"You could imagine this as a communication material. It communicates with its environment and reacts to stimuli", explains Liu. With her team she discovered that the liquid-crystal molecules react to radio waves. When the waves are turned on, the molecules twist to orient with the waves' direction of travel. By knowing this, Liu and her team decided to insert numerous micrometer-sized pores in the coating. Once filled with the desired liquid, the system acts like a sponge. "When the radio waves are turned on, the liquid-crystal molecules move in one direction and therefore wring the liquid out of the pores," explains Liu. “The coating even sweats more as the radio signal becomes stronger."
The reabsorption properties make this material even more special. An integrated capillary function can reabsorb the droplets on the surface within several seconds.
"With this feature, we can make surfaces that on command can be 'wet' or 'dry', for example to vary the level of lubrication or surface adhesion", continues Liu.
Liu worked with triggers like heat, light and electricity before, to switch her material on and off. But radio signals are a first: "In my field of material sciences, we often use the same electricity as in houses (50-60 Hz), but this frequency is deadly to touch. As we want to apply the material to robots and dressings, we needed to develop a material that is safe to handle."
That's why Liu uses radio frequency alternating electric fields (20 kHz to 300 GHz), similar to a Wi-Fi signal, which is low-energy and non-ionizing radiation.
"The high frequency we use was invented for the electrical car. It cannot penetrate the body, and therefore it is completely safe to touch. It is already a common frequency in the health care industry, for example in electrical therapies."