Mixing and internal dynamics of droplets impacting and coalescing on a solid surface

摘要

The coalescence and mixing of a sessile and an impacting liquid droplet on a solid surface are studiednexperimentally and numerically in terms of lateral separation and droplet speed. Two droplet generators arenused to produce differently colored droplets. Two high-speed imaging systems are used to investigate the impactnand coalescence of the droplets in color from a side view with a simultaneous gray-scale view from below.nMillimeter-sized droplets were used with dynamical conditions, based on the Reynolds and Weber numbers,nrelevant to microfluidics and commercial inkjet printing. Experimental measurements of advancing and recedingnstatic contact angles are used to calibrate a contact angle hysteresis model within a lattice Boltzmann framework,nwhich is shown to capture the observed dynamics qualitatively and the final droplet configuration quantitatively.nOur results show that no detectable mixing occurs during impact and coalescence of similar-sized droplets, butnwhen the sessile droplet is sufficiently larger than the impacting droplet vortex ring generation can be observed.nFinally we show how a gradient of wettability on the substrate can potentially enhance mixing.