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How does viscosity affect the way a liquid changes shape over time?
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Figure 1: Phase diagram of argon.
Viscosity is the measure of a fluid's resistance to a change in shape or the movement of neighboring portions relative to one another. Essentially, it acts as internal friction between molecules, which opposes the development of velocity differences within the liquid.12
Because liquids are fluids, they cannot sustain a shearing force when at rest and will undergo a continuous change in shape when subjected to such stress. Viscosity dictates how quickly or easily this change occurs:3
- Resistance to Flow: A higher viscosity indicates greater opposition to flow, meaning the liquid will change shape more slowly when subjected to a force. For example, molasses has a higher viscosity than water and therefore flows more sluggishly.12
- Energy Dissipation: Viscosity determines the amount of friction encountered during movement. This internal friction dissipates energy, which is a critical factor in processes like lubrication, spraying, and injection molding.42
- Temperature Sensitivity: In most liquids, viscosity decreases as temperature increases, allowing the liquid to change shape more easily when heated.42