表面张力Surface tension

液体内部的分子受到周围分子各方向的吸引，合力为零；表面分子只受到内侧和侧面分子的吸引，合力指向液体内部，因此表面分子能量更高。液体总是倾向于减小表面积，所以小液滴呈球形。分子间作用力越强，表面张力越大：汞（约 485 mN/m）＞水（72）＞乙醇（22）。温度升高，表面张力降低，到临界温度时为零。

A molecule inside a liquid is pulled equally in all directions, but one at the surface is pulled only inward and sideways, so surface molecules have higher energy. A liquid therefore minimises its surface, which is why small drops are spherical. Stronger intermolecular forces mean higher surface tension: mercury (about 485 mN/m) > water (72) > ethanol (22). Surface tension falls with temperature and vanishes at the critical point.

杨-拉普拉斯方程 Δp = 2γ/r 给出弯曲液面内外的压力差，解释了小气泡内压更大和毛细现象（毛细上升高度 h = 2γcosθ/(ρgr)）。杨氏方程 γ_SG = γ_SL + γ_LG·cosθ 描述接触角与润湿：θ < 90° 为润湿，θ > 90° 为不润湿，荷叶表面 θ > 150° 为超疏水。开尔文方程说明小液滴的蒸气压比平面液体高，这是过饱和与奥斯特瓦尔德熟化的原因。吉布斯吸附方程把溶质的表面吸附量与表面张力随浓度的变化联系起来。

The Young–Laplace equation Δp = 2γ/r gives the pressure difference across a curved surface, explaining why small bubbles have higher internal pressure and capillary rise, h = 2γcosθ/(ρgr). Young's equation γ_SG = γ_SL + γ_LG·cosθ links contact angle to wetting: below 90° a liquid wets, above 90° it does not, and lotus leaves exceed 150°, superhydrophobic. The Kelvin equation shows small drops have higher vapour pressure than flat liquid, the cause of supersaturation and Ostwald ripening. The Gibbs adsorption equation relates a solute's surface excess to how surface tension changes with concentration.

1805 年杨和拉普拉斯分别建立了毛细现象的理论；1878 年吉布斯建立了界面热力学；1891 年波克尔斯在家中厨房发明了测量表面张力的槽法，为朗缪尔膜天平奠定基础。表面张力现象应用于洗涤、喷墨打印、泡沫和乳液稳定、矿物浮选、微流控和防水涂层，水黾能在水面行走也依靠它。

Young and Laplace independently developed the theory of capillarity in 1805, and Gibbs founded interfacial thermodynamics in 1878. In 1891 Agnes Pockels, working in her kitchen, invented the trough method for measuring surface tension, the forerunner of the Langmuir film balance. Surface tension matters in washing, inkjet printing, stabilising foams and emulsions, mineral flotation, microfluidics and water-repellent coatings, and it lets pond skaters walk on water.