The dynamics of impacting and coalescing Newtonian droplets*

An experimental arrangement is described to study impact, coalescence and mixing between a sessile and an impacting droplet of glycerol/water striking a flat transparent substrate in air. We used high speed imaging to visualize and study the impact and coalescence of droplets from the side and from beneath. The impact parameters and liquid characteristics were chosen to match the typical dynamical conditions, based on Reynolds and Weber numbers, found in commercial drop-on-demand printing. The images were processed by particle image velocimetry and image processing algorithms to obtain velocity fields near the liquid-substrate interface, droplet geometries and contact line positions. The experimental results are compared with numerical simulations by the lattice Boltzmann method and good agreement is found. The experimental setup and its instrumentation are simple to reproduce and can be used in other practical applications.