Surface Tension and Viscosity

Definition

Two properties of fluids: surface tension is the tendency of a liquid surface to behave like a stretched elastic sheet, and viscosity is a fluid's internal resistance to flow.

Key points

• Surface tension arises because molecules at the surface are pulled inward by their neighbours; it lets small insects walk on water, forms spherical droplets, and causes water to bead up.
• Surface tension drives capillary action, the rise of liquid in a thin tube, which helps water travel up plant stems and soak into a cloth or wick.
• Adding soap or detergent lowers water's surface tension, which is why soapy water cleans better and spreads more easily.
• Viscosity measures how thick or sticky a fluid is; honey and oil are highly viscous, while water and petrol flow easily; the viscosity of liquids decreases as temperature rises, whereas the viscosity of gases increases with temperature.
• Lubricating oils are chosen for the right viscosity, and viscosity matters in pipelines, engines, and blood flow.

Why it matters for CAPF

Surface tension explaining droplets, insects on water, and capillary rise in plants, and viscosity as resistance to flow with its temperature dependence, are recurring everyday-physics facts.

Common confusion

Surface tension and viscosity are different properties: surface tension is about the surface behaving like a film, while viscosity is about resistance to flow throughout the fluid. Soap reduces water's surface tension (helping it clean), and the viscosity of a liquid falls, not rises, when it is heated, which is why warm oil pours more easily.

Related concepts

concept heat transfer modes, concept archimedes principle, concept atmospheric pressure belts

Parent note

physics everyday