Researchers at the Indian Institute of Technology (IIT) Guwahati have developed a new approach to anti-counterfeiting measures. They have developed a smart interface embedded with two different water wettabilities — extremely water repelling (superhydrophobic) and extremely water loving (superhydrophilic) — which can hide information or make it visible under certain definite conditions. Adding a layer of complexity, the researchers used a molecular printer to imprint a pattern of micron size (500 x 500 microns) that will become visible only when dipped in water or when moist air is blown.
The team led by Uttam Manna from the Department of Chemistry and Centre for Nanotechnology at IIT Guwahati first dip-coated a glass surface for 10 minutes in a mixture of two chemicals and air-dried it to make it hydrophobic.
Reactive coating
The coating is not only hydrophobic but is also reactive. Taking advantage of the reactive surface of the coating, the researchers used a chemical (glucamine) to write letters on the coated surface. The glucamine-treated region becomes selectively and extremely water-loving (superhydrophilic) and hence becomes visible to the naked eyes when dipped in water or when moist air is blown.
Since the rest of the hydrophobic region of the coated surface is still chemically reactive, they modified it using another chemical (octadecylamine) to make the surface chemically inert. “At the same time, the octadecylamine makes the surface extremely water-repelling (superhydrophobic). So we introduced two extremely different water wetting behaviours to the surface to make only the pattern visible when immersed in water while the rest of the surface remains opaque as it is highly water-repelling,” Prof. Manna says.
The printed letters or pattern become visible when dipped in water or when most air is blown. It once again becomes invisible when the pattern becomes dry. The transient and reversible identification of the hidden information was tested 100 times by exposing the pattern to mouth mist. The pattern becomes visible in 99% humid air.
“The pattern which is highly water-loving allows water to get in thus allowing light to pass through without much scattering. This makes the pattern visible when it is wet,” explains Supriya Das from IIT Guwahati and first author of a paper published in the journal ACS Applied Materials & Interfaces. “The rest of the region is highly water-repelling so there is trapped air that scatters light and hence remains opaque.”
“We initially tested the concept using a glass surface and then tested it using a filter paper. Since it is surface-independent, we can achieve similar results even when plastic, cloth or any other materials are used,” he says.
Using micropatterns
Initially, the researchers use a pen containing glucamine chemical to write letters on the coated surface. To make the invisible text more complex and restrict it to a small region the IIT team turned to Dr. Michael Hirtz and his team at the Institute of Nanotechnology & Karlsruhe Nano Micro Facility at the Karlsruhe Institute of Technology in Karlsruhe, Germany. Dr. Hirtz produced micropatterns using a molecular printer.
The team found the patterned coating to be physically durable even when exposed to a very high (100 degree C) and very low temperature (10 degree C). Using an adhesive tape the researchers tested the durability when exposed to wear and tear. Though the top portion of the pattern was abraded during the peeling of the adhesive tape, the pattern continued to exhibit the same properties and became visible when exposed to water.
