Overview: Fluids


Fluids are any material that changes its shape easily in response to forces. For simple kinds of uniform matter, this includes liquids and gases. Since liquids and gases have a lot of properties in common (and even blend into each other indistinguishably at high temperatures and pressures), we'll talk about both of the together as fluids — states of matter that can change their shape easily and quickly. 

Fluids are key to all biological systems. All known organisms either live in fluids or have fluids moving within their bodies (mostly both). 

There are many states of matter, not just solids, liquids, and gases. For the simplest forms of uniform matter, the separation into solid, liquid, and gas can be made precisely and there can be discontinuities in their properties when they change into one another. It is very important to know about this (think about the transitions from ice to water to steam). But this behavior is not true for all types of complex matter. Many parts of biological systems are complex mixtures of different kinds of substances, parts of which look like solids, parts like liquids, and parts like gases — but they are organized into a coherent object like a bone, a cell, or a vacuole. Other parts of biological systems look like solids in one direction and liquids in others!

Gases are fluids that are at a low enough density or high enough speeds that the interatomic attractive forces are not sufficient to hold the atoms or molecules together in a coherent whole. As a result, they will expand or compress to fill any container. They can consist of a single substance or a mixture. The most common gas that biological systems interact with is air, since it is the primary source of oxygen for most organisms, and oxygen is a primary agent in the extraction of energy from chemical compounds.

Liquids are fluids in which interatomic forces are strong enough to hold the molecules of the substance together against external forces that are not so strong. that they lock the molecules into a fixed structure (solid). But since they are strong enough to hold the substance together, liquids maintain an approximately constant volume (in contrast to gases) and can change their shape.

Liquids can be made of a single compound, in which case they are either made of atoms or molecules.  (Only a few elements are liquid at or near room temperature (mercury, bromine, and gallium), so most commonly we are considering molecules when we talk about liquids.)  Liquids can also be mixtures of different compounds.  Although liquids can take on the shape of whatever container holds them, they still have inherent properties. Just like solids, liquids have density and resist compression (have a bulk modulus). The molecules of a liquid tend to stick together, or have some internal cohesion. This gives them interesting properties like surface tension (surface cohesion), adhesion (sticking of the liquid to a solid surface), and viscosity (internal resistance to flow). They also will move or flow, and their behavior when they flow plays an important role in many biological processes.

To understand the properties of biological systems, the properties of fluids are of great importance. Here are links to the discussions of the most important principles and foothold ideas.

  • Pressure — As a result of the thermal motion of the molecules in fluids, they will bounce off any surface in the fluid creating a force. Pressure is the emergent concept that allows us to understand these forces.
  • Buoyancy — In response to gravity, the pressure in fluids increases with depth, leading to a net upward force on objects imbedded in them.
  • Internal cohesion — Intermolecular forces in liquids are strong enough to pull them together, leading to important phenomena such as surface tension.
  • Quantifying fluid flow — The motion of fluids in critical for many biological systems. In this section, we quantify rate of flow and see the implications for incompressible fluids (such as water).
  • Internal flow -- the HP Equation — This principle shows how a rate of flow arises from a push on the fluid (a pressure differential) and the internal forces (viscosity) resisting motion.


Mark W. Denny, Air and Water (Princeton U. Press, 1995) chapters 4 and 5.
Steven Vogel, Comparative Biomechanics: Life's Physical World (Princeton U. Press, 2003) chapters 5 and 6.  

Karen Carleton and Joe Redish 10/23/11


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Last Modified: February 12, 2019