Experimental And Numerical Studies On Dynamic Characteristics Of Droplet Impingement On Micropillar Array Textured Surfaces

Droplet impingement on textured surfaces has been widely applied in various industrial and commencial areas,including mechanical engineering,chemical engineering,agriculture,biomedicine and aviation.The microscale structure on the textured surface has a great impact on the spreading area and contact time during the droplet impact.It is of great significance to investigate the dynamic characteristics of droplets collision with textured surfaces having different microstructures for practical applications.In this thesis,micropillar array textured surfaces with differnet pitchs were fabricated on smooth silicon substrates.The experiment was conducted to compare the dynamic behavior of droplet impact on a smooth silicon surface and two micropillar array silicon surface.Simultaneously,2D numerical simulation was performed to study the droplet impact behavior on micropillar arrays using COMSOL Multiphysics.The effects of pillar pitch,sidewall inclination,pillar width,and pillar height on the spreading factor,contact time,and pinning extremum of droplet impact were numerically investigated.The main conclusions are drawn as follows:(1)Based on a solvent etching method,two micropillar arrays with pillar pitchs of about 500 nm and 200 nm were fabricated on silicon substrates,the diameter and height of micropillars are all about 1 and 12 ?m,respectively.After using FAS coating on the two micropillar array surfaces with different micropillar pitch,the above surfaces all showed superhydrophobicity.(2)High-speed CCD camera was used to record the dynamic evolution of droplet impact on each surface at Weber numbers ranging from 1.4 to 173.1.In the experiment,the droplet on the smooth silicon surface showed adhesion and partial rebound,and on the micropillar array surfaces showed complete rebound and crushing.Besides,the droplet is accompanied by a top fracture during the complete rebound on the micropillar array surfaces.The experimental results show that the splash and crush phenomenon of the droplet on the micropillar array surface with larger micropillar pitch were weaker.By analyzing the contact time of droplet impact on the two micropillar array surfaces,it was found that the droplet pinning on the micropillar array surfaces can effectively reduce the contact time.(3)The simulation of the droplet impact on the micropillar array surfaces with a pitch of 20~60 ?m was conducted.In the numerical simulation,the Level set method and the Kistle-Blake contact angle model were used,and completing the reliability verification of the simulation.In the calculation results,the impact spreading factor,contact time and pinning extreme value of droplets on different surfaces are analyzed.Calculation shows that the droplet spreading factor decreases with the increase of the microcolumn spacing in the retraction stage;the droplet contact time decreases with the increase of the microcolumn spacing;the pinning extremum of droplet impact is in proportion with the increasing micropillar pitch and We.Finally,the calculation result shows the droplet pinning provides both upward inertial force and additional viscous dissipation,which has a two-sided effect on the contact time of the droplet retraction.(4)The simulation of the droplet impact on the micropillar array surface under different micropillar sidewall inclination angles(70~110 °),widths(30~50 ?m),and heights(20~40 ?m)were investigated.The calculation shows that properly increasing the width of the micropillars on the surface of the micropillar array or reducing the height of the micropillars can effectively promote the spreading of the droplets with the same pitch and wettability.However,when the width of the microcolumn is too large,the spreading factor of the droplet impact decreases due to the increase of viscous dissipation.The droplet contact time decreases with increasing angle of micropillar sidewall.In addition,the larger the height of the micropillar,the stronger the lift of the air vortex within the micropillar gap during the impingement,resulting in a shorter contact time.