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Nanocrystal Liquid Identification
By: Katie Anderson (Schaefer), Cosmetics & Toiletries magazine
Posted: October 5, 2011, from the October 2011 issue of Cosmetics & Toiletries.
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The team chemically patterns the surfaces of the inverse opal with repeated iterations of alkylchlorosilane functionalization and masked oxygen plasma exposure. “When liquid is forced through the bottleneck of these ordered pores, it produces an energy tradeoff replacing solid air with solid liquid, but also creating liquid air interfaces,” explains Burgess. “This threshold can be decreased to lower surface tension. The high symmetry of the structure allows these transitions of wetting to be really sharp. A technique to put different surface chemistries in different regions was created. The surface chemistry is moving the location of the threshold in terms of surface tension or wettability, relative to another surface chemistry somewhere else. The more of these thresholds that were added, the more distinct patterns are seen, but because the structure is so highly regular, if I can pick surface chemistries really close to each other, then two really similar liquids can be made to produce different patterns,” he says.
The structure identifies different liquids with a color change, whichis produced as a result of the refractive index changes resulting from wetting, and a pattern is seen by comparing the refractive index-contrast between the wetted and non-wetted areas. “A green color or red color results depending on what angle you look at it from coherent scattering off of these air/glass interfaces that are regularly spaced on a nano scale,” Burgess explains. “However, when a liquid that has a higher refractive index than air and closer to that of glass fills the pores, the scattering becomes much weaker and the film appears transparent. The apparent color resembles that of the underlying substrate.”
According to Burgess, W-Ink falls into three catergories of applications: encryption, liquid authentication and liquid identification. He also notes that this order corresponds to their ease of success, with encryption being the easiest.
The encryption aspect of the device relates to packaging. “This film can be grown on a sticker or package, and with application of secret liquids such as alcohol, an individual can validate the package,” notes Burgess. For example, this can be used to validate cosmetic, pharmaceuticals, fragrances and money packages. “Hitting the quality control and standards testing early with commercializing the device in this category is easy because we control the liquids,” adds Burgess.
The second category involves authenticating the liquid itself. This category is a quality control test for a specific liquid formulation, for example, determining gas grade, liquid pharmaceutical or liquid cosmetic forgeries. “For this category, we would design a strip or film using the opal that upon being swabbed with the liquid, will produce two stripes in the correct surface tension, three stripes in a higher surface tension and one stripe in a lower surface tension,” notes Burgess. “We can tailor the chemistries to the particular liquid needs to be authenticated.” This application could possibly be used for quality control in scale-up to identify variations of a formulation before disposing a batch or sending it off for testing.