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With most of the focus to date having been on the coalescence of freely suspended droplets, much less is known about the coalescence of sessile droplets, especially in the case of droplets laden with surfactant. Here, we employ large-scale molecular dynamics simulations to investigate this phenomenon on substrates with different wettability. In particular, we unravel the mass transport mechanism of surfactant during coalescence, thus explaining the key mechanisms present in the process. Close similarities are found between the coalescence of sessile droplets with equilibrium contact angles above 90° and that of freely suspended droplets, being practically the same when the contact angle of the sessile droplets is above 140°. Here, the initial contact point is an area that creates an initial contact film of surfactant that proceeds to break into engulfed aggregates. A major change in the physics appears below the 90° contact angle, when the initial contact point becomes small and line-like, strongly affecting many aspects of the process and allowing water to take part in the coalescence from the beginning. We find growth exponents consistent with a 2/3 power law on strongly wettable substrates but no evidence of linear growth. Overall bridge growth speed increases with wettability for all surfactant concentrations, but the speeding up effect becomes weaker as surfactant concentration grows, along with a general slowdown of the coalescence compared to pure water. Concurrently, the duration of the initial thermally limited regime increases strongly by almost an order of magnitude for strongly wettable substrates. [ABSTRACT FROM AUTHOR]

A decline in natural snow due to global warming increases the reliance of ski resorts on artificial snow. To improve the formation rate and snow amount of artificial snow under any environmental conditions, it is necessary to use atomization technology that maintains the size of liquid drops of the snow-making spray field below a critical size, thus maximizing the number of freezing droplets into ice crystals. Here, we study the effects of the gas–liquid mass-mixing ratio (GLR) and the spacing (d) between two nozzles on the characteristics of the snow-making spray field produced by atomization technology for GLR of 0.05, 0.10, 0.15, and 0.2 and d of 5, 10, 15, and 20 cm. The measurement of the morphology, velocity, and particle-size distribution of the snow-making spray field is conducted by a laser particle-size measuring instrument, particle image velocimetry, and a high-speed camera. Our results show that both the spray cone and the intersection angles increase with the increase in the GLR in a two-nozzle arrangement, while the degree of interference between the nozzles increases with the decrease in spacing, d. We also find that the inward offset of the spray is the main reason for the increase in the intersection angle and the interference area and the deviation of the spray midline. In addition, we compare the characteristics of the spray field produced by the single and the two-nozzle systems. We conclude that the flow-field velocity of the single nozzle is smaller than that of the two nozzles and the velocity gradient of the liquid drops in the two-nozzle flow field varies greatly. The maximum vorticity (Max = 147) in the case of the two-nozzle arrangement is much larger than that of the single nozzle. Moreover, by analyzing the two-nozzle flow field at different spacings, we find that the particle-size distribution is most uniform for GLR = 0.10 and d = 15 cm. Thus, our study elucidates the characteristics of spray fields suitable for snow-making toward achieving higher quality and more efficient snow-making processes. [ABSTRACT FROM AUTHOR]