Lu Tie

pH-Manipulated Underwater–Oil Adhesion Wettability Behavior on the Micro/Nanoscale Semicircular Structure and Related Thermodynamic Analysis

Controlling oil of wettability behavior in response to the underwater out stimulation has shown promising applications in understanding and designing novel micro- or nanofluidic devices. In this article, the pH-manipulated underwater-oil adhesion wetting phenomenon and superoleophobicity on the micro- and nanotexture copper mesh films (CMF) were investigated. It should be noted that the surface exhibits underwater superoleophobicity under different pH values of the solution; however, the underwater-oil adhesion behavior on the surface is dramatically influenced by the pH value of the solution. On the basis of the thermodynamic analysis, a plausible mechanism to explain the pH-controllable underwater-oil adhesion and superoleophobic wetting behavior observed on a micro- and nanoscale semicircular structure has been revealed. Furthermore, variation of chemistry (intrinsic oil contact angle (OCA)) of the responsive surface that due to the carboxylic acid groups is protonated or deprotonated by the acidic or basic solution on free energy (FE) with its barrier (FEB) and equilibrium oil contact angle (EOCA) with it hysteresis (OCAH) are discussed. The result shows that a critical intrinsic OCA on the micro- and nano- semicircular texture is necessary for conversion from the oil Cassie impregnating to oil Cassie wetting state. In a water/oil/solid system, the mechanism reveals that the differences between the underwater OCA and oil adhesive force of the responsive copper mesh film under different pH values of solution are ascribed to the different oil wetting state that results from combining the changing intrinsic OCA and micro-/nanosemicircular structures. These results are well in agreement with the experiment.

Optimal design of superhydrophobic surfaces using a paraboloid microtexture.

Due to the crucial role of surface roughness, it has been recently proposed to design optimal and extract geometrical microstructures for practical fabrications of superhydrophobic surfaces. In this work, a paraboloid microtexture is employed as a typical example to theoretically establish a relationship between surface geometry and superhydrophobic behavior for a final optimal design. In particular, based on a thermodynamic approach, the effects of all the geometrical parameters for such a paraboloid microtexture on free energy (FE) and free energy barrier (FEB) as well as equilibrium contact angle (ECA) and contact angle hysteresis (CAH) of a superhydrophobic surface have been systematically investigated in detail. It is interestingly noted that the droplet position for metastable state is closely related to the intrinsic CA of the surface. Furthermore, the paraboloid base steepness plays a significant important role in ECA and CAH, and a critical steepness is necessary for the transition from noncomposite to composite states, which can be judged using a proposed criterion. Moreover, the superhydrophobicity depends strongly the surface geometrical dimension for noncomposite state, while it is not sensitive for composite state. Additionally, both vibrational energy and geometrical dimension affect the transition from noncomposite to composite wetting states, and a comprehensive criterion for such transition can be obtained. Finally, using such criterion, it is revealed that the paraboloidal protrusion is the most optimal geometry among the three typical microtextures for ideal superhydrophobicity.

Anisotropic wetting properties on various shape of parallel grooved microstructure.

It has been revealed experimentally that some superhydrophobic surfaces in nature, such as rice leaf, show strong anisotropic wetting behavior. In this work, based on a thermodynamic approach, the effects of profile shape of parallel grooved microstructure on free energy (FE) with its barrier (FEB) and equilibrium contact angle (ECA) with its hysteresis (CAH) for various orientations of different parallel micro texture surface have been systematically investigated in detail. The results indicated that the anisotropy of wetting properties strongly depended on the specific topographical features and wetting state. In particular, a paraboloidal profile of parallel micro-texture surface is used as an important example to theoretically establish the relationship between surface geometry and anisotropic wetting behavior for optimal design, showing that the wetting behavior of the composite state is similar to that of the non-composite state and the anisotropy will possibly be appeared with the decrease of height or intrinsic contact angle of paraboloidal profile of micro texture.

Inorganic adhesives for robust, self-healing, superhydrophobic surfaces

Superhydrophobic surfaces demonstrate remarkable advantages involving interfacial issues but limited practical applications due to their poor mechanical robustness and environmental durability. The required micro-/nano-hierarchical structures and low-surface-energy nanocomponents are very vulnerable to physical and chemical destruction. Moreover, harsh conditions fundamentally weaken the mechanical strength of already-constructed robust superhydrophobic surfaces. In this work, inorganic adhesives are proposed to strengthen the bonding force between superhydrophobic coatings and various substrates. A simple spray-coating method is adopted to fabricate superhydrophobic surfaces using an all-in-one suspension that contains an aluminum phosphate binder, titanium dioxide nanoparticles, and alkylsilane. The surfaces benefitting from inorganic adhesives still extremely repel water after physical abrasion, and greatly endure harsh conditions including hot oil (80 °C), hot water (80 °C), and hot acetone (50 °C) for 24 h to preserve their high mechanical strength. The prepared coatings also have a self-healing ability against boiling-water treatment, O2-plasma etching, and amphiphilic pollution. Superhydrophobicity can be rapidly regenerated after multiple cycles of high-temperature repairing for 5 min. In addition, the robust interfacial materials exhibit a very reliable performance in oil–water separation after 100 abrasion cycles. Benefiting from the distinctive advantages of inorganic adhesives, interfacial materials will be broadly developed for practical applications in related fields.

Underoil superhydrophilic surfaces: water adsorption in metal–organic frameworks

Highly hydrophilic surfaces in oil are usually difficult to prepare due to the high surface tension of water. In nature, sarcocarps such as Chinese yam can reserve water to keep itself fresh. Inspired by the unique wetting properties of sarcocarps, metal–organic frameworks (MOFs) that can capture moisture from the atmosphere spontaneously have been proposed as building blocks for construction of underoil superhydrophilic surfaces. Herein, a mussel-inspired preparation method was adopted to coat HKUST-1 tightly on stainless-steel meshes. The MOF-based surfaces showed remarkable self-cleaning properties to crude oil under water, and could realize high-efficiency, on-demand separation of oil-in-water and water-in-oil emulsions via selective water filtration and adsorption, respectively. Water adsorption in MOFs could be extended to other extreme wettability and interfacial issues.

Dual superlyophobic surfaces with superhydrophobicity and underwater superoleophobicity

Lotus leaf-inspired superhydrophobic, fish scale-inspired underwater superoleophobic, and the switchable superwetting surfaces have been broadly developed by entire modification with water-repellent, water-loving, and smart components, respectively. Inspired by beetles, here a strategy of fractional modification is proposed to construct dual superlyophobic surfaces that have both superhydrophobic and underwater superoleophobic properties. Specifically, copper-based coatings on various substrates are fractionally modified by adjusting the concentration of perfluorinated mercaptan. The obtained dual superlyophobic surfaces display mutual advantages like using either superhydrophobic or underwater superoleophobic materials without any continuous external stimulus, for example on-demand oil–water separation. In theory, the distinctive dual superlyophobic state exists in a narrow range of surface chemistry, and thus needs elaborate surface modification. This discovery will facilitate the extension of the surfaces with completely opposite superwettability to enjoy the superiority in interfacial issues and applications.

An all-water-based system for robust superhydrophobic surfaces

Superhydrophobic surfaces with micro-/nanohierarchical structures are mechanically weak. Generally, organic solvents are used to dissolve or disperse organic adhesives and modifiers to enhance the mechanical strength of superhydrophobic surfaces. In this work, an all-water-based spraying solution is developed for the preparation of robust superhydrophobic surfaces, which contains ZnO nanoparticles, aluminum phosphate as an inorganic adhesive, and polytetrafluoroethylene with low surface energy. The all-water-based system is appreciated for low price and less pollution. Importantly, the prepared superhydrophobic surfaces are durable enough against various harsh conditions (such as UV irradiation for 12 h, pH values from 1 to 13, and temperatures from -10 to 300 °C for 12 h) and physical damages (including sandpaper abrasion and sand impact tests for 50 cycles). In addition, the obtained interfacial materials show promise for practical applications such as anti-icing and oil-water separation.

Organic Media Superwettability: On-Demand Liquid Separation by Controlling Surface Chemistry

Superwettability involving water affinity has demonstrated prominent advantages in oil-water separation. However, superwetting surfaces in nonpolar liquid-polar liquid systems are rarely explored for the separation of organic liquids. In this work, a protocol of elaborately controlling surface chemistry is presented to construct dual superlyophobic surfaces for polar or nonpolar liquids in opposite organic media. On two kinds of silver-roughened copper coatings, a polar hydroxyl group is subtly integrated with nonpolar perfluoroalkyl chain at the nanoscale. Prewetted by one organic liquid, the obtained dual superlyophobic mesh can selectively intercept other immiscible organic liquids, realizing high-efficiency on-demand separation. In theory, the dual superlyophobic surfaces in organic media are strongly dependent on their affinity toward polar liquids and the surface roughness. The discovery may promote the development of organic liquid-related interfacial materials.