the tapered surface with a size of 2.0  2.0 cm was created based on type 101 copper plate with a thickness of 3.18 mm by combining a wire - cutting method and multiple chemical etching . square posts arranged in a square lattice were first cut with a post centre - to - centre spacing of 200 m . the post edge length and height are 100 m and 800 m , respectively . then the as - fabricated surface was ultrasonically cleaned in ethanol and deionized water for 10 min , respectively , followed by washing with diluted hydrochloric acid ( 1 m ) for 10 s to remove the native oxide layer . to achieve a tapered surface with post diameter of 20 m at the top , six cycles of etching were conducted . in each cycle , the as - fabricated surface was first immersed in a freshly mixed aqueous solution of 2.5 mol l sodium hydroxide and 0.1 mol l ammonium persulphate at room temperature for ~ 60 min , followed by thorough rinsing with deionized water and drying in nitrogen stream . as a result of chemical etching , cuo nanoflowers with an average diameter ~ 3.0 m were produced . note that the etching rate at the top of the posts is roughly eight - fold of that at the bottom of the surface due to the formation of an etchant solution concentration gradient generated by the restricted spacing between the posts . to facilitate further etching , after each etching cycle the newly - etched surface was washed by diluted hydrochloric acid ( 1 m ) for 10 then another etching cycle was performed to sharpen the posts . in preparing the straight post arrays , all the surfaces were modified by silanization immersing in 1 mm n - hexane solution of trichloro(1h,1h,2h,2h - perfluorooctyl)silane for ~ 60 min , followed by heat treatment at ~ 150 c in air for 1 hour to render surfaces superhydrophobic . the mts surface is fabricated on a copper foam with density 0.45 g / cm , porosity 94 % , and thickness 0.16 cm . the nanostructure formation on the mts surface and silanization were conducted using the same procedures described above . the static contact angle on the as - prepared substrate was measured from sessile water drops with a ram - hart m200 standard contact angle goniometer . deionized water drops of 4.2 l , at room temperature with 60% relative humidity , were deposited at a volume rate of 0.5 l s. the apparent , advancing ( a ) and receding contact angles ( r ) on the tapered surface with centre - to - centre spacing of 200 m are 165.6  1.3 , 167.2  1.1 and 163.9  1.4 , respectively . the apparent ( equivalent to the intrinsic contact angle on tapered surface ) , advancing ( a ) and receding contact angle ( r ) on the surface with nanoflower structure alone are 160  1.8 , 162.4  2.8 , and 158.8  1.7 , respectively . the whole experimental setup was placed in ambient environment , at room temperature with 60% relative humidity . water drops of ~ 13 l and 6 l ( corresponding to radii ~ 1.45 mm and 1.10 mm , respectively ) were generated from a fine needle equipped with a syringe pump ( kd scientific inc . ) from pre - determined heights . the dynamics of drop impact was recorded by a high speed camera ( fastcam sa4 , photron limited ) at the frame rate of 10,000 fps with a shutter speed 1/93,000 sec , and the deformation of drops during impingement were recorded using imagej software ( version 1.46 , national institutes of health , bethesda , md ) . the tapered surface with a size of 2.0  2.0 cm was created based on type 101 copper plate with a thickness of 3.18 mm by combining a wire - cutting method and multiple chemical etching . square posts arranged in a square lattice were first cut with a post centre - to - centre spacing of 200 m . the post edge length and height are 100 m and 800 m , respectively . then the as - fabricated surface was ultrasonically cleaned in ethanol and deionized water for 10 min , respectively , followed by washing with diluted hydrochloric acid ( 1 m ) for 10 s to remove the native oxide layer . to achieve a tapered surface with post diameter of 20 m at the top , six cycles of etching were conducted . in each cycle , the as - fabricated surface was first immersed in a freshly mixed aqueous solution of 2.5 mol l sodium hydroxide and 0.1 mol l ammonium persulphate at room temperature for ~ 60 min , followed by thorough rinsing with deionized water and drying in nitrogen stream . as a result of chemical etching , cuo nanoflowers with an average diameter ~ 3.0 m were produced . note that the etching rate at the top of the posts is roughly eight - fold of that at the bottom of the surface due to the formation of an etchant solution concentration gradient generated by the restricted spacing between the posts . to facilitate further etching , after each etching cycle the newly - etched surface was washed by diluted hydrochloric acid ( 1 m ) for 10 then another etching cycle was performed to sharpen the posts . in preparing the straight post arrays , all the surfaces were modified by silanization immersing in 1 mm n - hexane solution of trichloro(1h,1h,2h,2h - perfluorooctyl)silane for ~ 60 min , followed by heat treatment at ~ 150 c in air for 1 hour to render surfaces superhydrophobic . the mts surface is fabricated on a copper foam with density 0.45 g / cm , porosity 94 % , and thickness 0.16 cm . the nanostructure formation on the mts surface and silanization were conducted using the same procedures described above . the static contact angle on the as - prepared substrate was measured from sessile water drops with a ram - hart m200 standard contact angle goniometer . deionized water drops of 4.2 l , at room temperature with 60% relative humidity , were deposited at a volume rate of 0.5 l s. the apparent , advancing ( a ) and receding contact angles ( r ) on the tapered surface with centre - to - centre spacing of 200 m are 165.6  1.3 , 167.2  1.1 and 163.9  1.4 , respectively . the apparent ( equivalent to the intrinsic contact angle on tapered surface ) , advancing ( a ) and receding contact angle ( r ) on the surface with nanoflower structure alone are 160  1.8 , 162.4  2.8 , and 158.8  1.7 , respectively . the whole experimental setup was placed in ambient environment , at room temperature with 60% relative humidity . water drops of ~ 13 l and 6 l ( corresponding to radii ~ 1.45 mm and 1.10 mm , respectively ) were generated from a fine needle equipped with a syringe pump ( kd scientific inc . ) from pre - determined heights . the dynamics of drop impact was recorded by a high speed camera ( fastcam sa4 , photron limited ) at the frame rate of 10,000 fps with a shutter speed 1/93,000 sec , and the deformation of drops during impingement were recorded using imagej software ( version 1.46 , national institutes of health , bethesda , md ) .
engineering surfaces that promote rapid drop detachment1,2 is of importance to a wide range of applications including anti - icing35 , dropwise condensation6 , and self - cleaning79 . here 
 we show how superhydrophobic surfaces patterned with lattices of submillimetre - scale posts decorated with nano - textures can generate a counter - intuitive bouncing regime : drops spread on impact and then leave the surface in a flattened , pancake shape without retracting . 
 this allows for a four - fold reduction in contact time compared to conventional complete rebound1,1013 . 
 we demonstrate that the pancake bouncing results from the rectification of capillary energy stored in the penetrated liquid into upward motion adequate to lift the drop . 
 moreover , the timescales for lateral drop spreading over the surface and for vertical motion must be comparable . in particular , by designing surfaces with tapered micro / nanotextures which behave as harmonic springs , 
 the timescales become independent of the impact velocity , allowing the occurrence of pancake bouncing and rapid drop detachment over a wide range of impact velocities .