For colours, dunk clay in water

It’s not exactly that simple, but it’s still a relatively simple way to produce a lovely palette of colours. Researchers from Norway and Germany have reported that when a synthetic clay called Na-fluorohectorite is suspended in water, the material separates out in thin nanosheets – i.e. nanometre-thick layers of Na-fluorohectorite separated by water. And these sheets produce structural colours.

Colour is the frequency of light that we see, after other objects have absorbed all the other frequencies of light. For example, if you have a green-coloured bottle in front of you in a well-lit room, you see that it’s green because the bottle has absorbed all other frequencies in the visible light, leaving only the green frequency to reach your eyes. On the other hand, structural colours are produced when the structure of an object manipulates the incoming light to pronounce some frequencies and screen others.

When light enters between the Na-fluorohectorite layers in water, it bounces between the layers even as some beams of light interfere with other beams. The layer’s final colours are the colours that survive these interactions.

The amazing thing here is that class 10 physics allows you to glean some useful insights. As the researchers wrote in their paper, “The constructive interference of white light from individual nanosheets is described by the Bragg-Snell’s law”. The equation for this law:

2d(n² − sin²θ)^1/2 = mλ

d is the distance between the nanosheet layers. θ is the angle of observation of the layers. m is a constant. λ is the wavelength of the light “enhanced by constructive interference”, according to the paper.

When the colour visible changes according to the angle of observation, θ, the phenomenon is called iridescence. However, the researchers found that Na-fluorohectorite layers were non-iridescent, i.e. the colour of each layer looked the same from different angles of observation. They attributed this to bends and wrinkles in the nanosheets, and to turbostratic organisation: the layers are slightly rotated relative to each other.

Similarly, the effective refractive index of the light, interacting with two distinct materials, is given by this equation:

n = (n₁²Φ₁ + n₂²Φ₂)^1/2

n₁ is the refractive index of one material and Φ₁ is the amount of that material in the overall setup (by volume). So also for n₂ and Φ₂.

Taking both equations together, by controlling the values of n and d, they researchers could control the colour of light that survives its interaction with the water-clay composite. In fact, as we’ll see later, the volume of clay suspended in the water is very low (around 1% at a time), so the effective refractive can be approximated to be the refractive index of water – around 1.33. So if n is fixed, the researchers would only have to change d – the distance between the – to change the structural colours that the clay produced!

Here’s a short video of the team’s efforts:

The researchers found that some white light still survives and dulls the colours on display, which is why they’ve used a dark substrate (in the background). It absorbs the white light, accentuating the other colours.

This is a simple workaround – but it’s also inefficient and limits the applications of their discovery. So they found another way. The researchers dunked Na-fluorohectorite in water along with atoms of caesium. Within “seconds to minutes”, the Na-fluorohectorite formed double sheets – two layers of Na-fluorohectorite sandwiched together by a thin layer of caesium atoms. And these double layers produced bright colours.

The double layers form so rapidly because of a phenomenon called osmotic swelling. The surfaces of the Na-fluorohectorite single-layers are negatively charged. The caesium ion is positively charged, and gets attracted to these surfaces. If two layers, called L1 and L2, are closer to each other than to other layers, then the concentration of caesium ions between these two layers will be significantly higher than in the rest of the water. This prevents the water from entering the gap between L1 and L2, and allows them to practically stick to each other.

There’s more: the researchers also found that they could change the colours by adding or removing water. This is wonderfully simple, but also to be expected. The separation between two nanosheets – i.e. between L1 and L2 – is affected by the concentration of caesium ions in the water. So if you add more water, the concentration of ions drops, the separation increases and the colour changes.

An edited excerpt from the paper’s discussion section, on the findings’ implications:

Because of the sustainability and abundance of clay minerals, the present system carries considerable potential for upscaled applications in various areas ranging from pigments in cosmetics and health applications to windows and tiles. The results and understanding obtained here on synthetic clays should be transferred to natural clays, where vermiculite … presents itself as the most suitable candidate for upscaling the concept presented here. … our results could break new ground when embedding appropriate amounts of these clay nanolayers into transparent but otherwise mechanically weak matrices, providing structural coloration, mechanical strength, and tunable stability at the same time.

Featured image credit: Tim Mossholder/Pexels.