When the fabric is clean we see collection via impaction. The fabric can be considered a random mat of fibres through which air is pulled. The flow streams bring it through a circuitous route. At each turn there is the potential to collect particles.

Imagine a particle being drawn towards a fibre. The air will flow around it but the particle has greater inertia and goes straight on.

If the particle strikes the fibre it will often stick to it through the combination of electrostatic and inter-atomic forces. Note that this tends to collect larger particles better than smaller particles but once the filter has been on line for some time we see the second process coming into play.