Numerical Simulation Of Droplets Impingement And Mixing Behavior On A Solid Surface

The interaction between droplets and solid surface generally exists in various industrial and technological processes,such as micro-scale chemical engineering,inkjet printing and spraying.It is necessary to accurately understand the spreading,collision and mixing mechanism of single or multiple droplets on a solid surface,especially when it comes to the precise control of microdroplets morphology.Based on many-body dissipative particle dynamics(MDPD),the impingment and mixing behavior of droplets on the solid surface has been simulated in this study,including single droplet wetting on the solid surface,single droplet impingement on the solid surface,and multiple droplets collision and mixing on the solid surface.In the study of single droplet wetting on the solid surface,the accuracy of MDPD simulation method was verified firstly,and the time evolution of the droplet dimensionless diameter was found to meet the power law in the case of the droplet wetting on a homogeneous solid surface,which was consistent with the literature.When the droplet was wetting on the surface with a single hydrophilic stripe,it was found that the ratio of stripe width to droplet diameter(?)significantly affected the morphology of the droplet after wetting.The isotropic wetting was found when?<0.04;and when??[0.04,0.5],the aspect ratio of the droplet increased with the increase of the?and the increase of the hydrophobicity of the surface.When the droplet was wetting on periodically hydrophilic and hydrophobic stripes with?ranging from 0.5 to 1.0,the droplet deformed violently.It was found that on the multi-striped surface with superhydrophobic stripes,the values of the droplet perpendicular contact angles and the aspect ratios decreased with the?.The contact angles of the droplet in the direction parallel to the stripes matched closely with the intrinsic contact angle of the hydrophilic stripe.In the simulation of single droplet impingement on the striped surface with?ranging from 0.5 to 1.0,an inertial droplet can be splited by a hydrophilic surface coating with a single superhydrophobic stripe.The effect of?and droplet weber number(We)on droplet split behavior was studied.A correlation for critical split-off conditions of impinging droplet was propoesed.In addition,the impingement of a droplet on the multi-striped surface with superhydrophobic stripes was simulated.Three morphologies were found and the energy of droplet during impingement was analyzed.The influence of?,We and the surface wettability on the eventual droplet morphology was studied.Finally,we found that the correlation for the critical splitting boundary in single-striped cases is also valid for multi-striped cases.In the case of Newtonian liquid droplets collison on the surface,the hydrodynamics and mixing of a droplet with an impingment angle and a sessile droplet of the same fluid was studied.Based on the the internal velocity fields of the droplet,a recirculation pattern was found in non-head-on collosion cases,and this internal recirculation lasted longer in the hydrophobic droplet than in the hydrophilic droplet.Based on the concentration distribution,the mixed area was found increasing around the impingement interface.Specifically,a nonaxisymmetry impingement results in the mixed area was also asymmetrically with the time.By dividing the collision and mixing process in a convection and a diffusion stage,the effect of the Weber number(We),the Ohnesorge number(Oh),the impingement angle(?_i)and surface wettability on the total mixing time,convection time and diffusion time was studied.The results showed that the diffusion dominant period is 5 to 10 times of the convection dominant period;the dimensionless total mixing time hardly changed with We and Oh when We ranged from 5.68 to 22.7 and Oh ranged from 0.136 to 0.214.What's more,the effect of the impingement angle on the dimensionless total mixing time was found to be related to the surface wettability;decreasing the impingement angle was to be an effective mean to shorten the mixing time for hydrophilic droplet,which was mainly due to the influence of impingemtment angle on the concentration distribution at the end of the convection dominant stage,and this determined the diffusion distance in diffusion dominant stage,and further affected the duration of diffusion dominant stage.By using two types of MDPD particles with certain proportion and force parameters,non-Newtonian fluid droplet with the shear thinning properties can be generated.The study of the head-on collision between one or several shear thinning droplets and a sessile droplet with the same properties on a hydrophobic surface has been simulated.In the case of single impinging droplet collision with the sessile droplet,the effects of droplet We and droplet apparent viscosity on the morphology of the droplet at at equilibrium state were studied.When it comes to the successive multiple impinging droplets collision with the sessile droplet,the effects of the time gap and droplet We on the spreading of the sessile droplet were considered.When the particles entered the diffusion dominant stage,the particles with higher diffusion rate behaved differently from those in the mixing of Newtonian fluid droplets.This phenomenon could be explained by the difference of the diffusion coefficient of the two types of particles and the different distribution of two types of particles in a shear thinning droplet.