A Computational Study of High-Speed Microdroplet Impact onto a Smooth Solid Surface

Numerical solutions of high-speed microdroplet impact onto a smooth solid surface are computed, using the interFoam VoF solver of the OpenFOAM® CFD package. Toward the solid surface, the liquid microdroplet is moving with an impinging gas flow, simulating the situation of ink droplets being deposited onto substrate with a collimated mist jet in the Optomec Aerosol Jet® printing process. For simplicity and computational efficiency, axisymmetric incompressible flow is assumed here for the free-surface fluid dynamic problem. The computed values of maximum spread factor ξ, for the range of parameters relevant to Aerosol Jet® printing, are found in good agreement with some of the correlation formulas proposed by previous authors in the literature. A formula of improved accuracy is then obtained for evaluating ξ of Aerosol Jet® deposited droplets, by combining selected formulas from different authors with appropriate modifications. The computational results also illustrate droplet impact dynamics with lamella shape evolution throughout the spreading, receding-relaxation, and wetting equilibrium phases, consistent with that observed and described by many authors. This suggests a scale-invariant nature of the basic droplet impact behavior such that experiments with larger droplets at the same nondimensional parameter values may be applicable for studying microdroplet impact dynamics. Significant free surface oscillations can be observed with low viscosity droplets. The border line between free surface oscillations and aperiodic creeping to the capillary equilibrium shape appears at Oh ∼ 0.25. Droplet bouncing after receding is prompted with large contact angles at solid surface (as consistent with findings reported in the literature), but can be suppressed by increasing the droplet viscosity.