Liquid film stability and contact line dynamics of emulsion liquid films in curtain coating process

Previous studies reported that single-phase liquid curtains become more stable with increasing Ohnesorge number. Many liquid films used in the coating industry are types of emulsion samples. Despite this fact, the effect of multiphase liquids on the dynamics of the curtain breakup has not yet been considered. This study explores the stability of emulsion curtain coating via high-speed visualization. The critical condition at the onset of curtain breakup was determined by finding the flow rate below which the curtain broke. Curtain breakup was observed via a hole initiation within the curtain. The results reveal that curtain breakup dynamics is governed by the characteristic dynamic viscosity and the surface tension. The emulsion curtain stability, defined by Weber number, increases as Ohnesorge number rises, similar to the single-phase liquid (i.e., Newtonian and pure shear thinning) curtain stability. The critical web speed at which the contact line moves upstream of the curtain, a phenomenon called heel formation, and that at which air entrainment occurs, were determined for emulsion solutions at different flow rates. The results reveal that the surface tension increase delays the onset of air entrainment which could help to conduct faster curtain coating substrates with emulsion liquid films.

Automated summary

This study investigates the stability of emulsion curtain coating through high-speed visualization, determining the critical conditions of curtain breakup and the influence of dynamic viscosity and surface tension on the process. The findings show that the surface tension increase can delay air entrainment, potentially allowing for faster curtain coating with emulsion liquid films.