"Everything flows and nothing abides; everything gives way and nothing stays fixed."
-Greek philosopher Heraclitus of Ephesus
We live in a world of fluids, i.e., substances that can flow. Unlike solids, fluids have no definite form but instead assume the shape of their containers. Fluids include all liquids and gases and a rather strange state of matter called plasma, an ionized gas that scientists believe accounts for 99% of the matter in the visible universe.
The importance and pervasiveness of fluids cannot be overstated. The Earth's atmosphere, oceans and core are fluids. We breathe and drink fluids. We sail ships in them and fly planes in them. We are entertained by images on our plasma televisions, illuminated by the glowing plasma in fluorescent lights, and awed by the amorphous streams of charged particles found in lightning.
Fluids are described by properties such as density, viscosity, and compressibility and are responsible for familiar phenomena that include pressure, buoyancy, and aerodynamic lift. The characteristics associated with fluids derive from the relatively weak interactions between their constituent particles. Atoms and molecules found in fluids are not bound to fixed positions
Some common materials known as non-Newtonian fluids don't follow conventional laws of flow. With cornstarch and water, bread dough and peanut butter, resistance to flow changes with applied force. Silly Putty will ooze like a viscous liquid but, when pulled apart quickly, it will stiffen up and break cleanly in two. Other non-Newtonian fluids such as paint and mayonnaise flow more readily when disturbed.
The advances made in the understanding of fluids have been substantial, but we have much to learn. Understanding the transport of fluids across biological membranes, the airflow around the outer surfaces of airplanes and rockets, and the dynamics of our oceans, atmosphere, and convection in the Earth's mantle provide ongoing challenges.
The study of fluids is one of the oldest branches of the physical sciences. Despite its long history, it continues to fascinate scientists and lay people alike. In this edition of CoolStuff we offer ways to engage your students in the study of fluids.