Gasses and liquids are considered fluids. If you have ever watched smoke rise from a fire or a stick float down the river, you can see the way the fluid moves can be very complicated. The good news is that not all the types of motion of fluids are complex and complicated, but some types of motion can be modeled and well understood.

The description of how fluids move is important in many areas of biophysics. You may be interested in understanding the motion of air within a person’s lungs, the motion of blood within the body, or the flight of birds to mention a few situations.

For example, companies put a lot of research into developing the sharkskin swimsuit. These swimsuits are made of special materials and designed to reduce the drag between the swimmer and the water. The result: Most of the swimmers in international competitions wore the swimsuits by 2008. Swimmers wearing these swimsuits broke more than a hundred world records during the 2008–2009, which caused the International Swimming Federation to ban their use in 2010.

When a biophysicist studies a fluid, he or she needs to determine the properties of the fluid, such as which forces are dominant and the type of fluid under consideration. Ask yourself these questions when determining the properties of a fluid:

• Is the fluid stationary (static) or moving (dynamical)?

• Is the viscosity of the fluid important (viscous fluid) or not (nonviscous fluid)?

• Is the fluid Newtonian or non-Newtonian (and what type of non-Newtonian fluid)?

• If the fluid is viscous and moving (dynamical), is the motion chaotic (turbulent) or non-chaotic (laminar flow)?

• What forces and laws are relevant?

Some of the forces and laws relevant for fluids are

• The cohesive force, adhesive force, surface tension, and Laplace’s law

• Pascal’s principle, Archimedes’ principle, and the buoyancy force

• Continuity equation (conservation of mass), Bernoulli’s equation (work-energy theorem for fluids), and Poiseuille’s law (the net work done on the fluid is zero).

Some of the forces and laws relevant for objects within a fluid are

• The drag force, which is proportional to the speed (liquid resistance) or speed squared (air resistance)

• Diffusion equation and Fick’s law

• Osmosis and osmosis pressure

• Michaelis-Menten kinetics