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A team of scientists from MIT and Penn State
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.
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Color drops

46 Folder Collection
jbsatvtac1 published on August 22, 2019
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