液滴碰撞Janus颗粒球表面的行为特征

Behavioral characteristics of droplet collision on Janus particle spheres

为研究液滴碰撞Janus颗粒(双亲性)球表面的独特行为特征,以粒径为5.0 mm铜球为材料制备了Janus颗粒,用直径为2.0 mm的液滴,在韦伯数(We)为2.7,10,20,30的测试情况下对Janus颗粒球表面进行了碰撞实验.结果表明:液滴碰撞Janus颗粒球表面后的运动可分为铺展、回缩、振荡和回弹4个过程.在不同We下,液滴碰撞Janus颗粒后的运动状态主要与表面润湿性相关,在Janus颗粒亲水侧表现为铺展特性且铺展系数γ随着时间t的增大而逐渐增大并趋于稳定;但在疏水侧,表现为回弹现象,铺展系数γ会出现类似"抛物线"形状;当液滴碰撞Janus颗粒球表面亲-疏水分界线时,液滴铺展和回弹同时发生.基于能量平衡和受力分析发现,液滴动能和表面能的互相转化是液滴铺展的关键,液滴会在重力、惯性力、表面张力、黏性力、接触力等力的综合作用下展现其独特的行为特征并最终达到平衡状态.

To acquire the unique behavioral characteristics that droplets impact the Janus particle(amphiphilicity)sphere surface,a series of collision experiments is conducted by using Janus particles with a diameter of 5.0 mm.These Janus particles are prepared by chemical treatment of the copper particles.Water droplets with a diameter of 2.0 mm are used to impact hydrophbilic surface,hydrophobic surface and hydropholic-hydropholic boundary of Janus particle,under four Weber numbers which are 2.7,10,20 and 30,the corresponing Reynold numbers are 621.8,1191.9,1589.2 and 2185.1.The results show that the collision process can be divided into four stages:spread,retraction,oscillation and rebound.Under different Weber numbers,the behavioral characteristics of droplets are mainly affected by the surface wettability.On the hydrophbilic surface,the droplets exhibit the spreading characteristics,with increasing time the spreading coefficient gradually increases and finally tends to be stable.As Weber number increases,the difference in spreading coefficient for droplet under adjacent Weber number gradually decreases,indicating that droplets spreading is mainly affected by inertia.On the hydrophobic surface,the spreading coefficient on the figure presents a"parabola"shape.Droplets spreading takes the same time to reach the maximum spreading coefficient under different Weber numbers.However,when droplets impact the hydropholic-hydropholic boundary,droplets show spreading and rebound behavioral characteristics simultaneously.At the beginning of droplets spreading,the spreading coefficient has almost the same value on both sides of the hydropholic-hydropholic boundary.With the increase of time,part of droplets on the hydrophobic are attracted by the hydrophbilic side surface and go into hydrophbilic side zone.In order to explain this phenomenon,the concept of line tension is introduced and the line tension on the hydrophilic side is found to be less than that on the hydrophobic side by analyzing the forces on both sides of the droplets.Based on energy balance and force analysis,it is found that the mutual conversion of droplet kinetic energy and surface energy are the key factor to make droplets spread.The droplets possess the unique behavioral characteristics and reach an equilibrium state under the combined influence of gravity,inertial force,surface tension,viscous force,and contact force.