> and still no explanation how a thin film of water can be forced through a thick piece of highly water repellent fabric
A water repellent fabric is only water repellent to a degree, and that is determined by the magnitude of the force generated by capillary action. Since this force is proportional to the surface tension and cosine of the contact angle, and inversely proportional to the diameter of the capillary tube (aka weave insterstice), we could, if we knew these figures, calculate the force.
But, whatever value it is, there's a limit, and, the finer the weave, the higher the force.
If we apply water to the fabric at a pressure that results in a force greater than the capillary force, the water will pass through the capillary tube.
Rainwater resting on the fabric surface, or raindrops falling onto the fabric may not provide enough force to overcome the capillary force.
Putting your bodyweight onto a small area of fabric (e.g. sitting, kneeling) may just provide enough force to overcome the capillary force.
Time for you to do another of your experiments, Parky; put some water in the bath, put on some Paramo trousers, and kneel in the bath, and move your weight from knee to knee. Do your knees get wet? Is water pressed through the outer fabric? Is water pressed through the liner fabric?
Posted: 20/11/2009 at 14:51
