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How does siphoning work?

📓 The short answer

A siphon is a tool to transfer liquid from a higher elevation to a lower one, primarily by using gravity. This flow is also influenced both by atmospheric pressure and molecular characteristics of fluid dynamics.

📚 The long answer

You probably know the scene in the movies when someone drops a tube into a full gas tank, sucks on the other end (and spits out the gasoline that went into their mouth), and watches as the tube starts to fill a bucket on the ground. That is siphoning.

The key reason why this works is gravity. As liquid travels up and over the crest of the tube, the weight of the liquid going into the lower container starts to pull the rest of it over the crest, causing the upper container of liquid to drain. No pump needed! This gravitational pull creates a continuous flow until the upper container is empty or the liquid level of the lower container is even with the upper container.

Gravity explains how liquid goes down the tube, but let's take it a step further into the two leading scientific explanations that help explain how liquid goes up the tube.

The Dual Theories of Siphoning

Theory #1: Atmospheric Pressure

When you suck on the end of a straw, you lower the pressure within the straw. Since the pressure outside of the straw is greater, liquid is forced up the straw and into your mouth. The same thing is happening with a siphon. When you suck on the end of the tube, the higher atmospheric pressure on the outside causes the liquid to push the liquid into the siphon and promote flow.

What's interesting is that a siphon can also function in a vacuum (where there's basically no atmospheric pressure)! So Theory #1 doesn't fully explain how siphons are able to continuously pull liquid up, indicating there's another mechanism at work. Enter, Theory #2: Cohesion and Adhesion.

Theory #2: Cohesion and Adhesion

Zooming way down to the molecular level, liquids have the characteristics of cohesion and adhesion. Cohesion is the tendency of liquid molecules to stick together (think beads of water). Adhesion is the tendency of liquid molecules to stick to a surface (think water on concrete). Both of these forces aid in liquids being siphoned. Cohesion results in a chain-like force where liquid is pulled together up and over the crest of the siphon tube. Adhesion results in the attraction of the liquid to the tube's material, which is necessary to ensure the liquid column in the tube remains unbroken.

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In summary, the combination of gravity, atmospheric pressure, and the forces of cohesion and adhesion all factor into how siphons can siphon. Now go forth and learn how siphons make toilets possible.

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--- Thanks for reading this week's newsletter! If you have any thoughts, questions, or favorite GIFs, my inbox is always open. Just hit reply to send me a note! :) ​ All my best, Caitlin ​Click here to read five facts about Caitlin​

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​Sources for this week's newsletter​

📖 Book of the week

​The Library Book by Susan Orlean​

This book is an ode to libraries, librarians, and books. Centered around the fire that engulfed the Los Angeles Public Library in 1986, destroying and damaging over 1.1 million books, The Library Book delves into the history, evolution, and cultural significance of libraries, celebrating them as enduring symbols of knowledge and community. Orlean weaves personal stories, historical anecdotes, and interviews with librarians and researchers, painting a poignant portrait of a beloved institution and its integral role in society. Bonus points if you rent it from the library.

👀 Catch up on other curious questions

• ​How does a gas pump know when to stop pumping?​​
• ​Why are seconds of time called seconds​​
• ​Why is “W” pronounced double U and not double V?​​

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