For the manufacture of OLED displays, it is vital for the various layers of material to be as uniform as possible in order to optimise display quality, product lifetime and energy consumption.
However, an experimental investigation into the drying of a droplet containing OLED material deposited into a pixel-shaped well by inkjet printing showed a tendency to form a protuberance, or accumulation, of fluid and material in centre of the well, characterised as a ‘W-shaped’ profile. Experiments also showed that ‘M-shaped’ profiles developed in some cases, with an accumulation of fluid and material near the walls of the well, and very little in the centre. In order to produce efficient OLED displays, with uniform layers of deposited material, a ‘U-shaped’ profile is desirable, where the majority of the material is distributed evenly across the base of the well.
In this work, a numerical model is developed to simulate the evaporation of an axisymmetric droplet from within a well in order to better understand the physical mechanisms behind these experimentally observed droplet profiles.
The model is based upon the Arbitrary Lagrangian-Eulerian approach and the Finite Element Method (FEM) with local evaporation rates coupled to the free surface, giving the model the ability to produce non-spherical cap droplet profiles, representing a considerable advantage over the “Classical model”, and is thoroughly verified and validated against published numerical, analytical and numerical works.
Studies of evaporation rate, surface tension, viscosity, thermocapillary and solutocapillary effects, bank structure depth, angle and aspect ratio, and the effect of having the viscosity and mass flux dependent on the concentration of a solute are conducted.
It was found that the formation of a central protuberance is due to the ability of the flows from the centre of the droplet to maintain the radial outwards flows caused by enhanced evaporation at the contact line. When the outwards flows are sustained, a U-shaped profile will occur, and when they cannot, fluid in the intermediate region of the droplet will not be replenished, leading to a thinning of that region, and a W-shaped profile developing. This was the case in droplets with high evaporation rates, low surface tension or high viscosity, with a critical aspect ratio of the well of 30. M-shaped profiles were only seen with a surface tension gradient across the droplet surface intended to simulate the solutocapillary effect.
Investigation was also carried out into the impact of a concentration-dependent viscosity, in which a novel oscillatory behaviour was observed in the protuberance height at increasing evaporation rates.
