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.