Dynamics Of Compound Droplets Impact Onto Solid Substrates

The interaction between compound droplets and solid wall is relevant to a broad range of industrial and biomedicine processes,and a deep understanding of the underlying mechanisms is of great scientific significance.In this thesis,by using a three-phase diffuse-interface method,we numerically investigate the core-shell compound drop impact dynamics,flow regimes and the spreading of impacting Janus compound drops.The results and conclusions are briefly given as follows:(1)The dynamics of compound drops impacting on a flat substrate is numerically investigated using a ternary-fluid diffuse-interface method,with the aim of assessing the effect of density difference between the inner and outer droplets(denoted byλ)on the evolution of the interfaces.With the help of numerical simulations,we find that at the intermediate stage of drop impact,the inner droplet exhibits a self-similar deformation at λ=1 and relatively high Weber number,and experiences more or less a uniform acceleration for various λ.In particular,the acceleration magnitude at λ≠1 can be correlated by the acceleration at λ=1 and the Atwood number.When the inner droplet is denser than the outer one,a lamella occurs at the spreading front of the inner droplet.We present a scaling analysis of the thickness of the lamella,and the resultant theoretical prediction is in good agreement with numerical results.At the maximal spreading of the compound drop,a bulging structure is formed around the symmetry axis due to the presence of the inner droplet,thereby effectively reducing the liquid supply to the spreading front and leading to a decrease of maximal spreading ratio βmax as compared to a pure drop.We proposed a corrected Weber number Weλ*by taking account of the combined effects of λ,volume fraction of the inner droplet,Weber number and morphology of the compound drop.Integrating Weλ*with the universal model of βmax for impacting pure drops,we successfully build up a new model for predicting the maximal spreading ratio of impacting compound drops with various λ.(2)The flow regimes in the core-shell compound drops impacting on a solid substrate at moderate Reynolds number(Re)and low We are numerically investigated.The fate of the impacting compound drop is seen to follow three different flow regimes:complete rebound,partial rebound and adhesion,depending on the density and volume fraction of the inner droplet,the surfaces tension coefficient of the inner interface,and the wettability.The detailed numerical simulations validate a mechanistic model based on energy-balance analysis,delimating the critical conditions for the complete rebound and the other two regimes.For partial rebound and adherence regimes,the pinch-off of the liquid column due to the RayleighPletau instability is the key distinction between the two regimes.Finally,the non-dimensional contact time in complete rebound regime is determined by balancing inertia with capillary and compared well with the numerical results.(3)The dynamics of Janus compound drops impacting on a solid substrate are numerically investigated.The maximal spreading factor of the two parts of Janus drop for a wide range of We and Re is analyzed in theory.We proposed the theoretical prediction models of the maximal spreading factor,which are in good agreement with the numerical results.In addition,based on the energy budget and dissipation mechanisms during Janus compound drop impact on solid surfaces,we studied the effects of the viscosity ratio between two fluids in the viscosity dominated regime,and the effect of wettability in the capillary dominated regime on the droplet spreading characteristics.