Capturing the Colors of a Plastic Bubble

Christine Connolly

Everyone loves the bright rainbow colors of a soap bubble, and young children often cry when these delicate objects burst. However, Fumiyoshi Ikkai of Nihon L’Oreal KK in Kawasaki, Kanagawa, Japan, now has found a way to capture and preserve this beauty in long-lasting polymer bubbles. The effect could give an eye-catching appearance to liquid cosmetics and could create exciting paints.

This is his first exploration in the field of color. Originally, he was investigating polymer microcapsules as a container of various pigments, fillers or solutions.

“One day, when I was observing those microcapsules under the microscope, I noticed a few red and green microcapsules in the field of vision,” he said. “These were actually failed microcapsules with insufficient surface thickness.”

Tiny capsules of polystyrene produce a rainbow of bright colors and remain stable for over a year in water-based products. Courtesy of Fumiyoshi Ikkai, Nihon L’Oreal KK.

But Ikkai continued to think about his observation. “As I was wondering where the colors came from, there flashed the idea that this was the same as colored balloons.” He therefore undertook a detailed study of the phenomenon, varying the conditions under which he prepared the microcapsules and measuring their color and their dimensions.

In initial studies, Ikkai used a Canon Eos Kiss digital camera to record images through the A-Plan objective lens of a Carl Zeiss inverted microscope. He then analyzed the pictures with Mitani Corp. Mac Scope software to measure the microcapsule diameters.

Looking through the microscope, he carefully positioned an Ocean Optics Inc. miniature fiber optic spectrometer so that it measured the color of light from each individual particle in turn. He also measured the thickness of each capsule’s surface with a cryoscanning electron microscope between –160 and –180 °C.

To Ikkai’s surprise, he found no relationship between the size of the bubble and its color. Instead, the color depends on the thickness of the bubble’s surface. He showed that the source of the color is the interference effect between rays of light bouncing from the outer and inner sides of the thin surface of the capsule. It is “structural color,” an effect that occurs in nature in the iridescence of peacock tails, dragonfly bodies and fish scales. Sir Isaac Newton studied the bright purity of this thin-film structural color in 1704, and a few papers published since the late 1990s report polymer gels whose color depends on interparticle distance or particle size.

The structural colored balloons result from a double-emulsification procedure using polystyrene, gelatin and dichloromethane. The final emulsion is stirred for four hours and then filtered, and the structural colored balloons are washed. The resulting balloons retain their stability for more than a year in water and do not stick to each other.

To study them in more detail required a painstaking procedure. To measure the surface thickness of the different colored balloons, Ikkai observed them through the optical microscope, selected bubbles of a particular color and then carefully picked them up, one by one, with tweezers. He transferred them to the cryoscanning electron microscope, where he discovered that a green structural colored balloon had a surface thickness of 720. A red one was 550 nm thick.

Ikkai then worked out the detailed mathematics of the effect, showing that the color depends on the path difference between the rays reflecting from the upper and lower boundaries of the balloon’s surface, on the refractive indices of these boundaries and on the order of interference.

The thinnest surface capable of producing a color is 100 nm, at which zeroth-order interference produces violet. For structural colored balloons with thicker surfaces, the colors work through blue, cyan, green, yellow and orange to a zeroth-order red at a thickness of 200 nm. No color is produced by surfaces between 200 and 350 nm thick, but then the first-order interference appears from 350 nm (again producing violet) through to red at 620-nm thickness. The colors at higher orders of interference overlap with each other, producing mixtures of colored light at greater surface thicknesses.

The plan is to use the effect to add an eye-catching dimension to liquid products. Ikkai points out, “It is still difficult to control the surface thickness of each structural colored balloon so we can get whole rainbow colors at once.

Langmuir, April 1, 2008, pp. 3412-3416.