Breakthrough in liquid-repellent surfaces makes them much better and 1000 times cheaper

A piece of ordinary cloth (left) can become liquid repellent (right) simply with a layer of porous surface material (middle).

The dream of research and development on liquid-repellents is a structure that has robust liquid repellency, strong mechanical stability, and is inexpensive to produce on a commercial scale.

On liquid-repellent surfaces, liquid droplets bounce away instead of being stuck. These surfaces are important in many fields, such as water-repellent clothes and anti-fouling kitchenware. Used as drag-reduction coatings for water vehicles, these surfaces can even help with speeding up cargo ships and military equipment so as to save energy. The dream of research and development on liquid-repellents is a structure that has robust liquid repellency, strong mechanical stability, and is inexpensive to produce on a commercial scale. However, the functional outcomes of existing liquid-repellent surfaces have not been satisfactory, because of inadequacies of conventional structural design and fabrication approaches in engineering microstructures and properties of such surfaces.

The challenge was recently overcome by breakthrough research led by Professor Wang Liqiu at the Department of Mechanical Engineering, Faculty of Engineering, the University of Hong Kong (HKU) through the development of a robust liquid-repellent structure and the fabrication of porous surfaces by an innovative microfluidic-droplet-based technique. Materials such as textiles, metals, and glasses covered by a layer of this robust porous surface can then become liquid-repellent. The paper was recently published in academic journal Nature Communications (Zhu P. A., Kong T. T., Tang X. and Wang L. Q. 2017. Well-defined porous membranes for robust omniphobic surfaces via microfluidic emulsion templating, Nature Communications 8, 15823). With the new technology developed by the team, clothes would never get wet on rainy days in the future.

The team resolves effectively the conflict between liquid-repellency and mechanical stability by the springtail-cuticle-inspired design of liquid-repellent structures. Springtails are soil-dwelling arthropods whose habitats often experience rain and flooding. As a consequence, springtails evolve their cuticles with strong mechanical durability and robust liquid repellency to resist friction from soil particles and to survive in watery environments, respectively. Inspired by springtail cuticles, the research team designed porous surfaces composed of interconnected honeycomb-like micro-cavities with a re-entrant profile: interconnectivity ensures mechanical stability and re-entrant structure yields robust liquid-repellency.

Figure 1 Soil-dwelling springtails with dew (body length: ~2.5 mm).

Figure 1 Soil-dwelling springtails with dew (body length: ~2.5 mm).
(Source: https://www.flickr.com/photos/lordv/302831402/in/album-72157594371411831/).
*For the press using this photo, please credit Brian Valentine.

Figure 2 Bio-inspired design of liquid-repellent structures with robust liquid-repellency. Schematic (a) and image (b) of the designed porous surface. (c) Photo of a water drop suspended on top of the porous surface. (d) repellency of 10 different liquids by the porous surface.

Figure 2 Bio-inspired design of liquid-repellent structures with robust liquid-repellency.
Schematic (a) and image (b) of the designed porous surface. (c) Photo of a water drop suspended on top of the porous surface. (d) repellency of 10 different liquids by the porous surface.

Robust liquid-repellent structure shows a 21-fold enhancement in mechanical stability
The robust liquid-repellent surfaces repel at least 10 types of liquid, including water, surfactant solutions, oils, and organic solvents (Figure 2d) and show an astounding over 21-fold enhancement in mechanical stability compared with discrete structures (Figure 3). The porous surfaces are capable of recovering their non-wetting state as well even if micro-cavities are partially wetted by water. The flexible surfaces can also be readily coated onto various objects for liquid-repellency.

Figure 3 Bio-inspired design of liquid-repellent structures with enhanced mechanical stability. (a) Intact structure of interconnected porous surface. (b) Intact discrete structure. (c-d) Damaged interconnected structures at (c) 8.6 kPa (kilopascal, the unit of pressure) and (d)11.5 kPa respectively. (e-f) Damaged discrete structures at (e) 0.4 kPa and (f)2.9 kPa respectively.

Figure 3 Bio-inspired design of liquid-repellent structures with enhanced mechanical stability.
(a) Intact structure of interconnected porous surface.
(b) Intact discrete structure.
(c-d) Damaged interconnected structures at (c) 8.6 kPa (kilopascal, the unit of pressure) and (d)11.5 kPa respectively.
(e-f) Damaged discrete structures at (e) 0.4 kPa and (f)2.9 kPa respectively.

Porous surface material just costs about HKD1 per square metre
The research team also developed an innovative microfluidic-droplet-based technique for the fabrication of porous surfaces
 which is very much similar to shaped-cookies made by baking molds. Here the molds are uniform micron-sized droplets that are produced by microfluidics technology with precise control over their size, structure, and composition. Molded by microfluidic droplets, commercial-scale uniform microstructures are produced at low cost. The material cost is in a range of HKD 0.7 to 1.3 per square metre, only one thousandth of that in purchasing commercialized products such as PTFE water-repellent film. This technique has high accuracy and effectiveness in engineering surface structures, ensured by the precision and controllability of microfluidic-droplet generation that is low in cost and readily scaled up as well.

Figure 4 Fabrication of bioinspired liquids-repellent surfaces by microfluidic method. (a) Process of microfluidic fabrication method, involving emulsion deposition, solvent evaporation, and template removal. (b) Droplet assemblies after emulsion deposition. (c) Dry film after solvent evaporation. (d) Porous surface after template removal. (e-f) Images of porous surfaces with different pore sizes. (g) Transparency of porous surfaces. (h) Wafer-scale fabrication of the porous surface.

Figure 4 Fabrication of bioinspired liquids-repellent surfaces by microfluidic method.
(a) Process of microfluidic fabrication method, involving emulsion deposition, solvent evaporation, and template removal.
(b) Droplet assemblies after emulsion deposition. (c) Dry film after solvent evaporation.
(d) Porous surface after template removal. (e-f) Images of porous surfaces with different pore sizes.
(g) Transparency of porous surfaces. (h) Wafer-scale fabrication of the porous surface.

The breakthrough will change the way liquid-repellent surfaces are fabricated for robust liquid-repellency, strong mechanical stability, and economical production at a commercial scale. It has also paved the way for further progress in creating surface structures by design, and in tailoring their morphology, repellency and mechanical stability to suit a desired application in various fields, including energy, buildings, automobiles, chemical engineering, electronics, environments, bio-medical industry, advanced manufacturing, water vehicle and military equipment.

Learn more:  No more laundry? Innovative and ideal liquid-repellent surfaces developed by HKU scientists could make the dream come true!

 

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An Engineered Surface Unsticks Sticky Water Droplets

Xianming Dai, Chujun Zeng and Tak-Sing Wong Schematic showing a new engineered surface that can repel liquids in any state of wetness

Xianming Dai, Chujun Zeng and Tak-Sing Wong
Schematic showing a new engineered surface that can repel liquids in any state of wetness

The lotus effect has inspired many types of liquid repelling surfaces, but tiny water droplets stick to lotus leaf structures. Now, researchers at Penn State have developed the first nano/micro-textured highly slippery surfaces able to outperform lotus leaf-inspired liquid repellent coatings, particularly in situations where the water is in the form of vapor or tiny droplets.

Enhancing the mobility of liquid droplets on rough surfaces has applications ranging from condensation heat transfer for heat exchangers in power plants to more efficient water harvesting in arid regions where collecting fog droplets on coated meshes provides drinking water and irrigation for agriculture to the prevention of icing and frosting on aircraft wings.

“This represents a fundamentally new concept in engineered surfaces,” said Tak-Sing Wong, assistant professor of mechanical engineering and a faculty member in the Penn State Materials Research Institute. “Our surfaces combine the unique surface architectures of lotus leaves and pitcher plants, in such a way that these surfaces possess both high surface area and a slippery interface to enhance droplet collection and mobility. Mobility of liquid droplets on rough surfaces is highly dependent on how the liquid wets the surface. We have demonstrated for the first time experimentally that liquid droplets can be highly mobile when in the Wenzel state.”

Liquid droplets on rough surfaces come in one of two states, Cassie, in which the liquid partially floats on a layer of air or gas, and Wenzel, in which the droplets are in full contact with the surface, trapping or pinning them. The Wenzel equation was published in 1936 in one of the most highly cited papers in the field; yet until now, it has been extremely challenging to precisely verify the equation experimentally.

“Through careful, systematic analysis, we found that the Wenzel equation does not apply for highly wetting liquids,” said Birgitt Boschitsch Stogin, a graduate student in Wong’s group and coauthor on a paper titled “Slippery Wenzel State,” published in the August 28 online edition of the journal ACS Nano.

“Droplets on conventional rough surfaces are mobile in the Cassie state and pinned in the Wenzel state. The sticky Wenzel state results in many problems in condensation heat transfer, water harvesting and ice removal. Our idea is to solve these problems by enabling Wenzel state droplets to be mobile,” said Xianming Dai, a postdoctoral scholar in Wong’s group and the lead author on the ACS Nano paper.

In the last decade, tremendous efforts have been devoted to designing rough surfaces that prevent the Cassie-to-Wenzel wetting transition. A key conceptual advance in the current study is that both Cassie and Wenzel state droplets can retain mobility on the slippery rough surface, foregoing the difficult process of preventing the wetting transition.

In order to make Wenzel state droplets mobile, the researchers etched micrometer scale pillars into a silicon surface using photolithography and deep reactive-ion etching, and then created nanoscale textures on the pillars by wet etching. They then infused the nanotextures with a layer of lubricant that completely coated the nanostructures, resulting in greatly reduced pinning of the droplets. The nanostructures also greatly enhanced lubricant retention compared to the microstructured surface alone.

The same design principle can be easily extended to other materials beyond silicon, such as metals, glass, ceramics and plastics. The authors believe this work will open the search for a new, unified model of wetting physics that explains wetting phenomena on rough surfaces such as theirs.

Read more: An Engineered Surface Unsticks Sticky Water Droplets

 

 

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