Science1 publisher2 min readPublished
A silica shell on each silicon nanosphere made the structural color glossy on 3D parts
Kobe University's group reports glossy, non-iridescent coatings that hold their hue under a protective top coat, and projects an airliner's paint weight falling from several hundred kilograms to several hundred grams.
The Scientist · Science desk
What happened
- Hiroshi Sugimoto's group at Kobe University reports in Small Structures that it produced glossy structural color, and control over the gloss, while coating large three-dimensional surfaces.
- Earlier versions of the same silicon-nanosphere coating reflected light diffusely, so the surfaces lacked luster, and the trick that had enabled inkjet printing pointed the group toward a fix.
- Adding a protective coating over the nanosphere layer did not significantly change the hue. That combination has been out of reach for ordered structural-color arrays.
- Sugimoto projects that the coating on a wide-body airliner would weigh several hundred grams, against the several hundred kilograms that conventional paint adds now.
- The particles are silicon, a nontoxic material that can be sourced from the semiconductor industry, and silica, one of the most abundant materials in Earth's crust.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- capability Colour that survives a protective top coat can sit under the layer that takes the abrasion. That is what Sugimoto means when he calls it a single platform for real-world use.
- constraint A coatings engineer specifying this would need gloss units, a hue-shift curve and exposure hours, and qualitative descriptions of glossiness will not clear a specification review.
- cost Weight is a cost line for an aircraft operator, and the figure on offer is extrapolated from one monolayer's opacity, so a buyer would be pricing a projection.
- precedent Pitching compatibility with spray, slot-die and roll-to-roll lines moves the next credible test from a journal figure to pilot-line yield on curved parts.
Ordered arrays need precise alignment and change color with the viewing angle, and they cannot take a protective layer on top [9]. Those two properties have kept structural color in the laboratory. Sugimoto's silicon nanospheres dealt with the first problem: the coloration is angle-independent, and his group had already managed to inkjet-print it [10].
"Color coatings are expected to provide vivid color, brightness, gloss and long-term durability, yet no existing technology satisfies all of these requirements simultaneously," Sugimoto said [12].
The shell is the new part. "Packing the nanospheres in silica shells not only protects the particles but also controls their spacing and should improve self-assembly into neat arrays," he said [2]. Reducing diffuse reflectance is what makes the color glossy [3]. The verb in his sentence is "should", and the array quality it predicts is what the paper had to test.
Gloss control on a curved surface is the harder demonstration, and the team reports both gloss and control over it on large 3D objects [1]. The measurements are in the paper by Jialu Song and colleagues [17]. The announcement describes the gloss and the hue stability in words. It stops there: no weathering, erosion or adhesion testing [19].
High opacity and brightness come from a single layer of nanospheres [5], and Sugimoto scales that up to an airframe [6]. That is the claim likeliest to reach a purchasing decision. Take the middle of his range: 300 kilograms is 300,000 grams, and 300 grams goes into that 1,000 times [18]. The figure follows from what one monolayer covers optically.
Fade resistance here is a property of the class of color: structural color is not subject to the photofading that affects organic pigments [14], while conventional coatings rely on relatively thick pigment layers that fade and add significant weight [13].
Sugimoto is involved in setting up a company to scale up production for industrial use [7]. "The process is compatible with scalable coating methods such as spray coating, slot-die coating and roll-to-roll processing," he said [8].
What to watch
- The gloss and hue-shift values in the Small Structures paper, and whether they hold at the steep angles a curved part presents.
- Whether the company Sugimoto is setting up runs a slot-die or roll-to-roll pilot line, and at what coverage rate.
- Any accelerated weathering or rain-erosion result for a shelled monolayer sitting under its protective top coat.