Surface tension is a cohesion effect. It comes from a liquid’s molecules pulling on one another, not from the liquid sticking to anything else.1Lumen Learning. Cohesion and Adhesion in Liquids Adhesion still matters, because it decides how a liquid behaves where it touches a glass, a leaf, or a straw. Most of the familiar “surface tension” tricks in a glass of water are really a contest between the two forces.
Cohesion and Adhesion in Plain Words
Cohesion is the attraction between molecules of the same kind, such as water molecules holding onto other water molecules.2Circuit Globe. Difference Between Adhesion and Cohesion Adhesion is the attraction between molecules of different kinds, such as water molecules and the glass of a cup.3Circuit Globe. Difference Between Adhesion and Cohesion
In water, cohesion comes mainly from hydrogen bonds, a strong attraction between neighboring water molecules.4AU Biomedical Engineering. Surface Energy, Contact Angle and Young`s Equation Experiment Handout Those bonds are unusually strong, so water holds together more tightly than most everyday liquids.
A small memory trick helps. “Co-” means together, as in co-worker, so cohesion is a substance sticking together with itself. “Ad-” means to or toward, as in add, so adhesion is sticking to something else. Adhesive tape is the everyday reminder: it sticks to other things. A raindrop holding its round shape shows cohesion, and the same drop clinging to a window shows adhesion.
Neither force is stronger in general. Adhesion belongs to a pair, so water can stick strongly to glass, weakly to wax, and hardly at all to a greasy surface. That is why the same water can spread out on one surface and bead up on another without changing at all.
Why Surface Tension Comes From Cohesion
A molecule deep inside a liquid is pulled equally in every direction by its neighbors, so the pulls cancel out.5Wikipedia. Surface tension A molecule at the surface has liquid neighbors below and beside it but almost none above.6Wikipedia. Surface tension Cohesion therefore pulls surface molecules inward, toward the rest of the liquid.7Lumen Learning. Cohesion and Adhesion in Liquids The surface contracts to the smallest area it can, and that tightening is called surface tension.8Lumen Learning. Cohesion and Adhesion in Liquids The inward pull also creates a slight pressure inside the liquid.9Wikipedia. Surface tension
The “skin” is a way of describing how the surface resists being stretched or pushed through. Press gently on water with a fingertip and the surface dents before it breaks, which is the same inward pull being stretched. Push hard or fast enough and the surface gives way, because the pull is real but modest.
Surface tension can also be described as an energy cost. Creating new surface means moving molecules from the comfortable interior to the edge, and that takes extra energy.10University of Cambridge. Surface Energy So a liquid tends to settle into shapes with less surface. A sphere holds the most liquid for the least surface area, which is why small free-floating drops are round. Bigger drops flatten and wobble because their weight starts to matter more than the surface pull.
Surface tension shows up on its own in several everyday ways. A paper clip can rest on still water if you lay it down gently, because the surface resists being stretched. Water can bulge slightly above the rim of a full glass before spilling. Drops of water in space float as near-perfect balls because nothing but cohesion shapes them.
Where Adhesion Comes In The Meniscus
The curved surface of a liquid in a glass or tube is called the meniscus, a word you may see in measuring instructions.11Wikipedia. Meniscus (liquid) It is a visible tug-of-war between adhesion and cohesion. When the liquid’s pull toward the wall is more than half as strong as its pull on itself, the liquid climbs the wall and the surface curves down in the middle, a concave meniscus.12Wikipedia. Meniscus (liquid) When the pull toward the wall is less than half the liquid’s pull on itself, the liquid pulls away from the wall and the surface bulges up, a convex meniscus.13Wikipedia. Meniscus (liquid) The same balance decides how well a liquid spreads on a surface, known as wetting.14Wikipedia. Surface tension
Water in glass forms a concave meniscus because water’s attraction to glass is strong.15Vaia. Problem 95 The meniscus of water in a glass tube is concave, but that of mercury is convex. Explain why Look at water in a clear glass from the side and you will see the edge climb slightly up the wall while the middle sits lower. The effect is only a thin rim, because it fades quickly toward the center where the water feels no wall.
Both forces are needed. The water touching the glass clings to it, and other water molecules cling to those, so cohesion helps drag the liquid up the wall.16USGS. Water Meniscus Without strong cohesion, only a thin film would touch the glass. The climb stops when gravity outweighs what water’s cohesion can hold.17USGS. Water Meniscus
Mercury in glass does the opposite. It forms a convex meniscus because mercury’s pull on itself is far stronger than its pull toward glass.18Vaia. Problem 95 The meniscus of water in a glass tube is concave, but that of mercury is convex. Explain why
This matters when you read a measuring cylinder. For water, read the volume at the bottom of the curve, with your eye level with it. For mercury or similar liquids, read at the top of the bulge. Reading from above or below the line throws the number off.
Capillary Action Uses Both Forces
Capillary action is the rise or fall of a liquid in a narrow tube, driven by the balance between cohesion and adhesion.19Introductory Physics for the Health and Life Sciences. Fluid Statics Water rises in a glass tube because its attraction to the walls is strong enough, compared with its pull on itself, to draw it up the sides.20USGS. Capillary Action and Water The height it reaches is limited by surface tension and by gravity.21USGS. Capillary Action and Water The rise also depends on the liquid’s weight and the width of the tube.22arXiv. Capillary phenomena: New fundamental formula
In a thinner tube, a larger share of the water is touching or near the wall, so adhesion has more to work with relative to the weight being lifted. A wide tube holds much more water for the same wall contact, so gravity wins at a lower height. Dip a very thin tube and a wider one into the same water and the thin one shows the higher rise.
You can see this in daily life. A paper towel corner dipped in water soaks up liquid well above the surface, because the gaps between the paper fibers act like countless tiny tubes. Water wicking up a sugar cube or a cotton wick works the same way.
What Weakens Surface Tension
Soaps and detergents contain surfactants, soap-like molecules that crowd onto a liquid’s surface.23Brighton Science. What are Surfactants and How Do They Impact Surface Tension They push water molecules away from the surface, which weakens the cohesion there.24KRÃœSS Scientific. Surfactant Less energy is then needed to stretch the surface, so water spreads and wets more easily.25Brighton Science. What are Surfactants and How Do They Impact Surface Tension
You can watch it in the kitchen. Sprinkle pepper on a plate of water and touch the middle with a soapy finger. The pepper rushes outward as the surface tension near the soap collapses, showing a pull that can be weakened in one spot.
Heat weakens surface tension too. It generally drops as a liquid gets warmer.26Wikipedia. Surface tension So hot water spreads into fabric and grime more readily than cold water.
Surface Tension in Living Things
Water striders push on the water lightly enough not to break through the surface.27Journal of Bionic Engineering. Water Striders: The Biomechanics of Water Locomotion and Functional Morphology of the Hydrophobic Surface (Insecta: Hemiptera-Heteroptera) Their bodies and legs are covered in tiny water-repelling hairs.28Journal of Bionic Engineering. Water Striders: The Biomechanics of Water Locomotion and Functional Morphology of the Hydrophobic Surface (Insecta: Hemiptera-Heteroptera) Water does not stick to their legs, so cohesion keeps the surface intact beneath them.
Each leg presses a small dimple into the surface rather than piercing it, and the stretched surface pushes back up. The hairs trap tiny bits of air and repel water, so the leg stays dry. Spreading its weight over six legs keeps the push on each dimple small. If water sticks to the legs, the surface can no longer hold the dimple and the insect breaks through.
Trees use cohesion and adhesion to lift water. Water travels from roots to leaves through thin tubes in the wood, pulled up by cohesion and adhesion.29LibreTexts Biology. Transport of Water and Solutes in Plants As water evaporates from leaves, it pulls on the water column below.30Tree Physiology. Let’s not forget the critical role of surface tension in xylem water relations Because water molecules hold together so strongly, the column can be pulled under tension without breaking.31Tree Physiology. Let’s not forget the critical role of surface tension in xylem water relations Adhesion holds water to the walls of the tubes and keeps it from sliding back down, while cohesion links the molecules into an unbroken chain.
In our lungs, surface tension has to be kept low. The tiny air sacs are lined with a natural surfactant.32Canadian Respiratory Journal. Pulmonary Surfactant Function in Alveoli and Conducting Airways When you breathe out, it drops surface tension to near zero.33PubMed Central. The Biophysical Function of Pulmonary Surfactant The air sacs stay open instead of collapsing.34Canadian Respiratory Journal. Pulmonary Surfactant Function in Alveoli and Conducting Airways Premature babies may not make enough of this surfactant.35Canadian Respiratory Journal. Pulmonary Surfactant Function in Alveoli and Conducting Airways They can struggle to breathe and may need treatment.
When Surface Tension Takes Over
On Earth, surface tension is usually too small to matter in big engineering problems.36PubMed Central. Corner angle effects on capillary rise along curved interior corners The weight of a liquid pulls it flat and into the shape of its container, which overrides the small pull of the surface. You notice surface tension only where weight is small, like a drop or an insect’s leg. For tiny things, surface tension outweighs gravity. For large amounts of water, gravity wins.
In orbit, where liquids are effectively weightless, surface tension controls where liquid sits in tanks and pipes.37PubMed Central. Corner angle effects on capillary rise along curved interior corners With no “bottom” to settle in, a liquid can drift around or stick to walls. Engineers use internal structures that hold fuel in place by attraction and surface pull, so it can be drawn out. Astronauts see water float in wobbling spheres, shaped by cohesion alone, while spills cling to surfaces and spread along them by adhesion.