Liquid crystals 'could replace colour-shifting inks'; study

Scientists in South Korea have developed a material based on liquid crystals that they claim could replace colour-shifting inks as a security feature for banknotes, ID cards and other items.

The team – headed by Dr Sang-seok Lee (pictured right) from the Korea Institute of Science and Technology (KIST) – has discovered a technique that allows liquid crystals to be made in several layers that can reflect light in different colours without the use of added pigments.

Writing in the journal Advanced Materials, the researchers say that the material is created through the use of an additives – called a chiral dopant – which makes the liquid crystals rotate spontaneously to form a spiral structure known as a ‘cholesteric’ liquid crystal.

Light reflected from these crystals has a circular polarisation property in that it rotates in only one direction. Using this property, it is possible to make colours appear and disappear by changing certain polarisation conditions, according to KIST.

Producing multiple layers of the liquid crystals using a novel technique detailed in the research paper can create diverse optical effects that could have anti-counterfeit uses, they suggest.

“What we've developed is a simple method of creating multi-layered liquid crystals and we expect it will serve as the basis for adding unique optical characteristics to materials,” said Lee.

“Based on this new technology, we plan on developing diverse functional particles to develop composite materials.”

The team describe the production process as follows: added a cosolvent that dissolved in both oil and water as a way to mix organic alcohol, a hydrophilic moisturizing agent, and the hydrophobic liquid crystal material for all three substances to become evenly mixed together. Then, the mixture was emulsified* in water to form microemulsion drops. With the exchanges occurring among the cosolvent, moisturizing agent, and water molecules through the surfaces of the emulsion drops, this resulted in a separation of the hydrophobic and hydrophilic layers.

Depending on the initial mixing ratio of the substances, they separated into multiple layers ranging from one to five, and these layers could be freely controlled. Also, with the phase separation occurring continually within each emulsion drop, the concentration of the chiral dopant inside the liquid crystals changed, resulting in multiple structural colors. This is a new technology to fabricate liquid crystals of multiple layers through a simple process of emulsifying the mixture that has never before been reported.

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