Study On The Dynamic Behavior Of Shear-thinning Fluid Droplets Interacting With The Wal

Droplet impingement on surfaces is widely found in inkjet printing,additive manufacturing,anti-icing,self-cleaning,biomedics,etc.The impingement behavior and maximum dimensionless diameter of droplets are important factors in these applications.Fluids in industrial processes commonly exhibit non-Newtonian properties such as shear thinning,viscoelasticity and yield stress due to various additives.The existing researches mainly focus on Newtonian fluids,the dynamic behavior of interaction between non-Newtonian fluid droplet and solid surface requires further investigation.This paper proposes to conduct experimental and numerical simulations on the spreading and rebound behavior of shear-thinning fluid droplets impacting on solid surfaces,and aims to explore the mechanism underlying the effect of shear-thinning properties on the droplet impact dynamics.In this paper,we firstly built a droplet impact visualization platform based on high-speed camera technology.Xanthan solutions,classified as pure shear thinning fluids,were prepared by mixing the small amount of xanthan gum with deionized water（DI water）.The motion and morphological changes of droplets impact on hydrophobic surfaces were observed by high-speed imaging.Non-Newtonian droplets of dilute solution of xanthan gum（xanthan gum mass fraction 0.005～0.03%）were compared with Newtonian droplets of various viscosities（μ_l=0.89～18.89 Pa·s）within a range of Weber number（We）from 12 to 221.In contrast to Newtonian droplets,which undergoes either partial rebound or deposition occurs,the full rebounding behavior on hydrophobic surface at the same We has been observed in non-Newtonian droplets.Based on the effective viscosity(μ_eff)model,a prediction model of the maximum dimensionless diameter(β_max)of shear thinning fluid was proposed.The prediction model of the maximum dimensionless height(ξ_max)for a shear-thinning fluid droplet impact onto hydrophobic surface and the theoretical value of the critical dimensionless height（ξ_c）for droplet rebound were obtained by energy conservation law.A correlation equation to differentiate partial rebound and full rebound was also developed based on the relationship betweenξ_max and We.These models achieved good agreement with the experimental measured value over a wide range of We.In order to understand the bouncing mechanism of non-Newtonian fluid droplets,a numerical model of droplet impacting on solid substrate has been constructed based on phase field method（PFM）and finite element method（FEM）.Moreover,the Carreau model and dynamic contact angle function have been employed for accurate emulation of viscosity variations and impacting dynamics.The convergence and accuracy of the model were verified through grid-independent and experimental simulation comparison validation.Based on the model,the mechanism underlying the effect of shear-thinning properties on the droplet spread and rebound was investigated.The simulation results indicated that shear-thinning properties influence the bouncing behaviors of droplets dramatically.The high shear rates during spreading stage resulted in low viscosity,which reduced the viscous dissipation,and led to little differences inβ_max between xanthan gum aqueous droplets and DI water droplets.As we change the power index n in the Carreau model,the non-Newtonian droplets came into partial rebound（n≤0.4）,full rebound（n=0.5）and deposition（n≥0.6）respectively.The shear rates at the bottom part（at the location z/H<0.3）in the rebounding droplets are always greater than the deposition droplet at identical We.Nevertheless,if the high shear rates concentrate on the regions adjacent to the contact line（at the location z/H<0.05）,the disequilibrium pressure field would push the droplets off the surface thoroughly.Finally,the effects of consistency coefficient（k）,power-law index（m）,gas-liquid surface tension(σ_lg)and surface wettability on droplet impact process and rebound behavior were preliminarily explored by using power-law viscosity model.The simulation results showed that decreasing the rheological parameters k or m would increaseβ_max and promote the rebound behavior.In contrast,decreasingσ_lg or increasing the surface wettability also promoted spreading stage yet suppressed recoiling stage and rebound.Phase diagram of power-law fluids droplets rebound behavior in term of effective Reynolds(Re_eff)and dynamic receding contact angle(θ_dr)was presented.The influence of non-Newtonian fluid properties on the dynamics of droplet-surface interactions were recognized and clarified from mechanism through the experimental and simulation results of this study.This investigation will provide theoretical basis and research foundation for development of additive manufacturing,anti-icing,and epidemic prevention.