Superomniphobic surfaces (i.e., surfaces that are extremely repellent to both high surface tension liquids like water and low surface tension liquid like oils) can be fabricated through a combination of surface chemistry that imparts low solid surface energy with a re-entrant surface texture. Recently, surface texturing with lasers has received significant attention because laser texturing is scalable, solvent-free, and can produce a monolithic texture on virtually any material. In this work, we fabricated nanostructured omniphobic and superomniphobic surfaces with a variety of materials using a simple, inexpensive and commercially available CO laser engraver. Further, we demonstrated that the nanostructured omniphobic and superomniphobic surfaces fabricated using our laser texturing technique can be used to design patterned surfaces, surfaces with discrete domains of the desired wettability, and on-surface microfluidic devices.
surface, have attracted tremendous interests in wide variety of applications such as self-cleaning, [2,3] chemical shielding, [4] corrosion resistance, [5] water energy harvesting, [6][7][8][9] membrane separation, [10] and lab-on-chip devices. [11,12] As such, superomniphobic surfaces have been fabricated on numerous substrates such as metals, polymers, glass, and paper. [13][14][15][16][17][18][19][20][21][22][23][24] Among these, paper-based superomniphobic surfaces are of great importance because paper is flexible, inexpensive, lightweight, breathable, and recyclable. However, prior reports [25][26][27][28][29][30][31] that relied on tuning the inherent heterogenous texture of papers to achieve superomniphobicity have failed to demonstrate low roll-off angle (or low contact angle hysteresis, indicative of negligible solidliquid adhesion) with low surface tension liquids. [32] To overcome this issue, in this work, we developed a superomniphobic paper through a simple technique of growing nanofilaments on the microfibers of the paper. Unlike prior work, [16,21,[33][34][35] our superomniphobic paper displays very low roll-off angle, indicative of ultra-high droplet mobility, even with low surface tension liquids (e.g., n-hexadecane). Further, the required hierarchical texture is formed using a grow-from approach on inherent microfibers of the paper, without noticeably altering the microscale features (i.e., diameter and distance of the microfibers). We also developed a facile method to control the motion and adhesion of the droplets on the superomniphobic paper. Utilizing the liquid mobility in a controlled manner on our superomniphobic papers, we fabricated a simple on-paper pH sensor. We envision that due to simple fabrication technique, flexibility, lightweight, breathability, selective permeability, and ultra-high droplet mobility, our superomniphobic papers will have numerous applications including lab-on-paper devices, water-oil separation, and devices (e.g., water drones and microrobots) with enhanced weight-bearing capacity.When a liquid droplet contacts a nontextured (i.e., smooth) solid surface, it displays an equilibrium (or Young's) contact angle θ Y at triple-phase contact line. [36] Whereas, if the droplet contacts a textured solid surface, it displays an apparent contact angle θ * and adopts either the Wenzel (or fully wetted) state [37] or the Cassie-Baxter state [38] to minimize its overall free energy. In the Cassie-Baxter state, which is preferred for designing super-repellent surfaces, an air layer is trapped between the surface texture and the contacting liquid droplet.
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