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Posted: Jan 14, 2014

Water pinning nanostructures inspired by nature

(Nanowerk Spotlight) In the field on controlling liquid movement on surfaces, super water-repellent surfaces have been well-documented (see for instance: "Extremely water repellent graphene foams"). In contrast, comparatively fewer reports are available on the design of water pinning surfaces.
"Creating synthetic surfaces with water pinning property has broad technological implications ranging from dew collection as water source for residence in arid regions, anti-drip function for greenhouse films in agricultural countries, liquid transport and control in microfluidics and spectroscopy, and structural genomics as well as for the reduction of 'coffee-ring effect' in printing and coating technologies," Dr. Jaslyn Bee Khuan Law and Dr. Hong Yee Low, research scientists in the Patterning & Fabrication Group at Singapore's Institute of Materials Research and Engineering (A*STAR IMRE), explain to Nanowerk.
Law is first author of a new paper in Langmuir ("Bioinspired Ultrahigh Water Pinning Nanostructures") where she and her colleagues achieved polymer films with exceptionally high water pinning forces through nanoimprinted surface structures, without the incorporation of any chemical treatment.
Lund
Images demonstrating the water pinning ability of the synthetically fabricated nanoprotrusion topography and comparison of the quantified water pinning forces exhibited by various topographies patterned on transparent free-standing polycarbonate film. (a, b) Photographs of the fabricated nanoprotrusion patterned polycarbonate film showing water droplet staying pinned onto the surface when the sample is tilted vertically (90° of inclination) and upside down (180° of inclination). Inset in (b) shows the corresponding shape and pinning ability of the water droplet when the sample is tilted upside down. (c) Photograph showing an array of water droplet staying pinned onto the nanoprotrusion patterned polycarbonate film when tilted nearly upside down.(Reprinted with permission from American Chemical Society)
This work contributes to the field on water pinning surfaces by providing a simple geometrical rule-of-thumb design of nanostructures to engineer polymeric surfaces with tunable water pinning ability. The nanostructures are designed to significantly increase the solid-liquid contact line length of a water droplet on its surface, thereby achieving a high water-pinning force.
Through a systematic variation of the surface structures, we demonstrated an ultrahigh water pinning force on synthetic polymers by combining two surface topographical characteristics of nanostructures to ensure a continuous solid-liquid contact line: 1) conical or parabolic-shaped nanoprotrusion; and 2) isotropic and continuous nanoprotrusion.
Equally important, these topographical design rules allow easy and controllable scaling up onto synthetic films that further opens up the likelihood for industrial adoption.
"Our work was inspired by the water pinning property of the rose petal that is the result of the cone-shaped continuous microprotrusion texture on its natural surface, reported by Jiang’s group in 2008 ("Petal Effect: A Superhydrophobic State with High Adhesive Force")," says Law. "Extrapolating from the rose petal surface microstructures and contrasting with the surface topographies of the lotus leaf – which demonstrates the extreme opposite of water pinning property – we postulated that the use of nanostructures with characteristic surface designs could increase the length of the solid-liquid contact line that governs the total pinning force of the droplet on the surface to achieve a ultra-high water pinning surface. Our experimental results verify this hypothesis."
In their report, the team shows a practical application of their synthetic film in the mitigation of the 'coffee-ring effect' in solution-based deposition – a challenge faced in many practical and industrial applications ranging from coating/printing techniques to microarray analysis where control of a uniform distribution of the solute in an evaporating liquid is needed. Methods to reduce this ring-like effect have thus far focused mainly on solvent chemistry, or particle shape and composition in liquid suspension. The IMRE team showed that through the use of topography on the substrate, which is an environmentally friendly approach, the coffee-ring effect can be mitigated.
Law notes that, besides this direct application, two other applications that can directly benefit from this type of synthetic film are anti-drip greenhouse films and structural genomics.
In greenhouses, condensation of water droplets from the roof top occurs during early dawn and sunset. Water droplets dripping directly onto plants are very detrimental to plant growth in this controlled environment. To mitigate this problem, anti-drip films are employed on the rooftop of greenhouses. These films utilize chemical additives to alter the surface tension to achieve anti-drip function; however, the additives wear out over time and the film needs to be replaced routinely.
The new approach – using topographical means to impart good water-pinning onto films – may potentially be used in this application. This provides a more convenient and cost-effective solution as the topography that imbue the film with water pinning property is also part of the film and permanently embossed onto it.
In structural genomics, high quality protein crystals are commonly grown using the 'hanging drop' method – where a protein-containing droplet dispense onto a substrate is hung upside down. The team's synthetic film with a large water pinning force could potentially be used in this application to yield a large quantity of high quality protein crystals.
This is just one recent example from the team's work on biomimetic surfaces, taking inspirational cues from nature to engineer enhanced functional surfaces that can be scalably fabricated for industrial adoption.
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