Tuesday, July 30, 2013

RIPE: Rheology

I just came back from a workshop, and one of the things I "re-learnt" was the need to communicate research in plain english (RIPE) to a broad audience. I am going to try to do a pitch on some aspect of my research hopefully on a monthly basis.

Here is the first installment. It is an attempt to introduce my field of rheology to an audience with a high-school level science background.

You probably know that there are three states of matter: solid, liquid and gas (there are a few more exotic ones too).

Take water for example. At room temperature it is a liquid. You can drink it, splash it, swim in it etc. Put some water in a freezer and cool it below melting temperature, and you get cold solid ice-cubes. You can use them to chill your soda, preserve food in an ice-box, or as cold-compress to heal a bruise. Instead of cooling water, you could heat it until it e-“vapor”-ates into steam, which, as it turns out, is the life-blood of the chemical industry.

Okay. So you know your solids, liquids and gases.

So let me ask you this: Is toothpaste solid or liquid?

It holds its shape like a solid, but you can easily “deform” it like a liquid.

How about shaving gel? Mayonnaise, silly-putty, jam?

It turns out that a wide class of materials that you encounter frequently in the food and cosmetic industry, defy this simple binary classification into solids or liquids. The size of this class of materials – unimaginatively dubbed “complex fluids” - is actually even larger.

For instance, most synthetic polymers or plastics that you see around you are processed in the molten state and are called polymer melts (I single them out because I have been studying them for over a decade now, and often feel that they don’t get the attention they deserve).

Polymer melts are also complex fluids.

The study of how such complex fluids respond to deformation is called rheology.

Let me restate the statement above in plain English.

When you try to deform or strain matter, it gets “stressed out”. Different materials, like people, react to stress differently.

Solids usually resist strain by opposing the forces that try to deform or disfigure it. Solids are like people who really don’t like to be pushed around!

Fluids (liquids and gases), on the other hand, go with the flow. Like Zen monks, they don’t resist. They bend, assimilate the imposed force, dissipate its energy, and return to a state of calm.

Complex fluids are like children with “solid” and “liquid” parents. They inherit traits like elasticity from their “solid” parent, and viscosity from their “liquid” parent. It is therefore no surprise that they are very frequently called “viscoelastic fluids”.

Studying the rheology of viscoelastic fluids like polymer melts, helps us devise better methods of processing them.

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