'Not-so-silly' putty

"What do you study?", one of the first questions people ask when they meet me, elicits the same response from me, and the most varied reactions from them. "Rheology", I say. From "What's that?" to "Geology?",  "Theology?", and  "Oh, I've heard that, it's another word for microbiology! ", replies never cease to amuse me. Though I quickly begin to explain, I almost always fail to keep their attention.

Last week I came across something that could hopefully make my answer a little more interesting. A colleague brought along a box of silly putty to work, aptly named 'thinking putty' by the Firebox store.

Apart from making me feel like a six year old, I realised that this gooey material, amongst many others, has changed so much of my thought process and understanding over the past two years. Some of these thoughts, I intend to now pen down.

I'm sure most of us have come across silly putty as kids. The 'real solid liquid' that bounces like a ball when flung on the floor, and yet, flows like a liquid when at rest. What gives this ingenious material it's two- faced behaviour? (solid AND liquid, really? )

Silly putty is one of many materials that are termed 'viscoelastic fluids' in the scientific community because of their dual- behaviour. We would expect that a 'normal' liquid flows on the application of a force, and instantly stops flowing when the force is removed. Yes, water and oil do behave this way. In contrast, when the force applied on a 'viscoelastic fluid' is removed, it continues to deform for a while. This behaviour arises from the materials' capacity to sustain stresses for some time, termed 'fluid memory'. The time it takes for the material to completely release all the built up stresses is called 'relaxation time', and varies widely among materials. When exposed to quick force inflections, such materials are not provided with enough time to completely release the acquired stresses. So they behave like solids, and respond elastically. When given much more time, they relax, release the stresses, forget what they have last been through, and are liquid all over again. So you can deform it to any shape you want, and with time, silly putty lets it all out and reforms to its original gooey state.

Juvenile trial: On the left, a semi- cube at the start, and  on the right, its shape two hours later

In essence, this is what I deal with. The flow behaviour and deformation of such materials. And that is where the word 'Rheology' comes from! 'Rheo' being the Greek word for 'flow'. Viscoelastic fluids are just one kind of a large number of materials that behave strangely. Ironically, the unusual behaviour of materials is not uncommon to come across. Most of the stuff we use in our daily lives, from toothpastes and shampoos, to ketchup and paints, are materials that display such complex characteristics. Water is the exception! Rheologists try to understand the deceptively enticing behaviour of such materials to the best of their abilities, and delve into the niceties of such issues, in a hope to better predict them.

Sometimes it's fun to forget the intricacies involved in dealing with such materials, and just spend the entire day entertaining myself with silly putty. I must admit, I was completely absorbed by how my firmly placed fingerprint quickly faded away into nothingness. Fleeting memory is a lesson in itself. Putty certainly doesn't seem silly to me any more!