Topic 4: Flow Rate & Viscosity

Viscosity – how quickly fluid flow The thicker the fluid is the more viscous it is, it has a higher viscosity. Orange juice has a low viscosity. Ketchup and mustard have a high viscosity.

Recall from the particle model that the particles in a liquid slide around and roll over each other. The greater the friction or rubbing between particles in any fluid, the higher the viscosity. Fluids with high viscosity do not flow as easily as fluids with a low viscosity.

A fluid with a high viscosity has a large amount of internal resistance or friction. As the temperature of a liquid increases, its viscosity decreases. The opposite is also true. As the temperature of a liquid decreases, its viscosity increases. The particle model of matter can help you understand why this change in viscosity happens.

A liquid is made of particles that can slide and roll over each other. When energy or heat is added to the liquid, the particles slide and roll more quickly. As a result, the fluid flows more readily – its viscosity decreases. As the temperature of the liquid drops, the particles slow down. The result is that the viscosity increases – the fluid flows more slowly.

Viscosity in gases increases and decreases in a different way than it does for liquids. Gas particle flow differently than liquid particles. Gas particles are so far apart, and the attractive forces are so low, that the type of gas particle is less important than in a liquid. Particles of gas are more likely to collide with each other than rub against each other.

The temperature of a gas has a direct effect on viscosity. As gases are heated, the particles gain energy and move faster. The number of collisions between gas particles increases. Cooler gases contain particles that are not colliding as much or as often. Therefore, the viscosity of a gas increases as it is heated and decreases as it is cooled. Temperature has the opposite effect on the viscosity of gases as on liquids.

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