Making a dome out of ... water!

What is surface tension – and why don’t water meters sink?

Safe­ty pre­cau­tions



  • coin;
  • sy­ringe or pipette;
  • glass;
  • mar­bles.

Step-by-step in­struc­tions

Care­ful­ly fill a glass to the brim with wa­ter. Ob­vi­ous­ly, the wa­ter won't spill out just yet; it can’t get over the rim of the glass. What do you think – how many mar­bles can you add with­out spilling any wa­ter? When you add mar­bles one by one, you’ll no­tice that a dome of wa­ter grad­u­al­ly forms above the glass as each mar­ble dis­places a cer­tain amount of wa­ter. It’s in­cred­i­ble how many mar­bles you can add be­fore the wa­ter spills over the edge! To bet­ter un­der­stand this process, use a sy­ringe or pipette to grad­u­al­ly place drops of wa­ter on the sur­face of a coin. A “cap” of wa­ter will grow on the coin! The ex­pla­na­tions for both of these ef­fects are ground­ed in sur­face ten­sion.

Process de­scrip­tion

In liq­uids, mol­e­cules are lo­cat­ed rel­a­tive­ly close to each oth­er. Each mol­e­cule in a liq­uid is at­tract­ed to its neigh­bors on all sides. The mol­e­cules on the sur­face, how­ev­er, don’t have any neigh­bors above them. The at­trac­tive forces act­ing on them are all di­rect­ed in­ward, and as a re­sult, the sur­face of the liq­uid as a whole tends to con­tract. This ef­fect leads to the force of so-called sur­face ten­sion, which acts along the sur­face of the liq­uid and leads to the for­ma­tion of an in­vis­i­ble, thin, elas­tic “film”. Con­se­quent­ly, it takes me­chan­i­cal work to in­crease a liq­uid’s sur­face area, as these sur­face ten­sion forces must be over­come. If the liq­uid is not sub­ject to oth­er forces, it will strive to min­i­mize its sur­face area, adopt­ing a spher­i­cal shape. This is why dew­drops col­lect in al­most-spher­i­cal droplets on the grass. More­over, wa­ter strid­ers skill­ful­ly use sur­face ten­sion to walk along bod­ies of wa­ter. They don’t sink be­cause they weigh less than the forces of sur­face ten­sion they rely on.