Mechanical Properties of FluidsIntroduction
Holi morning. Riya pushes the plunger of her pichkari, and a thin, fast jet shoots across the lane. Pushing harder makes the jet faster, a narrower nozzle makes it go farther, and the drops that break off fly as tiny round balls. The thick sugar syrup for the gujiya, meanwhile, crawls out of its pan as if it did not want to move at all.
Every one of those things is about fluids: substances that can flow. Liquids and gases both flow, so both are fluids. The Earth wears a blanket of air, two-thirds of its surface is under water, and most of our own body is water. Plants lift sap, blood carries oxygen, and both depend on how fluids behave.
So what separates a fluid from a solid, and a liquid from a gas? Sort each thing by how it behaves.
A fluid has no shape of its own. A liquid keeps a fixed volume (at a given pressure) and settles into a free surface; a gas spreads into all the space it is given. Squeeze them and another difference shows up: the volume of a solid or a liquid hardly changes, while a gas is easily compressed. We say liquids and solids have a much smaller compressibility than gases.
The deepest difference is how they respond to a shear stress, a force that tries to slide one layer over the next. A solid resists it and changes its shape only a little. A fluid gives way to even a tiny shear stress and simply flows: the shear stresses fluids can hold are about a million times smaller than those of solids. In this chapter we first study fluids at rest (pressure), then fluids in motion (streamlines and Bernoulli's principle), then the internal friction of a flowing fluid (viscosity), and finally the special behaviour of a liquid's surface (surface tension).