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Transparent, Superhydrophobic Surface with Varied Surface Tension Responsiveness in Wettability Based on Tunable Porous Silica Structure for Gauging Liquid Surface Tension.

Any solid surface can spontaneously exhibit variational wettability toward liquids with varied surface tension (γ). However, this correspondence has seldom been proposed or used on an artificial superhydrophobic surface, which should be more remarkable and peculiar. Herein, we fabricated robust, transparent superhydrophobic surfaces utilizing acid- and base-catalyzed silica (AC- and BC-silica) particles combined with candle soot template for structural construction and the CVD process for chemical modification. Three types of porous silica structures were devised, which presented distinctive surface tension responsiveness in wettability. Interestingly, all types of surfaces (i.e., AC-, AC/BC-, and BC-silica) show high repellence to high surface tension liquid (γ > 35 mN/m), and small differences are observed. With decreasing γ of the ethanol-water mixtures (γ < 35 mN/m), the static contact angles (SCAs) on all surfaces have an evident decline, but the features of the decreases are fairly different. As γ decreases, the SCA on the AC-silica surface decreases gradually, but the extent of decline becomes larger when γ < 27.42 mN/m. However, the SCA on the BC-silica surface decreases gradually except for γ ≈ 30.81 mN/m, and the SCA undergoes a sharp decline at γ ≈ 30.81 mN/m. The SCA on the AC/BC-silica surface has a similar variation as that of the SCA on the BC-silica surface, but a lower rate of BC-silica particles, e.g., 1/16, 1/8, 1/1 (AC/BC), further diminishes the critical γ values (where a sharp SCA drop occurs) to 30.16, 29.56, and 28.04 mN/m, respectively. The diversity is believed to be ascribed to the structure-induced selectivity of pore infiltration for the liquid. The tunable responsiveness can be generalized to various classes of organic aqueous solutions including methanol, acetic acid, acetone, and N,N-dimethylformamide. Benefiting from this, we can estimate organics concentration of an organic aqueous solution as well as its liquid surface tension by detecting its wettability on all of the diverse superhydrophobic surfaces.