Reversibly switchable wettability

Abstract

This critical review outlines the current state-of-the-art research on the reversibly switchable wettability of surface brought about by external stimuli and the exchange of counterions. Chemical composition and surface topography are the two key factors in the wettability of solid substrates. Applying external stimuli and exchanging counterions of ionic liquids and polyelectrolyte films are valuable approaches for rendering the change in surface chemistry and/or topography, and for driving the transition between hydrophilicity and hydrophobicity of surfaces. Through the combination of stimuli-responsive films and micro-/nanostructural surfaces, smart surfaces with reversible switching between superhydrophobicity and superhydrophilicity have been achieved. As an important advancement in reversibly switchable wettability, this review briefly introduces ionic liquids (ILs) as on-off systems to obtain reversibly switchable wettability and then discusses in more detail the methods to induce the reversibly switchable wettability of surfaces modified by ILs, additives, or thin films. In addition to reversibly switchable wettability mechanisms, open problems and potential solutions are discussed (157 references).

Effect of surface morphology on wettability conversion

“…Among the large family of oxide semiconductors, TiO 2 , with the direct band gap energy of 3.55-3.75 eV, has been widely used in photocatalysts, hydrogen generation, gas sensing, solar cells, and so on [23][24][25]. However, the wettability of TiO 2 has not been fully studied, especially its wettability conversion [17]. Few of publications on wettability conversion of TiO 2 films have been reported, in contrast to the huge number of literatures on TiO 2 films [26,27].…”

“…Therefore, they have practical applications as a key component for the fabrication of piezoelectric transducers, transparent conductive oxides, sensors, light-emitting diodes, and optoelectronic devices. In recent years, there has been increasing interest in the wettability of oxide semiconductors as smart materials [16][17][18][19][20][21][22]. Among the large family of oxide semiconductors, TiO 2 , with the direct band gap energy of 3.55-3.75 eV, has been widely used in photocatalysts, hydrogen generation, gas sensing, solar cells, and so on [23][24][25].…”

External‐Field‐Induced Gradient Wetting for Controllable Liquid Transport: From Movement on the Surface to Penetration into the Surface

“…[1][2][3][4][5][6][7][8] The liquid-wetting behavior on a solid or liquid surface is mainly dependent on the chemical composition and surface roughness, which play important roles in liquid transport. [9][10][11][12][13][14][15][16] The wettability gradient of a surface for a liquid droplet with an asymmetrical contact angle (CA) can produce a driving force for liquid motion, which is generally generated by introducing a chemical or structure gradient. [17][18][19][20][21][22][23][24][25][26][27][28] External-field-responsive liquid transport has received extensive research interest owing to its important applications in microfluidic devices, biological medical, liquid printing, separation, and so forth.…”

Noncovalent Spiropyran Coatings for Photoinduced Wettability Switching

“…They switch between these states upon irradiation with light of a specific wavelength. Due to the different dipole moments of each isomer, different contact angles of water droplets are realized on surfaces [1,[3][4][5]. For surface stability reasons, covalent binding of photochromic molecules to surfaces is typically preferred [6].…”