have observed that, under the right conditions,

ordinary clear water droplets on a transparent surface

can produce brilliant colors

without the addition of inks or dyes.

This iridescent effect is due to what is known

as structural color, by which an object

generates color simply by the way light

interacts with its geometric structure.

In this case, the researchers were able to observe

and ultimately model how light travels through droplets

of a particular size when it enters at a particular angle.

The model they developed allows them to predict

the color a droplet will produce

given those specific optical and structural conditions.

The researchers imagined their model

could be used in the future as a design guide

to produce droplet-based litmus tests,

or color changing powders and inks

in art and makeup products without the need

for potentially unhealthy synthetic dyes.

At first, the researchers thought the color they observed

might be due to the effect that can cause rainbows,

but they soon realized it was in fact

something quite different.

They observed that droplets on a flat surface

were hemispheres rather than spheres,

like the raindrops that cause rainbows.

They found that a hemisphere's concave surface

allows an optical effect called "total internal reflection"

that is mostly not possible in perfect spheres.

The researchers found once light makes its way

into a droplet, it can take different paths,

bouncing two, three, or more times

before exiting at another angle.

The way light rays add up as they exit

determines whether a droplet will produce color or not

and what color is produced.

The color that droplets produce

also depends on structural conditions

such as the size and curvature of the droplets.

To test their model, the team produced a layer

of bi-phase oil droplets of the exact same size

in a clear Petri dish, which they illuminated

with a single, fixed, white light.

They then recorded the droplets with a camera

that circled around the dish,

and observed that the droplets exhibited brilliant colors

that shifted as the camera circled around.

This demonstrated how the angle at which light

is seen to enter the droplet affects the droplet's color.

The team also produced droplets of various sizes

on a single film, and observed that

when viewed in a microscope, each droplet

produces a different color depending on its size,

and the color always emanates

from the contact lines between the various liquids.

When viewed macroscopically, these droplets together

just appear a glitter-white color.

The team expects that their model

may be used to design droplets, particles, and surfaces

for an array of color-changing applications

where one could tailor a droplet's size, morphology,

and observation conditions to create a specific color.