Air entrapment in coatings by way of a tip-streaming meniscus

Entrapment of small air bubbles is a problem for continuous liquid-film coatings processes. The coating of any surface requires that the surrounding air in contact with it be displaced by an advancing liquid interface. Studies of dynamic wetting suggest that if the interface motion is too rapid, the air is not completely removed and it becomes entrained in the coating material. This process, which can lead to undesirable flaws in the form of bubbles, blemishes or voids, limits the speed at which the substrate can be moved in the production of uniform precision coatings. However, the entrapment process is not understood in detail. Here we report an experimental investigation of air entrapment in high-speed coating operations. Tip streaming--a phenomenon well known in emulsification technology, involving the ejection of a fine filament from the cusped interface between two immiscible fluids--is shown to be the precursor of air entrainment. We demonstrate that tip-streaming air filaments emanating from the contact zone of a dynamic liquid interface give rise to minute (approximately 10 microm) bubbles.