(Photo : Image from Physical Review Letters)

In the world of physics, invisibility cloaks are not necessarily just devices that make objects invisible to the naked eye. Scientists have created devices that hide object from light waves under conditioned environments, and others that can hide the effects of an object on other objects that produce sound waves. Now aside from light and sound waves, researchers have come up with a cloaking device that can mask the presence of an object in a fluid so that the current is redirected away from the object.

Two research teams, one from China and the other from Korea, have published separate researches in a recent issue of Physical Review Letters. They explained how the techniques can also reduce drag force, then reducing the effect of waves on the object.

Similar to how cloaking devices for light and sound waves work, this one for water waves will steer the waves so that they travel around the object rather than allowing it to spread, making it seem invisible. The waves would then take a detour around the object and proceed to a conformation as though the object were not there. Otherwise, if allowed to spread, the wave paths would reveal the object's presence.

Describing the process makes it seem easy but actually making it happen is "tricky" as described by John Pendry, Imperial College of London physicist, who was not involved in the research. This was backed by Zhejiang University in Hangzhou engineer Zhenyu Wang, one of the authors from China. "The structure appears simple, but sometimes simple is very powerful," explains Wang. "When you understand the theory, you do not need to use a very complex method."

The cloaking process was done by using a small toy boat inside a wave tank. The research group from China built a structure along each side of the wave tank using steel beams that sloped upward to a flat region and then sloped back down. Doing this changed the water depth at the edges so that the incoming waves adjusted their speed and direction. The researchers reported that the miniature boat remained almost motionless as the waves rolled in their experiment. They proposed to build such beams along ports to stop boats from continuously moving while being loaded with cargo.

Researchers from Korea suggest a different approach. In their experiment, they safeguarded an object in a flowing stream of water using a more intricate design. They designed a material using more than 500 pillars of 50 microns in width. The device encircled the object in the water, and caused the water to act as though it had a higher viscosity, flowing as though there was no object in its path. "The force cannot penetrate into the inner cloaked region," coauthor of the study and materials scientist Juhyuk Park says as he explains how the material decreases drag forces on the object. He mentioned how reducing drag force using their design could allow for more fuel-efficient vehicles in the future.