pH-Induced Reversible Wetting Transition between the Underwater Superoleophilicity and Superoleophobicity

Abstract

Surfaces with controlled oil wettability in water have great potential for numerous underwater applications. In this work, we report a smart surface with pH-responsive oil wettability. The surface shows superoleophilicity in acidic water and superoleophobicity in basic water. Reversible transition between the two states can be achieved through alteration of the water pH. Such smart ability of the surface is due to the cooperation between the surface chemistry variation and hierarchical structures on the surface. Furthermore, we also extended this strategy to the copper mesh substrate and realized the selective oil/water separation on the as-prepared film. This paper reports a new surface with excellently controllable underwater oil wettability, and we believe such a surface has a lot of applications, for instance, microfluidic devices, bioadhesion, and antifouling materials.

Oil‐Water Separation: A Gift from the Desert

“…Following this strategy, many mesh and porous materials that are underwater superoleophobic have been fabricated, and these materials have been successfully applied in oil-water separation. [18][19][20][21][22][23][24][25] For example, Jiang and co-workers coated microscale porous stainless steel mesh with a nanostructured hydrogel to give a novel underwater superoleophobic rough mesh. [ 18 ] This hydrogelcoated mesh effi ciently removed water from water-oil mixtures and did not become fouled with oil.…”

“…Problems that have not been adequately addressed are the cost of the materials, the equipment required, and the preparation processes required -all of which prevent large-scale applications. Another issue is that the most widely used materials are metal meshes and porous polymers that have rough microscale-nanoscale hierarchical structures, which are generally formed using chemical corrosion or other chemical methods, [20][21][22][23][24][25] meaning that solving one environmental problem (oil pollution) could cause another (disposing of the waste produced to create the superoleophobic material). It is therefore very important that we develop or fi nd stable materials that are underwater superoleophobic and can be used to simply, cheaply, and effi ciently separate large amounts of oil-water mixtures.…”

“…"Water-removing" superhydrophilic and superoleophobic materials have been developed to address the problems described above, and they have proved extremely effective in practical applications. [15][16][17][18][19][20][21][22][23][24][25] However, it is harder to produce a superoleophobic material (with an oil contact angle (OCA) greater than 150°) than to produce a superhydrophobic material because the surface tensions of oils are lower than the surface tension of water. [26][27][28][29][30] A surface that is superoleophobic several metal elements, as shown in Figure S1 of the Supporting Information.…”

Facile transformation of superhydrophobicity to hydrophilicity by silica/poly(ɛ-caprolactone) composite film

“…The smart surfaces are typically being responsive toward the change of certain environmental cues such as temperature [1,16,17], light illumination [18,19], pH [2,[20][21][22], chemical compositions [23][24][25][26], solvents [27,28], and applied bias voltage [29][30][31]. However, the surfaces with reversible wettability controlled by some of the previously used approaches require a long time conversion, even several weeks [32], which limits their practical applications.…”

Self-patterning, pre-applied underfilling technology for stack-die packaging

“…Microfluidic channels and oilwater separation are a few examples [1], [2]. Kobayashi et al have shown that the combination of hydrophilic-hydrophobic surfaces can lead to surface wettability differences and liquid manipulation.…”