Flow Rate and Viscosity

• Flow rate is the measurement of how quickly fluids move. It is measured in volume per unit time (ex. L/s)
• Flow rate depends on several factors:

  • The type of fluid (thick or thin)
  • The force pushing on the fluid (stronger forces = faster flow rate)
  • The size of the pipe / opening (larger openings = faster flow rate)
  • The type of surface the fluid is flowing over (smooth surface = faster flow)
• Viscosity is the measure of how easily a fluid’s particles are able to slide past each other
• The viscosity of a fluid refers to its “thickness”
• The two factors of viscosity are cohesion and adhesion
• Cohesion is the force of attraction between the particles of a substance
• Fluids with slow flow rates have particles with greater cohesion which means that the particles stick together and don’t flow past each other very well
• The cohesion (force of attraction between particles) of particles on a liquid’s surface is called surface tension. Insects like water striders use surface tension to walk on water. The force of attraction between the water particles is stronger than the force of gravity on the insect.
• Sometimes the cohesion of water needs to be reduced (like during a forest fire, a wetting agent can be added to water to reduce cohesion so the water can disperse and spread out more easily)
• Adhesion is the force of attraction between the particles of a fluid and particles of other substances (ex. after drinking milk, there’s a small film of milk on the sides and bottom of the glass)
• Adhesion between water particles and the container is responsible for the curved top surface (meniscus) you see when water touches the sides of a cup, graduated cylinder, or other container.
• Adhesion causes gases and liquids to travel faster near the centre of pipes and tubes than at the edges