Let it flow: how flowing water can appear to be completely frozen

Do try this at home. Image courtesy of RTE Brainstorm
Feb 13 2019 Posted: 14:39 GMT

Author: Nathan Quinlan

Analysis: Here's the engineering behind that recent viral video and optical illusion 

You may have seen the video which recently did the rounds billed as "laminar flow". Water pours from a tap so steadily that it looks like a solid twisted icicle. It’s utterly still, until it’s disturbed and it splashes like ordinary water.

Is it a camera trick or an outright digital fake? Could the water be rapidly freezing and thawing? Or it it a liquid with more exotic properties? If it is actually what it seems, this weird behaviour goes to the heart of fluid dynamics, an old branch of physics that that still holds mysteries and keeps engineers busy designing everything from heart valves to wind turbines.

Is this video genuine? To flip the question - why not? Why does it look so very wrong for water to flow with a convoluted but rock-steady surface?

Two effects dominate our intuition about water. One is turbulence. If the speed of a flowing fluid (liquid or gas) increases, it transitions at some point from predictable laminar flow to chaotic swirls and vortices. The chaos is turbulent flow. It’s what you what you feel on a windy day and what you see in the early stages of milk mixing into tea. Turbulence isn’t just more turbulent than laminar flow; it’s fundamentally different. Turbulence is almost universal in our daily lives. The easiest way to find laminar flow is to look at more viscous liquids, for example by mixing colour into thick paint or icing.

The internet debate on the video isn’t helped by the fact that "laminar" is often lazily defined as "parallel". There is some truth in this, but the reality is more subtle. This is a video of very slow laminar water flow past a cylinder, with dye injected to visualise it:

The flow swirls and tumbles, but it’s regular, and the marker dye stays in well-defined streaklines. Though it’s laminar, it’s anything but parallel.

The other effect in play is surface tension, which enables insects to walk on ponds, and pulls liquid drops into a round shape. In some ways, a water surface is like a very weak balloon skin. Because of surface tension, a cylinder of water in air (falling from a tap, for example) is inherently unstable. The slightest irregularity in its surface will be amplified by surface tension, and will grow until the stream breaks into drops. This was discovered by Joseph Plateau, who spent a lot of time staring at the sun for science. Later he went blind and, later again, did classic experiments in fluid dynamics.

This video shows real high-speed footage of a water jet breaking up under surface tension, even though it’s laminar (this so-called Plateau-Rayleigh instability is explained beautifully here):

These two phenomena - turbulence, and instability driven by surface tension - combine to make water splashy, wobbly and erratic in our daily experience. In theory, the convoluted but steady flow in the video should be possible if we remove turbulence, and remove other sources of any disturbances that surface tension can latch onto.

One of the most thoughtful responses to the video came from Dr Andrew Steele, a biologist and author based in London. He argued that the water flow might be unsteady but oscillating in a regular way. If it oscillates at exactly the same frequency as the camera’s frame rate, it would appear to be frozen on video. He pointed to a previous example where a water stream was blasted with a speaker to synchronise it to a camera, with spectacular results. That would be a very pleasing explanation, if true, but I was still hoping the phenomenon was real.

Along with Dr Nobuhiko Izumi, a physicist working on nuclear fusion in California, Andrew and I got into a conversation on Twitter and Andrew went on a mission to track down the original photographer Dario Bonzi. In the end, Andrew and Nobuhiko set about recreating the effect in their kitchen sinks. This is scientific method and the internet at their joyous best: people coming together to debate ideas, keeping sceptical but open minds, and testing those ideas with experiments, all for no reason but curiosity. Here’s my own effort:

The verdict is in: it’s real. Theory is all very well, but nothing beats an experiment. Unlike Plateau’s tests of the human eye, you are strongly encouraged to do this one at home. (There are some tips under the video on YouTube).

Why does it work? We’re feeding the flow from a still pool instead of pipes and valves so there is no turbulence. We’re eliminating other disturbances that might trigger breakup into droplets (for example, in my setup, the effect can be spoiled by ragged edges on the opening, probably because they feed vibration into the flow). These pure conditions are very unusual in our everyday dealings with water - and that alone is why it looks so very strange. I was prepared to accept that this phenomenon required some flukeish conditions that occurred where Dario stumbled across it in the Italian Alps, but I was stunned to find that it’s so easy to recreate.

The same fluid dynamics are exploited in other fun applications such as laminar fountains and urinal technique. There are more serious uses too: the paper industry uses stable sheet-like laminar flows to jet a solution of fibres onto a bed. In our research in the CÚRAM centre in NUI Galway with medtech company Aerogen, we explore the same principles in technology that uses microscopic liquid flows to generate droplets that can be inhaled for medication purposes. If you’d like to explore further in the world of fluid flow, two good places to start are Dr. Nicole Sharp’s FY Fluid Dynamics and the Gallery of Fluid Motion.

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