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THE LAST WORD

Repulsive coffeeWhat causes coffee grounds to dry into the pattern shown at the bottom of this coffee cup (see photo)?

n The coffee dregs essentially consist of two components – very large coffee grains which remain settled in one place and very fine colloidal material that moves with the liquid. Because of surface-tension effects, the large grains retain a pool of liquid around them. As this liquid droplet dries, evaporation happens preferentially at the edge of the droplet because it is more exposed. This creates a flow of liquid towards the edge which carries with it the fine colloidal particles. As a result, the majority of them end up being deposited at the droplet periphery, which remains anchored in place some distance from the particle until nearly all of the moisture has evaporated.

This explains the resultant pattern of large particles, with

a clear space around them, then a very concentrated ring of fine material. A New Scientist article many years ago called “The thrill of the spill” (25 October 1997) explains this effect in detail. Simon IvesonSchool of Engineering, Faculty of Engineering and Built EnvironmentUniversity of NewcastleCallaghan, New South Wales, Australia

Rise and fallWhy are the largest tide ranges in the world – of up to 17 metres – found in the Bay of Fundy, on Canada’s Atlantic coast?

n To understand what happens in the Bay of Fundy, start with a hand basin half-full of water. Push down on the surface on one side with the palm of your hand and the water will rise on the other, after which it will slosh back and forth like a liquid see-saw. By pushing down even more on

each side at the same time as the level on that side is falling, the rise and fall of the surface will increase – and can be made to overflow the rim of the basin. The sloshing of the water has a natural frequency and your additional input resonates with it, increasing the amplitude of the see-saw

wave. At the central axis of the basin the level remains unchanged, although water moves to and fro horizontally.

Now imagine the basin cut vertically down that central axis and consider just half of it. The half that is left corresponds to the Bay of Fundy, the axis-edge marks

the opening of the bay, and the missing half of the basin is replaced by the open Atlantic Ocean. An incoming tide effectively appears in the ocean as a huge wave advancing towards the bay. As it reaches the continental shelf at the bay’s opening it plays the role of the high half of the see-saw and happens to coincide with the low water level at the far end of the bay. By the time this wave has moved to the innermost section of the bay – raising its water level to a peak – the dip in the ocean surface corresponding to the low tide has reached the continental shelf.

The exceptionally high tides occur because the successive incoming tides appear at nearly the same frequency as water sloshing into and out of the bay, just as happened in the wash basin. It is a resonance effect.

If the Bay of Fundy opened directly into the Atlantic it would not have such high tides because the natural period of water moving to and fro in it would

be only about 9 hours, which is not close enough to the 12.5-hour period of the tides to lead to significant amplification of the wave motion. However, the Bay opens into the Gulf of Maine and together they have a natural frequency of just over 13 hours.Richard Holroyd Cambridge, UK

This week’s questionsPiles of ReAsonsWhy do rugs placed on carpets appear to move of their own volition? How can a bunch of inert fibres exert the force needed to move my rug 15 centimetres in just two weeks when there is a substantial armchair on it?Ann DuncombeFalkirk, Stirlingshire, UK

shooTing sTARYesterday I tripped over a fence post I had just pulled from the ground. When my face hit the concrete around the bottom of the post, I literally saw stars. Today I have a shiny new black eye and a few questions. What are those stars and what causes them? Victor StanwickStaten Island, New York, US

Revision TAcTicsIs it better to stay up late on the night before an exam, learning those last-minute facts, or to get up early and revise in the morning before the exam?Andrew MaeerExmouth, Devon, UK

“An incoming ocean tide effectively appears as a huge wave advancing towards the bay”

last words past and present, plus questions, at www.last-word.com

“if the bay opened into the Atlantic it would not have such tides, but it opens into the gulf of Maine”

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