Tunable Scattering of Immersions

A mild temperature change radically alters the degree to which a solid-fluid mixture bends light (Source: MIT)

A mild temperature change radically alters the degree to which a solid-fluid mixture bends light (Source: MIT)

By immersing glass particles in a fluid, researchers at MIT’s Media Lab and Harvard University are exploring a new mechanism for modifying an optical device’s diffu­sivity, or the extent to which it scatters light. In its current form, the new diffuser could be used to calibrate a wide range of imaging systems, but the researchers believe that their mechanism could ultimately lead to holographic video screens or to tunable optical devices with appli­cations in imaging, sensing, and photo­graphy.

In experiments, the solid-liquid mixture demon­strated much more dramatic changes in diffusivity than existing theory would have predicted, so the researchers also deve­loped a new computer model to describe it. That model could help them devise more complex applications for the basic technology. The fluid and the glass in the proto­type were chosen because they have very similar refractive indices, meaning light travels through them at similar speeds. When light moves from a material with a high refractive index to one with a lower refractive index, it changes direction; this is the phenomenon behind the familiar illusion of a straw’s appearing to bend when it’s inserted into a glass of water.

The researchers’ prototype exploits the fact that changes in tem­perature alter materials’ refractive indices. “It’s hard to find a solid and liquid that have exactly the same refractive index at room tem­perature,” says Barmak Heshmat, a postdoc in the Media Lab’s Camera Culture group. “But if the speed at which the refractive index changes for solid and liquid is different then at a certain tem­perature they will exactly match, to the last digit. That’s why you see this giant jump in trans­parency.”

In their experiments, the researchers found that a tempe­rature change of 10 degrees would increase the diffusivity of their device tenfold, and a change of 42 degrees changed it a thousand­fold. Heshmat believes that a tempe­rature-modulated version of his team’s filter could be used to calibrate sensors used in the study of material flows, the study of cells, and medical imaging.

For instance, medical-imaging systems are typically calibrated using devices called “tissue phantoms”, which duplicate the optical properties of different types of biological tissues. Tissue phantoms can be expensive, and many of them may be required to calibrate a single imaging device. Heshmat believes that a low-cost version of his team’s filter could mimic a wide range of tissues. But the fun­damental principle illus­trated by the researchers’ prototype could have broader ramifications. The effect of heat on the refractive index of either the solid or the fluid, taken in isolation, is very subtle. But when the two are mixed together, the effect on diffu­sivity is dramatic.

The same would be true, Heshmat argues, of other types experi­mental materials whose refrac­tive indices change in response to either light or an electric field. And optical or electrical activation would broaden the range of applications for tunable optical devices. “If you have photo­refractive changes in a solid material in a solid phase, the amount of change you can get between the solid and itself is very small,” he explains. “You need a very strong field to see that change in your refractive index. But if you have two types of media, the refractive index of the solid is going to change much faster compared to the liquid. So you get this deep contrast that can help a lot.”

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.

Liquid-solid mixtures could also be used to produce tunable diffraction gratings, which are used in some sensing appli­cations to filter out light or other electro­magnetic radiation of particular frequencies, or in tunable light diffusers of the sort photo­graphers use to make the strongly directional light of a flash feel more like ambient light. The computer model that the researchers describe in their paper predicts the diffusivity of a liquid-solid mixture on the basis of the physical charac­teristics of the solid particles and on their concen­tration in the liquid. That model, Heshmat says, could be used to develop solid particles tailored to specific appli­cations. (Source: MIT)

Reference: B. Groever et al.: Tyndall Windows: Tunable Scattering of Disordered Solid–Liquid Matching Mixtures, ACS Phot. 3, 930 (2016), DOI: 10.1021/acsphotonics.6b00199

Link: Camera Culture Group, Media Lab, Massachusetts Institute of Technology, Cambridge, United States

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