Research On Superhydrophobic Columnar Array Structures Capable Of Droplet Pancake Bouncing

Superhydrophobic surface has a large contact angle and a small slide angle.When droplets impact on superhydrophobic surface,bouncing will happen,the solid-liquid contact time can effectively reduced compared to the common surface,which has received considerable attention in self-cleaning,anti-icing,corrosion resistance,droplet directional transportation fields.Researches in these years figured that when droplets impact on superhydrophobic surface with special structures,contact time will reduce further.Especially droplets can generate pancake bouncing when impacting on superhydrophobic columnar array structures,with contact time significantly reducing,which has an important significance in freezing rain resistance respect.However,the existing columnar array structures capable of droplet pancake bouncing whose diameter is less than 100 μm and height-diameter ratio is over 10 are difficult to be large-area fabricated.For large-area fabrication,this paper analyzed the influence law of columnar array sizes on droplet bouncing,optimized the sizes and proposed to use the template method to fabricate pancake bouncing capable superhydrophobic columnar arrays.And based on the research above,this paper also proposed a new method of reversible droplet bouncing state and contact time controlling.Based on the bouncing results on superhydrophobic columnar array structures with different sizes,this paper optimized the sizes and obtained the optimal sizes of columnar arrays on which droplets of 17.9 μL can generate pancake bouncing.Column diameter is 1.05 mm,spacing is 0.25 mm,and height is 0.6~1.0 mm.When 17.9 μL droplets impact on the columnar arrays with the sizes above,pancake bouncing will happen and contact time is 8.625 ms,about 40% of the bouncing time on the flat superhydrophobic surface.In order to make the droplets of larger volume can generate pancake bouncing,this paper chose 1.0 mm as the column height and according to the diameter and spacing sizes above,fabricated superhydrophobic columnar arrays on PDMS and PP by vacuum pouring process and hot-press process respectively,with a contact angle of 164°.The materials and processing method provide theoretical and experimental support for large-area fabrication of pancake bouncing capable superhydrophobic columnar arrays.Experimental results of droplets bouncing on columnar arrays showed that droplet bouncing is determined by the column diameter,spacing,height,droplet size and impacting velocity,among these,the minimum column spacing influences the most.Columnar arrays whose column diameter is 0.40 mm and height is 1.0 mm on shape memory polymer were fabricated through pouring process.Taking the advantage of shape memory polymer characteristics of the thermal deformation and recovery,droplet bouncing state,bouncing direction and bouncing time controlling was achieved by changing the columnar array inclination angles to change the minimum column spacing indirectly.In addition,this paper analyzed the relationship between droplet bouncing on inclined columnar arrays and on vertical columnar arrays,and also proposed the fitting formula of bouncing time and superhydrophobic columnar array inclination angles,which agrees well with actual values,proving the influence of minimum column spacing on droplet bouncing.