3D Bioinspired Microstructures for Switchable Repellency in both Air and Liquid

Abstract In addition to superhydrophobicity/superoleophobicity, surfaces with switchable water/oil repellency have also aroused considerable attention because of their potential values in microreactors, sensors, and microfluidics. Nevertheless, almost all those as‐prepared surfaces are only applicable for liquids with higher surface tension (γ > 25.0 mN m−1) in air. In this work, inspired by some natural models, such as lotus leaf, springtail skin, and filefish skin, switchable repellency for liquids (γ = 12.0–72.8 mN m−1) in both air and liquid is realized via employing 3D deformable multiply re‐entrant microstructures. Herein, the microstructures are fabricated by a two‐photon polymerization based 3D printing technique and the reversible deformation is elaborately tuned by evaporation‐induced bending and immersion‐induced fast recovery (within 30 s). Based on 3D controlled microstructural architectures, this work offers an insightful explanation of repellency/penetration behavior at any three‐phase interface and starts some novel ideas for manipulating opposite repellency by designing/fabricating stimuli‐responsive microstructures.

hatching distances were 300 nm and 200 nm for the pillar part and the cover part, respectively. The laser power was set at 92% for all the process. After the 3D printing, the samples were immediately immersed into 15 mL propylene glycol methyl ether acetate (PGMEA) (99.0%, Shanghai Aladdin Co., Ltd.) for 15 minutes. This process was repeated twice. Later, the sample was transferred immediately into 15 mL isopropyl alcohol (IPA) (AR, Sinopharm Chemical Reagent Co., Ltd.) for 15 minutes. This process was repeated twice, too.
After the four developing steps, the sample was pick out and dried in air. These four developing steps of post-processing are designed to remove the unreacted photoresist.

Fabrication of deformable doubly re-entrant microstructures.
The same substrates were used as above. To create deformable doubly re-entrant microstructures, the cross section of the pillar is different (a= 3.5 μm, b= 5 μm, see Figure 4b) from the above mentioned stable doubly re-entrant microstructures. The laser power, the scan speed, the height and the diameter of the top cover vary to create different microstructures.
The slicing distance was 1 μm. The hatching distances were both 150 nm for the pillar part and the cover part. In our experiment, it took about 20 h to fabricate one array with a size of 1 cm × 1 cm (the center-to-center distance is 80 μm). After the 3D printing, similar developing process in PGMEA as above was conducted. Later, the sample was immediately transferred into 15 mL isopentane (AR, Sinopharm Chemical Reagent Co., Ltd.) for 15 minutes. When the developing process ended, the sample was picked out. To study the deformation behavior, the sample was immersed in IPA, picked out after 15 min and dried in air. To study the recovery behavior, the substrates with collapsed microstructures were immersed in different liquids and observed with a microscope.

Surface modification
The superhydrophilic modification was conducted by on O 2 plasma treatment (DT-01, Opsplasma, China) for 200 s. The fluorination treatment was subjected to a process similar to previous report. [1] Namely, the sample firstly experienced superhydrophilic modification and then immersed in a 5 ml Falcon tube containing 5 ml of dichloromethane (DCM), 40 μL of triethylamine (TEA), 10 mg of 4-(Dimethylamino)pyridine (DMAP) and 20 μL of Trichlorovinylsilane (TCVS) for 30 min. Later, the sample was rinsed with ethanol.

Measurements and Characterization.
The microarchitectures were characterized by SEM images using a field emission scanning electron microscope (FESEM, Ultra Plus, Zeiss). The static contact angle was characterized using a JC2000D measuring instrument equipped with a CCD camera at room temperature and the size of liquid droplets were controlled at around 3 μL (Scheme S1a). The advancing angle (θ A ) and the receding angle (θ R ) were evaluated by pumping in and pumping out the liquid according to early report. [2] Generally speaking, the distance between the tip of the nozzle and top of the microstructures was set at about 300 μm and the number of microstructures deposited by droplet in view was set as 15 (Scheme S1b-c). The advancing angle θ A and receding angle θ R were measured with the help of ImageJ software. nhexadecane (AR, Sinopharm Chemical Reagent Co., Ltd.), silicone oil with viscosity of 50 CS (Dow Corning) and water (18.25 MΩ) were used. The optical microscope images were obtained by an Olympus MVX10 microscope (OLYMPUS GmbH, Japan).         According to the analysis, the possible bending direction is along the short side. Another parameter determining the bending direction is the scanning direction. Generally, the bending along the scanning direction is difficult due to the comparable larger elastic restoring force. Besides the shape of the cross section and the scanning path, the actual bending direction is also determined by the liquids. The liquids influence the bending direction through the surface energy (γ) as well as the retreating behavior of the environmental liquids when the evaporation occurs. As a contrast, isopentane can decrease the possibility of bending because of lower γ and faster evaporation (corresponds to a smaller difference between θ 1 and θ 2 ), which ultimately lead to a smaller net capillary force.