How to make something clean itself

The secret of perpetually stainless stuff hid for centuries in plain sight—at least for those of us living around lotus leaves. Now physics can help you never to clean again, no matter what you spill.

It all comes down to how water sticks to stuff, in other words, how stuff gets wet.

Water molecules are made of oxygen and hydrogen atoms. Oxygen loves electrons, and bullies them away from the tiny hydrogens. So one side of the molecule gets a little negative charge, while the other side has a tiny positive one.

The average density of electrons in a water molecule. Oxygen is protective of its and greedy of other’s, so electrons tend to stay around it. credit: Lawrence Livermore Laboratory

When water approaches a surface oxygen-side-on, its electrons push the ones in the surface a little bit away (and the opposite happens if it comes hydrogen-on). The surface gets temporarily a little bit charge, and water immediately sticks to it. Stuff gets wet because of electricity.

CC-BY-NC-ND Thomas, via Flickr.

Water molecules also stick to each other. If they like sticking to each other better than to the surface, they curl up in a ball and roll off. Materials that do that are called hydrophobic, meaning that they fear water… Though it’d be more accurate to say that water is scared of them.

Chemical coatings, like those on non-stick pans, can make a surface hydrophobic because their electrons are harder to push around. But there’s a better way—the physics way.

Take the lotus leaves. They are covered in tiny bumps and ridges, a few atoms big. Water droplets rest on the tip of just a couple of bumps, too little to properly stick. Simplifying a bit, this means that cohesion forces within water “win”, keeping the drops rolled in tidy balls that simply roll away, raking up all the dirt.


A sketch of how a water droplet rests on the mictrostructure of a lotus leaf

Studying the leaves, scientists found the trick, and recently managed to create surfaces with the right kind of tiny bumps and ridges. These materials never get wet or dirty, for surgical tools that blood can’t stain, or toilets that don’t need cleaning, saving precious water in places where it’s needed.

If you want more

Cover photo: CC0 yang pin/pixabay

Jazz and the atmosphere of exoplanets

The atmosphere of a planet holds the keys to make it habitable, so we need to look at them to figure if exoplanets are habitable. They are too far to send probes to measure them directly like we do with Mars or Jupiter’s moons, but scientists can study them from right here, looking at how they block light.

When infrared light (the invisible electromagnetic waves that also carry heat) hits the atoms in a molecule, it makes them vibrate. Depending on the chemical bonds that keep a molecule together and the geometry of its atoms, each molecule vibrates differently. The molecules absorb the light with the same frequency as those vibrations (the resonant frequencies) and let the rest through.

How much light of each frequency passes through a sample of material. Each dip in the graph is light blocked by this particular molecule (called pentene). The labeling indicates the vibrations scientists recognized. credit:

It’s kind of the opposite of what a musical instrument does. Instruments only produce sounds of the frequencies their shape resonates with; these sounds combine to build the instrument’s characteristic voice.

What a graph of how much sound a guitar emits at various frequencies could look like. It’s clear only some actually come out and if you flip it, it looks like the light absorbed from a molecule. credit:

Just as we can distinguish the sound of a piano from that of a trumpet, it is also possible to tell molecules apart by shining light through samples and looking at the blocked light.

Tatyana Kazakova/pixabay

To study the atmosphere of an exoplanet, scientists measure what light comes from the star it orbits. Then measure it again when the planet is passing in front of it, so the light appeared filtered by the planet’s atmosphere.

Like a trained ear can make out all the instruments playing in an orchestra, the scientists can figure what molecules make up the planet’s atmosphere looking at the missing light.

Whether or not jazz really is about the notes you don’t play, searching for life in the universe surely is about the light distant exoplanets don’t play.

If you want more

Cover photo: CC0 Ahkeem Hopkins/pixabay

What makes a planet habitable

A couple of days ago, NASA announced the discovery of seven-count-them-seven rocky planets orbiting a small star called TRAPPIST1. Three of them seem to even be in the “habitable zone”. So, have we found the aliens’ home?

Nope. However, words are a bit confusing, so let’s review what it takes for a planet to be habitable.

On planets too close to the star it’s too hot and water evaporates, on the far ones it freezes. Only those in the sweet spot stay just the right amount of wet. credit: NASA/JPL/Caltech

The key is liquid water. All life we know—from bacteria to cats, from carrots to Lionel Messi—hinges on chemical reaction that only take place in water. No water, no life.

The habitable zone of a star is the space around it where planets can have sustained liquid water on their surface. Closer, and the heat from the star will fry the planet with all the water (think Mercury), farther and the surface will freeze (think Pluto).

Now we need a surface to collect this water on. That’s why rocky planets are interesting: no rocky surface, no place for water. And again, no water, no life.

Location isn’t everything, though: the atmosphere is key too. Earth’s atmosphere keeps water on the surface, and temperatures friendly (for planetary standards). Less of it and we’d risk turning into Mars, which is in the habitable zone, but is a frozen desert, where water is more like a killer sludge. More atmosphere, and we might become Venus (also in the habitable zone), which is effectively hell, molten lead lakes and deadly acid rains included. Winds in the atmosphere also favor habitability on some exoplanets.

Then you must hold on to your atmosphere. Earth has a cozy magnetic field that deflects part of the Sun’s radiation. Mars probably had an atmosphere, when it also had oceans, but has no magnetic field. Atom by atom, the constant barraging of energy and particles from the Sun eroded it. With the atmosphere gone, so was the water. No water, no life.

How Google’s doodle portrayed the discovery

The planet discovered around TRAPPIST1 are important not because they might be habitable (though some are promising), but because TRAPPIST1 is the first star of its kind we study, and it immediately delivered several promising planets. That means it should be relatively easy to find interesting planets around that kind of star, which multiplies the chances of finding actually habitable ones.

Moreover, they are close to Earth (“just” 40 light years), so we can study their atmospheres with existing telescopes, or with the James Webb space telescope slated to launch next year. We’ll train on these for the multitude of planets we are about to find.

For now, as usual, no aliens.

If you want more
  • Do you remember what a fuss was made that time NASA found the first Earth-sized planet in a habitable zone? This time they were three at once.
  • NASA put together an impressive amount of information, graphics, and even an app. You can find everything here
  • The real star of the bunch is this phenomenal 360 video (fullscreen highly recommended!) of the view standing on TRAPPIST1d

Cover photo: CC0 David Mark/

The thirsty dragon and other capillarity magic

The mighty thorny dragon. Credit: wikimedia

There once was a dragon, who lived in the desert and loved to eat ants. He liked it so much that he gave up the ability to drink for that: he made his mouth perfect for eating, but useless when it came to sipping.

Instead, he learned something way cooler: how to conjure water from sand itself, summoning a force even stronger than gravity.

It’s actually a true story: the thorny dragon is a majestic, 20cm long Australian lizard. A recent study revealed that, to drink, it uses a complicated network of narrow channels that start from its feet, suck water from the ground. Then, snaking between its scales, they deliver the water directly into the lizard’s throat.

We bottled that force into wondrous technologies such as… er… paper towels.

Seriously, though, paper towels do suck water against gravity. Since nothing is pushing the water up, that’s more or less sorcery. However, scientists prefer to call it capillarity, and it’s fascinating.

You probably noticed that water sticks to stuff. It’s scientifically called—I kid you not—wetting. But water also sticks to itself, and these two things combined make capillarity work.

In a way it’s like keeping up a hammock. The net is made to stay together, it’s suspended from the poles, and it sags in between under its own weight. In a tube with water it’s much the same: the tube walls act as many tiny poles around the water surface hammock.


Water in a capillary tube is like a hammock, but the poles around it grow a little on their own. credit: Stefanie Laubscher/pixabay

Water has an important difference from the hammock net: it self-sticks to the tube walls. That means it can climb (a little) up the wall. That’s why capillarity works so well in narrow channels, with walls close to each other. Basically, the distance between the “hammock poles” is small, so there’s less space for the net to sag.

A paper towel and its maze of narrow tubes under the microscope. credit: Mattheyses Lab/Emory University

Though it seems a little more mundane, paper towels work the same way. Inside their mesh hides an intricate network of tiny tubes through which water can climb. A little dragon magic in our kitchens.

If you want more
  • Water sticks to stuff because of van der Waals forces. They’re very cool, but they deserve their own thing.
  • Now that we know how to get stuff wet, can we do the opposite? Well, yes… stay tuned!!

Cover photo: CC0 Tom Mathews/pixabay

2 deep questions on gravitational waves (with puppies!)

It seems like yesterday, but it’s already been a year since LIGO announced they found the elusive gravitational waves, that stretch and squeeze space (a tiny bit) in their wake.

But what does it mean “they stretch and squeeze space”? and how do you actually measure something like this? Let’s answer these important, very deep questions!

What does it even mean to “stretch and squeeze space”?

That is kind of a hard question, and I also struggle a bit with it. I’ll try my best, and please do correct me if I get it wrong.

Let’s consider an adorable universe where all of space consists of the picture of this puppy:

credit: Torsten Dettlaff/

So cute. If we squeeze space in one direction, we make all distances smaller, we literally squeeze points together. Like this:Notice that we are scaling down distances, not chopping space off. The puppy is still all there, its ears got closer together. All the universe we started with is still there. Gravitational waves squeeze bits of space, while stretching others. Kind of like this¹:

Basically, stretching a portion of space means pulling everything in that portion of space away from everything else, by the same proportion.

Needless to say, spacetime is more complicated than puppies, but this should give an idea.

How do you measure a change of space itself?

If every length changes the same way, you can’t use rulers to measure changes in space, because they change too. To explain how it’s done, I might need some math. I promise it won’t hurt and I’ll use puppies to make it better.

Let’s say we have two perfectly identical puppies², Brian and Stella.

credit: Chiemsee2016/pixabay

They run to fetch a ball 50m away from us. They always run at 10 meters per second (freakin fast puppies), so it takes them 10 seconds to fetch the ball: 5 seconds to cross the 50m to the ball, and another 5 on the way back.


How Stella and Brian breathe as they run to fetch the ball. Since their paths are the same length, and the puppies start identically, they come back perfectly in sync.

These puppies also breathe very regularly, taking a breath every two seconds (one second breathe in, one breathe out). So, if they both start running with a breath in, by the time they come back with the ball, each of them will have taken exactly 5 breaths (one every two seconds), and they both arrive breathing out.

Good girl! Good boy!

Ok, now we throw the ball again, but an amazingly strong gravitational wave passes through, stretching Brian’s way path to 100m, and squeezing Stella’s to 25m. If they leave at the exact same time, breathing in, and running at the same speed, they will not come back together anymore: Stella arrives first, after 5 seconds (half as much as before, because she has half the distance), breathing in, whereas Brian arrives after 20 seconds breathing out.


What changed now that a gravitational wave stretched Brian’s path and squeezed Stella’s. Stella has less distance to cover and breathes fewer times than Brian: now they come back out of sync.

And that is how we measure if space itself changed. We can tell the two paths are not the same length anymore, because identical puppies come back out of sync.

Real dogs, of course, don’t breathe that regularly, or change speed instantly, regardless of how much you train them. You know what behaves like that, though? Light. Light travels at the same speed no matter what, and each “color” has its constant frequency (it breathes regularly in time).

LIGO sends trains of perfectly identical “laser puppies” on perfectly identical paths and measures any teeny tiny difference in their “breathing” (their wave phase) when they come back.

Bonus (without puppies): doesn’t the laser get blue- or red-shifted?

It should indeed. Like the stretching of space between us and distant galaxies shifts their light towards longer wavelengths, so the passing of gravitational waves shifts the light of the laser inside LIGO.

However, despite the scientists’ best efforts, lasers at LIGO can only produce a range of wavelength. They’re extremely close to each other, like between 1063.99999999999999999nm and 1064.00000000000000001nm. Yet, gravitational waves change them by ten times less than that, so it’s impossible to distinguish the effect from the instrument uncertainty.

Also, the effect the time it takes light to cross space is waaaaaay larger, so it’s “easier” to measure.

A huge thank you to my friend Leila, who patiently and graciously explained all this stuff to me

If you want more

… then you must really be into gravitational waves! I got a few posts for you, and this great video on how freakin hard it is to make the measurements.


¹Gravitational waves don’t quite work like that. It’s just an example, please don’t take it literally.

²These are not real puppies, these are example puppies. Please, for goodness’ sake, PLEASE don’t take this literally.

Cover photo: CC0 Chiemsee2016/pixabay

Why o why are rockets painted that way?

A few months ago, during a break at a conference, I met a very interesting young engineer. She told me she worked for Space X (you know, the Elon Musk ones, with the reusable rockets); particularly, in the team that takes care of painting the rockets. Our conversation was brief and left me wondering: do rockets need engineering teams just for the paint?

As it turns out, they always have.

In the old days, paint was used to keep an eye on the rocket’s roll (its spin around its length). Instruments on board don’t necessarily realize, amidst the phenomenal launch forces, the enormous acceleration, and the vibration.

Looking at how the large white and black checkered or striped patterns moved, you could check on it, just by looking, from the control center.

Different parts were also painted differently. Should any problem arise in flight, technicians could see in what part of the rocket it originated. Also, the rocket probably exploded as a result of the breakdown, so they only had a few seconds of grainy footage to go on. So they took any and all help.

Nowadays computer image analysis got much better (probably better than humans) and paint lost part of these functions. NASA still uses paint schemes at times, but most recent ESA and SpaceX rockets are just white.

A 2009 picture of a piece of NASA’s Ares. The Z-pattern is particularly useful to track roll. credit: NASA

Discovery launching, now you know why the tank is orange. credit: NASA, via commons

Why white? Because the rockets have to stand a long time in (hopefully) good weather in typically warm locations—like Florida. The white coat of paint keeps the frigid fuels from boiling off the tank. However, if it’s not strictly necessary, the tanks stay unpainted to save weight. It was the case, for example, of the iconic orange tanks for the shuttle.

When it’s used, paint must withstand being close to the freezing fuels, then the launch, then the rough conditions in space, and finally the extreme heat of reentry.

New materials must be designed for the job, and techniques must be developed to apply them so that they don’t shed or burn off.

That’s why, to paint a rocket, you needed—and still need—engineers, like the young lady I met.

If you want more
  • Wanna know more than you ever need on SpaceX? There’s a subreddit for that! It gave me many useful tips for this post
  • As for so many things space, NASA has a very interesting page about painting rockets too
  • Why should a rocket want to roll? Vintage Space explains that

Cover photo: CC0 kaboompics/

Wormholes: digging tunnels through space

Sometimes science fiction tells us stories of technology we can almost grasp already, like traveling to Mars. Other times it’s much more far-fetched and outrageous stuff, like wormholes. Since general relativity doesn’t explicitly, entirely forbid them, they have fascinated scientists and authors alike.

An example of a wormhole connecting regions of two-dimensional space. credit:

A wormhole is a tunnel, a shortcut between two far-apart regions in spacetime. The movie Interstellar had many flaws, but at least plausible science (thanks to the supervision of star physicist Kip Thorne). They also explain quite effectively the idea of wormholes: take a sheet of paper and fold it in half, then punch a hole through it. You just created a wormhole in your paper universe.

The entrance should look like a black hole, an inescapable sink where light and matter disappear forever. The exist should be the opposite: a source from which matter and light spring eternal—a white hole. Through a wormhole, you’d be able to cross immense distances in relatively short times. But, probably, not travel in time*.

So do they exist?

For sure we can’t make them. Making wormholes with paper is cute, but it only works because the sheet is two-dimensional and we are comfortable handling three. To create a real wormhole we’d need to work in four dimensions which is a non-starter for now.

It’s also unlikely that naturally-occurring, large wormholes exist. First, at least observing a white hole would give some indication of the existence of wormholes, but we’ve never seen them. Secondly, keeping a macroscopic wormhole open requires something that turns gravity from a force that pulls things together to one that pushes them apart. And we’ve never seen that either.

Still, I find really cool that we can imagine such an outlandish thing and actually reason about them, make sound arguments on how it could or could not work.

A simulation of what a wormhole from the university Tübingen (Germany) to the dunes of Boulogne (France) would look like. CC-BY-SA CorvinZahn/Gallery of Space Time Travel, via commons

If you want more
  • There’s plenty of semi-accurate explanations around about wormholes. But I liked this more serious one on Chalkdust
  • NASA does a great job seriously answering all sorts of wormhole questions on this page
  • Some say black holes are actually entrances of wormholes to other universes. Maybe, maybe not. Black holes are freakin’ weird.

* MINOR SPOILER: In Interstellar, Cooper does sort of travel in time, too. But that only happens after stepping into other dimensions: we’ve already crossed into magic.

Cover photo: CC0 Pexels/pixabay