UV-mediated coalescence and mixing of inkjet printed drops

In this study, we experimentally investigated the material flows of coloured dyes in coalescing small inkjet printed droplets of different volume ratios. With two differently coloured dyes, one in each droplet, we were able to distinguish the time-resolved contributions in dye transport across the coalescence bridge due to convection and diffusion. Droplets with differently coloured dyes were inkjet printed onto a glass substrate at a sufficiently large pitch such that they do not touch each other. Under UV exposure, the wetting of the substrate improves, causing the droplets to coalesce. Filmed at 50 fps, the coalescence and mixing of the droplets of volume ratios 1:1, 2:1 and 4:1 was followed. For equally sized drops, the mixing of the dyes shows good agreement with a 1D approximation of Fick’s second law along the central axes of the coalescing droplets with a diffusion coefficient D = 2 × 10⁻⁹ m² s⁻¹. For unequally sized droplets, convective flows from the small to the large droplet were demonstrated. The convective flows increase in size with increasing volume ratio, but only enter the droplet over a small distance. Complete mixing of the dyes in the unequally sized droplets is only reached after a long period and is diffusion controlled. At the initial moment of coalescence of unequally sized droplets, a small convective flow is observed from the large into the small droplets. Further investigation in this phenomenon is recommended.