Droplet Spreading Characteristics On Ultra-slippery Solid Hydrophilic Surfaces

Dynamic interactions of droplet impacting on a solid surface are essential to many emerging applications,e.g.,electronics cooling,ink-jet printing,water harvesting/collection,anti-frosting/icing,microfluidic and biomedical devices.Understanding the physical phenomena and mechanism of liquid spreading on the solid surface is of utmost importance in improving the process efficiency and developing advanced technology for various industrial systems.Despite extensive studies on the kinematic features of droplet impacting over the last two decades,the role of low contact angle hysteresis as the main parameter of surface wettability during the droplet impact process is still not clear.Previous research has demonstrated that ultra-slippery hydrophilic solid surface with low contact angle hysteresis exhibits excellent heat/mass transfer performance in condensation and defrosting.However,the underlying mechanism of droplet spreading dynamics on the ultra-slippery solid hydrophilic surfaces remains poorly understood yet.This work reports rapid spreading of droplet impacting without contact line pinning on an ultra-slippery hydrophilic solid surface having an equilibrium contact angle of 37°and low contact angle hysteresis of 3°.The spreading characteristics are systematically compared with that on the plain hydrophilic,hydroxylated hydrophilic,and plain hydrophobic surfaces.The results reveal that the maximum spreading factor of impacting droplets is mainly dependent on the contact angle while the corresponding residence time is closely related to contact angle hysteresis.The low contact angle hysteresis can decrease the time to reach the maximum spreading diameter and the time interval to maintain the maximum spreading diameter when the contact angles are similar.For the droplet impacting on the ultra-SHI surface,the maximum spreading factor of the impacting droplet conforms to D_max/D₀～We^1/5 on the horizontal substrate.During the spreading and retraction process of the droplet impacting the ultra-SHI surface,the three-phase lines are smooth,and the waves in the center pit propagate regularly.In the process of retraction,the liquid film in the center pit rises smoothly and keeps synchronous retraction.This is because there is no local free energy minimum to pin the three-phase line on the ultra-SHI surface due to physically and chemically uniform everywhere,and the energy barriers overcome at each point in the process of spreading and retracting are the same.The evaporation mode of droplets on the ultra-SHI surface is a constant contact angle mode,because the droplets have a very weak three-phase contact line pinning effect.Compared with the dependence of contact diameter and sliding distance on the static contact angle,the contact angle hysteresis affects the motion state of a droplet impacting the inclined surface after the retracting regime.The droplets impacting on the ultra-slippery hydrophilic surface can continue to slip at a constant speed on the inclined substrate.For the droplet impacting on the ultra-SHI surface,the maximum contact diameter of the impacting droplet conforms to D_max/D₀～We^1/4 on the inclined substrate.The droplets wetting state on the ultra-SHI surface is regulated by the addition of ethanol.With the increase of ethanol concentration,the surface tension of the droplet decreases,the contact area increases,and the retraction speed slows down when the ethanol water droplet impacts the ultra-SHI surface with low speed at room temperature.During the film evaporation stage,the center of the ethanol water droplets retracted first,and then the edge of the liquid film retracted.When the droplets are in the nucleate boiling stage and the transition boiling state,the addition of a small amount of ethanol makes the atomization and fragmentation of the droplets more severe than that of the pure water droplets,which inhibits the droplet rebound.In the transition boiling state,the ethanol water droplets explosive boiling during the impact process.The addition of ethanol promotes droplet spreading,atomization and breakup,which is conducive to the advance of boiling phenomenon and takes away heat,thus improving the cooling efficiency.The findings of this work not only show the important role of surface wettability on droplet spreading characteristics but also present a pathway for controlling dynamic interactions of impacting droplets with ultra-slippery hydrophilic surfaces.