Why does a falling raindrop flatten and burst?
Air drag flattens the bottom of a falling drop faster than surface tension can restore a sphere, so the drop becomes a disc with a thick rim — and when that rim gets too thin to balance the pressure difference, surface tension fails and the drop bursts.
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Why does a falling raindrop first flatten into a hamburger-bun shape and then break apart?
A high-speed photo shows a raindrop that looks like a tiny hamburger bun, mid-air, just before it shatters — a shape nobody expects from 'water'.
We learn in school that surface tension pulls drops into spheres, so why does a falling drop look like a flat disc with a rim instead of a stretched teardrop?
Compare side-by-side high-speed frames of drops at different fall stages, then use a force-balance diagram showing air drag squeezing the bottom while surface tension pulls the rim outward.
Air pressure on the falling drop's underside flattens it into a hamburger shape, and once that rim grows thick enough, surface tension can no longer hold it — so the drop breaks into a bag-and-doughnut splash.
A common guess is that gravity alone stretches the drop into a teardrop and it simply snaps at the thin end.
- Detailed numerical models of drop oscillation frequencies
- The full Bag-and-Plate breakup regime across many Weber numbers
- Rain formation inside clouds and coalescence of tiny droplets
- Effects of surfactants and charged droplets
- 01A raindrop that looks like a hamburger bunslideQuestion
Open with a high-speed photograph of a falling water drop caught mid-fall, shaped like a flat bun with a thick rim. Pose the driving question: why does it flatten this way, and why does it break right after?
- Surface tension usually makes small drops spherical
- Yet falling drops are photographed as flattened discs
- Two questions: why flat, and why does it burst?
- 02What is squeezing the falling drop?quizPrediction
Ask the learner to commit to one cause for the hamburger shape before any evidence is shown.
- One focused prediction, not a survey
- Commit before seeing the explanation
- 03High-speed frames: sphere, then disc, then burstslideEvidence
Show a sequence of high-speed frames: a spherical drop just released, a slightly oblate drop, a clear hamburger-bun shape with a thick rim, and finally a torn bag with small satellite droplets. Highlight how the underside is flat while the top stays rounded.
- Drop leaves the nozzle nearly spherical
- Bottom becomes visibly flat during fall
- Rim thickens, then a bag tears off the rim
- 04Pressure on a falling dropinteractiveEvidence
Let the learner see how pressure changes around a drop by toggling between 'sphere' and 'flattened disc' shapes, with arrows showing stronger pressure under the flat underside than on the rounded top.
- Flat underside: air pressure stacks up and pushes up
- Rounded top: pressure slides around the curve
- Net force squeezes the drop flat
- 05Air drag, not gravity, does the flatteningslideExplanation
Explain that gravity accelerates the drop until air drag roughly balances it, then the underside hits still air and feels strong pressure while the curved top deflects air around it. Surface tension tries to pull the shape back to a sphere but the pressure difference wins, producing a disc with a thick rim.
- Air piles up under the flat bottom → upward push
- Surface tension pulls the rim outward
- Pressure beats tension → stable hamburger shape
- 06When the rim gets too thin, surface tension losesslideBoundary
Explain that the rim is the only thing holding the drop together against the pressure difference. As the disc widens, the rim thins, and at a critical point the curvature pressure inside the rim can no longer balance the outside pressure — so the rim pinches off and a thin bag of water flies ahead.
- Thicker rim = stronger tension = stable
- Thinner rim = curvature pressure rises but material runs out
- Past the critical size, surface tension cannot hold the drop
- 07Try a different liquid or a different speedinteractiveTransfer
Let the learner test the idea by changing fall speed and liquid type: a slower fall or a more viscous, more surface-tension-rich liquid should hold a stable shape longer, while a faster fall or lower-tension liquid should burst sooner.
- Faster fall → stronger pressure → bursts earlier
- Higher surface tension → rim holds longer
- Same mechanism, different outcome
- 08Answer: hamburger shape, then breakupslideResolution
Tie it together: air pressure on the flattened underside squeezes the drop into a hamburger-bun disc, surface tension builds a thick rim to fight back, and when the rim thins past the point where curvature pressure can match the outside push, the drop tears into a bag-and-doughnut splash.
- Hamburger shape = pressure flattened a sphere
- Thick rim = surface tension's last defense
- Burst = rim too thin to hold the pressure
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