The effect of substrate geometry on dynamic contact angles in surface wetting

Dynamic contact angles play a central role in the problem of wetting of surfaces. A solid surface is moving steadily through the free surface of a liquid. The angle between the plunging solid surface and the liquid free surface at the line of solid-liquid contact is the dynamic contact angle. This work compares experimentally measured dynamic contact angles of horizontally rotating rolls of different diameters with those of circular fibers, and tapes. The comparison also includes dry and pre-wet surfaces. Dynamic contact angles depend on the geometry of the wetted substrate. Specifically the geometry through its curvatures affects the surface tension forces at the contact line. Smaller diameter rolls generate smaller angles. In wetting of circular fibers the angles are the smallest compared to tapes and rolls. Flat dry tapes form the largest angles when they are wetted. This implies that the curvatures of the circular rolls and fibers contribute to the balance of surface energies at the contact line. Pre-wet surfaces generate considerably smaller angles at the same wetting speeds. In contrast with that, the diameter of rolls does not affect the critical speed of air entrainment.