Xinjie Liu

Extreme wettability and tunable adhesion: biomimicking beyond nature?

In this critical review, we summarize the recent developments of extreme wettability in nature and biomimetic examples, and then we focus on surface wetting behavior beyond nature, which means surface wetting properties that cannot be found in nature. They are: switchable wettability between (super)hydrophobicity and (super)hydrophilicity, switchable water/oil droplet adhesion between superhydrophobic pinning states and superhydrophobic rolling states, superoleophobicity at the air–solid interface or even under vacuum, and self-healing (super)amphiphobicity at the air–solid interface.

Towards a tunable and switchable water adhesion on a TiO2 nanotube film with patterned wettability

Tunable water adhesion was realized on a TiO(2) nanotube film with patterned wettability formed via selective illumination through a mask. Meanwhile, the adhesion can be switched between sliding superhydrophobicity and sticky superhydrophobicity by masked illumination and heat annealing.

Switching Water Droplet Adhesion Using Responsive Polymer Brushes

Two stimuli-responsive polymers, poly(N-isopropylacrylamide) (PNIPAM) and poly(dimethylamino)ethyl methacrylate (PDMAEMA), were grafted from initiator-modified anodized alumina substrates with irregular micro/nanoscale surface topography. The resulting polymer-coated surfaces exhibited highly unusual wettability properties, as spherical water/acid/alkali/salt droplets could be reversibly switched between pinned states and rolling states due to the changes of temperature, pH, and electrolytes. The key to this effect is the combination of a mixed monolayer which provides initiator points for brush growth as well as a permanently hydrophobic substrate and a surface roughness.

Spray-coated fluorine-free superhydrophobic coatings with easy repairability and applicability.

The present paper reports a very simple and low-cost fluorine-free superhydrophobic coating prepared by spray-coating metal alkylcarboxylates, for example, Cu[CH(3)(CH(2))(10)COO](2), onto virtually any substrates. Superhydrophobicity with a static water contact angle of about 160 degrees and a sliding angle of 5 degrees was achieved from the proper precursor concentration. The advantages of the present approach include the cheap and fluorine-free raw materials, environmentally benign solvents, an industrial implementation method, and easy repairability and applicability so as to make a great application potential in practice. The hydrophobicity of coatings and the adhesion to water were found to be dependent on the surface morphology that was governed by the precursor concentrations from which coatings were prepared. The static wetting behavior of water droplets with different sizes gentlly deposited on the coatings was studied in more detail and correlated to theories, i.e., Wenzels and Cassies models. The results indicated that nanoribbon-textured coatings prepared from low precursor concentration (0.02 M) exhibited a transition from the metastable Cassie-Baxter state to the Wenzel state with increments in the droplet volume, and eventually droplets firmly stick to the surface even when the droplet was gently deposited on the surface. Surface coatings with dual roughness at both microscale and nanometer scale were formed as the concentration (0.04 M) was increased and conferred a stable Cassie state, even for increased droplet size and increased droplet deposit speed.

A replication strategy for complex micro/nanostructures with superhydrophobicity and superoleophobicity and high contrast adhesion

The present work reports a replication approach to imprint complex micro/nanostructures into polymeric coatings and materials, such as silicone elastomers, polyurethane, ultra-high-molecular-weight polyethylene and polytetrafluoroethylene etc., with applications in engineering. Al and Al2O3 molds with terraced micro/nanostructures were produced using an industrially compatible anodization method. To assist replication, the molds were coated with a water-soluble polymer layer as the sacrificial layer, to reduce the adhesion between replica and mold. Polymeric replicas with complex terraced structures were successfully obtained, which were otherwise impossible to achieve with conventional perfluorinated molds. The as-prepared replicas all exhibited superhydrophobicity without further modification with low-surface-energy coatings. Interestingly, residual hydrophilic sacrificial layer resulted in high water-adhesion without losing superhydrophobicity; these surfaces turned extremely slippery after removal of the residual sacrificial layer. A molecular mechanism is proposed to interpret the contrast adhesion. After being coated with a sticky perfluoroalkyl, containing methacrylate, the surfaces convert to superoleophobicity and low adhesion to a number of oils.

Responsive wetting transition on superhydrophobic surfaces with sparsely grafted polymer brushes

We demonstrate here that surface wetting transitions and contact angle hysteresis can be significantly altered by manipulating the droplet–surface interaction, which has never been reported before. The dynamic wetting behavior of a pressed water droplet on responsive polymer brushes-modified anodized alumina with pre-modified dilute initiator is shown. The wetting transition between superhydrophobicity and hydrophilicity can or cannot be achieved depending on the responsiveness between droplets of different pH, the concentrations of electrolytes and the environmental temperature and surface grafted stimuli-responsive polymer brushes. The contact angle changes are rather apparent, giving the surface double-faced wetting characteristics. The responsive surface composition regulated wetting will be very useful in understanding wetting theory, and will be helpful experimentally in designing smart surfaces in, for example, microfluidic devices.

Photo-regulated stick-slip switch of water droplet mobility

Water droplet mobility can be reversibly manipulated by using a photo-responsive coating on a rough surface. The surface coating consists of silicone elastomer as the basic hydrophobic material and the incorporated azobenzene compound as photosensitizer that assumes trans/cis conformation change under visible and UV illumination. The surface thus could switch between slippery and sticky states when the azo-compound assumes trans- and cis- conformation, while the surface superhydrophobicity doesnt change apparently.

Low surface energy surfaces from self-assembly of perfluoropolymer with sticky functional groups.

Fluorinated methacrylic copolymer containing catechol and perfluoroalkyl pendant side groups is synthesized by the free radical polymerization of a catechol-containing methacrylate monomer N-(3,4-dihydroxyphenyl)ethyl methacrylamide and 1H,1H-perfluorooctyl methacrylate using alpha,alpha-azobisisobutyronitrile (AIBN) as initiator. The fluorinated copolymer can assemble onto surfaces of a wide variety of materials including Ti, Al, Cu, steel, silicon, glass, mica, polyimide, polystyrene, and polymethylmethacrylate using catechol groups as multivalent H-bonding anchors. X-ray photoelectron spectroscopy and contact-angle data provide solid evidence of the formation of the assembly films that exhibit the surface free energy as low as 8.0 mJ m(-2).

Bulk metallic glasses based on binary cerium and lanthanum elements

The authors reported the formation and dynamic and thermodynamic properties of bulk metallic glasses (BMGs) based on binary Ce and La elements, in which the compositions of Ce and La can be gradually changed in the range from 0 to 100at.% without deteriorating the glass-forming ability. The properties of the binary base glasses can be tuned by modification of the Ce and La compositions. The BMGs with unique features and tunable properties might provide a model system to investigate some long-standing issues in BMG-forming alloys. The approach has implication for designing BMGs with tunable properties.

Superoleophobicity under vacuum

By using superoleophobic alumina and low vapor pressure oils we have been able to study wetting behavior at high vacuum. Here, we show that a superoleophobic state can exist for some probe liquids, even under high vacuum. However, with other liquids the surfaces are only superoloephobic because air is trapped beneath the droplet and the contact angle decreases dramatically (150°–120°) if this air is removed. These observations open up the possibility of designing materials which fully exploit the potential of physically trapped air to achieve extreme oleophobicity and/or hydrophobicity.