The effect of substrate roughness on air entrainment in dip coating

Abstract

Dynamic wetting failure was observed in the simple dip coating flow with a series of substrates, which had a rough side and a comparatively smoother side. When we compared the air entrainment speeds on both sides, we found a switch in behaviour at a critical viscosity. At viscosity lower than a critical value, the rough side entrained air at lower speeds than the smooth side. Above the critical viscosity the reverse was observed, the smooth side entraining air at lower speed than the rough side. Only substrates with significant roughness showed this behaviour. Below a critical roughness, the rough side always entrained air at lower speeds than the smooth side. These results have both fundamental and practical merits. They support the hydrodynamic theory of dynamic wetting failure and imply that one can coat viscous fluids at higher speeds than normal by roughening substrates. A mechanism and a model are presented to explain dynamic wetting failure on rough surfaces.

Particle-assisted dynamic wetting in a suspension liquid jet impinged onto a moving solid at different flow rates

“…An increase in surface speed promotes the spontaneous breaking of a dynamic contact line into two or more inclined segments, leading to an onset of intermittent bubble entrainment from downstream vertices of the contact lines (Burley and Kennedy, 1976;Blake and Ruschak, 1979;Gutoff and Kendrick, 1982;Cohu and Benkreira, 1998;Benkreira and Cohu, 1998;Benkreira, 2004). The flow region where the liquid successfully wets the surface is usually referred to as the coating window.…”

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

“…This phenomenon has been observed in several film coating processes, e.g., dip coating, 6-8 roll coating, 9,10 and curtain coating. 3,4, 11,12 Many empirical relationships for the onset of air entrainment exist, 8 for example the equation of Burley and Jolly, 13…”

The sol–gel route: A versatile process for up-scaling the fabrication of gas-tight thin electrolyte layers

“…Again, the microstructure of the YSZ layer is unchanged, but a twice thinner YSZ layer is obtained: the thickness is 25 m for an immersion rate of 2.0 cm s −1 instead of 45 m for an immersion rate of 1.0 cm s −1 . The support immersion rate is consequently a key parameter for controlling the thickness of the deposited coating This result brings out the importance of the hydrodynamic parameters during the deposition process, which are related both to the substrate surface quality (roughness and porosity) and to the composite sol formulation (viscosity, ceramic loading) [24]. In our experiments, increasing the immersion rate of the anode in the composite sol strongly modifies the process hydrodynamics and consequently the coating thickness.…”