the classical capillary rise curve. Capillary rise models were fitted to the data to predict an equivalent capillary diameter, dynamic contact angle, and height of the liquid column as time approaches infinity, using the known filter and oil properties. Different capillary models were examined, and it was found that the modified Washburn equation was the simplest model to use and produced an acceptable agreement between theory and experiment. It was found that the adjustment phase of the curve from the fibrous media was generally more rapid than for a typical capillary, presumably since the fibrous media can be wetted not only from directly below (as with a capillary), but also through the meniscus at the sides of the media. It was found that a linear correlation existed between the effective capillary diameter of the media, and the packing density of the media divided by the fibre diameter. The results appear to be relatively independent of the material type. These results allow capillary diameters to be determined for fibrous media from easily measured parameters, without the requirement of conducting wetting experiments or ‘bubble-point’ tests.