Abstract
For P-OLED display fabrication, it is important to control the final film shape, arising from drying of volatile droplets containing polymer. Due to peripheral pinning and subsequent outward capillary flow, a coffee-ring typically develops. This is inconvenient since a spatially uniform height, above the substrate, is required to ensure uniform current across the device. Typically the droplets are deposited inside a trough-like structure on the substrate. We present a thin-film lubrication model that tracks the drying dynamics through to the final film shape. The governing equations are derived and solved numerically. We investigate the effect of the trough’s depth and the slope of the walls. Increasing the depth or the wall’s gradient increases coffee-ring formation. This is due to an increase in horizontal velocity, caused by the substrate’s shape as well as delayed gelation of the polymer. The latter allows the outward capillary flow to act for a longer time, before the height becomes fixed.
Highlights
There is significant interest in, and research into, the drying of volatile liquid droplets containing polymer, or other non-volatile⇑ Corresponding author.species
We solely focus on the impact of the trough geometry on final film shape
We find that the substrate profile can alter the internal flow within the droplet and change the resultant film shape
Summary
There is significant interest in, and research into, the drying of volatile liquid droplets containing polymer, or other non-volatile. This is due to the array of industrial and practical applications for which drying is an important step. There are several applications for biological fluids [1], including pattern analysis of dried blood, for crime scene investigation [2] and disease diagnostics [3], DNA microarrays [4,5,6,7] and herbicides/pesticides on leaves [8].
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