Dynamic wetting in metering and pre-metered forward roll coating

This experimental study of dynamic wetting in metering and pre-metered forward roll coating shows that the dimensionless applicator flow rate lambda(A), is a key parameter as it defines whether dynamic failure occurs by air entrainment (lambda(A) > lambda(A,trans)) or cascade (lambda(A) < lambda(A,trans)). lambda(A,trans) is the flow rate when the regime switches from metering to pre-metered and; A,mm, the minimum flow rate in the pre-metered regime. When Ca was varied in the range 0.2-5, lambda(A,trans) and lambda(A,trans) lambda(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;,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. (C) 2002 Elsevier Science Ltd. All rights reserved.