11.8 Cohesion and Adhesion in Liquids: Surface Tension and Capillary Action

Liquids exhibit interesting behaviors due to cohesive and adhesive forces. These molecular attractions create surface tension, which is why water striders can walk on ponds and raindrops hold their shape. They also drive capillary action, the process that helps plants transport water and makes paper towels absorbent.

Cohesion and Adhesion in Liquids

Cohesive vs adhesive forces

Cohesive forces are attractions between molecules of the same substance. They're what hold a liquid together. Water molecules in a droplet, for example, pull on each other and keep the drop from flying apart. Liquids with stronger cohesive forces have higher surface tension and tend to bead up on surfaces rather than spread out.

Adhesive forces are attractions between molecules of different substances. These occur at the boundary between a liquid and whatever it's touching. Water clinging to the inside of a glass is a classic example of adhesion at work. When adhesive forces are strong enough, the liquid "wets" the surface and spreads across it.

The balance between cohesion and adhesion determines a liquid's behavior on any given surface:

• If cohesion dominates, the liquid beads up (water on a waxed car).
• If adhesion dominates, the liquid spreads out and wets the surface (water on clean glass).

Both cohesive and adhesive forces arise from intermolecular forces like hydrogen bonding and van der Waals attractions.

Surface tension in liquids

Surface tension is a direct result of cohesive forces. Molecules deep inside a liquid are pulled equally in all directions by their neighbors, so the net force on them is zero. Molecules at the surface, though, have no liquid neighbors above them. They experience a net inward pull, which causes the surface to contract and behave like a stretched elastic membrane.

This "membrane" effect has several consequences:

• Liquids naturally minimize their surface area, which is why free-falling drops are spherical.
• Small, dense objects like a paperclip can float on water if placed gently, even though steel is denser than water. The surface tension supports the weight.
• Soap bubbles form because the thin film of soapy water has surface tension on both its inner and outer surfaces.

A few factors affect surface tension:

• Temperature: Higher temperatures weaken intermolecular forces, reducing surface tension. Hot water spreads more easily than cold water.
• Surfactants: Substances like dish soap lower surface tension by disrupting cohesive forces at the surface. That's why adding soap to water breaks the "membrane" and causes a floating paperclip to sink.
• Interfacial tension is a related concept that describes the tension at the boundary between two immiscible liquids (like oil and water) rather than between a liquid and air.

Capillary action and applications

Capillary action is the ability of a liquid to flow through narrow spaces without (and even against) external forces like gravity. Dip a thin glass tube into water, and the water rises inside the tube on its own. This happens because the adhesive forces between the water and the glass are stronger than the cohesive forces within the water itself, so the liquid is pulled upward along the walls.

The narrower the tube, the higher the liquid rises. The exact height is given by Jurin's Law:

$h = \frac{2\gamma \cos\theta}{\rho g r}$

where:

• $\gamma$ = surface tension of the liquid
• $\theta$ = contact angle between the liquid and the tube wall
• $\rho$ = density of the liquid
• $g$ = acceleration due to gravity
• $r$ = radius of the tube

Notice that $h$ is inversely proportional to $r$. A tube with half the radius will produce twice the capillary rise. Also, the contact angle $\theta$ matters: if $\theta < 90°$, the liquid wets the surface and rises (like water in glass). If $\theta > 90°$, the liquid is actually pushed down (like mercury in glass), because cohesion dominates over adhesion.

Capillary action shows up in many real-world situations:

• Plants: Water travels from roots to leaves through narrow xylem vessels, partly driven by capillary action.
• Absorption: Paper towels and soil soak up water because of the tiny spaces between their fibers or particles.
• Ink delivery: Fountain pens and inkjet printers rely on capillary action to move ink through narrow channels.
• Microfluidics: Lab-on-a-chip devices use capillary action to move tiny volumes of blood or other fluids through channels for medical diagnostics.

Fluid properties and behavior

Viscosity measures a fluid's resistance to flow. Honey has high viscosity; water has low viscosity. Viscosity affects how quickly capillary action occurs: a liquid might still rise to the same height in a thin tube, but a more viscous liquid will get there more slowly. Both viscosity and surface tension are temperature-dependent, which is why warm liquids generally flow and spread more easily than cold ones.