Impact Dynamics Of Non-Newtonian Droplets On Solid Surfaces

The impact dynamics of droplets on solid surfaces has a wide range of applications including internal combustion engine combustion,spray cooling,pesticide spraying,and inkjet printing et al.The complex flow behaviors during droplets impacting on the solid surfaces directly influences the implementation effect of the above industrial and agricultural applications.Nanofluids is a new class of fluid containing nanoparticles.Due to its excellent thermal and magnetic conductivity,nanofluid has significant application prospects in the field of high-efficiency heat transfer and new material manufacturing.The polymer materials have wide use in microfluidics and material deposition preparation due to their special rheological behavior.However,the addition of nanoparticles or polymer materials often causes the base fluid to exhibit a variety of non-Newtonian fluid properties including shear thinning,viscoelasticity and yield stress.These non-Newtonian properties play an important role in the fluid flow behavior.The existing research about the impact dynamics of droplets on solid surfaces mainly concerns the Newtonian fluid droplets.There is still a lack of in-depth understanding on the impact of non-Newtonian shear-thinning droplets on solid surfaces.This work focuses on the effects of shear-thinning properties on droplet impact dynamics.We prepared nanofluids and polymer solutions that exhibit shear-thinning properties and analyzed their rheological properties using a rotary rheometer.A visual experiment system was built based on high-speed camera technology to capture the process of droplets impact.Based on the finite element method coupled with level set method to capture the interface movement,a numerical model was established to simulate the droplet impact process.The impact dynamics of shear-thinning droplets on solid surfaces was investigated by experimental,numerical and theoretical methods.We investigated the impact dynamics of nanofluids on solid surface.The nanofluid containing graphene nanoparticles and carbon nanotubes was prepared with by dispersing nanoparticles into base fluid.The rheological behavior of the prepared nanofluids was analyzed using a rotary rheometer,and it was found that nanofluids containing nanoparticles of various types and different mass fractions exhibit different degrees of shear-thinning properties.Based on high-speed camera technology,a visual experiment system was built.The impact dynamics of nanofluid droplets on solid surfaces was experimentally studied.It was found that the addition of nanoparticles significantly inhibited the spreading and retraction behavior of droplets.Based on the finite element method coupled level set method to capture the movement of the phase interface,a truncated power law model was used to describe the effect of shear-thinning characteristics.The effect of shear-thinning properties on droplet impact dynamics was investigated.A numerical model is implemented through finite element method coupled with level set method to investigate the impact dynamics of a shear-thinning droplet on the horizontal surface.The shear-thinning property was considered and characterized by a truncated power-law model.The effects of rheological parameters(power-law index m and consistency coefficient K),impact velocities,surface tension,as well as the surface wettability on impact behaviors of the droplet have been studied numerically.We show that the shear-thinning property might lead to various physical outcomes including more drastic shape deformation,lamella rupture and bouncing,since the alternating of flow fields by shear dependent viscosity and the diminishing of viscous dissipation during the impact process.For both Newtonian and shear-thinning droplets,the effects of surface tension are found to be more pronounced during receding phase.The bouncing phenomenon was reported as increasing the shear thinning degree(smaller power-law m)on the surface with relatively higher contact angle.In addition,an empirical model based on classical viscous model has been proposed to predict the maximum dimensionless spreading diameter and shows good agreement with simulation results.The impact dynamics of water and aqueous polymer droplets on superhydrophobic surfaces is investigated.The rigid chain polymer material Xanthan gum is selected as polymer additive to prepare polymer solutions with different mass fraction.The rheological properties of prepared fluids are analyzed using a rotary rheometer,and it is found that the aqueous Xanthan gum solutions show significantly shear-thinning characteristic.The superhydrophobic surfaces are prepared by spraying nano-silica particles on the glass surfaces.By means of high-speed visualization,we show the existence of singular jet when droplet impact on a superhydrophobic surface under moderate Weber number.In addition,for a certain range of Weber number,the jet formation is accompanied by entrapment of air bubbles.We also reveal that by adding very small amount of Xanthan into deionized water droplets,a complete suppression of the singular jet is observed.To get a deeper insight into the generation and suppression mechanism of singular jet.We build a numerical model based on the finite element and level set methods.In this work,we conducted a series of experimental,numerical and theoretical study to investigate the effect of non-Newtonian shear-thinning properties on impact dynamics of droplets on solid surfaces.This work aims to provide theoretical understanding and technical guidance for applications based on the impact dynamics of non-Newtonian droplets on solid surfaces.