By examining the gel, which is formed from an exfoliating solution with added silica nanoparticles and solidifies as the temperature increases, scientists discovered an unusual and previously unknown optical phenomenon. Temperature dependence of the visible light transmission band of
gel material . At 29 degrees, the gel transmits only red light, and at 27 degrees - blue. The remaining wavelengths are strongly scattered and are evident in the form of a diffuse glow at and around the location of the inscription. At 20 degrees, the material turns into a clear liquid.
One of the tasks frequently encountered in various technical fields of daily life and science is to pass electromagnetic radiation of certain wavelengths and frequencies through any device, but not all others. Simply put, make an electromagnetic radiation filter, which includes filters for cameras and tuned circuits in radios. The most important characteristic of the filter is its band - the range of wavelengths it transmits or absorbs.
Light transmittance of gel materials at different temperatures
Radio wave filters usually pass exactly the desired wavelength range. In addition, they are not difficult to customize: radio wave filters are composed of electronic components, the parameters of which can be adjusted.
Making tunable filters is much more difficult at shorter wavelengths of electromagnetic radiation, such as visible light. Electronic components cannot operate at such frequencies. Simple filters use dyes, but they have fixed absorption bands. The width and position of these bands are determined by their molecular structure, and in molecules often things can't simply be taken away and tweaked.
Since the absorption band set of dyes is limited and fixed, tunable filters are made based on physical phenomena such as interference phenomena. These are quite complex devices.
A team of scientists from the National Institute of Standards and Technology (USA), led by Yuyin Xi, has created a material for tunable optical filters whose bandwidth position can be adjusted by simple heating and cooling . They report their development in the journal Nature.
This discovery is somewhat unexpected. The authors of this work studied the properties of a gel material similar to silicone that could be used in batteries, water filters, the creation of artificial biological tissues, and many other technologies.
The recipe for this magical material is very simple. It consists of three parts: the organic solvent 2,6-lutidine (dimethylpyridine), water and spherical silicon dioxide (silica) nanoparticles with a diameter of 27 nanometers.
The first part of the gel's unusual properties is that it hardens as the temperature increases. At temperatures below 26 degrees Celsius, lutidine is mixed with water. When heated, the solubility of and decreases and the liquid separates into two layers or phases - aqueous lutidine and aqueous lutidine. Chemists know of many systems that behave in this way, but the components here were 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 to form a clear colloidal solution. Stratification causes them to "clump" in the volume of the water phase - half of what it was before. The particles contact and adhere to each other, immobilizing the aqueous phase areas at the moment they form and preventing them from mingling with each other. As a result, a solid structure is formed in which aqueous and organic phases alternate on a microscopic scale.
Structure of the gel material. Blue represents the aqueous phase, yellow represents the organic phase, and gray represents silica nanospheres. The size of the depicted area decreases from left to right, with approximately 25 μm on the left, 0.3 μm in the middle, and 0.08 μm on the right. The molecules are not shown to scale; they are ten times smaller than the nanospheres.
We noticed in particular that the size of the silica particles (27 nanometers) is much smaller than the wavelength of visible light (400 - 760 nanometers), so they form an integral body with the water. The dimensions of the phase sections reach three to four microns, so light "notices" that they are not strongly scattered, repeatedly crossing their boundaries.
Water, silica and lutidine are colorless, so what gives the gel its color? It turns out it's all about refractive index and dispersion - they are wavelength dependent, so a substance refracts blue light more than red light. In a solution, the refractive index depends on the composition, whereas in a layered liquid, the composition of each layer depends largely on the temperature, just as the solubility of salt in water changes.
Dimethylpyridine and silica have high refractive index, while water has low refractive index.
When heated, the organic phase contains more lutidine and less water, and its refractive index increases. In contrast, in the water-silica phase, the concentration of lutidine decreases with heating, and the refractive index also decreases. At a certain temperature they become equal and scattering disappears, since the deflection of light at phase boundaries only occurs if the refractive index is different.
Dependence of the refractive index (Refractive Index) on wavelength (Wavelength) in the aqueous phase (blue curve) and organic phase (yellow curve). The solid line corresponds to low temperature, and the dashed line corresponds to high temperature. As the temperature increases, the yellow curve moves upward and the blue curve moves downward (indicated by the black arrow). The intersection point moves from left to right, that is, toward the red light.
And this zeroing only occurs at a certain wavelength, because the wavelength dependence of the refractive index of the two phases is also different. At one end of the spectrum, the lutidine phase refracts slightly less light than the water-silica phase, while at the other end it is slightly more refracted, reaching equality in the middle. At different temperatures, the intersection points are at different wavelengths.
The transmitted wavelength has a very strong dependence on temperature. At plus 27.1 degrees, the material transmits blue light, and at 27.7 degrees, it is already green. The bandwidth in the prototype was also far from ideal, reaching tens of nanometers. But discovery is one thing, practical application another: even in such a simple case, the second does not follow the first. The search for the best materials and designs for filters that change color when heated is still ahead of the curve.
