It’s an intriguing notion, an invisibility cloak that’s effective in milk and I suspect that I’ve never entirely understood the implications of the research featured in a June 6, 2014 news item on Nanowerk,

Real invisibility cloaks are rather complex and work in certain situations only. The laws of physics prevent an optical invisibility cloak from making objects in air invisible for any directions, colors, and polarizations. If the medium is changed, however, it becomes much easier to hide objects. KIT physicists have now succeeded in manufacturing with relatively simple means and testing an ideal invisibility cloak for diffusive light-scattering media, such as fog or milk. …

A June 6, 2014 Karlsruhe Institute of Technology (KIT) press release, which originated the news item, provides more details,

In diffusive media, light does no longer propagate linearly, but is scattered permanently by the particles in the medium. Examples are fog, clouds, or frosted glass panes that let the light in, but hide the light source. “This property of light-scattering media can be used to hide objects inside,” says Robert Schittny, first author of the study. “The new invisibility cloaks have a rather simple structure.”

In the experiment, Schittny used an extended light source to illuminate a Plexiglas tank of a few centimeters in width from the back. The tank was filled with a white, turbid liquid. Objects inside cast a visible shadow onto the tank wall. Simple metal cylinders or spheres of a few centimeters in diameter were used as test objects. To hide them, they were first coated with a white dispersion paint, such that the light was reflected in a diffusive manner. To pass the light around the object, the researchers applied a thin shell made of the transparent silicon material PDMS, to which a certain concentration of light-scattering melamine microparticles was added. The silicon/melamine shell caused a quicker diffusion than in the environment and, thus, passed the light around the objects. Hence, they did no longer cast a shadow. “Disappearance of the shadow indicates successful cloaking.”

“Ideal optical invisibility cloaks in air have a drawback,” Martin Wegener points out. He conducts research at the KIT Institute of Applied Physics and the KIT Institute of Nanotechnology. “They violate Albert Einstein’s theory of relativity that prescribes an upper limit for the speed of light. “In diffuse media, in which light is scattered several times, however, the effective speed of light is reduced. Here, ideal invisibility cloaks can be realized.”

The researchers have provided this image to illustrate their work,

In a diffusive light-scattering medium, light moves on random paths (see magnifying glass). A normal object (left) casts a shadow, an object with an invisibility cloak (right) does not. (Image: R. Schittny / KIT)

I’m not convinced that there’s actually a cloaked object in that image but I appreciate the call to use my imagination.

Here’s a link to and a citation for the paper,

Invisibility cloaking in a diffusive light scattering medium by Robert Schittny, Muamer Kadic, Tiemo Bückmann, and Martin Wegener. Science DOI: 10.1126/science.1254524 Published Online June 5 2014

This article is behind a paywall.