Numerical Simulation And Dynamics Of Viscoelastic Drops Impacting On Hydrophobic Solid Substrates

The viscoelastic fluid is a typical kind of non-Newtonian fluids,which is very com-mon in daily life and industry,such as blood and polymer solution.Multiphase flows of viscoelastic fluids with moving contact line has a wide use in agricultural and indus-trial application,such as pesticide spray and inkjet printing,and forensic analysis.In this paper,viscoelastic drops(polymer solution drops)impacting on hydrophobic sol-id substrates is investigated through numerical simulation and experiments.Firstly,Oldroyd-B constitutive model combined with Diffuse Interface method is implemented to simulate the multiphase flow of viscoelastic fluids with moving contact line,and the validness and accuracy are verified.Secondly,the dynamics of viscoelatic drops impact-ing on hydrophobic solid substrates and mechanism of viscoelasticity inhibiting drop rebound are investigated with the help of numerical simulation.At last,by conducting a series of experiments,different regimes and special structures of drop impacting on super-hydrophobic solid substrates are studied;and the influence of the interaction be-tween polymer chains and substrates on the dynamic behaviors of moving contact line is studied.The main work and research achievement are as below:(1)Numerical method is established to simulate multiphase flow of viscoelastic flu-ids with moving contact line.Oldroyd-B constitutive model is implemented to simulate elastic stress,combined with Diffuse Interface method to capture the interface.To verify the validity of Oldroyd-B model,planar channel flow with an-alytical solution,lid-driven cavity flow(Martins et al.,2015;Yapici et al.,2009;Su et al.,2013)and drop deformation in shear flow Pillapakkam&Singh(2001)with numerical benchmarks are compared quantitatively.At last,the numerical results of axisymmetric programs are compared with experimental data,which is viscoelastic drops impacting hydrophobic solid substrates the physical problem we studied here.Numerical results are in good agreement in these cases.Besides,the numerical results have good mesh convergence.(2)The spreading process of viscoelastic drops impacting on hydrophobic solid sub-strates are investigated numerically.The nonlinear effect of viscoelasticity(represented by Deborah number)on drop spreading under various Weber number are studied.At low Weber(We)number,the spreading is dominated by inertial force and sur-face tension,viscoelasticity has no significant influence on drop spreading.In the range of our numerical simulation,the spreading time and the maximum spread-ing radius of both viscoelastic drops and Newtonian drops fit the same scaling law:Rm?Tm3/2.At last,the effect of contact angle and ratio of solvent viscosity on the dynamics of drop impact are also studied.(3)The retraction process of viscoelastic drops impacting onto hydrophobic solid sub-strates are studied numerically,and the mechanism of viscoelasticity inhibiting drop rebound is also investigated.There are three regimes for viscoelastic drops after impacting on hydrophobic surface,which is bounce,oscillation and bounce,and oscillation respectively,under rather low We(We<10)and Re(Re<500).In the phase diagram of De—We,the critical line of bounce and oscillation is:De?We.And the critical separation is explained by the balance of differ-ent timescales,which is the relaxation time of elastic force and surface tension.Besides,by analysing the distribution of elastic force inside the drop and the di-rections of elastic force near the contact line,and the comparison of elastic force with surface tensile,inertial force,the mechanism of viscoelasticity inhibiting drop rebound is discovered.At last,the oscillation period of viscoelastic drops on sub-strate is studied,and the same period with Newtonian drops are explained by stress relaxation theory.(4)Experiments of drops of polymer solution impacting on super-hydrophobic solid substrates are conducted.The Drop-filament and Beads On A String(BOAS)structure are formed during the retraction process,which could be explained but the balance of elastic force and surface tension.The stretching of polymer chains has dramatic influence on the movement of contact line,which form zigzag during retraction process due to the inhibition of polymer chains.By modeling and theoretical analysis,the effect of polymer chains on the dynamic behaviors of the contact line.