Boeing recently launched a new line of aircraft, the 787 Dreamliner, that they claim uses 20% less fuel than existing, similarly sized planes.

How did they pull off this sizeable bump in fuel efficiency? And can you always build a more fuel-efficient aircraft? Imagine a hypothetical news story, where a rival company came up with a new type of airplane that used half the fuel of its current day counterparts. Should you believe their claim?

More generally, do the laws of physics impose any limits on the efficiency of flight? The answer, it turns out, is yes.

There’s something about flying that doesn’t sit well with us. If we never saw a bird fly, it may never have occurred to us to build flying machines of our own.

Here’s where I think this sense of unease comes from. It takes stuff to support stuff. Everyday objects fall unless other things get in their way. Take the floor away, and you’ll plummet to your doom – the air below your feet isn’t going to do much for you. We move through air so effortlessly, that we barely notice it’s there. So what keeps a plane up? There doesn’t seem to be enough ‘stuff’ there to hold up a bird, let alone a Boeing aircraft weighing up to 500,000 pounds. To put that last number in context, its more than the weight of an adult blue whale!

Why is it that planes fly and whales typically don’t? The answer is easy to state, but its consequences are rather surprising. Planes fly by throwing air down. That’s basically it. It’s an important point, so I’ll say it again. Planes fly by throwing air down.

As a plane hurtles through the air, it carves out a tube of air, much of which is deflected downwards by the wings. Throw down enough air fast enough, and you can stay afloat, just as the downwards thrust of a rocket pushes it up. The key is that you have to throw down a lot of air (like a glider or an albatross), or throw it down really fast (like a helicopter or a hummingbird).

A physicist’s two-step guide to flight (it’s simple, really!)

Let’s make this idea more quantitative. Following David MacKay’s wonderful book on Sustainable Energy, I’m going to build a toy model of flight. A good model should give you a lot of bang for the buck. The means being able to predict relevant quantities about the real world while making a minimum of assumptions.

Step 1: Sweep out a tube of air

As a plane moves, it carves out a tube of air. This air was stationary, minding its own business, until the airplane rammed into it. This costs energy, for the same reason your car’s fuel efficiency drops when you speed up on the highway. Your car has to shove air out of its way.

Exactly how much energy does this cost? You might remember from high school physics that it takes an amount of energy equal to to bring stuff with mass up to a speed .

In our case, we have

There’s still this mysterious factor of the mass of the air tube. To work this out, we can use a favorite trick in the toolbox of a physicist – unit cancellation. We can re-write the humble kilogram as a seemingly complicated product of terms.

What we’ve done here is to express an unknown mass of air in terms of other quantities that we do know. Each of these terms makes sense. Air that’s more dense will weigh more. A fatter plane (larger cross-sectional area) sweeps out more air, as does a faster plane. We’ve arrived at a meaningful result, just by playing around with units. In the words of Randall Munroe, unit cancellation is weird.

Put these two ideas together and here’s what you find:

Here’s a graph of what that looks like.

If you’re with me so far, we just found that for a plane to plow through air, it has to expend an amount of energy proportional to the speed of the plane to third power. (The extra factor of v comes from the fact that faster planes sweep out a larger mass of air.) If you want to go twice as fast, you need to work 8 times as hard to shove air out of your way.

We’ve arrived at a general rule about the physics of drag. This holds true for a car on the highway, or for a swimmer or cyclist in a race. It’s why drag racing cars get only about 0.05 miles to a gallon! If we want to reduce overall energy consumption by cars, one option is to lower the speed limits on highways.

What does this mean for our toy plane? It would seem that the slower the plane, the higher its efficiency. So are airplane speed limits also in order? Absolutely not! To see why, read on to the second half the story..

Step 2: Throw the air down

In order to fly, a plane must throw air downwards. This generates the lift that a plane needs to stay up. It turns out that slower planes have to throw air harder to stay afloat. That’s why slow moving hummingbirds and pigeons have to flap their wings frenetically. It’s also why planes extend flaps while landing – they’re not throwing the air fast enough, so they compensate by throwing more of it.

More precisely, for a plane to stay afloat, the speed of the air jettisoned downwards must be inversely proportional to the speed of the plane. (You can take my word for this, although if you want to see where it comes from, take a look at David MacKay’s book.)

So we can now work out the second part of the puzzle. How much energy does it take to throw air down? As before, this is given by

Just as we did in the first step, let’s express things in terms of the speed of the plane.

In words, the energy spent in generating lift is inversely proportional to the speed of the plane. Here’s what this looks like on a graph.

You can see from the plot that, as far as lift is concerned, slower flight is less efficient than faster flight, because you have to work harder in throwing air downwards.

There’s a lot to chew on here. To summarize, we’ve discovered that in making a machine fly, you have to spend energy (really fuel) in two ways.

1. Drag: You need to spend fuel to push air away. This keeps you from slowing down.
2. Lift: You need to spend fuel to throw air down. This is what keeps the plane afloat.

The total fuel consumption is the sum of these two parts.

If you fly too fast, you’ll spend too much fuel on drag (think of a drag racer or an F-16). Fly too slow, and you’ll have to spend too much fuel on generating lift, like a hummingbird furiously flapping its wings, powered by high calorie nectar. However, at the bottom of this curve there is a happy minimum, an ideal speed that resolves this tradeoff. This is the speed at which a plane is most efficient with its fuel. Be it through the ingenuity of aircraft engineers, or the ruthless efficiency of natural selection,  airplanes and birds are often fine-tuned to be as energy efficient as possible.

Here’s a plot of experimental data of the power consumption of different birds, as their flight speed varies.

You can see that it matches the qualitative predictions of the toy model.

But we can do more than this, and actually extract quantitative predictions from the model. An undergraduate schooled in calculus should be able to work out that special optimal speed at which energy consumption is a minimum. David MacKay plugs in the numbers in  his book, and finds that the optimal speed of an albatross is about 32 mph, and for a Boeing 747 is about 540 mph. Both these numbers are remarkably close to the real values. Albatrosses fly at about 30-55 mph, and the cruise speed of a Boeing 747 is about 567 mph. 

That’s a lot of mileage from a toy model!

And with this physicsy interlude into the world of albatrosses, hummingbirds, and jet planes, we come back to the question of the fuel efficiency of Boeing’s new aircraft.

You can actually use the model to work out the fuel efficiency of a plane. What you find is that it really just depends on a few factors: the shape and surface of the plane, and the efficiency of its engine. And of these factors, the engine efficiency plays the biggest role. So we would predict that engine efficiency, followed by improvements in body design might drive Boeing’s fuel savings.

This agrees with Boeing’s own assessment.

New engines from General Electric and Rolls-Royce are used on the 787. Advances in engine technology are the biggest contributor to overall fuel efficiency improvements.

New technologies and processes have been developed to help Boeing and its supplier partners achieve the efficiency gains. For example, manufacturing a one-piece fuselage section has eliminated 1,500 aluminum sheets and 40,000 – 50,000 fasteners.

Try as we like, we can’t squeeze a lot of improvement out of airplanes. Engines are already remarkably efficient, and you certainly can’t shrink the size of a plane by much, as economy class passengers can well attest. New manufacturing techniques could cut the amount of drag on the plane’s surface, but these improvements would only raise fuel efficiency by about 10%.

To quote David Mackay,

The only way to make a plane consume fuel more efficiently is to put it on the ground and stop it. Planes have been fantastically optimized, and there is no prospect of significant improvements in plane efficiency.

A 10% improvement? Yes, possible. A doubling of efficiency? I’d eat my complimentary socks.

References

I based this blog post on material I learnt from David MacKay’s fantastically clear book, Sustainable Energy without the Hot Air. It’s available online for free, and is highly recommended for anybody looking to use numbers to understand energy.

David MacKay (2009). Sustainable Energy – Without the Hot Air UIT Cambridge Ltd

I used this tip to make those XKCD style plots.

Take the case of the mantis shrimp. These incredible crustaceans come in two varieties: stabbers, and smashers. Sheila Patek is a biologist who studies them for a living. In a fascinating TED talk from 2004, she describes how mantis shrimp have the fastest blow in the animal kingdom. Their strike force is so great that it creates a visible shock wave in water, in a bizarre phenomenon known as cavitation. Patek goes on to describe the engineering solutions that these animals use to create and sustain their powerful smash.

Since 2004, the list of nature’s fastest has had more than a few additions. It’s the time of the year for holiday lists, so I decided to list some of the most impressive record holders in this regard. To do this, I relied mainly on references I found on the wonderful website of Patek’s lab.

The life forms that follow are pushing the limits of physics and engineering. Typically, they are doing this to rein death and terror onto hapless prey. They are the Terminator 2’s of our world. So please join me, as we descend down this list towards the most lethal of all blows. This is a quest for the fastest draw in nature.

But first, let’s start with something fast that we’re familiar with. When talking about short intervals of time, we often use the phrase ‘in the blink of an eyelid’. The time it actually takes us to blink an eyelid is about 3 tenths of a second or 300 milliseconds.

A blink of an eye  (300 milliseconds)

So, our first point of reference is 10 milliseconds, or 1/30th of a blink of an eye

The ballistic tongue of the salamander (< 10 milliseconds)

The explosive tongue of the giant palm salamander of Central America bursts out in under 10 milliseconds, targeting flying bugs that don’t know what hit them. To achieve this feat, the tongue of this cold blooded sniper needs to output energy at the rate of a whopping 18,000 Watts per kilogram of muscle.

It stores this energy like a tightly coiled spring. As it relies on the principle of a slingshot, it can even operate in cold temperatures when muscles are slow to contract.

This tongue has been called the world’s most powerful muscle, but it’s no comparison to what follows.

The vacuum suction of the anglerfish (<5 milliseconds)

An anglerfish has what seems like a rather improbable fishing strategy.

It lures its prey in with a shiny dangling object attached to its head. All of a sudden, its mouth expands to more that 12 times its original size. The low pressure region thus created sucks in water at great speed, as well as whatever unfortunate fish happens to be swimming nearby. It’s a process that looks alarmingly like this.

And this strange kiss of death can take place in less than 5 milliseconds, or 1/60th of a blink of an eye.

The blinding strike of the mantis shrimp (2.7 milliseconds)

This has to be one of the most impressive punches in nature.

Sheila Patek and collaborators measured that the blow of the mantis shrimp can reach a peak speed of 51 mph (23 m/s), in less than 1/100 of the blink of an eye. All this while underwater! It’s so fast that it actually creates a visible shock wave. Meanwhile, its limb experiences over 10,000 g of acceleration.

To put this number in context, think of this: a typical person can handle an acceleration of about 5 g before losing consciousness, while decelerations of 100 g are about the highest that humans have survived, in Indy car racing accidents. A bullet shot out of a Beretta gun is accelerated by about 40,000 g.

If you were a snail or a clam, this could well be the last thing that you see:

Needless to say, a mollusk doesn’t stand much of a chance against this punch. The muscle that powers this impressive blow is delivering a mind-numbing 470,000 Watts per kilogram. It’s quite literally blowing the competition out of the water.

Well.. not quite. Read on.

The trapdoor stomach of the bladderwort plant  (2 milliseconds)

The first plant to enter our list is, of course, carnivorous, and it has an insidious method of devouring its prey. Bladderworts are a genus of over 200 carnivorous plants, all of whom capture tiny animals with a bladder-like trap. I’ve been frightened of the bladderwort ever since I watched my favorite of all David Attenborough documentaries, the Private Life of Plants, that uses time lapse footage to demonstrate their chilling strategy.

Picture this. A plant with a tiny transparent capsule. The inner walls of the capsule pump water out, and in so doing create a partial vacuum inside. The outer walls of this capsule are lined with tiny, sensitive hairs. The trap is set.

If a mosquito larvae has the misfortune of brushing against these hairs, it triggers a trapdoor that opens in 2 thousandths of a second. The insect is sucked in with an acceleration of 600 g, so escape would literally be a miracle.

The swirl of water closes the trapdoor. One set of glands then secretes juices that digest the prey, while the other set sucks the water out. In just two hours, the trap is ready to be used once again and the prey has been dissolved.

Here is a video of scientists describing this process, with some bizarrely gratuitous sci-fi sounds thrown in at appropriate moments.

By now, we’ve moved down in time by an order of magnitude, to the level of 1 millisecond. Keep in mind, 1 millisecond is 1/300th of a blink of an eye

At this point, we reach the first (and only) life form in our list that isn’t using its speed to hunt.

The pollen cannon of the bunchberry dogwood (0.3 milliseconds)

At only 2 millimeters in size, you wouldn’t normally notice this tiny flower. But this inconspicuous flower is like a loaded gun, waiting for the right conditions to go off.

When triggered, its petals unfurl with incredible force, jettisoning its pollen out in a thousandth of the blink of an eye. In this time, the pollen is accelerated by about 2,400 g, and shoots up to an inch, or about 10 times the height of the flower.

Most flowers don’t rely on wind pollination, instead opting to use insects as the distributors of their pollen. The bunchberry dogwood prefers to diversify its strategy. If a large pollinator like a bumblebee were to land on it, the pollen cannon will fire, showering the bee. However, if a smaller, less-mobile insect such as an ant were to climb onto the flower, the flower will not waste it’s precious pollen. Ants are not heavy enough to trigger the cannon. And if no insects come by, it’s not a problem, as the wind can carry the pollen a meter away.

Not bad at all, for a tiny flower.

This is not the only plant to use explosive ejaculation. At this point, I should also mention the marvelous squirting cucumber. As this cucumber shaped fruit ripens, it fills with a liquid that builds up at an immense pressure. Eventually, the pressure reaches a point where the cucumber bursts open, and the seeds shoot out with speeds over 30 miles per hour. (See the Private Life of Plants for some slow-mo action.) The squirting cucumber deserves its own entry, but I couldn’t find a reference with accurate timing information, so this is what it gets.

We’re now at a tenth of a millisecond, or 1/3000th of a blink of an eye.

The multi-purpose ballistic jaw of the trap jaw ant (0.13 milliseconds)

If you’ve ever spent some time playing a first-person-shooter computer game, you’ll know that weapons can have multiple uses. Sure, you can use that rocket-propelled grenade to attack. But you can also use it for propulsion. A well placed shot to the ground can launch you high up into the air.

The trap jaw ant understands this. Its jaws are a lethal weapon, snapping shut with an explosive force that can equal 500 times the ants weight. In a tenth of a millisecond, the jaws reach a peak speed of 143 mph (64 m/s). A quick calculation puts the acceleration of this strike in at over 50,000 g. That’s the same acceleration that a bullet experiences as it leaves a gun!

With touch sensitive hairs that serve as a trigger, and an internal latch mechanism, they can control this formidable explosive force. But what is truly incredible is how they wield it. Not only does the ant use its trap jaw for attack, it can also use it for escape.

Scientists have documented two unconventional uses of its jaw, that go by the technical names of ‘bouncer defence’ and ‘escape jump’. The latter is pretty much what it sounds like. When the ant finds itself cornered in the ant equivalent of a dark alley, it can launch itself vertically 10 cm into the air and leap to safety. The other strategy, bouncer defence, would be familiar to anyone who, like me, has wasted their childhood playing violent video games. Essentially what you do here is strike the enemy, while using the recoil to propel yourself to a safe distance.

Really, this is all just an excuse to show you this Matrix style ant video:

I’m sure you’d agree that this is a pretty sophisticated ant.

Moving down the list, we have now reached a hundredth of a  millisecond. We’re talking about a duration of time that happens 30,000 times in the blink of an eye. To put it another way, a hundredth of a millisecond is to a blink of an eye, what a blink of an eye is to two and a half hours.

Next on our list, we have:

The scissor-like jaws of the soldier termite (< 0.025 milliseconds)

The soldiers of the termite species termes panamaensis (Panama termites) are somewhat oddly shaped fellows. Their considerable, sword-like jaws and large, muscular heads take up more than half their bodies. The reason for this unwieldy headgear becomes abundantly clear when an unfriendly termite passes by. More than 70% of the time, this encounter results in death for the visitor (the number becomes 85% if you only consider visiting worker termites).

So how is this soldier termite butchering its foes with such ruthless efficiency? The key lies in speed. Its jaws snap open like a scissor, reaching a peak speed of 150 mph (67 m/s) in under 25 millionths of a second.

To power this absurd feat of strength, it relies on muscle that is delivering a peak power output of  11 Million Watts per kilogram. As far as I know, this is the most powerful muscle ever studied.

And now, we finally arrive at the end of the list, crossing a new threshold of speed. We have reached a thousandth of a millisecond, or a microsecond. This is to a blink of an eye, what a blink of an eye is to a day.

And the title of nature’s fastest draw (so far) goes to:

The retractable stingers of the jellyfish (0.0007 milliseconds, or 700 nanoseconds)

There’s fast, and then there’s mind-bogglingly, overwhelmingly, blazingly fast.

When a jellyfish detects its prey, it extends a kind of venomous vein. Like fiery filaments of doom, the job of these hair-like barbed structures is to inject neurotoxins into its prey. Here’s a video of this happening, shot under a microscope at 400x magnification.

Just how fast does a jellyfish arm itself? It turns out that the acceleration of these stingers as they emerge is 5,410,000 g. That’s not a typo.

Let me put it this way. The speed of light is a foot per nanosecond. So, in the time it takes for a jellyfish to whip out its stingers, light has travelled a distance of two football fields. It’s a timescale so fast, that the astronomical shifts down to the mundane.

And it is at this extreme scale where our journey ends, a scale where evolution is pushing up against the very laws of nature and against the speed limit of our universe. I’m excited (and a little afraid) to learn what we’ll discover next.

References:

In this article, I’ve focused on a specific way in which you can measure the fastest motion – the acceleration of an appendage relative to the body. There are, however, many other ways in which you might do this, each giving you a different champion. Here’s a splash of cold water from the lab of Dr. Patek:

Looking at peak sustained speeds – cheetahs might be the fastest. Or, focusing on peak unpowered speeds, diving falcons may be the fastest. On the other hand, if duration of the movement were the criterion of interest, nemtocysts and fungal spores would be the fastest. Lastly, considering acceleration of an appendage relative to the body through power amplification, then trap-jaw ants and termites come out on top.

Papers referenced:

Deban SM, O’Reilly JC, Dicke U, & van Leeuwen JL (2007). Extremely high-power tongue projection in plethodontid salamanders. The Journal of experimental biology, 210 (Pt 4), 655-67 PMID: 17267651

Grobecker DB, & Pietsch TW (1979). High-speed cinematographic evidence for ultrafast feeding in antennariid anglerfishes. Science (New York, N.Y.), 205 (4411), 1161-2 PMID: 17735055

Patek SN, Korff WL, & Caldwell RL (2004). Biomechanics: deadly strike mechanism of a mantis shrimp. Nature, 428 (6985), 819-20 PMID: 15103366

Vincent O, Weisskopf C, Poppinga S, Masselter T, Speck T, Joyeux M, Quilliet C, & Marmottant P (2011). Ultra-fast underwater suction traps. Proceedings. Biological sciences / The Royal Society, 278 (1720), 2909-14 PMID: 21325323

Edwards J, Whitaker D, Klionsky S, & Laskowski MJ (2005). Botany: a record-breaking pollen catapult. Nature, 435 (7039) PMID: 15889081

Patek SN, Baio JE, Fisher BL, & Suarez AV (2006). Multifunctionality and mechanical origins: ballistic jaw propulsion in trap-jaw ants. Proceedings of the National Academy of Sciences of the United States of America, 103 (34), 12787-92 PMID: 16924120

Seid MA, Scheffrahn RH, & Niven JE (2008). The rapid mandible strike of a termite soldier. Current biology : CB, 18 (22) PMID: 19036330

Nüchter T, Benoit M, Engel U, Ozbek S, & Holstein TW (2006). Nanosecond-scale kinetics of nematocyst discharge. Current biology : CB, 16 (9) PMID: 16682335

Bacteria have busy social lives. You might get a glimpse of this the next time you take a shower. The slimy discolored patches that form on bath tiles and on the inside of shower curtains are the mega-cities of the bacterial world. If you zoom into these patches of grime, you’ll find bustling microcosms that are teeming with life at a different scale.

That we can see these microbial communities with our naked eye is testament to the scale of their achievement. Perhaps the most spectacular examples are the giant mats of bacteria that lend life to the Grand Prismatic Spring in Yellowstone National Park. These macroscopic structures are just as impressive as our cities that are visible from outer space. Microbes have colonized practically all moist surfaces on earth, from the inside of our mouths (they’re responsible for dental plaque) to hot vents at the bottom of the ocean. And it all started from small beginnings.

The first wave of bacterial settlers that arrived on your shower curtain were few and far apart. They would try to hold on using the molecular adhesion between themselves and the shower curtain. Those that couldn’t get a grip were flushed down the drain plug.

Bacteria have an adaptation that serves them well in such tricky situations. It’s a sort of multi-purpose prong, technically known as a type IV pilus (plural: pili). These wonderful filament-like structures extend out from the bacteria, and grab on to the surface like a suction cup on a bathroom tile. What happens next is straight out of science fiction.

Once these settlers have their ‘feet’ firmly planted on the ground, the next step is to build a home. They begin to excrete a polymer substance, forming a grid that locks them into place. Many different microbes can co-inhabit these homes, from bacteria and archaea to protozoa, fungi and algae. Each species performs a specialized metabolic function, neatly occupying a niche in this city. Together these interlocked communities, or biofilms, are the beginnings of a thriving multicultural microbial civilization.

Why do bacteria congregate into cities? It’s basically for the same reasons that we do. By collecting together in large numbers, they can more effectively share resources. The grid offers them protection from antibiotic enemies, and helps them share resources. Some biofilms even have their own utilities and telephone system (that’s right, bacteria can talk). These grids have water channels running through them, which the bacteria use to share nutrients and send signals to each other.

But as city dwellers are well aware, moving to the grid comes with its disadvantages. The bacteria pay a price in mobility – their cities have no public transportation. It’s hard enough for bacteria to move in water, and being embedded in an organic glue makes matters considerably worse. Their winding propellers, the bacteria flagella, are of little use here.

However, the bacteria have a clever way out. Their pili (the hair like appendages pictured above) are more than just suction cups. They can also work like a grappling hook. The bacteria shoots them out to hook onto the surface, and then reels itself in. By repeating this motion, it can slowly crawl across the biofilm in a lengthwise motion that biologists delightfully refer to as twitching.

Here’s a video that shows bacteria (Pseudomonas aeruginosa) twitching along a surface as they keep dividing:

and a slowed down version of the same process:

You can see that the motion is jerky, because the bacteria are using their pili to pull themselves forwards or backwards. This crawling strategy was widely accepted as the explanation for how bacteria move in a biofilm.

But there were always some pieces that didn’t quite fit. Scientists knew that bacteria can sometimes make sharp turns, but they never quite understood how. The grappling hooks are mostly in the front and back of the bacteria, and aren’t much use for turning.

In an innovative solution to this problem, some bacteria instead use their pili like a walking stick. Rather than pulling themselves forward, they prop themselves up from the ground, stand upright and flop over. By repeating this motion, they can walk across the terrain. You can watch this strategy at work:

These walkers are not as energy-efficient as the crawlers, but they can move faster and are more meandering, both good ideas if you want to quickly explore new territory.

And a recent paper published by scientists from UCLA and University of Houston adds a new twist to the story. Fan Jin and colleagues describe an experiment where they track the motion of the bacteria Pseudomonas aeruginosa, the star of the twitching videos shown above.

They recorded videos of these bacteria moving under a microscope, and used software to track the positions of the two ends on their rod-shaped body. This process looked something like this:

Near the end of the video, you can see the bacteria make sideways leaps.

By analyzing this motion over many steps of the bacteria, they discovered a consistent pattern to the data. The following figure from the paper shows the horizontal and vertical position of the bacteria, as it crawls along the surface.

From the data, they worked out the speeds of the leading and trailing ends of this bacteria. You can see this plotted as the blue skyline in the figures above. What it shows is that the bacteria are constantly switching between short, furiously fast bursts of motion, and slower, more methodical crawls.

These two motions are quantitatively very different. The scientists found that although the bacteria spend only about 1/20 or 5% of their time in these leaps, they move 20 times faster than their normal crawling pace. Put the two together, and it means that the bacteria cover just as much distance leaping as they do crawling.

This tracking video from the paper shows this sudden move in action:

How do the bacteria manage to propel themselves through these considerable distances? The researchers realized that the bacteria must be using their pili as a slingshot. They use one pilus to tether themselves to the surface, like an anchor. By trying to pull the bacteria forward, the other pili become stretched like taut rubber bands. And as the bacteria severs its anchor, the rubber bands uncoil and it shoots out like a pellet from a slingshot. As it slides away, it can skid to one side like a car that’s taking a turn too quickly. This is the mechanism behind the sudden turns.

But there’s still a puzzle remaining, and it has to do with the physics of the small. In my previous post I talked about how bacteria move in a world of a low Reynolds number. What this means is that a bacteria feels its environment to be thick and viscous, robbing it of its tendency to maintain its speed (inertia). If you try to fling a bacteria forward, it should immediately come to a dead stop. So how are these slingshotting bacteria managing to coast through the slime? The solution comes from the physics of ketchup.

Let’s start with pouring honey out of a bottle. It doesn’t matter much if you squeeze the bottle or not. That’s because honey is a Newtonian fluid, meaning that its viscosity (or syrupy-ness) is independent of how much force you apply. You can’t rush such fluids, they’ll just stubbornly keep doing what they’re going to do.

On the other hand, there are some strange fluids like quicksand. These thicken up if you squeeze them, a fact used as a gag in countless hollywood films (quicksand had its heyday in the 1960s, when 3% of all films showed someone sinking in mud, sand or clay!)

Such fluids in which the viscosity increases with the applied force are known as shear thickening fluids. Silly putty has this property, as does cornstarch mixed with water, much to the amusement of kids everywhere.

And then there are fluids whose viscosity decreases as you squeeze them. These are the shear thinning fluids. This is like ketchup, that flows when you squeeze or shake the bottle, but won’t flow off your burger. Paints work on the same principle. They will flow across the canvas when applied with the force of a brush, but won’t drip when left alone.

And biofilms fall into this latter class of fluids. In the case of our bacteria, the researchers estimate that force of the slingshot is enough to lower the viscosity of the surrounding goo by three-fold.

By launching themselves forward, the bacteria are taking advantage of this quirk of physics to effectively slice through the slime. This is in contrast to the strategy adopted by the stomach bacteria Helicobacter pylori, that solves the problem using chemical engineering. H. pylori lives in the mucus lining of our stomachs, an alarmingly inhospitable environment for a life form. To help it move, it releases a chemical that thins out the surrounding mucus.

These bacterial communities are the results of countless failed experiments in the annals of evolution. In the game of life, success follows a seemingly endless line of heavy losses and incremental gains. And yet, from our shower curtains to the linings of our stomach, these microbes have arrived at strikingly clever solutions to the problem of getting around in a sticky situation.

References

Jin F, Conrad JC, Gibiansky ML, & Wong GC (2011). Bacteria use type-IV pili to slingshot on surfaces. Proceedings of the National Academy of Sciences of the United States of America PMID: 21768344

Gibiansky ML, Conrad JC, Jin F, Gordon VD, Motto DA, Mathewson MA, Stopka WG, Zelasko DC, Shrout JD, & Wong GC (2010). Bacteria use type IV pili to walk upright and detach from surfaces. Science (New York, N.Y.), 330 (6001) PMID: 20929769

Image References
All images link to the source, except those taken from the paper.