Fabrication Of Gradient Wettability Aluminum Alloy Surface By Femtosecond Laser And Study On Droplet Motion Mechanism

In recent years,with the rapid development of surface science and micro nano fluid control technology,more and more attention has been paid to the application of gradient wettable surface to realize the directional movement of droplets.Because gradient wettability surface can realize droplet self-driving without external energy input,so it has been widely used in microfluidic field.In this paper,the femtosecond laser etching method and the laser processing parameters and micro texture parameters are used to construct various gradient wettability surfaces,and the motion characteristics of droplets on different surfaces are studied.The following work has been completed:(1)The effect of laser fluence,scanning passes and spot overlap rate on the wettability of the surface was studied by etching the square micro pit array structure on the surface of 6061 aluminum alloy by femtosecond laser.Three gradient wettability surfaces were prepared by changing laser processing parameters.The droplet motion characteristics were studied by drop experiments.The experimental results show that the side length and depth of square micro pits increase with the increase of laser single pulse fluence and scanning passes,and the change of depth is larger than the side length.The change of surface 3D roughness parameters and chemical elements explains the reason of surface contact angle change under different processing parameters.The displacement of the droplets on the surface of gradient wettability with the largest change of contact angle is the longest,and the peak velocity and average velocity are the fastest.(2)The effects of different laser processing parameters and micro texture density on the wettability of three kinds of micro texture surfaces were studied by etching the square micro concave array,grating like micro groove array and surface scanning array on the surface of 6061 aluminum alloy by femtosecond laser.The surface wettability of different gradients was constructed by changing the edge length and spacing of the microstructure.The divergence between the calculated contact angle and the actual contact angle is compared with the one obtained by Wenzel and Cassie wetting model.The experimental results show that the trend of contact angle can be explained by Wenzel model,and the divergence between the calculated contact angle and the actual contact angle can be explained by the change of the micro nano bulge on the surface of micro texture and the content of carbon and oxygen in the micro texture surface.The change of the micro texture edge length and the contact angle of the spacing surface are different,and then various gradient wettability surfaces can be prepared.The results show that the volume loss of the surface droplets is the smallest and the spreading time is the shortest and the average speed is the fastest.(3)The coordination between laser processing parameters and micro texture parameters is needed to control the surface wettability accurately.After the successful preparation of gradient wettability surface,the influence of droplet volume and surface inclination angle on droplet motion characteristics was further studied.The experimental results showed that the larger the droplet volume,the slower the droplet velocity and the smaller the peak velocity of droplets.The larger the surface inclination angle,the greater the resistance of the droplets,the smaller the average velocity.