Numerical simulations and experiments on droplet coalescence dynamics over a liquid-air interface: mechanism and effect of droplet-size/surface-tension

Present study is on partial/complete coalescence dynamics of a droplet (surrounded by air) over a horizontal pool of the same liquid. Experimental and numerical studies are presented for both isopropanol and glycerol droplet of a constant diameter. Numerical study is presented in more detail for the isopropanol droplet to study the effect of diameter (D = 0.035 - 6.7mm) and surface tension coefficient (gamma = 2 - 200mN/m) on the coalescence dynamics. For partial coalescence of an isopropanol droplet and complete coalescence of a glycerol droplet, excellent agreement is demonstrated between our numerically and experimentally obtained interface dynamics; and a qualitative discussion on the mechanism of the partial and complete coalescence is presented. Three regimes of partial coalescence - viscous, inertio-capillary and gravity - proposed in the literature for a liquid-liquid system are presented here for the present liquid-air system while studying the effect of diameter of the isopropanol droplet. Probably for the first time in the literature, our numerical study presents a flow and vorticity dynamics based quantitative evidence of the coalescence-mechanism, analogy with a freely vibrating Spring-Mass-Damper System, the gravity regime for a liquid-gas system, and the effect of surface tension coefficient gamma based coalescence dynamics study. The associated novel gamma based droplet coalescence regime map presents a critical Ohnesorge number Oh(c) and critical Bond number Bo(c) for a transition from partial to full coalescence; and such critical values are also presented for the transition under effect of the droplet diameter. The critical values based transition boundaries, obtained separately for the varying D and varying gamma, are demonstrated to be in excellent agreement with a correlation reported in the literature.

Automated summary

The study focuses on the partial/complete coalescence dynamics of a droplet surrounded by air over a horizontal pool of the same liquid. Both experimental and numerical studies were carried out for droplets of constant diameter in isopropanol and glycerol. The numerical study, particularly on isopropanol droplets, investigated the influence of droplet diameter and surface tension coefficient on coalescence dynamics. The study presented a flow and vorticity dynamics-based quantitative evidence of the coalescence mechanism, identified critical values for transition from partial to full coalescence, and demonstrated excellent agreement with existing literature correlations for transition boundaries under varying droplet parameters.