Dual‐Responsive Surfaces That Switch between Superhydrophilicity and Superhydrophobicity

Abstract

In general, superhydrophobic surfaces [1,2] with a water contact angle (CA) greater than 150°can be obtained by controlling the topography of hydrophobic surfaces, while superhydrophilic surfaces with a CA of about 0°can be realized through a three-dimensional (3D) [3] or two-dimensional (2D) capillary effect [4] on hydrophilic surfaces. The surface roughness dramatically enhances the CA on the hydrophobic surface but decreases the CA on the hydrophilic surface owing to the capillary effect, which is consistent with Wenzel's equation. [5] The fundamental mechanism of these phenomenaproposes that a combination of a hierarchical micro/nanostructure is essential for superhydrophilicity/superhydrophobicity. Recently, with the development of the combination of responsive materials and surface roughness, [6,7] several thermally, pH, or optically responsive smart interfacial materials that can switch between superhydrophilicity and superhydrophobicity have been reported: for example, a temperature-responsive polymer poly(N-isopropyl acrylamide (PNIPAAm); [6] photoresponsive materials, such as ZnO, [8] spiropyram, [8] two-level-structured self-adaptive surfaces, [9] the photoswitched wettability on an electrostatic self-assembled monolayer; [8] and a reversible pH-responsive surface. [10] However, all of these surfaces [11,12] are responsive to only one kind of external stimuli, such as temperature, [6] light, [8] or pH. [10] To the best of our knowledge, a dual-responsive surface that switches between hydrophilic and hydrophobic has never been reported, to say nothing of a dual-responsive surface that switches between superhydrophilic and superhydrophobic.In this communication, a dual-stimuli-responsive surface with tunable wettability, reversible switching between superhydrophilicity and superhydrophobicity, and responsivity to both temperature (T) and pH, is reported. Such surfaces are obtained by simply fabricating a poly(N-isopropyl acrylamide-co-acrylic acid) [P(NIPAAm-co-AAc)] copolymer thin film on both a flat and a roughly etched silicon substrate. Reversible switching between superhydrophilicity and superhydrophobicity can be realized over both a narrow temperature range of about 10°C and over a relatively wide pH range of about 10. This dual-responsive property is a result of the combined effect of the chemical variation of the surface and the surface roughness. In contrast to the roughness-enhanced homo-PNIPAAm film that is only responsive to temperature, the dual responsivity of the P(NIPAAm-co-AAc) films is due to the effective addition of the pH-sensitive component, acrylic acid (AAc). In addition, the lower critical solubility temperature (LCST) of the copolymer is tunable with increasing pH.The copolymer P(NIPAAm-co-AAc) thin films are fabricated on both a flat and a rough silicon substrate by a typical surface-initiated atom transfer radical polymerization. [13] Compared with Figure 1a (left), which shows the flat substrate, Figure 1a (right) shows a typical scanning electron microscopy ...

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