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

摘要

Present study is on partial/complete coalescence dynamics of a droplet (surrounded by air) over a horizontal pool ofthe same liquid. Experimental and numerical studies are presented for both isopropanol and glycerol droplet of a constantdiameter. 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 (γ = 2 − 200mN∕m ) on the coalescence dynamics. For partial coalescence of an isopropanol droplet and complete coalescence of a glycerol droplet, excellent agreement is demonstratedbetween our numerically and experimentally obtained interface dynamics; and a qualitative discussion on the mechanismof the partial and complete coalescence is presented. Three regimes of partial coalescence − viscous, inertio-capillary andgravity − proposed in the literature for a liquid-liquid system are presented here for the present liquid-air system whilestudying the effect of diameter of the isopropanol droplet. Probably for the first time in the literature, our numericalstudy presents a flow and vorticity dynamics based quantitative evidence of the coalescence-mechanism, analogy with afreely vibrating Spring-Mass-Damper System, the gravity regime for a liquid-gas system, and the effect of surface tensioncoefficient γ based coalescence dynamics study. The associated nove γ based droplet coalescence regime map presentsa critical Ohnesorge number Oh_c and critical Bond number Bo_c for a transition from partial to full coalescence; and suchcritical values are also presented for the transition under effect of the droplet diameter. The critical values based transitionboundaries, obtained separately for the varying D and varying γ , are demonstrated to be in excellent agreementwith a correlation reported in the literature.