Van der Waals interactions are the faint pulls that every atom and molecule exerts on its neighbors, even ones with no electric charge at all. They exist because electrons are constantly shifting around, so no particle is ever perfectly neutral from moment to moment.1Wikipedia. London dispersion force A single pull is tiny. What makes them matter is their numbers, since millions of small pulls together can hold up a gecko.2PubMed Central. Evidence for van der Waals adhesion in gecko setae
Where Van der Waals Forces Fit Among Chemical Forces
Atoms inside a molecule are held together by chemical bonds, which share or transfer electrons and are hard to break. Separate molecules also attract one another, but through weaker intermolecular forces, meaning forces between molecules that never join them into one particle. Van der Waals forces are the weakest of these.3TutorChase. How do hydrogen bonds differ in strength from van der Waals forces? A hydrogen bond, the attraction that holds water molecules to each other, can be up to about 30 times stronger.4TutorChase. How do hydrogen bonds differ in strength from van der Waals forces? What van der Waals forces have that hydrogen bonds lack is reach: they act between all atoms and molecules, with no special chemistry needed.5TutorChase. How do hydrogen bonds differ in strength from van der Waals forces?
Boiling water pulls molecules apart from one another without breaking the bonds inside each water molecule. Because van der Waals forces are so weak, many substances held together mainly by them melt or boil at low temperatures.
Because every molecule feels them, van der Waals forces are the only attraction available between things like oil molecules or noble gas atoms. Where stronger forces exist, van der Waals forces work alongside them and add to the total.
The other forces sit at different rungs. Ionic attractions between oppositely charged ions, as in table salt, are much stronger, which is why salt melts only at very high temperatures. Hydrogen bonds need a special setup, a hydrogen attached to a very electron-hungry atom such as oxygen or nitrogen, so they appear only in certain molecules.
Why Neutral Molecules Attract Each Other
The electrons around an atom are never still, so their spread shifts from moment to moment.6Wikipedia. London dispersion force For an instant, one side can carry a little more negative charge than the other. That lopsided charge, slightly positive on one side and slightly negative on the other, is called a dipole. The brief imbalance creates an electric field that nearby atoms feel.7Wikipedia. London dispersion force Their electrons shift in response, and the two atoms’ electron motions fall into step.8Wikipedia. London dispersion force The matching lopsided charges then attract.9Wikipedia. London dispersion force The full explanation needs quantum physics, but the flickering-dipole picture captures what happens.10Wikipedia. London dispersion force
You might wonder why the flicker doesn’t average out to nothing. Over time, an isolated atom looks perfectly symmetrical, so it has no lasting dipole of its own. Once a neighbor is nearby, the shifts in the two atoms line up more often than not, so the average attraction is not zero.
Van der Waals forces are usually described as three related attractions.11Wikipedia. Van der Waals force The flickering-electron attraction works between any two atoms or molecules.12Wikipedia. Van der Waals force A second arises when a molecule with a permanent lopsided charge induces one in a neighbor.13Wikipedia. Van der Waals force A third acts between two molecules that both have permanent lopsided charges as they tumble and turn.14Wikipedia. Van der Waals force
In molecules with permanent lopsided charges, the other two add to the flickering-electron force. For most large molecules, the flickering-electron part is the biggest contributor.
Size matters too. Larger electron clouds shift more easily, so the flickering-electron attraction grows with size. That is why heavier noble gases liquefy more easily than helium. Long chain molecules also have more contact area along their length.
Why the Pull Only Works Up Close
In 1930 the physicist Fritz London showed that the attraction’s energy shrinks with the sixth power of distance.15Wikipedia. Lennard-Jones potential Doubling the gap cuts the attraction to about one sixty-fourth. So the force only matters when surfaces are almost touching.
Halving the gap does the reverse and makes the pull about 64 times stronger. This is why the force is called short-range.
Van der Waals interactions include both an attractive part and a repulsive part.16Protein folding: Process, structure, and significance. Protein folding: Process, structure, and significance When electron clouds start to overlap, they push each other away hard. So there is a sweet-spot distance where atoms settle, as described by a simple formula proposed in 1924 by John Lennard-Jones.17PubMed Central. A double exponential potential for van der Waals interaction That formula is still used to estimate these forces in computer models of proteins and drugs.18PubMed. Van der Waals Potential in Protein Complexes At the right gap, atoms settle, and this sets how tightly molecules can pack in liquids and solids.
It also explains why surfaces don’t stick together in daily life. Most are rough at tiny scales, so only a few points actually touch, and dust and water layers keep them too far apart. That is why special shapes, like those geckos have, are needed to make the force useful.
How Small Forces Add Up to Big Effects
Evaporating a liquid means breaking all the attractions holding each molecule to its neighbors.19Chem. Sci.. van der Waals interactions in non-polar liquids So how much heat a liquid needs to evaporate shows how strong those attractions are.20Chem. Sci.. van der Waals interactions in non-polar liquids A molecule with more surface touches more neighbors, so more energy is needed to pull it free.
For oily, uncharged liquids, the total attraction rises roughly in step with each molecule’s surface area.21Chem. Sci.. van der Waals interactions in non-polar liquids Longer, larger molecules therefore boil at higher temperatures. In large flat molecules, the pulls can add up to slightly more than the sum of the parts, about 10% more in one measured case.22PubMed Central. Non-additivity of molecule-surface van der Waals potentials from force measurements
Look at the kitchen and garage. Methane is a gas, gasoline a liquid, and candle wax a solid, all similar oily molecules of different lengths. The difference is mostly how much surface each molecule offers for van der Waals contact. Wax melts with gentle heat because only these weak forces need breaking.
How Geckos Use Van der Waals Forces to Climb
Gecko toes are covered in tiny hairs whose grip comes from van der Waals forces, not glue or suction.23PubMed Central. Evidence for van der Waals adhesion in gecko setae Their toes stuck equally well to water-repelling and water-attracting surfaces, which rules out water-based sticking.24PubMed Central. Evidence for van der Waals adhesion in gecko setae So the grip depends on the size and shape of the hair tips rather than the surface’s chemistry.25PubMed Central. Evidence for van der Waals adhesion in gecko setae Many people assume geckos use suction cups or sticky secretions, but suction would fail on porous walls, and geckos still cling.
The toe pad has ridges, which carry thousands of hairs, and each hair splits into hundreds of tiny flat tips. Each level lets the toe adjust to bumps of a different size, from the wall’s visible roughness down to its microscopic texture. More contact points mean more of the close-range pull, and the grip comes from the enormous number of tips touching at once.
The grip is also directional. The gecko sticks when it drags its toes in one direction and lets go when it changes the angle. Because the hairs are slanted, pulling along their slant increases contact, while peeling against it reduces contact. Peeling the toe from the edge lets the gecko release instantly, so it can attach and detach with each step without muscling against a sticky surface.
Engineers copy this design in dry adhesives that can handle delicate tiny machine parts.26ECS Transactions. (Invited) Dry Adhesives for MEMS Assembly, Manipulation and Integration: Progress and Challenges These leave no residue because nothing chemical is left behind, and they can be reused many times. They lose grip on dusty or very rough surfaces, where contact is poor.
Van der Waals Forces Inside Your Body
The flat bases in DNA lie stacked like coins, touching through van der Waals contacts.27PubMed Central. Forces maintaining the DNA double helix Hydrogen bonds pair bases across the helix, but stacking supplies about 60% of the stability of each base pair.28PubMed Central. Forces maintaining the DNA double helix Stacking also accounts for essentially all the heat needed to melt DNA apart.29PubMed Central. Forces maintaining the DNA double helix Melting DNA means heating it until the two strands separate, which is not the same as the solid-to-liquid melting of ice. Stacking contributes more than the hydrogen-bond rungs the school diagram emphasizes, though both are needed.
The bases are rings of atoms with large electron clouds, so each pair touches its neighbors over a broad area. Many close contacts along the helix add up to strong total stability even though each contact is faint. The bases are also somewhat water-avoiding, so they cluster in the helix’s core, where stacking keeps them in close contact.
Van der Waals forces also help pack a protein’s building blocks tightly and shape its coils and sheets.30Protein folding: Process, structure, and significance. Protein folding: Process, structure, and significance They help drugs fit snugly into the pockets of their target proteins.31PubMed. Van der Waals Potential in Protein Complexes So drug designers screen candidates partly by how well their shape fits a pocket.32PubMed Central. Computer-Aided Drug Design Methods
A good drug fills its pocket with no gaps, and every close contact adds a little attraction. A poor fit leaves gaps, and because the pull falls off so steeply with distance, there is almost no attraction across them.
Van der Waals Forces in Materials and Nature
Materials such as graphite are made of sheets held to each other only by van der Waals forces. So the sheets can be peeled apart and restacked in new combinations to make new materials.33Materials Today. Heterostructures based on two-dimensional layered materials and their potential applications
Within each sheet, carbon atoms are held by strong bonds in a honeycomb pattern. Peeling can leave a sheet only one atom thick, called graphene. The same weakness explains pencils: graphite is stacked carbon sheets, and weak forces between them let layers slide off onto paper.
Whether tiny particles in a liquid stay spread out depends on van der Waals attraction balanced against electric repulsion.34PubMed. DLVO surface forces in liquid films and statistical mechanics of colloidal oscillatory structural forces in dispersion stability When attraction wins, particles clump and settle.35Interface Science and Technology. DLVO theory of colloid stability In rivers and seas, sticky fine grains attach to larger ones and, in simulations, settled up to 29% faster.36Cambridge University Press. Settling of cohesive sediment: particle-resolved simulations
These forces matter for computers, too. Some standard chemistry software methods underestimate them, so programs add a correction term, which improves predictions for materials and drugs.