Storm in a Teacup: The Physics of Everyday Life(24)
Except they don’t. Most of that droplet is water, and in the first few seconds in the outside air, that water evaporates. What was a droplet big enough for gravity to pull it through the viscous air now becomes a mere speck, a shadow of its former self. If it was originally a droplet of spit with a tuberculosis bacterium floating about in it, it’s now a tuberculosis bacterium neatly packaged up in some leftover organic crud. The gravitational pull on this new parcel is no match for the buffeting of the air. Wherever the air goes, the bacterium goes. Like the miniaturized fat droplets in today’s homogenized milk, it’s just a passenger. And if it lands in a person with a weak immune system, it might start a new colony, growing slowly until new bacteria are ready to be coughed out all over again.
Tuberculosis is treatable if the right drugs are available. That’s why it has mostly disappeared from the western world. But at the time of writing, TB is still the second greatest killer of our species after HIV/AIDS, and it’s a gigantic problem in the developing world. Nine million people developed TB in 2013, and 1.5 million of them died. The bacterium has changed in response to antibiotics, becoming resistant to so many waves of drugs that it’s obvious it can’t be eradicated using medicine alone. The number of multi-drug-resistant strains of TB is on the increase. Outbreaks are popping up in hospitals and schools. So recently the focus has shifted to those tiny droplets. Rather than cure TB once you have it, how about changing your buildings to prevent the spread of those disease-laden plumes so it never gets to you in the first place?
Professor Cath Noakes works in civil engineering at the University of Leeds, and she is one of the researchers chipping away at this particular coalface. Cath is very enthusiastic about the potential for relatively simple solutions to emerge from a sophisticated understanding of tiny floating particles. Engineers like her are now learning how these tiny vehicles for disease travel, and it turns out this has very little to do with what’s in them and how long they’ve been there. It has everything to do with the battle of forces on the particle, and the battle lines are drawn by the particle size. It’s been discovered that even the larger droplets can travel farther than anyone had thought, because turbulence in the air can keep them aloft.# The tiniest ones can stay in the air for days, although ultraviolet and blue light damage them. If you know where your particle sits on the size scale, you can work out where it’s going to go. So, if you are designing a ventilation system for a hospital, it’s becoming possible to plan to remove or contain specific particle sizes, and therefore control the spread of disease. Cath tells me that each airborne disease may require a different plan of attack, depending on how much of it you need to get sick (in the case of measles, very little) and where in your body the disease settles (the TB bacterium has different effects in your lungs and your windpipe). These studies are still in their early days, but they’re advancing very quickly.
Humans have been at the mercy of TB for generations, but now we can visualize its spread, and that gives us the chance to control it. Where our ancestors saw only a foul room of sickness, awash with mysterious miasmas, we now understand the subtle swirling of the air around each patient, the sorting and shunting of disease particles, and how the consequences take effect. The outcomes of this research will be incorporated into the hospital designs of the future, and many lives will be saved by engineering on the macro scale to influence particles on the micro scale.
Viscosity matters when something small is moving through a single fluid—fat globules rising through milk or a tiny virus falling through the air. Surface tension, its partner in the world of the small, matters at the place where two different fluids touch. For us, that’s usually where air touches water, and everyone’s favorite example of air mixing with water is a bubble.** So let’s start with a bubble bath.
The sound of a bathtub filling up is distinctive and jolly. It announces the imminent reward after a hard day, a soak to recover from a particularly tough tennis match, or just a bit of pampering. But the moment you pour in a bit of bubble bath, the sound changes. The deep rumble gets softer and quieter as the foam builds up, and the place where the water stops and the air starts gets harder to identify. Pockets of air are trapped inside watery cages, and all it took was a tiny amount of stuff from a bottle.
It was a group of European scientists in the late nineteenth century who picked apart the puzzle of surface tension. The Victorians loved bubbles. Soap production expanded dramatically between 1800 and 1900, and the white suds washed the workers of the Industrial Revolution. Bubbles provided the Victorians with good fodder for moralizing; they were the perfect symbol of pure cleanliness and innocence. And they were also a nice example of classical physics at work, just a few years before Special Relativity and quantum mechanics came along and poked a sharp pin in the ballooning idea of a neat, tidy and well-behaved universe. But even so, the serious men with top hats and beards didn’t work out the secrets of bubble science all by themselves. Bubbles were so universal that anyone could have a go. Enter Agnes Pockels, often described as a mere “German housewife,” but really a sharp-minded critical thinker, who used the limited materials available to her and a decent dose of ingenuity to examine surface tension for herself.
Born in 1862 in Venice, Agnes was of a generation very firmly convinced that the woman’s place was in the home. So that’s where she stayed while her brother went off to university to study. But she learned advanced physics from the material he sent to her, carried out her own experiments at home, and generally kept up with what was going on in the academic world. When she heard that the famous British physicist Lord Rayleigh was starting to take an interest in surface tension, something she had done many experiments on, she wrote to him. He was so impressed with the letter describing her results that he sent it to be published in the journal Nature so that it could be seen by all the greatest scientific thinkers of the day.