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Fluids (2013)

Page history last edited by Joe Redish 5 years, 1 month ago

Working ContentMacroModels

 

Fluids are any material that changes its shape easily in response to forces.  For simple kinds of uniform matter, this includes liquids and gases.  Fluids are key to all biological systems.  All known organisms either live in fluids or have fluids moving within their bodies (mostly both).

 

These definition of solids, liquids, and gases are very familiar to you. You probably know them from elementary school and have seen them in every science course you have ever taken. Often they are defined sharply: "solids have a fixed volume and shape, liquids have a fixed volume and can change shape, gases have neither and expand to fill any container."  I have stricken these out because I want you to not see these as precise definitions but as statements of approximate convenience. 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!  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 are 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.
 

 

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 move 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 too strong. Since the forces are not strong enough to lock the molecules into a fixed structure (solid), but 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.

 

Therefore, to understand the properties of biological systems, the properties of fluids are of great importance.  Some of those properties are discussed on the follow-on pages.

 

 

Resources:

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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