Influence of the flow field in curtain coating onto a prewet substrate

Abstract

The onset of air entrainment for curtain coating onto a surface prewetted with the coating fluid was studied. The substrate used was a polished, scraped steel wheel and coating was performed over ranges of dimensionless parameters observed in commercial coating processes ͑Reynolds number, 0.14Ͻ Re= Q / Ͻ 33.02; Capillary number, 0.19Ͻ Ca= U / Ͻ 25.07͒. The substrate velocity for the onset of air entrainment was obtained as a function of the curtain flow rate per unit width of curtain ͑1 Ͻ Q Ͻ 9 cm 2 s −1 ͒, fluid dynamic viscosity ͑0.0326Ͻ Ͻ 0.878 Pa s͒, curtain height ͑0.035Ͻ h Ͻ 0.095 m͒, and thickness of the prewet film ͑1 ϫ 10 −7 Ͻ c Ͻ 3 ϫ 10 −5 m͒. A remarkable and strong dependence of the onset of air entrainment on curtain flow rate was observed ͑hydrodynamic assist͒ and the general features of the hydrodynamics were very similar to those observed for previous works onto dry substrates. However, the presence of the prewet film led to higher maximum substrate velocities at the onset of air entrainment than observed for dry substrates. For high liquid viscosities, the air entrainment curve bifurcates; under these conditions, the maximum substrate velocity is no longer inversely proportional to the fluid viscosity and stable coating is possible at higher substrate velocities than would be predicted by conventional theory. This "intense assist" exhibits a complex relationship with the prewet film thickness. The results presented in this paper demonstrate that hydrodynamic assist is not exclusive to wetting, but is a generic phenomenon of fluid flows.

Thin film flow on surfaces containing arbitrary occlusions

“…The finite element method too has shown itself to be well-suited to the solution of coating and spreading flows, having been employed for more than two decades to solve a variety of viscous, free-surface industrial coating problems, [20][21][22][23][24][25]. Consequently, this type of numerical analogue is now gaining popularity as a method for solving the weak form of the lubrication equation for thin film free-surface flows.…”

Curtain coating of dilute suspensions

“…A flow visualization technique was used to observe the upstream profile of the falling curtain. To provide a better view of the curtain profile, the edge guides had to be bent slightly, as suggested by Marston et al [10]. A CCD camera was located strategically to record the images of the upstream profile.…”

“…Marston et al [10, 11] showed that the maximum coating speed could be increased substantially if the substrate was pre‐wetted. Pre‐wetting the substrate is similar to the situation of two‐layered coating where the bottom layer is a dilute liquid carrier and the upper layer is the more viscous coating solution.…”

Long-wavelength stability of an unsupported multilayer liquid film falling under gravity

“…This causes air bubbles to be entrained between the coating and the substrate and therefore defects such as bubbles, pin holes and streaks appear [11]. There is a strong dependence of the web speed at the onset of air entrainment on the curtain flow rate; this is termed "hydrodynamic assist" [12]. A detailed discussion of recent developments on the physics of moving contact lines is given in [13].…”