The evaporation of droplets on flat substrates has been studied widely since the seminal paper in 1997 by Deegan and coworkers on the origin of the 'coffee ring effect'. In many practical applications, however, one wants to confine the droplet and its deposit to a defined region on a surface. One way of achieving this aim is to pattern the substrate with raised banks to create wells into which droplets can be printed. In most cases, the contact line of the droplet is pinned at the top of the well. We have imaged the drying of picolitre droplets of organic solvents contained within square wells defined by banks of polymer resist. Interferometry has been used to observe the profile of drops during drying. For slowly evaporating pure liquids, the droplet shape starts off as a spherical cap, passes through a flat surface parallel to the top of the walls and then adopts the shape of a spherical bowl. Continued evaporation leads to flat region in the centre of the well with curved regions adjacent to the walls. The resulting capillary force accelerates drainage of the thin film, in a manner similar to the drainage of liquid from the lamella of a foam into the Plateau borders, with the added constraint that there is a no-slip boundary condition at the liquid-substrate interface. More complex dryng dynamics is also observed. For rapidly evaporating drops we observed the formation of a raised region in the centre of the well towards the latter stages of drying (see Figure 1). There is evidence for thermal Marangoni effects in fast-drying solvents, where the droplet never passes through a stage where the surface is flat. For binary mixtures of solvents, solutal Marangoni effects may influence the droplet dynamics during drying.

Evaporation of thin films on patterned substrates

254th National Meeting (2017)

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