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

Abstract

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 ($$D=0.035-6.7 mm$$ D = 0.035 - 6.7 m m ) and surface tension coefficient ($$\gamma =2-200 mN/m$$ γ = 2 - 200 m N / 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 quantitativeevidence 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 $$\gamma$$ γ based coalescence dynamics study. The associated novel $$\gamma$$ γ based droplet coalescence regime map presents a critical Ohnesorge number $$Oh_{c}$$ O h c and critical Bond number $$Bo_{c}$$ B o 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 $$\gamma$$ γ , are demonstrated to be in excellent agreement with a correlation reported in the literature.

Coalescence of surfactant-laden droplets

“…Numerical continuum approaches have also been employed to complement understanding of droplet coalescence. 5,7,15,17,42,69,[72][73][74][75][76][77][78] However, such methods suffer from inadequate resolution in capturing the mass transport mechanism of surfactant during coalescence or resolving the initial contact of the droplets. The singularity at the contact point still remains a challenge for continuum simulation.…”