Scientists have created a simple filter material with adjustable wavelength transmission

(ORDO NEWS) — By examining a gel that forms from exfoliating solutions with the addition of silica nanoparticles and solidifies with increasing temperature, the scientists discovered an unusual and previously unknown optical phenomenon.

One of the tasks often encountered in various fields of technology – both in everyday life and in science – is to pass electromagnetic radiation of certain wavelengths and frequencies through any device, but not to let everything else through.

Simply put, to make an electromagnetic radiation filter, which includes filters for the camera, and tuning circuits in the radio. The most important characteristic of a filter is its band – the range of wavelengths that it transmits or absorbs.

Scientists have created a simple filter material with adjustable wavelength transmission 2
Light transmission of Seedgel material at different temperatures

Filters for radio waves usually pass exactly the wavelength range that is needed. In addition, they are not difficult to make customizable: radio wave filters consist of electronic components, the parameters of which can be adjusted.

At shorter wavelengths of electromagnetic radiation, such as visible light, it is much more difficult to make a tunable filter.

Electronic components do not work at such frequencies. Simple filters use dyes, but they have fixed absorption bands. The width and position of these bands are determined by the structure of their molecules, and in molecules, as a rule, one cannot simply take and adjust something.

Since the set of absorption bands of dyes is limited and fixed, adjustable optical filters are made based on the phenomenon of interference and other physical phenomena, and these are quite complex devices.

A team of scientists from the National Institute of Standards and Technology (USA), led by Yuin Xi (Yuyin Xi) , created a material for a tunable optical filter, the position of the bandwidth of which can be adjusted by simple heating and cooling. They reported their development in the journal Nature.

This discovery was somewhat accidental. The authors of the work investigated the properties of the SeedGel material, which is similar to silica gel and can be used in batteries, water filters, the creation of artificial biological tissues, and many other technologies.

The recipe for this miracle material is quite simple. It has three components: an organic solvent 2,6-lutidine (dimethylpyridine), water, and spherical nanoparticles of silicon dioxide (silica) with a diameter of 27 nanometers.

The first part of SeedGel’s unusual properties is that it hardens as the temperature rises. At temperatures below plus 26 degrees Celsius, lutidine is mixed with water, and when heated, the solubility drops, and the liquid is divided into two layers, or two phases – a solution of lutidine in water and a solution of water in lutidine.

Chemists know many systems that behave in this way, but here the components are chosen so that the nanoparticles tend to be in one of two phases – in water.

Before separation, the particles are evenly distributed in the liquid, forming a transparent colloidal solution . Stratification causes them to “crowd” in the volume of the aqueous phase – half as much as before.

The particles come into contact with each other and adhere, fixing the areas of the aqueous phase at the moment of their formation and preventing them from merging with each other. As a result, a solid structure is formed in which the aqueous and organic phases alternate on a microscopic scale.

Scientists have created a simple filter material with adjustable wavelength transmission 3
The structure of the SeedGel material. The water phase is shown in blue, the organic phase in yellow, and the silica nanospheres in gray. The sizes of the depicted areas decrease from left to right and are about 25 micrometers on the left, 0.3 microns in the middle and 0.08 microns on the right. Molecules are not shown to scale; they are ten times smaller than nanospheres

We especially note that the size of silica particles (27 nanometers) is much smaller than the wavelength of visible light (400 – 760 nanometers), and for it they form a single whole with water.

And the size of the phase sections reaches three to four micrometers, so the light “notices” them and is strongly scattered, repeatedly passing through their boundaries.

Water, silica, and lutidine are colorless, so what gives the gel its color? It turns out that it’s all about the refractive index and dispersion – their dependence on the wavelength, due to which substances refract blue light more than red.

In solutions, the refractive index depends on the composition, and in layered liquids, the composition of each layer depends strongly on temperature, just as the solubility of salts in water changes.

Lutidine and silica have a high refractive index, while water has a low refractive index.

When heated, the organic phase contains more lutidine and less water, and its refractive index increases. In the water-silica phase, on the contrary, the concentration of lutidine decreases with heating, and with it, the refractive index.

At a certain temperature, they become equal to each other, and scattering disappears, because the deflection of light at the phase boundary occurs only when the refractive indices differ.

Scientists have created a simple filter material with adjustable wavelength transmission 4
Dependence of the refractive index (Refractive Index) on the wavelength (Wavelength) in the aqueous phase (blue curves) and the organic phase (yellow curves). Solid lines correspond to low temperatures and dotted lines to high temperatures. As the temperature rises, the yellow curves move up and the blue curves move down (indicated by black arrows). The intersection point in this case shifts from left to right, that is, towards the red light

And this zeroing occurs only at a certain wavelength, since the dependences of the refractive index on the wavelength for the two phases also differ.

At one end of the spectrum, the lutidine phase refracts light slightly less than the water-silica phase, at the other end it is slightly stronger, and equality is achieved in the middle. At different temperatures, the intersection point is at different wavelengths.

The dependence of transmitted wavelengths on temperature turned out to be very strong. At plus 27.1 degrees, the material transmitted blue light, and at 27.7 degrees it was already green. The bandwidth in the prototypes was also far from ideal and amounted to tens of nanometers.

But discovery is one thing, and practical application is another: even in such simple cases, the second does not immediately follow the first. The search for the optimal material and design of a light filter that changes color when heated is still ahead.

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