Selective evaporation at the nozzle exit leads to changes in the resonance behavior and an evaporative equilibrium in the piezoacoustic ink channel

In piezo acoustic drop-on-demand inkjet printing a single droplet is produced for each piezo driving pulse. The inks used in most applications are multicomponent, leading to selective evaporation at the nozzle exit during idling periods of the printhead. Here, we study the change in the water-glycerol composition in the nozzle due to the evaporation at the nozzle exit by measuring the ring-down signal of the acoustics in the ink channel. We find excellent agreement between the experiments and the analytical model, showing that the changes in the acoustic resonance frequency of the ink channel are a result of changes in both density and viscosity of the fluid within the nozzle of the printhead. Using this measurement method we observe that the evaporation process reaches an equilibrium situation with respect to the waiting time, where the evaporating water at the nozzle exit is diffusively replenished by water from the ink channel reservoir, as we demonstrate through direct numerical simulations.