Surface Tension

The Skin of a Liquid: Understanding Surface Tension

Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible. It is a physical property that makes the surface layer of a liquid behave like a thin, stretched elastic sheet. This phenomenon is caused by cohesion—the attraction between similar molecules. Within the bulk of the liquid, each molecule is pulled equally in every direction by neighboring molecules, resulting in a net force of zero. At the surface, however, the molecules are pulled inwards by the molecules below them but have no corresponding upward pull from above. This creates an inward force at the surface that causes the molecules to pack together more tightly, minimizing their exposed surface area.

Surface tension is responsible for many familiar phenomena. It's what allows small insects, like water striders, to walk on the surface of a pond. It's what pulls water droplets into a nearly perfect spherical shape, as a sphere has the smallest surface area for a given volume. It's also what causes the capillary action that allows water to be drawn up into the narrow tubes of a plant's stem. The concept is crucial in chemistry, biology, and material science, influencing everything from the way detergents work to the design of inkjet printers and medical devices. The SI unit for surface tension is Newtons per meter (N/m), which can also be expressed as energy per unit area (Joules per square meter).

Relevant Formulas and Concepts

• Definition of Surface Tension (Physics): Surface tension (γ, gamma) is defined as the force (F) per unit length (L) along which the force acts: γ = F / L.
• Laplace Pressure (Physics): Surface tension is responsible for the pressure difference between the inside and outside of a droplet or bubble. For a spherical droplet, this pressure difference is ΔP = 2γ / r, where 'r' is the radius. For a bubble (which has two surfaces), it is ΔP = 4γ / r.
• Capillary Action (Physics): The height (h) to which a liquid rises in a capillary tube is given by h = (2γcosθ) / (ρgr), where 'γ' is the surface tension, 'θ' is the contact angle, 'ρ' is the liquid density, 'g' is the gravity, and 'r' is the tube radius.

A Deep Dive into Common Surface Tension Units

• Newtons per meter (N/m): The standard SI unit for surface tension. It represents the force in Newtons required to break a one-meter length of the liquid's surface.
• Dynes per centimeter (dyne/cm): The CGS (centimeter-gram-second) unit for surface tension. It is a very common unit in older chemistry and physics literature. 1 dyne/cm is equal to 0.001 N/m.
• Millinewtons per meter (mN/m): Because the N/m is a fairly large unit for common liquids, the mN/m is frequently used for convenience. 1 mN/m is equal to 1 dyne/cm. The surface tension of water at room temperature is about 72 mN/m.