Developing mechanically robust, transparent, superoleophobic surfaces
The wetting of surfaces has generated immense interest in material scientists for decades because of its prominent role in a wide range of daily phenomena and commercial applications. The simplest measure of wetting on a smooth surface is the equilibrium contact angle (θ), given by the Young’s equation. Surfaces that display contact angles greater than 90° with water are considered hydrophobic, while surfaces that display contact angles greater than 150° and low contact angle hysteresis (i.e. the difference between the advancing θadv and the receding contact angle θrec) are generally considered to be superhydrophobic.
The most widely-known example of a natural superhydrophobic surface is the surface of the lotus leaf (Nelumbo nucifera). Numerous studies have suggested that it is the combination of surface chemistry and roughness on multiple scales on the lotus leaf’s surface that allows for the trapping of air underneath a water droplet, thereby imbuing the leaf with its characteristic superhydrophobicity. However, a liquid with a markedly lower surface tension like hexadecane (γlv = 27.5 mN/m) rapidly wets the lotus surface leading to a contact angle of ~0°, clearly demonstrating the leaf’s oleophilicity. Indeed, in spite of the plethora of superhydrophobic surfaces now available, there are no naturally occurring superoleophobic surfaces, i.e. surfaces that display contact angles greater than 150° with organic liquids such as alkanes having appreciably lower surface tensions than water.
In recent work, we developed the first-ever superoleophobic surfaces by considering the effects of re-entrant surface texture on surface wettability. Further, to aid the systematic engineering of non-wetting surfaces, we developed four design parameters that allow us to provide an a priori estimation of both the apparent contact angles, as well as the robustness of the composite interface, supported with a given contacting liquid. We are currently investigating various methodologies to develop mechanically robust and transparent superoleophobic surfaces. Such surfaces are expected to have a wide range of commercial applications, including the development of surfaces with enhanced solvent-resistance, stain-resistant textiles, ‘non-stick’ coatings, controlling protein and cell adhesion on surfaces, engineering surfaces with enhanced resistance to organic solvents, reduction of biofouling and the development of finger-print resistant surfaces for flat-panel displays, cell-phones and sunglasses.
Membranes for oil-water separation
There is an acute need for the development of new energy-efficient solutions to separate oil-water mixtures as both the production of oil and oil-transport engender a severe environmental risk in sensitive ecosystems. In many ways 2010 was a banner year highlighting this risk, as evidenced by the Deepwater Horizon oil-spill disaster off the coast of Louisiana and the Chinese tanker that ruptured on the Great Barrier Reef in the Indian Ocean.
Mixtures of oil and water are classified based on the size of oil droplet (doil) – free oil if doil > 150 microns, dispersed oil if 20 microns < doil < 150 microns and emulsified oil if doil < 20 microns. We have recently developed a novel solution for the separation of free oil, dispersed oil, and oil-water emulsions based on the design of hygro-responsive (from the Greek word ‘hygra’ meaning liquid) surfaces. These surfaces, counter-intuitively, are wet by water, but are still able to repel low surface tension oils like rapeseed oil or hexadecane . This makes these porous surfaces ideal for gravity-based separation of oil and water as they allow the higher density liquid (water) to flow through while preventing the flow of the lower density liquid (oil). We have also developed strategies that allow us to use these membranes for the continuous separation of surfactant stabilized oil-in-water and water-in-oil emulsions.
For more information see Kota et al., “Hygro-responsive membranes for effective oil-water separation”, Nature Communications, 2012, 3:1025, DOI: 10.1038/2027.
In addition, we have also recently developed a novel methodology that uses an electrical potential to separate out almost all types of oil-water mixtures. See Kwon et al., “On-demand separation of oil-water mixtures”,Advanced Materials, 2012, 24 (27), 3666-3671.
Wettability Engendered Templated Self-assembly (WETS)
Precise control over the geometry and chemistry of multiphasic particles is of significant importance for a wide range of applications including drug delivery, vaccines and inhalation biotherapeutics, biological sensors, optical devices, and nanomotors. We have developed one of the simplest methodologies for fabricating monodisperse, multiphasic micro- and nanoparticles possessing almost any composition, projected shape, modulus, and dimensions as small as 25 nm. The synthesis methodology involves the fabrication of a nonwettable surface patterned with monodisperse, wettable domains of different sizes and shapes. When such patterned templates are dip-coated with polymer solutions or particle dispersions, the liquids, and consequently the polymer or the particles, preferentially self-assemble within the wettable domains. Utilizing this phenomenon, we fabricate multiphasic assemblies with precisely controlled geometry and composition through multiple, layered depositions of polymers and/or particles within the patterned domains. Upon releasing these multiphasic assemblies from the template using a sacrificial layer, we obtain multiphasic particles. The templates can then be readily reused (over 20 times in our experiments) for fabricating a new batch of particles, enabling a rapid, inexpensive, and easily reproducible method for large-scale manufacturing of multiphasic particles.
See Kobaku et al., "Wettability Engendered Templated Self-assembly (WETS) for Fabricating Multiphasic Particles", ACS Appl. Mater. Interfaces, 2015, 7 (7), pp 4075–4080 for more details.
Drag Reduction in Turbulent Flow
One of the exciting applications of water repellent surfaces is their use for friction drag reduction. About 60% of the fuel consumed by large ships goes directly to overcoming the frictional drag of the water. By coating the side of a ship with a superhydrophobic surface, much of this friction can be avoided. Or, at least, this is the case for laminar flow. Recently, the Tuteja group became involved in a Multi-University Research Initiative (MURI) that aims to understand if and how superhydrophobic surface can reduce friction drag in turbulence. The team combines experts in experimental fluids research, rheology, computational fluid mechanics and of course, surface science. A website dedicated to the MURI can be found here. Steve Ceccio, chairman of the Naval Architecture and Marine Engineering Department at the University of Michigan, spearheads the effort.
During our MURI effort, we compiled a review article highlighting the recent attempts over the past decade at understanding turbulent drag reduction using superhydrophobic surfaces. The review article was published as part of a themed issue in the Philosophical Transactions of the Royal Society A, entitled, "Bioinspired hierarchically structured surfaces for green science."
Designing Durable Icephobic Surfaces
Ice accretion has a negative impact on critical infrastructure, as well as a range of commercial and residential activities. Icephobic surfaces are defined by an ice adhesion strength τice < 100 kPa. However, the passive removal of ice requires much lower values of τice, such as on airplane wings or power lines (τice < 20 kPa). Such low τice values are scarcely reported, and robust coatings that maintain these low values have not been reported previously. We show that, irrespective of terial chemistry, by tailoring the cross-link density of different elastomeric coatings and by enabling interfacial slippage, it is possible to systematically design coatings with extremely low ice adhesion (τice < 0.2 kPa). These newfound mechanisms allow for the rational design of icephobic coatings with virtually any desired ice adhesion strength. By using these mechanisms, we fabricate extremely durable coatings that maintain τice < 10 kPa after severe mechanical abrasion, acid/base exposure, 100 icing/deicing cycles, thermal cycling, accelerated corrosion, and exposure to Michigan wintery conditions over several months.
Open-Channel Microfluidic Devices based on Substrates with Patterned Wettability Microchannels
Conventional microfluidic devices are produced by complex, time-consuming photolithography techniques adapted from the semiconductor industry to produce microscale features in poly(dimethyl siloxane) (PDMS). PDMS-based closed-channel devices are vulnerable to clogging, may have a high hydraulic resistance to flow, and limit access to the liquid except at limited inlets and outlets. Open channel microfluidic devices aim to address these issues with their large vapor-liquid contact area, and by using less expensive, scalable production techniques.
Paper has recently emerged as a promising materials platform for microfluidic devices due to its low cost, easy disposal, high surface area, capillary-based wetting, flexibility, and compatibility with a wide range of patterning and printing techniques. We have developed a method of generating omniphobic paper surfaces that are resistant to wetting by a broad range of liquids, including numerous low surface tension solvents, rather than the aqueous systems previously demonstrated. Further, we have also developed a methodology to induce selective wetting of liquids with different surface tensions and polarities on a paper surface. Such selective extreme wettabilities of fluidic channels, combined with improved fluidic control, make several new applications of paper-based microfluidic devices possible. These applications rely on the separation or compartmentalization of liquids based on their surface tensions and/or polarities, and include oil–water separation, liquid–liquid extraction, droplet generation for microparticle fabrication, and the measurement of the surface tension/composition of a mixture.
More details may be found in Li, et. al. Paper-Based Surfaces with Extreme Wettabilities for Novel, Open-Channel Microfluidic Devices. Advanced Functional Materials, (2016), 26 (33) and Li, et. al. Open-channel, water-in-oil emulsification in paper-based microfluidic devices. Lab on a Chip, (2017), DOI: 10.1039/C7LC00114B