Dynamic wetting in metering and pre-metered forward roll coating

Abstract

This experimental study of dynamic wetting in metering and pre-metered forward roll coating shows that the dimensionless applicator flow rate λ A , is a key parameter as it defines whether dynamic failure occurs by air entrainment ( λ A > λ A,trans ) or cascade ( λ A < λ A,trans ). λ A,trans is the flow rate when the regime switches from metering to pre-metered and λ A,min , the minimum flow rate in the pre-metered regime. When Ca was varied in the range 0.2–5, λ A,trans and λ A,min were in the range 1.40–1.29 and 1.18–1.15, respectively. These values compare very well with theoretical predictions. The air-entrainment speeds V M ∗ were found to be similar to the corresponding speeds V p ∗ , measured in plunging tape flow but the cascade speeds V M ∗∗ can be smaller or larger depending on λ A . This led to exploring hydrodynamic assistance and drawing an analogy with curtain coating, which was supported by the data. It was also observed that cascade occurred in the pre-metered regime because the dynamic wetting line X D , moves near the minimum gap position just as in reverse roll coating. Indeed the cascade speeds conform to correlations developed for reverse roll coating. Finally, the study gives data on X D , and compares them with theoretical predictions. The agreement in this case is only qualitative and this points to deficiencies in current modelling of the dynamic wetting region.