Patterned monolayer collapse from constrained sessile droplets using virtual wells and pedestals

A web-like pattern is created when a rigid monolayer forms at the interface of a sessile droplet and subsequently buckles during evaporation. Prior work investigated diluted bourbon whiskey droplets on coverslips (1 microliter drops, 25% alcohol-by-volume or ABV), but such investigations were limited to relatively hydrophobic liquids. In order to apply this technique to more hydrophilic samples, the sessile droplet will need to be physically pinned to increase the droplet's surface area. This can be accomplished through the use of virtual wells (i.e. patterned hydrophobic surfaces) or microfabricated pedestals. For this work we compared commercially available 2 mm diameter Teflon wells with similarly-sized SU-8 pedestals (0.12 mm height). This work demonstrates that web-like patterns were created with both methods, though pedestals produced more consistent patterns (through digital image analysis). This approach enabled the investigation of patterns created by more hydrophilic liquids (i.e. > 25% ABV), larger volumes (> 1 microliters), and explore the impact of pedestal curvature on monolayer collapse.