Decoupling Mechanical And Wetting Stability For Robust Superhydrophobic Surfaces And Application

The ability of superhydrophobic surfaces to stay dry,self-clean and avoid biofouling is attractive for applications in biotechnology,medicine and heat transfer.The common assumption thus is that mechanical robustness and water repellency are mutually exclusive surface properties,but we show here that this need not be the case:unprecedented performance levels can be realized when structuring surfaces at two different length scales,with a nanostructure design to provide water repellency and a microstructure design to provide durability.The microstructure is an interconnected surface frame with‘pockets’that house highly water repellent and mechanically fragile nanostructures and prevent their removal by abradants greater than the frame size.We apply this armor concept to various substrates and show that the obtained superhydrophobic surfaces preserve their water repellency even after sharp steel blade and sandpaper abrasion.The research online is as follows:（1）While minimizing liquid-solid contact area is widely used to enhance superhydrophobicity,it results in fragile surface textures and poor wear resistance.To resolve this bottleneck,we divide the mechanical durability and non-wettability and implement them at two different length scales:the nanostructures provide excellent water repellency,whereas the microstructures act as armor to resist abrasion.Three design features have been built:the first is an interconnected frame that prevents abradants greater than the frame size from removing the nanostructures.The second and third design features for the armor are thus low f ^micro and?near 120°.We constructed the armor surface following these three design features with a framework consisting of microscale inverted pyramidal cavities.The inverted pyramidal microstructures withα¹25°were be manufactured on silicon substrates by photolithography and the structures can resist the highest pressure and have experienced only minor damage.We also fabricated inverted pyramidal structures on ceramic,metal,transparent glass and flexible PDMS substrates by embossing technology.From the engineering perspective,this armor approach can also be applied on curved substrates and is scalable by using roll-to-plate printing technology.To demonstrate that the interconnected frame architecture,i.e.,individual cavities designed with a large sidewall angle,is a generic concept to achieve the superior performances,we further fabricated inverted triangular pyramidal（tri-pyramidal）and inverted hexagonal pyramidal（hex-pyramidal）structures on silicon,metal and ceramic substrates,respectively.（2）The armored surface exhibits superhydrophobicity after integrating a nanostructured coating.Here,the fluorinated fractal nanoclusters of silica were used as a model superhydrophobic nanomaterial.After repeated scraping by a steel blade,the armor microstructure shows excellent resistance to the vertical pressure and shear force,and the fractal nanostructure in between the armor keeps itself intact.It is notable that the abrasion removes the fluorinated silane layer from the top of armor microstructures altering local wetting from hydrophobic to hydrophilic.Using laser scanning confocal microscopy,we confirmed that the air-water-solid composite interface at microscale was very stable,since the air-liquid-solid three-phase contact line is supported by nanoscale superhydrophobic materials.The water repellent nanostructures can prevent the sagging of the liquid/air interface caused by the Laplace pressure and the entire system stayed at the constrained equilibrium Cassie-Baxter state.To systematically evaluate the impact of abrasion on superhydrophobicity,a series of armored superhydrophobic surfaces with different open width of the cavities（l）and liquid-solid contact fraction(f ^micro)were prepared,andθ*andθ_roll-off on the surfaces before and after abrasion were measured.All experimental data are consistent with the theoretical model.Both the static contact angle and the roll-off angles show the armored surfaces can keep its super-repellency after abrasion if f ^micro was lower than 8%.Those results suggest that non-wettability was independent of the scale of inverted pyramidal structures.However,the smaller the scale of the armor structures,the more extensive the changes to the liquid-solid contact fraction(（35）f ^micro)after the same abrasion fracture.The suitable armor size can be tailored for various practical application situations.To further understand the effect of the liquid-solid contact fraction for controlling the non-wettability after abrasion,pull-off force maps of the armored superhydrophobic surfaces were measured by scanning droplet adhesion microscopy.After abrasion,the damage of the hydrophobic layer on the armor top resulted in a rise of pull-off forces at the same f ^micro.However,the pull-off forces on the high f ^micro（⁷.8%）rose more rapidly than on the low f ^micro（²%）surfaces,which corresponds with the trend ofθ_roll-off.The water jet impinging tests also agrees with our previous wettability measurement,with a lower f ^micro the deflected angles were higher,i.e.,less energy dissipation occurred when the water bounced away from the surface.Similar variations by water droplets impacting experiments also supported this principle.（3）In real-world applications,surfaces are exposed to repeated abrasion and we further examine the long-term mechanical durability of armored superhydrophobic surfaces with different microstructures.The abrasion was conducted by using a PP probe as the indenter with a defined vertical pressure and reciprocating linear abrasion.The armored superhydrophobic surfaces maintained the static contact angle above 150°and roll-off angle less than 12°even after 1000 abrasion cycles,and present an ideal resistance to the shear force and protection for the silica nanomaterials inside.To illustrate the mechanical durability of our armored superhydrophobic surfaces,we benchmark the critical fracture force,i.e.,the maximum force to destroy the superhydrophobicity and the maximum number of abrasion cycles against conventional superhydrophobic surfaces.Specifically,the maximum number of abrasion cycles are measured to be more than 1000,which is 10 times higher than for conventional superhydrophobic surfaces.The mechanical robustness of the armored surfaces was also demonstrated by tape-peeling tests,ASTM standard Taber abrasion tests and ultra-sharp object scratch tests.We have also conducted more severe durability tests,including thermal stability,chemical corrosion,high-speed jet impact and the tolerance of condensation-induced failure at high-humidity environments.It was found that the armored surfaces maintained their superhydrophobicity even under extremely harsh conditions.（4）These findings demonstrate the value of the armor concept for improving the mechanical stability of superhydrophobic surfaces.The decoupling strategy at the heart of this design framework allows us to perfectly balance mechanical robustness,non-wettability and optical transparency.We use this to create a robust and transparent self-cleaning topcoat for solar cells that maintains their high energy conversion efficiency through passive removal of dust contamination,which could save massive amounts of freshwater,labor and costs associated with the traditional cleaning process.Beyond this initial proof-of-concept illustration,the generality and effectiveness of our design principle and strategy promises to move superhydrophobic surfaces toward real-world applications.