Making a water vortex is simple: connect two plastic soda bottles with a small hole between them, fill one with water, flip it on top, and give it a circular swirl. The spinning motion creates a funnel-shaped vortex that lets water drain from the top bottle into the bottom one far faster than gravity alone. The whole setup takes about ten minutes with materials you probably already have.
What You Need
The classic version uses two empty 1- or 2-liter plastic soda bottles, washed with labels removed. You’ll also need one bottle cap, a drill or sharp tool to punch a hole in it, strong tape (duct tape or electrical tape), water, and optionally a few drops of food coloring. If you’d rather skip the DIY connector, plastic “tornado tube” connectors are sold at science museums and novelty stores that screw directly onto standard bottle threads.
Step-by-Step Assembly
Start by drilling a hole near the center of one bottle cap, roughly 7 mm (about a quarter inch) across. This opening is the key to the whole experiment. Too small and water barely trickles through. Too large and the vortex never forms because there isn’t enough restriction to organize the spinning flow.
Tape the cap upside-down onto the mouth of one empty bottle so the threaded side faces outward. Fill the second bottle about two-thirds full of water and add a few drops of food coloring if you want a more visible vortex. Screw or press the capped bottle assembly onto the top of the water-filled bottle, then wrap the joint tightly with several layers of duct tape. You want this seal completely watertight, so don’t be shy with the tape.
Flip the whole assembly so the water-filled bottle sits on top. Set it on a table and watch for a moment. Without any swirling, the water either drips very slowly or stops entirely as air pressure in the lower bottle has nowhere to escape. Now pick up the assembly and rapidly rotate it in a small horizontal circle a few times. Don’t shake it, just swirl. A funnel-shaped vortex will form almost immediately, and the water will flow smoothly and quickly into the bottom bottle while air spirals up through the center column.
Why the Vortex Makes Water Drain Faster
Without a vortex, water and air compete for the same narrow opening. Water tries to fall while air tries to rise, creating an alternating glug-glug pattern that slows everything down. The vortex solves this traffic jam. Spinning the water pushes it outward against the bottle walls, opening a hollow air core through the center. Water spirals down around the edges while air flows freely up the middle, and both move simultaneously instead of taking turns.
This is a free vortex, the same type that forms when water drains through a hole in any container. No external force keeps it spinning once it starts. The initial swirl you give the bottle provides the angular momentum, and then the draining water sustains the rotation on its own. In a free vortex, water near the center spins faster than water near the walls, which is why the funnel narrows to a tight point at the opening.
Getting a Better Vortex
If you’re using the DIY cap method, hole size matters more than anything else. NASA’s version of this experiment calls for a 7 mm hole, which produces a tight, long-lasting vortex. NOAA’s version uses a metal washer with a 3/8-inch (roughly 9.5 mm) hole. Either works, but smaller openings generally produce more defined funnels that last longer, while larger openings drain faster with a wider, less dramatic vortex. If the hole is too large, the water pours through before a stable rotation can form.
Adding three drops of dish soap to the water helps in two ways. It reduces surface tension, allowing the vortex funnel to form more easily, and it creates a slight foaminess that makes the spinning motion easier to see. Food coloring deepens the visual contrast.
For a truly striking effect, try making a rheoscopic fluid. Add about half a gram of cosmetic-grade mica powder (the glittery kind sold for soap-making or crafts) to the water. Mica flakes are close to water’s density, so they stay suspended rather than sinking, and they reflect light as they align with the flow. The result reveals every swirling current inside the bottle in shimmering detail. This technique has roots going back centuries. Scientists have used mica, graphite, and aluminum flakes to visualize fluid motion since the 1800s.
Common Problems and Fixes
The most frequent failure is leaking at the joint. If water seeps out where the bottles meet, your tape seal isn’t tight enough. Dry the bottles completely before taping, and wrap at least four or five layers of tape around the connection. Electrical tape conforms better to curved surfaces, but duct tape is stronger. Using both, with electrical tape against the bottles and duct tape over it, is a reliable combination.
If the water just sits in the top bottle without flowing, you likely have an airlock. This happens when the seal is actually too good and there’s no path for air to enter the upper bottle. Give the assembly a firm circular swirl to break the lock and establish the vortex. Once that central air column opens, the flow will sustain itself.
If the vortex forms but collapses within a second or two, you’re probably not generating enough initial rotation. Use a smooth, confident circular motion, like you’re drawing small circles in the air with the top of the bottles. Three to five rotations is usually enough. Shaking introduces turbulence that disrupts the organized spin.
Does Rotation Direction Matter?
You may have heard that water always spirals one direction in the Northern Hemisphere and the opposite in the Southern Hemisphere. This is a myth. The Coriolis effect, the force created by Earth’s rotation that steers hurricanes and ocean currents, is far too weak to influence water in a bottle or even a bathtub. According to the Library of Congress, you can find both clockwise and counterclockwise drains in either hemisphere. The direction your vortex spins depends entirely on which way you swirl the bottles. Spin it left, it goes left. Spin it right, it goes right.
Scaling Up: Open-Container Vortexes
The bottle method is the easiest starting point, but you can create water vortexes at larger scales too. A simple version uses a large clear container (a round fish bowl or cylindrical vase works well) with a drain hole in the bottom. Fill the container, use your hand or a paddle to stir the water in one direction, then open the drain. The water will form a visible funnel as it exits.
This open-drain version produces a forced vortex at the edges, where the water moves because you pushed it, and a free vortex near the center, where the draining action takes over. The speed profile is different in each zone: in the forced region, water farther from the center moves faster, while in the free-draining core, water closer to the center moves faster. That transition is what creates the dramatic funnel shape that narrows sharply toward the drain.
For a permanent display, some hobbyists build “vortex fountains” using a submersible pump that pushes water tangentially into a cylindrical container with an open top. The pump creates continuous rotation, and the vortex persists as long as the pump runs. These require more engineering, but the core physics are identical to what happens in a pair of taped-together soda bottles on a kitchen table.