Surface tension and capillarity.

Step 1 of 6

A liquid that climbs

Stand a thin glass tube in a glass of water and the water inside rises above the water outside. No pump, no suction: the liquid simply climbs and stops. This is capillary action, and it is why a paper towel soaks up a spill and a sugar cube drinks coffee from the bottom up. The Scene has a wide vessel joined at the bottom to three thin tubes, each twice as wide as the one before.

Try it: choose Molecules under Zoom to the molecules. The molecule inside the liquid is pulled equally every way; the one at the surface is not pulled upward, so its pulls add up to one arrow pointing in.

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Step 2 of 6

The tube’s width

The surface’s pull along the rim grows with the tube’s circumference, $2\pi r$⁠. The water it has to lift grows with the area of the section, $\pi r^2$⁠. The Liquid is fixed now: Water.

Step 3 of 6

Jurin’s law

The column stops where the pull along the rim holds up exactly the weight of the raised water. That is Jurin’s law:

$h = \frac{2\gamma\cos\theta}{\rho g r}$$h =$$\frac{2\gamma\cos\theta}{\rho g r}$⁠

$\gamma$⁠ is the surface tension, the force along each metre of the surface’s edge, in N/m. $\theta$⁠ is the contact angle, the angle at which the liquid’s surface meets the glass; $\rho$⁠ is the liquid’s density and $r$⁠ the tube’s inner radius. Water’s surface tension is unusually high, which is why a carefully placed paper clip can rest on water without sinking. Both Parameters are fixed now.

Step 4 of 6

Forty millimetres

Now you can change the Narrowest tube.

Try it: after answering, choose Soapy water under Liquid. The soap gathers at the surface and weakens its pull, and the columns climb less than half as high.

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Step 5 of 6

Mercury

Not every liquid climbs. What decides is which pulls harder: the glass on the liquid’s molecules, or the liquid on itself. Glass pulls water nearly as hard as water pulls itself, so water wets glass. Mercury is a liquid metal, and its atoms pull on each other very hard.

Step 6 of 6

Trees

Capillary action carries ink along the fibres of paper, lifts melted wax up a candle’s wick, and draws water through the gaps in soil. But trees grow tens of metres tall.