Studying how droplets, that contain solid particles, evaporate from a solid surface reveals a perplexing procedure. The combination of mass and heat transfer lead to intriguing phenomena, such as the coffee-ring effect, which describes the evaporation of colloidal droplets on a solid surface, generating non-uniform ring-like depositions. The results of this process have an impact, in many research areas, where a uniform particle deposition is a crucial requirement, such as biosensors, inkjet-printing, photonics and others. Recently, Butt et al. reported that control and regulation of the evaporation of water droplets, on oil-coated surfaces, can suppress the coffee-ring effect and produce asymmetric supraparticles.

How can the coffee ring effect be avoided, and a uniform deposition guaranteed?

In order to understand the processes of droplet evaporation, Butt et al. studied the droplet evaporation on surfaces, with and without an oil film. The droplet evaporated more slowly on the oil-coated surface and formed a homogenous pattern. During the evaporation process on an oil coated substrate two contact lines (interfaces) are present on the droplet surface: a liquid−liquid−air contact line (LLA-CL) and a liquid−liquid−solid contact line (LLS-CL). In contrast, the droplet evaporation on the bare glass surface was significantly faster, with only a single contact line, a liquid-air-solid contact line (LAS-CL), and generated a typical coffee ring, due to the outward capillary flow inside the droplet.

Moreover, the figure illustrates that there is a tendency for colloidal particles to be transported to the upper interface (LLA-CL) and accumulate there. This phenomenon is a result of hindered evaporation, both on the oil-coated water drop surface, and from the bottom edge LLS-CL. Only at the uncovered area of the droplet evaporation can occur freely. This localised evaporation results in an upward flow, of particulate matter inside the drop, which, in the first instance, concentrates the dispersion towards the upper surface of the drop, during the evaporation and facilitates a uniform deposition on the substrate. The oil viscosity, film thickness, droplet size, and concentration of the colloids were all found to influence the deposition pattern.

Fluorescent polystyrene particles were used as tracers and observed by confocal microscopy in order to get a deeper understanding of the particle flow during evaporation. The results show that the droplets deposition radius on the oil-coated surfaces decreases with time, while the contact angle at the droplet edge changes only slightly. This phenomenon indicates that the droplet edge is continuously receding and the pinning of particles, to the substrate surface, is not an immediate effect.

For comparison, on the non-oil-coated surface, the deposition radius did not change, while the contact angle is much smaller than on the oil-coated surfaces and decreases during the whole evaporation process. This is caused by the fact that for the non-oil coated surface the particles are already pinned to the surface at the beginning of the evaporation.