Experimental Study On Directional Transport Of Droplets On Superheated Surfaces With Asymmetric Structures

Regulating the directional transport of droplets on high-temperature solid surfaces has broad application prospects in the fields of heat transfer,microfluidic devices,and drag reduction.When the surface temperature exceeds a certain value,the droplet placed on the surface will float on the vapor layer generated by evaporation,which is called Leidenfrost effect,and the corresponding temperature is called Leidenfrost temperature(LFT).With the help of Leidenfrost effect,the existing research on the directional transport of droplets on high temperature surfaces mainly focuses on the design of asymmetric surface structures.However,the directional transport speed of high-temperature droplets in existing research still needs to be improved.In addition,manufacturing such structures is usually time-consuming and complex,which is difficult to meet the requirements for large-scale preparation in industrial production.Therefore,this paper designs and uses wire electrical discharge machining and laser processing technologies to efficiently fabricate three kinds of surfaces with fast directional transportation functions,and conducts research on the behavior and mechanism of the directional transport of high-temperature droplets on them.The contents are as follows:Firstly,in order to simplify the processing method and improve the preparation efficiency,flat samples were prepared on a variety of common metal substrates by wire electrical discharge machining(WEDM),and experiments on directional transport of droplets were carried out on the surface of high-temperature samples.It is found that the droplets can perform directional transport spontaneously on the surface of the high-temperature flat samples,and the directional transport velocity increases with the increase of the Weber number of the droplet.Further research found that continuous scale-like structure can be formed spontaneously on the sample surface due to the discharge of the electrode wire in WEDM.This structure is similar to the ratchet structure with smaller scale,which can guide the airflow generated by the evaporation of Leidenfrost droplets to escape in a specific direction,thus dragging the droplets to move directionally.Based on this,dual-scale ratchet structure was fabricated on 304 stainless steel substrate by WEDM.It is found that under the same geometric parameters,the directional transport speed of the droplet on the surface of the dual-scale ratchet structure is much higher than that on the surface of the traditional ratchet structure.And this velocity first increases and then decreases with the increase of the surface temperature.In addition,the directional transport speed of the droplet increases with the increase of the inclination angle of the dual-scale ratchet structure.Finally,the relationship between the initial acceleration of the droplet on the surface of the dual-scale and traditional ratchet structure sample is obtained by theoretical derivation,which is equal to the ratio of the initial effective contact area of the droplet on the two surfaces.The experimental data are in good agreement with the theoretical value.Secondly,in order to achieve the directional transport of droplets with large Weber number,laser etching technology was used to construct two adjacent egg tray-like microarray regions with different densities on the surface of 304 stainless steel,and the bouncing experiment of impacting droplets was carried out on the surface of high-temperature samples.At high temperature,when the droplet hits the boundary between the two regions,it will spontaneously bounce towards the region with larger roughness.In addition,with the increase of temperature and Weber number,the directional rebound distance of the impinging droplet will increase first and then decrease.Further research found that when the droplet impacts at the boundary,due to the different Leidenfrost temperature of these two adjacent regions,the two parts of the droplet will be in contact boiling and film boiling states respectively.The resulting asymmetric Young's force is the main reason for driving the directional bouncing of the droplet.This research has important reference value for the design of surface with directional transportation functions of droplets at high temperature.