Construction And Properties Of SiO₂-Based Super Slippery Surfaces

Superhydrophobic nano-coatings inspired by the‘lotus effect’have been widely studied in the fields of self-cleaning,anti-foggy/anti-icing,anti-fouling and anti-corrosion.However,superhydrophobic surfaces have multiple defects,such as repulsiveness only for droplets with high surface tension.And the irreversible transition from Cassie to Wenzel state leads to short-term stability underwater.Moreover,low mechanical stability,low transparency,weak pressure stability and so on.To overcome the shortcomings of the lotus-leaf-inspired materials,a new type of Nepenthes pitcher plant-inspired materials called slippery liquid infused porous surfaces（SLIPS）has been introduced recently that exhibit anti-wetting behavior to almost all fluids and show extreme temperature and pressure stability.Unlike superhydrophobic surfaces,SLIPS do not rely on air trapped inside highly porous structures to repel foreign fluids.Instead,a thin layer of a liquid lubricant is trapped inside the porous structures that present an ultrasmooth,continuousand chemically homogeneous overlying liquid interface,which provides an extremely slippery,low-hysteresis and nonwetting surface to a broad range of fluids and solids.However,most of the practical applications of non-wetting surfaces are subject to environmental shear stresses of the airflow or other foreign fluids that resist capillary action and compromise the robustness of SLIPS.Therefore,how to retain the lock lubricant for a long time is the key to prolong the application time of SLIPS.The influence of length and roughness of porous solids on the long-term fixation of lubricants was studied.Based on the method of using surface layered structure to enhance the effect of the capillary and then increase the storage capacity of lubricants,a simple and scalable micron and nano-scale roughening method was introduced to prepare the SLIPS.A double-layer Zn O slippery surface consisting of plate-like and rod-like Zn O was prepared on an aluminum substrate by combining the sol-gel and hydrothermal methods.The self-corrosion current density of the surface was 7.49×10^-7A/cm²,which is 3 orders of magnitude lower than that of the untreated aluminum alloy substrate.After three months of immersion in 3.5%Na Cl solution,the low frequency impedance modulus(|Z|_0.01Hz)is about1.5×10⁵Ω·cm²,which still one order of magnitude higher than the blank sample.The experimental results show that the structure can store lubricant and has good anti-fouling and self-cleaning properties.The complex liquids such as fruit juice and jam could slide down at an inclined angle of<5 degrees.In order to further enhance the oil storage capacity,Zn O/Co₃O₄/SiO₂ hierarchical structure was synthesized on aluminum alloy.Compared with double-layer Zn O,the self-corrosion current density is further reduced by 1-2 orders of magnitude.During the same immersion period,|Z|_0.01Hz is about 1.78×10⁶Ω·cm².The results show that even under the high shear test conditions of 8000 rpm/min,the excellent repellent performance is maintained,and the sliding angle of ethanol is always less than 5 degrees.In order to overcome the disadvantage of poor mechanical stability of the as-prepared samples by hydrothermal method,the effects of different surface morphologies（smooth SiO₂microspheres,wrinkled SiO₂ microspheres,bayberry-like hollow SiO₂ microspheres and urchin-like Cu SiO₃ hollow microspheres）on the properties of the corresponding non-wetting surfaces were studied.The experimental results show that the contact angle lag of the slippery surface with urchin-like structure is always less than 4 degrees at a high shear rate of 8000rpm/min,owning the best liquid resistance,shear resistance and corrosion resistance.The self-corrosion current density is about 8.05×10^-10A/cm²,which shows the best stability in the soaking process.The reason is that lubricant also exists inside the urchin-like Cu SiO₃ hollow sphere,and the spherical shell composed of nanotubes presents a superimposed capillary effect.So,a new idea is provided for designing of long-term robust slippery liquid-infused surfaces.In order to further increase the storage capacity of lubricant,the favorable hollow multi-stage structure was enlarged,the surface folded porous tubular SiO₂ was fabricated by electrospinning template method to obtain a new slippery surface.The self-corrosion current density of the surface is 1.01×10^-10A/cm²,which is 2 orders of magnitude lower than the superhydrophobic surface and 5 orders of magnitude lower than the untreated aluminum alloy substrate.|Z|_0.01Hz is about 10⁹Ω·cm² in the early period.After three months of immersion in3.5%Na Cl solution,the low frequency impedance|Z|_0.01Hz is about 5.43×10⁶Ω·cm²,which is2 orders of magnitude higher than that of bare aluminum alloy.The as-prepared surfaces have self-healing,self-cleaning and anti-icing properties.The porous tubular SiO₂ structure was extended to transparent substrates,and the slippery surface with 90.8%transmittance was prepared.The evaporation rate of the surface lubricant is 0.08%/day.The slippery surface also possesses high liquid repellency and excellent anti-icing performance.During 24 freezing/deicing cycles,the ice adhesion strength was about 17 k Pa,which is 1/52 of the blank glass sample.And the freezing temperature was reduced from-5°C to-24℃.Compared with the untreated matrix,the freezing delay time was increased by 27 times.The supercooled droplets on the blank glass set a time of 84 s,while the icing time on the ultra-slip surface was extended to 2352 s and the icing delay time was increased 27 times.The constructed surface has the advantages of self-repairing and self-cleaning at the same time.