Teacup microdroplets and colours

Look for them in hot tea or coffee (no milk) or indeed just hot water. Best when the sun is low. View the liquid at a low angle. Iridescent colours shift and dance, flicker, they are gone, then come again. Some are merely grainy, others intensely bright specks. There is colour on the surface, there is colour coiling upwards.

In shade, when colours are largely gone, the surface seems to have a skin. A 'skin' ever changing. It evokes ice floes with cracks (leads, the curse of early Arctic explorers) of clear water between. The cracks open and extend. Floe edges explode, disappear. All is change.

The colours

Above the arrayed droplets floes are the sheets and coils of ‘steam’ – droplets condensed out of the rising water vapour evaporate as it mixes with air and cools.

The steamy coils famously give shifting iridescent colours. Their drops diffract light to give the iridescence of sky borne clouds but with greater vivacity and grace.

The floe colours are harder to fathom. There is sometimes a temporal and spatial coherence of colour across several floes. The shifting patterns suggest the motion and colours of the overlying vapour coils. How are floes and upward vapour streams related? Does the same liquid convection pattern structure both? Or is some of apparent floe iridescence from the vapour coils?

Floe droplets themselves can diffract light either directly (1) or involving reflection from the water beneath (2)(3). Their similar sizes and high concentration will generate vivid and regular colour patches.

Qu�telet colours are an extra possibility (4). Waves scatter both from direct light and that reflected upwards from the tea. They overlap and interfere to give a coloured diffraction pattern.

The quasi-regular triangular droplet arrays might even act as diffraction gratings. That is especially so in hot tea when the droplets are closer together.

Many possibilities. Much scope for more teatime gazing laced with video and even high speed video.

The Floes

Given the ubiquity of tea and coffee cup gazing, there has been surprisingly little curiosity about the true nature of the 'skin'. Now, microscopy and high speed video have revealed that it is stranger than we might imagine. The skin is a layer of tiny similar sized droplets around 20 micron (1/50 mm) diameter. Their size decreases as the tea cools. They align themselves in triangular patterns clustering more densely the hotter the liquid.

But the droplets are not on the surface. They levitate some .5 to 5 drop diameters above it. The upward flow of vapour from surface evaporation might support them. Or aerodynamic and surface tension effects on spinning drops could do so. The droplets and tea surface could be charged and the drops held up by electrostatic repulsion. That would also nicely account for their alignment into arrays.

I tried making tea with an added teaspoon of salt to render the liquid conducting and reduce any surface charge. Salty tea had no floes. On its own, though, that�s not conclusive evidence for electrostatic levitation. The salt could have other effects.

Salt in tea? Traditional Mongolian Suutei tsai is tea with milk and salt. Does it have droplet floes?

And the cracks?

The levitating drop arrays are stable until just one of them disappears. It�s not known to where. Then there is catastrophe. The drop's demise spreads outward wave-like. Surrounding drops disappear too in a fraction of a second. Entire floes vanish, cracks formed and propagate. The crack locations and the initial drop deaths might relate to the tea liquid convection patterns.

Why does a single droplet death afflict the floe so? It might fall to the liquid and create a shallow wave that engulfs the surrounding drops. The spread is actually at the speed of surface capillary waves. But more research is needed for the drops might sweep upwards instead.

Amazing behaviour raising many more questions about our everyday drinks.