Mechanical Properties of Fluids | Class 11 Physics Notes

9.6 Surface Tension

Surface tension is a property of liquids arising from the imbalance of intermolecular forces at the liquid-air interface. Molecules inside the liquid experience attraction from all sides, resulting in lower potential energy, while molecules at the surface experience net inward attraction, leading to higher potential energy. This causes liquids to minimize their surface area, forming shapes like spheres for drops and bubbles. Surface tension is defined as the force per unit length acting along the surface, or equivalently, the surface energy per unit area. It can be measured by the force required to hold a movable bar on a liquid film. Surface tension depends on the nature of the liquid and the adjacent medium, and decreases with temperature. The angle of contact between a liquid and a solid surface determines wetting behavior: acute angles indicate wetting, obtuse angles indicate non-wetting. Surface tension explains phenomena such as the shape of drops, bubbles, and capillary rise, where liquid climbs narrow tubes against gravity due to surface forces. The pressure inside drops and bubbles is higher than outside, proportional to surface tension and inversely proportional to radius.

📊 Diagram: Fig. 9.14 Schematic picture of molecules in a liquid, at the surface and balance of forces. (a) Molecule inside a liquid. Forces on a molecule due to others are shown. Direction of arrows indicates attraction or repulsion. (b) Same, for a molecule at a surface. (c) Balance of attractive (AI) and repulsive (R) forces; Fig. 9.15 Stretching a film. (a) A film in equilibrium; (b) The film stretched an extra distance; Fig. 9.16 Measuring Surface Tension; Fig. 9.17 Different shapes of water drops with interfacial tensions (a) on a lotus leaf (b) on a clean plastic plate; Fig. 9.18 Drop, cavity and bubble of radius r; Fig. 9.19 Capillary rise, (a) Schematic picture of a narrow tube immersed water. (b) Enlarged picture near interface.

🔗 Connection: This section concludes the chapter and connects to the summary and exercises for consolidation.