Working content >MacroModels > Fluids

One of the biggest differences between a gas and a liquid is the tendency for liquid molecules to stick together or be cohesive.  In contrast, gas molecules bounce off each other in elastic collisions.  While the molecules in liquids are also moving and exchanging mechanical energy with each other, the molecular attractions in a liquid such as water can be quite strong. (In water this is the result of hydrogen bonding between the molecules.) The attractions between molecules of the liquid that hold it together and help it retain a fixed volume are referred to as cohesion. The molecules of the liquid also attract solid surfaces that either form the container for the liquid or are imbedded in it. This attraction is called adhesion. The cohesive attractions can hold a column of water together such that you can “pull” on the column at one end, and suck the liquid up a straw or the xylem of a plant. The adhesive attractions are responsible for capillary action that draws liquids up small tubes against gravity.

The internal cohesion of liquids has several implications for biological systems.  It is particularly important for plants that need to move fluids, but don’t have pumps with moving parts to do it.  We will discuss how much fluid can be moved when we talk about flow.  For now, we just note that water moves up the xylem of a plant due to its internal cohesion.  The movement is driven by the evaporation of water from the leaves, causing an effective negative pressure, sucking the water from the roots up through the body of the plant to those leaves, based solely on water’s internal cohesion.

     We will discuss three different outcomes of molecular forces and internal cohesion in liquids:

• Surface tension
• Viscosity
• Capillary action

Karen Carleton and Joe Redish 10/25/11