Your fluids questions answered

Is it true that water goes down the plughole with the opposite spin in the Southern Hemisphere?

No, well maybe, but it has nothing to do with which hemisphere you are in.

It is true that because the Earth is rotating, the Coriolis force has an effect on the motion of water, and this effect is reversed between the northern and southern hemispheres. However, the importance of this effect is measured by a number known as the Rossby number, R, which is the ratio of the average speed of the fluid and the product of the rate of rotation of the Earth (at a particular latitude) and the length scale of the motion. Flows with small Rossby numbers are strongly affected by the rotation of the Earth, while those with large Rossby numbers are not. If we consider a sink that is 30cm wide in London and assume that the water flows at approximately 10cm/s, then the Rossby number of the flow would be close to 10000. Basically, this has no bearing on the direction of the flow. The biggest effect you'll see comes from the initial direction of the flow and the symmetry of the basin, which could very well be different depending where you are in the world.

If the wind is blowing on my back, in which direction should I walk for the nice weather?

We all know that high-pressure zones have better weather, but how can we find high-pressure zones? It turns out that for flows on the scales of winds, which are both "slow" and "steady" (don't change very much with time) the dominant balance of the equations of motion is: 2ρΩ×u=-∇P, where ρ is the density, Ω is the angular frequency of the earth, u is the velocity of the wind and P is the pressure. This may seem difficult to decipher for those who are not familiar with the notation, so I'll just explain that if you face in the direction of the wind, then Ω×u points to your left and that ∇P points in the direction that P increases. So our equation is telling us that P increases in the opposite direction to Ω×u, or in other words, to the right if we are facing the same way as the wind.

So if you want to find the best weather, walk with the wind blowing to your left! (Try to keep track of all these directions!)

Unfortunately, this will only really work if you're in the middle of a very flat plain, or rather high in the sky.

Why do chocolate fountains fall inwards, not downwards?

Perhaps you've noticed this when enjoying a chocolate fountain at a special event. When chocolate falls off the plastic domes, it falls inwards towards the next layer, not downwards. So why is this? Surely gravity will pull it downwards?

In fact, you can see this with all sorts of fountains, even with water. Chocolate is a special type of fluid, because it thins out as it flows ('its viscosity decreases as it shears'), but this doesn't explain this effect. One thing that is responsible is the shape of the plastic dome when it ends: it's very rounded so the chocolate shoots off inwards to start with. But that doesn't explain why the inwards slope is so much.

Is it a pressure difference between the inside and outside, maybe? Is the chocolate getting sucked inwards? Well, no, not really. Holes appear in the falling sheet of chocolate all the time, and this would definitely equalise the pressure.

Is it the teapot effect? Well, that definitely has something to do with it. The back-dribble that we see with teapots is similar to the way chocolate falls off the dome, but again this should only have an effect at the top.

Is it to do with speed? In a way, yes: if the chocolate is too slow it will drop off undramatically, and if it's too fast it will spray off everywhere. So in a way, the chocolate fountain speed is a 'sweet spot'.

Overwhelmingly, the answer is surface tension. At a fluid interface (e.g. where chocolate meets air), this is the force that keeps molecules of one type of fluid stuck to its fellow type, and not to the type it meets. Surface tension is a property of a type of fluid, so you can look up its value for chocolate, air or water. Increasing the surface tension will bring the falling sheet further in, and decreasing it will bring it further out.