(147a) Physicochemical Hydrodynamics of Droplets in Inkjet Printing

Inkjet printing is the most widespread technological application of microfluidics. It is characterized by its high drop productivity, small volumes and extreme reproducibility. In this talk I will give a synopsis of the fluid dynamics of inkjet printing and discusses the main challenges for present and future research. These lie both on the printhead side – namely the detailed flow inside the printhead, entrained bubbles, the meniscus dynamics, wetting phenomena at the nozzle plate, and jet formation – and on the receiving substrate side – namely droplet impact, merging, wetting of the substrate, droplet evaporation, and drying. In most cases the droplets are multicomponent, displaying rich physicochemical hydrodynamic phenomena. The challenges on the printhead side and on the receiving substrate side are interwoven, as optimizing the process and the materials with respect to either the printhead side or the substrate side is not enough: As the same ink (or other jetted liquid) is used and as droplet frequency and size matter on both sides, the process must be optimized as a whole. One example for conflicting requirements from the printhead side on the one hand and from the receiving substrate or more specifically the paper side on the other hand is the volatility of the ink: At the nozzle, it would be preferable if the evaporation of ink were avoided to prevent nozzle clogging, but on the paper side, fast evaporation of ink is desirable to enable productive printing and to prevent paper deformation.

Even such a seemingly simple process as the evaporation of multicomponent droplets keeps surprising us through its richness of phenomena. I will show and explain several of such phenomena, namely evaporation-triggered segregation thanks to either weak solutal Marangoni flow or thanks to gravitational effects. The domiance of the latter implies that sessile droplets and pending droplets show very different evaporation behavior, even for Bond number << 1. I will also explain the full phase diagram in the Marangoni number vs Rayleigh number phase space, and show where Rayleigh convections rolls prevail, where Marangoni convection rolls prevail, and where they compete, and why these processes are very important in piezoacoustic inkjet printing.

Paul Steen had been deeply interested in the physics and chemistry to inkjet printing and we went several times together to our industrial partner Canon. This has lead to a joint publication in Physical Review Applied, in which we explain the conditions for the entrainment of bubbles into the nozzle. Unfortunately, this was one of Paul’s last papers, and we deeply miss the intellectual discourse with him on this subject and on many other ones, and him as a scholarly, knowledgable, original, creative and dear colleague.