The joy of understanding: how ink works

Last week I blogged about Joy and wonder - the science behind mayonnaise and other things. Today I’ll discuss one of those things. Chapter 9 in Michel Mitov’s 2012 book Sensitive Matter tells about ink.

At its simplest, black Chinese or India ink is a suspension of small particles of soot (carbon black) in water. But that suspension is not stable for more than a few hours and a process called sedimentation occurs. The particles grow and fall to the bottom of the container.

Adding less than half a percent of gum arabic changes the behavior to produce a suspension stable for a year or more. Gum arabic contains long polymer molecules that coat the pigment particles. They act like little hairs, with one end attached to a particle surface and the other sticking into the liquid. Those particles look like little brushes. If too much gum is added, then those molecules get tangled up and form a thick gel rather than a liquid.     

Chapter 4 of the 1996 book Fragile Objects by Pierre-Gilles de Gennes and Jacques Badoz also discusses The Egyptian Scribe, Arabic Gum, and Chinese Ink. The book documents lectures that de Gennes gave to French high school students after winning the 1991 Nobel Prize in Physics. He  adds the detail that the long-chain polymer is polyhyaluronic acid. The mechanism by which it stabilzed the suspension was only explained in the mid 1980s. Dr. de Gennes adds that:

“I consider the story of Chinese ink a perfect example in more than one respect. It illustrates the properties of finely divided matter, which plays so prominent a role in our daily lives. Witness the products of the food industry (creams, margarine, mayonnaise), of the oil industry, of the cosmetic industry, and of many others. It also provides another example of the radical changes in physical properties which can be imparted by a seemingly weak action, in this case the addition of small quantities of polymers. This effect is the principal characteristic of what I have already called soft matter.

I also like the story of Chinese ink because it shows that even a product so trivial in appearance that we no longer accord it any attention has the power to amaze us by the marvelous quality of the invention which gave birth to it and by the subtlety of the physical phenomena which explain its behavior. We should be equally amazed by many of the industrial products which we use in our daily lives.”

The image from a Botticelli painting was on Wikimedia Commons.