Straw Atomiser Experiment: Build a Spray Bottle

Two straws, a glass of coloured water, and one hard breath. If you get the geometry right, a cloud of fine spray shoots across the room on your first attempt. If you get it wrong, nothing at all happens, which is why most people try once and give up. The difference is about two millimetres of gap. What makes a straw atomiser experiment worth more than a trick is that you can measure the thing doing the work: how far the water will climb tells you how much the pressure at the straw’s mouth dropped, and that number turns out to match the energy in your breath.

Key Takeaways

• You’ll build a working atomiser from two straws, tune the gap until it sprays, then measure how high it can lift water.
• It teaches jet entrainment, that atmospheric pressure does the lifting, and how a liquid column breaks into droplets.
• Best fit is ages 7 to 13. Younger children can blow but cannot usually manage the fine positioning.
• The usual failure is a gap that is too wide or a vertical straw standing too far above the water. Both are fixable in seconds.
• Water and food colouring only. No alcohol, no essential oils, and nobody inhales through the horizontal straw.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Drinking straws, plain, 6 mm	4	Kitchen	Paper straws work but soften; plastic or metal hold shape better
Scissors	1	Drawer	Adult cuts for younger children
Narrow glass or jar, 5–6 cm across	1	Kitchen	A tall shot glass is ideal; wide glasses make positioning harder
Water	100 ml	Tap	Nothing else needed
Food colouring	2 drops	Baking cupboard	Makes the spray visible on paper; optional
White paper or card	2 sheets	Printer	A tea towel you don’t mind marking
Ruler	1	School supplies	A strip of paper, marked
Modelling clay or sticky tack	A blob	Craft box	Tape, to hold the vertical straw still
Tape measure	1	Drawer	For spray range

Time: prep 10 min, experiment 25 min, cleanup 5 min Cost: everything is probably in your kitchen Mess level: medium, so work over a sink or outside

Do the Experiment

Step 1 — Cut the vertical straw and stand it in the water

Put 100 ml of coloured water in the glass. Cut one straw so that when it stands on the bottom of the glass, only about 1 cm of it pokes above the water surface. Hold it upright with a blob of clay on the rim.

What you should see: a straw standing vertically, almost entirely submerged, with just a short stub in the air. Short is the single most important adjustment in this whole experiment, and most instructions get it wrong by leaving far too much straw above the water.

Step 2 — Position the second straw at right angles, almost touching

Hold the second straw horizontally so its end is level with the very top of the vertical straw, pointing across the opening, with a gap of about 1 to 2 mm between the two.

What you should see: the two straws forming a T with a tiny gap at the junction. Get a sheet of white paper standing up about 30 cm away to catch the spray.

Step 3 — Blow hard and steadily, and tune the gap

Take a full breath and blow as hard as you can through the horizontal straw, for two to three seconds. If nothing happens, keep blowing and slowly move the horizontal straw a millimetre at a time, nearer, further, slightly higher, slightly lower.

What you should see: the moment it catches, water climbs the vertical straw and bursts into a visible fan of fine droplets that hits the paper. There is a clear sweet spot, and you will find it within a few attempts once you accept that it needs tuning rather than force.

Step 4 — Measure the maximum lift height

Now raise the vertical straw out of the water, a millimetre at a time, blowing each time, until it no longer sprays. Measure how far the top of the vertical straw was above the water surface at the last successful attempt.

What you should see: somewhere around 15 to 30 mm for a child’s breath, perhaps a little more for an adult. That height is the number that matters. Multiply it by 9.81 and by the density of water in kilograms per cubic metre and you have the pressure drop in pascals: 20 mm gives about 196 pascals.

Step 5 — Change the jet and see the droplets change

Flatten the last centimetre of the horizontal straw between finger and thumb to make a thinner, faster jet, and try again. Then try blowing gently. Catch each spray on separate sheets of paper held the same distance away.

What you should see: the flattened straw throws a finer, further-reaching mist. A gentle breath, if it sprays at all, produces big fat droplets that fall close by. Compare the dot patterns on the two sheets side by side; you have measured, crudely, that faster air makes smaller droplets.

What’s Actually Going On

Here is the part that most explanations get wrong, so it is worth being precise. The usual line is that fast air above the straw has low pressure, and Bernoulli’s principle sucks the water up. Bernoulli’s relation is correct physics, but it relates pressure and speed along a streamline in a single steady flow, and a free jet of breath blowing into a still room is not a flow you can trace a streamline through from the jet into the quiet air beside it. NASA Glenn’s discussion of Bernoulli and Newton exists in part because this misapplication is so common, and the aerodynamicist Charles Eastlake made the same point in The Physics Teacher in 2002.

The honest mechanism is entrainment. A fast jet of air drags the still air around it along by viscous friction at its edges, which is why a jet widens and slows as it travels. Your vertical straw’s mouth sits right in that shear layer, so the jet continually carries air away from it. Air is removed faster than it can be replaced, so the pressure at the straw’s mouth falls below atmospheric. Meanwhile, the full weight of the atmosphere, about 101 kilopascals, is still pressing on the surface of the water in the glass. The water is pushed up the tube. Nothing pulls it.

Step 4 measures exactly that pressure difference. A column of water h metres tall exerts ρgh at its base, so lifting water 20 millimetres requires a deficit of 1,000 × 9.81 × 0.020, which is 196 pascals. Now check it against your breath. Air moving at 20 metres per second has a dynamic pressure of ½ρv², which is 0.5 × 1.2 × 400, or 240 pascals. Those two numbers agree to within the precision of a kitchen, which tells you the pressure deficit at the straw’s mouth is of the same order as the energy density in your breath. That agreement is a genuine result, and it is the reason this article asks you to measure the lift.

The spray itself is a separate piece of physics. Once water reaches the top of the tube, the high-speed air shears across its surface and tears off thin ligaments of liquid, which are unstable: surface tension makes any long thin column of liquid break up into a line of drops, because that reduces its surface area. Lord Rayleigh worked out the mathematics of this in 1878 in the Proceedings of the London Mathematical Society, and the process still bears his name. The faster the air, the thinner the ligaments and the smaller the drops, which is what Step 5 shows. Engineers call this airblast atomisation and study it in detail for fuel injectors and spray dryers; Rizk and Lefebvre’s drop-size work from 1981 is a standard reference. A lung-powered atomiser makes coarse drops, tens to hundreds of micrometres; industrial airblast nozzles running at far higher air speeds reach a few micrometres.

One more honest note. This is the same device as an old-fashioned perfume atomiser and as a paint spray gun, and it is also, uncomfortably, a reasonable model for how a sneeze produces aerosol. The mechanism does not care what liquid is in the glass, which is exactly why this one should only ever contain water.

How to Explain It to Your Kid

Ages 5–8: The fast air steals the air out of the straw

Air is a real thing, and when you blow a fast stream of it across the top of the straw it drags away the air that was sitting in the straw’s mouth. That leaves less air pushing down inside the straw than outside, and the air pressing on the water in the glass pushes the water up to fill the gap. Then the fast air smashes the water into tiny bits.

Let them feel the entrainment directly: blow across the back of their own hand held a few centimetres to the side of the jet, and they will feel air moving even though the breath is not pointed at them.

Ages 9–12: Atmospheric pressure does the lifting

Make the correction explicitly, because they will hear the wrong version elsewhere. Nothing sucks. The jet lowers the pressure at the top of the straw, and the atmosphere, which is pressing on the water surface with about 101 kilopascals, pushes the water up. Then have them do the lift measurement and the ρgh calculation; 196 pascals out of 101,000 is a change of 0.2 per cent, which is a nice surprise. Our piece on floating a ping-pong ball on air corrects the same misconception in a different setting.

Ages 13+: Compare the two pressures and then vary one thing

Give them ½ρv² and ρgh, and set the task of checking whether they match. Estimating breath speed is the interesting part: time a light paper streamer being blown a measured distance, or use the spray range itself as a proxy. Then run a proper investigation: fix everything except the gap between the straws and measure the maximum lift height at gaps of 0.5, 1, 2, 3 and 5 mm. Plot lift against gap. There will be an optimum, because too small a gap blocks the jet and too large a gap puts the straw’s mouth outside the shear layer. Finding an optimum rather than a trend is what real engineering measurement usually looks like. Pair it with how a siphon really works, which is the other great “nothing sucks” demonstration.

The question to ask: “If we used a heavier liquid, like syrup, would it climb higher or lower, and would the spray be finer or coarser?”

Why Teaching This Early Pays Off

The idea that suction is a push from somewhere else is one of the highest-value corrections available in school physics. It makes straws, syringes, vacuum cleaners, siphons, pipettes, pumps, breathing and weather all follow from one principle instead of being separate pieces of folk knowledge. A child who has measured a 200 pascal deficit and compared it with 101,000 pascals of atmosphere has a quantitative grip on that idea.

Atomisation is the other quiet payoff. Spray behaviour decides how inhalers deliver medicine, how crops are sprayed, how fuel burns in an engine, how a fire sprinkler works, and how far an aerosol travels through a room. Having seen that faster air makes finer droplets, and having understood that surface tension is what breaks a stream into drops, a teenager has a real handle on all of it.

Keep the claim about learning narrow and honest. One session does not change a trajectory. What is well supported is that explaining and arguing beat watching: Chi and Wylie’s ICAP framework ranks constructive and interactive engagement above merely doing an activity, which means the ten minutes spent arguing about whether the water is pushed or pulled is the part carrying the learning. The National Research Council’s framework lists engaging in argument from evidence among eight core practices, and comparing the Bernoulli story with the entrainment story against your own lift measurement is exactly that.

Before You Start: What Parents Should Know

Best age	7 to 13. Younger kids can blow while an adult holds the straws, which works well and is genuinely fun.
Supervision	Light. Adult cuts the straws for under-eights and supervises where the spray is pointed.
Real hazards	None beyond normal care. Never inhale through the horizontal straw, which would draw water into the mouth or airway. Don’t spray at faces or eyes. Hard repeated blowing can make a child dizzy, so take breaks and stop if they feel lightheaded. Straw offcuts are a choking hazard for under-threes. Water on a hard floor is slippery.
Don’t	Don’t put alcohol, perfume, essential oils, cleaning fluid or anything other than water and food colouring in the glass. Don’t share a blowing straw between children; cut one each. Don’t substitute a compressed-air can for the breath, which produces a cold, high-pressure jet and a much more hazardous spray.

When It Doesn’t Work

• Nothing happens at all, no matter how hard you blow. Nine times out of ten the vertical straw sticks too far above the water. Cut it or push it down until only 5 to 10 mm is above the surface, and try again before changing anything else.
• Water climbs partway and stops. You are close. Nudge the horizontal straw 1 mm nearer and slightly lower, and blow for longer. The column has to reach the top before atomisation can start, and that takes a second of sustained flow.
• It sprays once and then stops. You ran out of breath, or the water level dropped below the straw’s bottom. Refill, and blow for a full three seconds rather than puffing.
• You get big droplets, not a mist. Your jet is too slow or too round. Flatten the last centimetre of the blowing straw to a thin slot, and blow harder. Soft paper straws collapse and ruin the jet; switch to plastic or metal.
• The vertical straw keeps moving. Fix it to the rim with modelling clay. This experiment is about millimetres, and a hand-held straw drifts far more than that.

Take It Further With the Straw Atomiser Experiment

Gap against lift height. Fix the breath as consistently as you can, and measure the maximum lift at gaps of 0.5, 1, 2, 3 and 5 millimetres, three trials each. Plot lift against gap and identify the optimum. This is a tuning curve, and real nozzle design consists of exactly this kind of measurement.

Droplet size by paper evidence. Spray onto sheets of paper held at 20, 40 and 60 cm, and compare the size of the stains at each distance with a magnifier or a phone macro lens. Smaller droplets slow down faster in air and fall out of the stream sooner, so the distant sheets should catch a different size distribution than the near ones. That is the same reasoning used to describe how aerosols travel.

What to Watch For Over the Next 3 Months

• Week 4: they take apart a trigger spray bottle to see the nozzle, or ask how a hairspray can makes a mist.
• Month 2: the word pressure or the phrase “the air pushes it” appears when they explain a straw, a plunger or a vacuum cleaner.
• Month 3: they correct a “suction pulls it” explanation, which is the single most valuable thing this activity can produce.

Frequently Asked Questions

Why do most instructions fail for us?

Because they say to stand the straw in a glass of water without saying how far above the surface the top should be. That one dimension decides everything. Five to ten millimetres of straw above the water works; three centimetres almost never does.

Can we use this to spray something other than water?

No. Keep it to water with a drop of food colouring. The same device would happily atomise alcohol, solvent or essential oil into a breathable mist, and none of those belong in a child’s spray. This is one of those cases where the mechanism is indifferent and the user has to not be.

Is this how a perfume atomiser works?

Yes, almost exactly. The squeeze-bulb atomisers used before pump sprays blew a jet of air across the top of a tube standing in the liquid, which is your straw rig with a rubber bulb instead of lungs. Modern trigger sprays use a pump and a swirl nozzle instead, which is a different and more controllable design.

Does the water get sucked up?

No, and this is the whole point. Nothing sucks. The jet lowers the pressure at the mouth of the straw, and the atmosphere, pressing down on the water surface with about 101 kilopascals, pushes the water up into the low-pressure region. Your lift measurement in Step 4 is a direct measurement of how much lower that pressure got.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. NASA Glenn Research Center. “Bernoulli and Newton.” Beginner’s Guide to Aeronautics. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/bernoulli-and-newton/
2. Rayleigh, Lord. (1878). “On the instability of jets.” Proceedings of the London Mathematical Society, s1-10(1), 4–13. doi:10.1112/plms/s1-10.1.4
3. Rizk, N., & Lefebvre, A. (1981). “Airblast atomization: Studies on drop-size distribution.” 19th AIAA Aerospace Sciences Meeting. doi:10.2514/6.1981-263
4. Eastlake, C. N. (2002). “An aerodynamicist’s view of lift, Bernoulli, and Newton.” The Physics Teacher, 40(3), 166–173. doi:10.1119/1.1466553
5. Georgia State University. “Fluid Concepts.” HyperPhysics. http://hyperphysics.phy-astr.gsu.edu/hbase/fluid.html
6. U.S. Geological Survey. “Surface Tension and Water.” Water Science School. https://www.usgs.gov/water-science-school/science/surface-tension-and-water
7. Chi, M. T. H., & Wylie, R. (2014). “The ICAP framework: Linking cognitive engagement to active learning outcomes.” Educational Psychologist, 49(4), 219–243. doi:10.1080/00461520.2014.965823
8. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. doi:10.17226/13165