Developments in Solid-Liquid Mixtures Suggests New Applications

Researchers within the Media Lab at the Massachusetts Institute of Technology (MIT) and Harvard University are exploring new means for adjusting the extent that a material scatters light. The researchers believe that such a capability can eventually lead to a variety of improved optical devices.

A recent article on this subject published by the team is entitled ‘Tyndall Windows: Tunable Scattering of Disordered Solid-Liquid Matching Mixtures.’ It was published on the website version of the American Chemical Society journal Photonics on the 23rd May. A copy of the article is available for purchase here.

In a material composed of two physically distinct components, the magnitude of the scattering depends, in part, on the difference in the indices of refraction of the components. The greater the difference, the greater the scattering. The size of the difference in the indices of refraction of the two components can be affected and/or controlled by several means, including the application of an electrical field or changing the temperature.

The compound material investigated by the team was a solid particle-liquid mixture. The solid was a glass particle. The reason that the researchers chose to investigate this type of material is the fact that the changes in refractive indices that can be induced in solid components is typically quite small. If, for example, an electric field is applied to a solid-solid material compound, then a very strong field is needed to induce even a small change in the refractive indices of the components and, hence, in the magnitude of the scattering. On the other hand, if the material is a solid-liquid mixture, the refractive index of the solid can be induced to change much faster than that of the liquid. The result can be a dramatic change in the magnitude with which the compound material scatters light.

It should be mentioned that PDLC devices are based on a similar configuration and operating principle.

The specific solid and liquid used in the prototype devices developed at MIT were chosen because they have values of refractive index that are almost the same. It is, however, difficult to find a solid and liquid that have exactly the same refractive index at room temperature. While this is true, it is also true that the rate at which the refractive indices of most solids and liquids change with temperature are different. It follows, then, that there will be some temperature at which they exactly match. When this occurs, the material will not scatter, but become transparent.

In their experiments, the researchers found that a temperature change of 10° would increase the scattering of a prototype device by a factor of ten. A change of 42° changed the scattering of the material by a factor of 1,000.

A video is appended at the end of this article in which one of the researchers discusses the technology.

The researchers believe that the fundamental principle illustrated in the prototype could have broad applicability. With this in mind, they suggest that the same effect could be observed in other types of materials, in which the refractive indices’ change is induced by light or an electric field. The ability to use optical or electrical activation could greatly extend the range of possible applications.

The researchers suggest that the technology could be used in tunable optical devices such as diffraction gratings and light diffusers with applications in imaging, sensing and photography. Potential applications also exist for use in architectural applications such as controllable privacy screens.

“In holographic displays, cells filled with a mixture of electrically responsive solid materials and a fluid could change their diffusivity when charged by an electrode, in much the way that cells filled with ionized gas change their color in plasma TVs. Adjacent cells could thus steer light in slightly different directions, mimicking the reflection of light off of a contoured surface and producing the illusion of three-dimensionality,” the team said.

In their paper, the researchers point out that the experiments with solid particle-liquid mixtures demonstrated much more dramatic changes in scattering than predicted by existing theory. To better understand this observed phenomenon, the researchers developed a new computer model. A unique feature included of their model is consideration of the physical characteristics of the solid particles.

The plan is to use the new model to develop solid particles tailored to specific applications and, thus, devise new and innovative applications for the basic technology. -Arthur Berman

Media Lab, Barmak Heshmat, 857-919-9503, [email protected]