Surface Tension

Interactive visualization of surface tension phenomena - Explore droplet formation, molecular forces, capillary action, and contact angle effects

Surface Tension Phenomena

Surface Tension Information

Surface Tension (γ): 72.8 mN/m
Temperature (T): 20 °C

Liquid Properties

Liquid Type: Water
Density (ρ): 998 kg/m³
Viscosity: 1.0 mPa·s
Boiling Point: 100 °C

Parameters

Freezing Boiling

Physical Equations

Surface Force: F = γL
Laplace Equation: ΔP = γ(1/R₁ + 1/R₂)
Capillary Height: h = 2γcosθ/(ρgr)
Young's Equation: γ_sg = γ_sl + γ_lg·cosθ

What is Surface Tension?

Surface tension is a phenomenon where the surface of a liquid behaves like an elastic sheet. It occurs because molecules at the surface experience different forces than molecules in the bulk. Surface molecules are only pulled inward and sideways by neighboring molecules, creating a net inward force that minimizes the surface area.

Molecular Level Explanation

At the molecular level, surface tension arises from cohesive forces between liquid molecules. Molecules in the interior are attracted equally in all directions by their neighbors. However, molecules at the surface have no neighbors above them, resulting in a net inward force. This causes the liquid surface to contract to the minimum possible area, explaining why water droplets are spherical (a sphere has the minimum surface area for a given volume).

Factors Affecting Surface Tension

Surface tension decreases with increasing temperature because thermal energy disrupts the intermolecular forces. At the critical temperature, surface tension becomes zero. Different liquids have different surface tensions depending on the strength of intermolecular forces. Water has high surface tension (72.8 mN/m at 20°C) due to strong hydrogen bonding, while mercury has even higher surface tension (485 mN/m) due to metallic bonding.

Capillary Action

Capillary action is the ability of a liquid to flow in narrow spaces without external forces. When a glass capillary tube is placed in water, water rises in the tube due to surface tension. The height is given by h = 2γcosθ/(ρgr), where γ is surface tension, θ is contact angle, ρ is density, g is gravity, and r is tube radius. This phenomenon is important in plants (water transport from roots to leaves), paper towels, inkjet printing, and many biological systems.

Contact Angle and Wetting

The contact angle is the angle at which a liquid-vapor interface meets a solid surface. It depends on the relative strengths of cohesive forces (within the liquid) and adhesive forces (between liquid and solid). Young's equation relates these forces: γ_sg = γ_sl + γ_lg·cosθ. Small contact angles (< 90°) indicate good wetting (hydrophilic surfaces for water), while large contact angles (> 90°) indicate poor wetting (hydrophobic surfaces). Lotus leaves are superhydrophobic with contact angles > 150°, causing water to bead up and roll off.

Applications and Examples

Surface tension has countless applications in nature and technology: soap bubbles and films use surface tension to minimize surface area; insects like water striders can walk on water due to surface tension; raindrops are nearly spherical; surface tension drives the formation of emulsions and foams; it's crucial in painting, coating, printing, and oil recovery. Understanding surface tension helps in designing detergents, cosmetics, pharmaceuticals, and many industrial processes.