Mechanical Properties of Matter

Surface Tension (γ) The force per unit length acting along the surface of a liquid, perpendicular to any line drawn in the surface, tending to minimize the surface area. Molecules in the bulk liquid experience cohesive forces equally in all directions. Surface molecules, however, have fewer neighbours above them, resulting in a net inward pull. This creates a "skin-like" behaviour, allowing insects like water striders to walk on water and causing small droplets to form spheres (minimum area for a given volume). Contact Angle (θ) The angle measured inside the liquid between the solid surface and the tangent to the liquid surface at the point of contact. If the liquid wets the solid (e.g., water on clean glass), θ < 90°; if it does not wet (e.g., mercury on glass), θ > 90°. The contact angle depends on the balance between adhesive forces (liquid-solid) and cohesive forces (liquid-liquid). A perfectly wetting liquid has θ = 0°, while a completely non-wetting liquid has θ = 180°.