杏吧原创

High brow

Why do people have eyebrows?

鈥� My father has alopecia so he doesn鈥檛 have eyebrows. In warm weather sweat runs into his eyes and makes them sore; in wet weather he has to keep wiping the rain out of his eyes. So your eyebrows divert sweat droplets and raindrops from running directly into your eyes. You would be very uncomfortable without them.

Valerie Higgins

Telford, Shropshire, UK

鈥� We use our exceptionally mobile eyebrows to communicate our emotions.

The position of the eyebrows emphasises expressions on the human face thus giving others an accurate picture of the individual鈥檚 mood. This gives a good indication of whether a person is friendly or whether they might be dangerous to approach.

Smiles come in many forms, from expressions of merriment or contentment to leers, smirks and even anger. The position of the brow, emphasised by the eyebrows, is what gives us a visual cue to what an individual is really feeling.

The importance of eyebrow position as a guide to mood was brought home to me when a friend had Botox injections in the lines on her forehead and couldn鈥檛 raise or lower her eyebrows. Talking to her became a disconcerting experience 鈥� the bottom half of her face remained mobile but her eyebrows did not move. I couldn鈥檛 deduce her mood accurately by looking at her expression, and needed to use other cues such as her actions and speech.

Alison Venugoban

Ngunnawal, ACT, Australia

Eyebrows are important in the expressing emotions. Perhaps most important is the 鈥渆yebrow flash鈥�, a rapid up-and-down flick of the eyebrows that conveys recognition and approval. The ability to telegraph friendly intentions from a safe distance would have had obvious survival value for our ancestors.

Eyebrow signalling of various kinds is widespread among primates, although only in humans are the eyebrows highlighted by setting them against bare skin 鈥� Ed

Water substitutes

Is water the ideal liquid in which to swim? Assuming there are no ill effects to your health, would a different liquid that was either denser and more viscous, or had some other property, enable one to swim faster or with less effort?

鈥� The speed of a body in fluid is limited by the sum of three factors. The viscous drag is the friction of the fluid against the wetted surface. The form or pressure drag is the force created by the pressure difference between the front and the rear of the body. Finally, there is the wave-making drag, which is the energy wasted in making waves on the surface of the water.

I would suggest two strategies to achieve a higher speed for a given power. Swim totally submerged in a liquid with a lower viscosity and lower density than water, such as acetone, methanol or ether. The lower viscosity would cause less friction and reduce pressure drag, which is proportional to the density of the fluid, cross-sectional area of the swimmer and square of their velocity. Swimming below the surface would totally eliminate the wave-making drag. This is what submarines do to achieve high speeds.

The alternative is to swim on a liquid that has a much higher density than water but low viscosity. Mercury, which has a density 13.6 times that of water, would be ideal.

Archimedes鈥� principle would ensure that just a small part of the body would be submerged, so the wetted surface and the viscous drag would be very small. The pressure drag would be about the same because while the cross-sectional area of the immersed part of the body would be reduced, the density of mercury is greater. The wave-making drag would remain. And of course you would have to invent a completely new style of swimming, probably something like paddling.

Radko Istenic

Ljubljana, Slovenia

鈥� Mercury has to be the best liquid in which swimmers can enhance their performance. A swimmer weighing 90 kilograms whose back has a surface area of 3000 square centimetres could do a modified backstroke with their torso displacing less than one inch of the mercury.

The swimmer could keep all of their limbs out of the liquid and use vigorous heel kicks into the mercury as an effective means of propelling themselves forward.

Mercury does not wet skin and the sharp shape of its meniscus would further reduce the drag. With less than an inch of immersion, the swimmer鈥檚 body would virtually 鈥渉ydrargyro-plane鈥� across the smooth surface.

The swimmer could use hand strokes for further power and steering, but the technique would require experimentation as limb immersion could slow things down.

In a ceramic-fibre bodysuit a swimmer could do even better in a pool of molten gold, platinum or uranium, displacing barely half an inch of liquid 鈥� before frying when the thermal insulation of the suit failed.

David Emanuel

Tulsa, Arizona, US

This week鈥檚 questions

Bluto strikes back

My Italian recipe book says that I should cut cooked spinach with a stainless steel knife to avoid discoloration. Which would become discoloured if I don鈥檛 鈥� the knife or the spinach? And what would be the chemistry at work?

Hans Hamich

Hull, East Yorkshire, UK

Spinning onwards

Erosion redistributes mass from the mountain heights to the bottom of the sea. This must reduce the Earth鈥檚 moment of inertia and so increase the planet鈥檚 rotational speed and decrease the length of the average day. On the other hand, humans dig holes, and build skyscrapers, shifting the mass-balance in the opposite direction. Is either of these effects of any significance?

Tim Threlfall

Shenton Park, Western Australia