Scientists don’t know how static electricity works

0:00 Static electricity is everywhere,

0:02 from balloons to lightning storms to giving yourself

0:05 a shock by touching a door handle.

0:07 This common phenomenon been studied since the time of

0:10 the Ancient Greeks so you might be surprised

0:12 to learn that scientists don’t know that

0:15 much about how static electricity actually works.

0:18 SCOTT WAITUKAITIS: I would say we know virtually nothing.

0:20 Some people will get mad at me for saying that if they watch this video.

0:24 I think we know some things about particular situations.

0:27 But kind of the overall picture of what happens most of the time,

0:32 for most materials, we don't know much.

0:34 VOICE OVER: The basic idea is that static electricity is an imbalance of charges

0:39 that occurs when charged ions or electrons move from one object to another.

0:43 This kind of charge transfer is often

0:46 called the triboelectric effect or tribocharging

0:48 and it’s caused by two surfaces rubbing against each other,

0:51 or even just coming into contact.

0:53 SCOTT: Whenever any two objects touch, they exchange charge.

0:57 It's literally unavoidable.

0:58 So, like, you know, right now sitting in my chair, I'm moving around.

1:02 I'm exchanging charge with the chair.

1:03 VOICE OVER: So we know it happens but we don’t really know why.

1:07 SCOTT: For example, if I give you rabbit's fur and a balloon,

1:10 and I rub the balloon on the rabbits fur,

1:13 I can't even tell you what is different

1:15 about the rabbits fur and the balloon that makes them exchange charge.

1:19 Could it be their electronic structure?

1:22 Chemicals on the surface?

1:23 Their thermoelectric properties?

1:25 Flexoelectricity?

1:25 So I literally can't even tell you what is the driving parameter.

1:29 VOICE OVER: And if we don’t understand what’s driving static charge

1:32 then we don’t really understand what

1:34 causes lightning in thunderclouds or volcanoes,

1:36 or what makes little bits of rock dust

1:39 come together around a star to form planets,

1:41 or how to avoid getting electric shocks off door handles.

1:44 SCOTT: Industries care a lot about static electricity

1:47 mostly because they're trying to avoid sparks.

1:50 A classic example that's maybe a bit older now is a coal mine explosion.

1:54 But even in high tech settings, for example,

1:56 in a clean room, where they make computer chips.

1:58 These are environments where you have to

2:01 have everything extremely free of airborne particles.

2:04 Because if that particle lands on your computer chip while you're making it,

2:08 it will mess everything up.

2:09 And the reason those particles attach is because of static electricity.

2:12 VOICE OVER:

2:13 Scott's one of a relatively small group of researchers trying to understand

2:16 this at the fundamental level, which is easier said than done.

2:20 SCOTT: And it's actually a really hard problem because, you know, when you,

2:22 when you rub two things together and they get really charged,

2:25 it seems like, oh, wow, they're really charged,

2:27 my hair is sticking to the balloon, it's so big!

2:29 But actually, you know, maybe 1 in 100,000 surface atoms,

2:33 gained or received a charge.

2:35 And so when you're trying to do any kind of

2:38 technique to see what that 1 in 100,000 was,

2:40 it's a lot like looking for a needle in a haystack.

2:42 VOICE OVER: So the team set out on a needle hunt.

2:45 And they started with an even more intriguing puzzle than balloons and hair…

2:50 why does charge still move from one object to

2:52 another when they’re both made of the same thing?

2:55 GALIEN GROSJEAN: Let’s say you have a grain and you have another grain,

2:58 and they're both made of exactly the same material,

3:01 and if you contact them and you measure the

3:03 charge maybe this one will be a bit positive,

3:05 and this one will be a bit negative.

3:07 And the question we wanted to answer is what's the difference?

3:10 VOICE OVER: Galien and Scott wanted to find the

3:13 difference between two apparently identical objects.

3:15 They were working with silicon dioxide,

3:17 the compound in quartz and a big component of sand,

3:20 and their approach to experimentation was to

3:23 simplify things as much as possible.

3:25 Take two objects— in this case a grain and a flat plate made of the

3:29 same silicon dioxide— hit them together then see how the charge has changed.

3:35 And do this over and over again in different controlled conditions.

3:39 Setting up the experiment was tricky.

3:41 GALIEN: You cannot touch the grains in any way,

3:43 because if you touch them, you're going

3:45 to give them some charge,

3:46 and that's going to be more charged than they gain through the collision.

3:50 So what we needed to do is isolate the grains.

3:53 So we trap them in an ultrasonic standing wave

3:56 and then you turn off briefly the acoustic field,

3:59 you let the grain fall on the plate, bounce, and then you capture it again.

4:02 VOICE OVER: After each collision they measured the charge.

4:05 Half the grains became more positive, half negative, seemingly at random.

4:11 But the same grain always went in

4:13 the same direction— either positive or negative.

4:15 Clearly there must be some invisible difference

4:18 that determines which way it goes.

4:21 GALIEN: Things that people have named as

4:24 being important in tribocharging include humidity,

4:26 roughness, surface chemistry, pH, mechanical properties, strain,

4:29 electronic structure, zeta potential, flexoelectric constant,

4:31 seebeck coefficient, dielectric constant, hydrophobicity, et cetera, et cetera.

4:34 And you can probably find a scientist that will tell

4:37 you that any of those is the most important one.

4:40 VOICE OVER: So the team started testing theories,

4:43 trying to find the thing that makes the difference.

4:46 And finally they landed on something that did seem to make a difference.

4:51 It was baking.

4:52 GALIEN: If you take the positively charging particle and

4:56 you bake it even at relatively low temperatures,

4:58 like below 100 degrees, and you do the measurement

5:02 again, it's going to charge more negative now.

5:04 VOICE OVER: Baking the grains was the key.

5:07 And it turns out that the magic ingredient in this cooking process was carbon.

5:12 All around you are tiny little carbon compounds

5:14 that float through the air and land on

5:17 everything— it’s often just a few molecules—

5:20 but this stuff is absolutely everywhere

5:22 and experiments showed that baking got rid of the carbon compounds,

5:27 cleaning the surface of the oxide grains.

5:30 This made each grain charge more negatively than it did before

5:34 until the carbon started slowly landing on the

5:36 surface again and the effect wore off.

5:39 So could bits of carbon explain the

5:43 invisible differences between otherwise identical samples?

5:45 GALIEN: What exactly goes on the surface is a little bit random.

5:48 It depends a little bit on whatever molecules were in the air at the time.

5:53 And so this tells you that if you take two

5:56 really clean samples and you store them in a box

5:58 for a few days, they're going to end up slightly different.

6:02 And the slight difference explains why certain samples want to charge

6:05 positive and others want to charge negative.

6:07 VOICE OVER: Finally, there was a clear signal,

6:10 a variable that actually impacted the static

6:12 charging of these little oxide grains.

6:15 But is this really the answer to the centuries old puzzle of static electricity?

6:19 Are carbon compounds the one thing that always makes the difference?

6:23 SCOTT: So we're very cautious just because,

6:25 this is such a messy topic,

6:27 you know, that I personally have learned my lesson not to,

6:32 say that, you know, the thing we saw here matters everywhere.

6:35 So I would not be surprised at all if

6:38 these molecules from the environment matter for other materials.

6:42 But until we get a big body of experimental evidence,

6:44 I'm not going to stick my neck out.

6:47 VOICE OVER: It could be that surface carbon only matters for

6:50 the oxides which they tested in this particular

6:52 experiment; different materials could have different key factors.

6:57 But knowing how it works in oxides is pretty useful.

7:00 SCOTT: They're actually one of the most common materials in nature.

7:03 So something like 60% of the Earth's

7:06 crust is made from silicon dioxide.

7:08 And the same is true for most other terrestrial planets.

7:11 So recently it was discovered that there's lightning on Mars.

7:14 And that was because of the

7:15 oxide particles on Mars colliding,

7:17 getting charged up and then somehow getting separated to create,

7:20 you know, lightning and dust storms.

7:22 VOICE OVER: Whether the lightning is on another planet,

7:25 or in the clouds pouring from an active volcano,

7:27 it all comes down to tiny static interactions between grains.

7:31 GALIEN: If you look at the system,

7:33 for example, like, volcanic lightning, you have these,

7:36 silicate particles that are thrown very violently in the air.

7:40 And, the temperature changes,

7:41 the pressure, humidity, there's all of these complicated things happening.

7:45 But if you don't understand just a little,

7:49 microscopic mechanism, just what's happening when

7:53 particle A and particle B collide, then how can you explain the big picture?

7:57 Right?

7:58 And there are so many little open questions that maybe you could answer

8:03 by just understanding fundamentally what's happening

8:04 at the scale of the particles.