Field Notes Interactive

Color Change

Eight ways a material or an animal changes colour, each animated at the level where the change actually happens. Apply heat, voltage or UV, fire a nerve, stretch a lattice, and watch why the colour moves — not just that it does.

01Pigment moving

Nothing about the pigment changes. What changes is where it is: driven across a capsule by a field, or pulled into a disc by muscle. The colour was always there; the mechanism decides whether you see it.

1.1

Electrophoretic ink

e-paper · pigment moving

Apply a voltage pulse

Each microcapsule holds a clear fluid and two pigments that carry opposite charges: white titanium dioxide, positive, and black carbon, negative. A voltage across the capsule is a field through the fluid, and the pigments migrate along it. Whichever pigment reaches the transparent top is the colour you see; the other is hidden behind it.

The point of the mechanism is what happens when the field is removed: nothing. The particles stay where they were driven, held by the fluid’s viscosity, so the pixel keeps its image with no power at all. That bistability is why e-readers last weeks on a charge and why the display cannot refresh quickly — every change is a physical migration.

State
Pixel
Field

1.2

Cephalopod chromatophores

octopus, squid, cuttlefish · pigment moving

Fire the motor neurons

A chromatophore is a tiny elastic sac of pigment with a ring of radial muscle fibres attached to its rim. Firing the nerve contracts the muscles, and they pull the sac out into a flat disc many times its resting diameter. The same pigment, spread thin over a wide area, is now what the skin shows. Cut the signal and the sac’s own elasticity snaps it back to a dot.

Nothing about the pigment changes; only its area does. Because muscle is fast, the whole patch can switch in tens of milliseconds, and because each cell has its own nerve, the animal paints patterns rather than flat colours. Beneath the pigment cells sit reflective iridophores, which show through whenever the sacs are retracted.

State
Coverage
Switch time

02Pigment changing chemically

A molecule with two stable forms, one coloured and one not, and a stimulus that pushes it from one to the other: heat unbinding a dye from its developer, ions turning an oxide into a bronze, a photon breaking one bond. The reverse reaction sets how the colour comes back.

2.1

Thermochromic leuco dye

colour-change inks and textiles · chemistry changing

Apply heat

A thermochromic microcapsule holds three things: a leuco dye, a weak-acid developer, and a waxy solvent with a sharp melting point. Below that point the solvent is solid and the dye and developer sit bonded together. The developer donates a proton, the dye’s ring opens, and the open form absorbs light: the fabric is coloured.

Warm the capsule past the melt and the solvent turns liquid. Dye and developer dissolve apart, the ring closes, and the closed form is colourless. Cool it and the solvent re-solidifies a few degrees lower than it melted, forcing the pairs back together — that lag is the hysteresis you can see in the thermometer. The threshold is set entirely by the solvent, which is why the same dye can be made to switch at 15 °C or 45 °C.

Temperature
Solvent
Dye

2.2

Electrochromic

smart windows, printed displays · chemistry changing

Drive the cell

Material

Same motion, same direction — the difference is what it’s printed on.

Tungsten oxide window

WO₃ on glass · clear when empty, blue when full

Apply a voltage

The active layer is tungsten oxide between two transparent conductors, with a lithium-ion electrolyte and a nickel-oxide counter electrode. Charged, it becomes the bronze Li_xWO₃, which absorbs across the visible. It is transmissive — you look through it — and slow, seconds to minutes for a pane, because every ion has to diffuse into the solid.

State
Transmission
Composition

Printed PEDOT display

PEDOT:PSS on PET · pale at rest, dark when driven

7 segments · each is its own cell · dark = electrons in

Digit

The active layer is PEDOT:PSS, a conductive polymer screen-printed as ink on PET, with a printed gel electrolyte. A fresh film rests in its pale, oxidised state. Drive a segment and electrons go in, with ions to balance them, and it darkens to deep blue; reverse the voltage and it clears. It is reflective — a display, not a window — so a digit reads as dark segments on a pale ground. Each segment is its own cell; small ones switch in about a second and hold for under 1 µW/cm². It is Ynvisible’s technology.

State
Driven segments
Power

Window vs display

: Window · WO₃; Display · PEDOT:PSS
Active layer: WO₃ (inorganic); PEDOT:PSS (organic polymer)
Mode: transmissive (window, lens); reflective (display)
Substrate: glass + ITO; PET, screen-printed
Process: sputtered thin film; roll-to-roll printing
Colored state: both: with electrons in
Speed: seconds to minutes; sub-second to seconds
Where it ends up: buildings, visors, aircraft windows; labels, packaging, wearables, e-textiles

2.3

Photochromic spiropyran

self-darkening lenses · chemistry changing

Apply UV

A spiropyran molecule is two ring systems joined at a single carbon and held at right angles, so their electrons cannot communicate and the molecule is colourless. A UV photon has enough energy to break the carbon–oxygen bond at that junction. Freed, the two halves rotate flat into one planar, conjugated chain, the merocyanine, which absorbs visible light strongly.

Nothing holds it there. Thermal energy alone re-forms the bond, so in the dark the population drifts back to the closed form over seconds. That asymmetry is the whole experience of a photochromic lens: it darkens in a second or two in sunlight and takes noticeably longer to clear indoors, and it cannot darken behind a windscreen that filters UV.

State
Converted
Form

03Structure changing

No colourant at all. A regular nanostructure reflects only the wavelength that fits its spacing, so changing a distance by tens of nanometres changes the colour. Structural colours are saturated, angle-dependent, and vanish when the structure is disturbed — the tells that separate them from pigment.

3.1

Chameleon guanine lattice

panther chameleon · structure changing

Excite the animal

Chameleons carry no blue or green pigment. Beneath a layer of yellow pigment cells lies a sheet of iridophores packed with guanine nanocrystals in a regular lattice. The lattice is a Bragg reflector: white light goes in, and only the wavelength that fits the crystal spacing reflects back. At rest the crystals sit close together and reflect blue, which through the yellow layer reads as green.

When the animal is excited it stretches the skin, and with it the lattice. The spacing widens, the reflected wavelength lengthens through green to yellow and orange, and the same yellow overlay now reads orange or red. No pigment is made or moved; a distance changes by a few tens of nanometres. Read the spacing and the wavelength together and the whole mechanism is one equation.

State
Reflected λ
Lattice spacing

3.2

Morpho butterfly lamellae

Morpho wing scale · structure changing

Viewing angle 0°

The scales of a Morpho wing are ridged, and each ridge carries a stack of chitin lamellae separated by air, a nanometre-scale Christmas tree. Light reflects from every lamella, and the reflections from successive layers travel different distances. At one wavelength those path differences are whole wavelengths, the reflections add, and that colour comes back strongly: about 450 nm, an intense blue from a brown material.

Two things prove it is structure. Tilt the wing and the path through the stack shortens, so the peak shifts bluer — the iridescence. Wet it with alcohol and the air gaps fill with a fluid of higher refractive index; the optical path lengthens and the wing turns green until the alcohol evaporates. A pigment would do neither.

State
Reflected λ
Gap index

04What the illustrations are, and aren’t

Each animation is a mechanistic sketch, not a simulation. Particle counts, layer thicknesses and rates are representative: a chromatophore expands in tens of milliseconds, a photochromic lens clears over seconds, an electrochromic pane over seconds to minutes. Where colour is set by structure it is computed from wavelength — a Bragg spacing for the chameleon, a two-layer interference condition for the Morpho — and converted to screen RGB, so the readouts show a wavelength rather than a chosen swatch. Real reflectance peaks are broader, angle effects involve refraction the sketch omits, and real chameleons combine several cell layers and a second, deeper lattice that reflects near-infrared.