- Hello, and welcome to the Physics World weekly
- podcast.
- Our guest in this episode is Scott
- Wietekaitis,
- who is a physicist at the Institute of
- Science and Technology
- Austria
- in Vienna.
- Scott studies
- soft
- electrified materials,
- and he's particularly
- interested
- in static electricity,
- which is one of those everyday phenomena that
- turns out to be much more complicated
- than you might think
- for reasons
- that you'll hear about in a moment.
- Physics World's online editor Margaret Harris
- spoke to Scott at the American
- Physical Society's
- Global Physics Summit, which took place in Denver,
- Colorado
- earlier this year.
- Here's that conversation.
- First off, big question.
- What is static electricity, and why is it
- important to study it?
- Static electricity is essentially
- it's the charge that's on objects after they've
- contacted and exchanged charge.
- So you can think of things like rubbing
- a balloon on your hair. The balloon and
- the hair,
- initially start out electrically neutral,
- but when you touch them together, they exchange
- some charge. And when you pull them apart,
- one is now positively charged and one is
- negatively charged.
- Why is it important?
- It's important because it occurs whenever
- any two objects touch. So literally, your
- coat's touching the surface right now, your pants
- touching the chair, your hand touching the phone.
- It's happening all the time. That means it
- happens in nature all the time,
- And therefore, there are tons of physical systems
- in nature whose behavior is highly governed by
- static electricity.
- How did you become interested in this topic?
- I became interested by chance. So during my
- PhD, I worked on problems in granular physics.
- So systems of lots of interacting grains, like,
- sand grains, for example.
- And these are systems where the constituents
- are constantly rubbing, sliding,
- rolling all over each other, and therefore, they
- get electrically charged.
- Most of the time in granular physics, people
- try to ignore this. But during my PhD,
- my advisor decided maybe we should pay attention
- to it.
- And we had no idea
- the kind of can of worms we were
- opening up.
- Why is it such a can of worms?
- Oh, man. It's a can of worms because
- we we don't really know what's going on.
- And if you look at the literature,
- the experiments are extremely difficult
- in the sense that you do one thing
- one day,
- and you can't reproduce it again.
- And there are all sorts of practical reasons
- why the irreproducibility
- is there.
- And as a result of the experiments not
- being able to get things very clean,
- the theorists have basically been unconstrained.
- There's nothing to prevent them from thinking
- all sorts of different fantastical ideas of how
- it works.
- So when you look at the literature, there
- are a bunch of experiments that are inconsistent
- with each other and a bunch of theories
- that are unconstrained.
- And so if you're a graduate student looking
- at it for the first time, it's a
- can of worms. It's just chaotic.
- So what is it we exactly don't understand
- about what happens when two objects contact each
- other? I mean, naively, you might think that
- this surely, there's some sort of microscope these
- days that can show us what's happening there.
- There is no such microscope,
- and I would pay a lot of money
- if there were.
- We don't know I I'm very, conservative, and
- I think we basically don't know anything.
- We don't know what charge is being exchanged.
- We don't know even what the difference is
- between two materials that causes them to exchange
- charge.
- So if I give you balloon and hair,
- for example,
- I don't know if it's the electronic structure
- of the balloon.
- I don't know if it has something to
- do with the chemistry of the balloon, maybe
- the acid base properties.
- Some people have suggested it has to do
- with piezoelectricity
- or flexoelectricity,
- but but we don't actually know. You know?
- Give me a a parameter of the balloon,
- like conductivity,
- electronic structure, work function. We don't know what
- parameter matters.
- So I'd you know, that's why I say
- we're basically clueless.
- So what do the experiments that have been
- done? What do they typically show?
- I think a typical experiment is to
- well, a very classical experiment is to take
- a group of materials, say five materials,
- ideally try
- touching
- every pair together
- and then seeing if there's kind of an
- ordering,
- which one charges the most positively,
- which one charges maybe negative to the first
- one but positive to the rest, etcetera.
- And so this is called a triboelectric series.
- There are lots of instances historically where people
- have found triboelectric series in the data they
- work with.
- And then, of course, the goal is to
- try to correlate that triboelectric series with some
- property of the materials so we can identify
- the property that matters.
- But the thing is when you look at
- those typical kinds of experiments,
- one group will say, hey. Look. We got
- this series. It orders this way. It's because
- of the acid based properties of the materials.
- And the next one will say, hey. Look.
- It has to do with the electronic structure
- of the materials.
- And so you you get these very, you
- know, classical kinds of experiments.
- The series from one lab to the next
- don't agree, and the interpretations from one lab
- to the next don't agree.
- Does that answer your question? I think it
- does. Yeah. Everybody is trying to do their
- best. It's just such a messy system that
- it's hard to see the forest for the
- trees, I guess. Yeah. I might get one
- set of results in my lab, and so
- I think it's the interpretation,
- that makes sense to me. But you do
- it in your lab, and you get a
- different triboelectric series, and you get a different
- interpretation.
- So what specific question have you been trying
- to answer in the latest run of work
- you've been doing for the past couple of
- years? So we've been concentrating
- on a very particular
- and quirky
- aspect of this topic.
- Most of the experiments I just described, people
- rub different materials together.
- We're working with always rubbing the same materials
- together.
- And it's been known for a long time
- that even if you touch, you know, two
- balloons together, so to speak, they always exchange
- charge.
- The rationale for working with identical materials
- is that when you work with different materials,
- you know, they differ in so many possible
- ways,
- and it's hard to nail down the one
- that matters.
- But if you work with materials that should
- be nominally identical,
- it might be easier to find the thing
- about them that's different. And then we can
- actually hone in on what is different faster.
- So, basically, we study why identical things exchange
- charge. And what specific identical things are you
- looking at? Can you describe your experimental setup?
- Sure. In the recent work, we work with
- oxides,
- specifically oxides that are electrical insulators.
- These are materials like silicon dioxide or quartz
- or alumina.
- They're actually very common materials
- on the surface of the Earth or on
- other planets. In fact, they're the most common
- material on the surface of the Earth.
- We're interested in these materials
- because they're so common, so they really matter
- in natural settings, like in the sand of
- a dust storm in the Sahara
- or in the ash cloud of a volcano.
- And what we do in our experiments is
- we take
- a particle
- of one material, so silica, say, and we
- actually make it float in the air with
- acoustic levitation.
- So acoustic levitation is a way to levitate
- objects with sound.
- We can then take that particle and make
- it collide
- with a plate
- made of the same material,
- also using the acoustic levitation.
- So we get a nice clean contact where
- charge is exchanged.
- And then finally, we can measure how much
- charge was exchanged in that contact
- also with the acoustic levitation.
- And the reason we use acoustic levitation is
- because a giant pitfall of these experiments is
- if you touch the thing you wanna measure
- the charge of to manipulate it, you change
- the charge.
- So it's Schrodinger's sphere, basically. You can't, yeah,
- you can't,
- interact with your system without changing it. So
- you need to somehow interact with your system
- in a touch free way.
- And like I said, historically, this is a
- big pitfall is adding charge to objects that
- is not wanted. And so acoustic levitation is
- a wonderful tool,
- because we can hold things without touching them.
- So how do you actually measure how much
- charge is on these little spheres that are
- levitating acoustically above a plate? So imagine I
- have a a little sphere and an acoustic
- trap.
- The acoustic trap is basically a speaker and
- a reflector plate,
- and the particle gets attracted to the node
- of the standing wave in that speaker reflector
- geometry,
- it looks like it's on a spring. Right?
- Because everything that's held at
- a potential energy minimum looks like it's on
- a spring. So what we actually do is
- we apply an external electric field to shake
- it on that acoustic spring. And then by
- looking at how much it shakes in response
- to the external field, we can measure the
- charge. And it's actually it turns out to
- be wonderfully precise.
- So I can show you a grain,
- a particle, a sphere that's, 500 microns in
- diameter. It's big enough to see with your
- eye. And we can tell you how much
- charge it has on it to plus or
- minus 500 electrons.
- So, you know, kind of a countable number
- of electrons on a macroscopic object.
- Wow.
- So I think one thing you've worked out
- is that there are history dependent effects. Like,
- it matters what's happened to this little silicon
- dioxide sphere before you do an experiment on
- it.
- It's extremely frustrating.
- It was extremely frustrating for a long time
- because we could not get consistent results,
- from one experiment to the next.
- In general, the idea that the history of
- an object matters in this effect
- had not really been proposed.
- And for the systems we've been working with,
- we found out that when we really pay
- attention to the history and keep track of,
- in the case with these oxides,
- the environmental history,
- then we can start to see patterns that
- make sense. But, indeed, it was extremely frustrating
- before we understood that the history was the
- the key variable.
- So what is it about the history that
- matters? How did you figure out what was
- going on? Well,
- we had been suspecting, like many people, that
- adsorbed water on the surface of, an oxide
- particle or an oxide plate
- was the thing that mattered. So that was
- a suspicion that a lot of people have
- had for a long time.
- And so we were looking for ways to
- kind of change the water on the surface.
- One of the simplest ways is actually to
- to bake an object, to heat it up
- to an elevated temperature, to get rid of
- the water.
- But when we did that, we saw, number
- one, that the changes we expected
- were not going in the right direction. And
- in fact, they were going kind of in
- random directions.
- And so this suggested that it maybe, you
- know the baking certainly had an effect, but
- it wasn't related to water.
- So what if it wasn't due to water,
- what was it about the history of these
- spheres that was causing this different patterns of
- charge?
- So what we learned is that when an
- object is sitting in some environment, like the
- air we're sitting in right now,
- molecules from the environment
- adsorb to the surface. They get stuck to
- the surface.
- The molecule that everyone thinks about the most
- is water. That's why we were thinking about
- it. But water is not the only molecule
- in town.
- In every environment, there are all sorts of
- small organic molecules,
- organic in the kind of carbon hydrogen chemistry
- sense, not like, you know, pieces of hamburger
- or something, but little molecules that exist naturally
- in our environment, in the environment of Mars,
- in the environment of space
- that also attach to the surface.
- And so while we were thinking so hard
- that it must be the water on the
- surface, it turns out it's these carbon based
- molecules that were the ones that actually mattered.
- And that's why following the water route led
- us off track.
- So is it the case, like, the more
- carbon on the surface means it's more likely
- to charge positively or negatively? Do you know
- the difference?
- That's the basic trend we see. If we
- have one surface
- of one material, say silica,
- that has a lot of carbon on its
- surface because we've left it in an environment
- for a long time
- and has therefore acquired a lot of this
- carbon, and we have another surface that by
- some experimental means, has been stripped of carbon.
- The one that has no carbon
- charges negatively to the one that has carbon.
- And that's the key thing we found.
- So would it make sense to if you're
- wanting to understand these charging relationships
- to put the particle in a in a
- vacuum, an ultra high vacuum, and remove all
- this carbon stuff from the environment?
- That's a funny question because we actually we
- actually were cued into the idea of these
- carbon molecules mattering
- from surface scientists who work in ultra high
- vacuum.
- Whenever they want to study a surface where
- they can, you know, really, for example, use
- a vacuum AFM to see every atom on
- a material
- or to, you know, do some kind of
- spectroscopy or something on a pure material.
- The first step they do after they put
- it in the vacuum chamber is get rid
- of all of the carbon.
- And so when we started working with surface
- scientists, they said, oh, yeah. We put your
- sample in the vacuum chamber and got rid
- of the carbon, and here's your data.
- And we had to do a double take.
- What? You got rid of the carbon?
- What do you mean?
- The funny thing is is that when they
- do their experiments,
- they put it in, they get rid of
- the carbon, they do their thing. If they
- leave the sample in the vacuum chamber overnight,
- the carbon's all back. So there's really not
- an environment where you can get rid of
- this,
- not even in the best ultra high vacuum.
- It's really I mean, every environment we know
- of has
- these adventitious molecules,
- that tend to cover a surface,
- you know, on a kind of hours to
- days time scale. So it just gets all
- schmutzed up Schmutz'd up. Yeah. In fact, that
- was the word we were using when we
- first
- were, you know, thinking what was on the
- surface. The word we had for it was
- schmutz.
- So what's the mechanism behind this schmutz affecting
- charging? Do you know what's going on at
- sort of a nanoscale level?
- We don't know. We don't know what the
- mechanism is that actually causes the schmutz to
- matter. We just know the schmutz matters.
- So the claim, you know, that we're making
- is we've identified
- the symmetry breaking parameter that causes two identical
- materials to be different.
- At least in the case of oxides. I
- wouldn't extend it beyond that.
- But, you know, we don't we don't know
- the mechanism. Now the the great goal is
- to, of course,
- use the fact that we know what matters
- to focus more on what how it matters.
- Yeah.
- You mentioned that it's just for oxides that
- you're saying that this is true. Are there
- other things that are responsible for different symmetry
- breaking and charging for other materials?
- Yeah. So the experiments we've done,
- in this current project are all with,
- oxides and specifically insulating oxides.
- So silica, quartz, alumina, sapphire,
- spinel,
- magnesium oxide,
- they're all materials that are electrically good insulators.
- They are very similar in the sense that
- they they have high surface energies, which means
- they readily absorb these these adsorbates, these molecules.
- And so they're all very similar, and we
- found this,
- you know, this carbon effect exists for all
- of them that we've tried. And that's about
- altogether, you know, probably close to 10 different
- of these oxides.
- This class of materials is it's, inorganic,
- meaning that the material itself does not really
- have a bunch of organic chemistry going on
- inside. There are other materials like polymers, for
- example, plastics,
- where we see very different effects.
- So in polymers, we actually see that the
- contact history of a sample matters. So literally
- a sample that has
- touched
- more things than another sample will charge with
- a sign based on how many things it's
- touched.
- So there are for sure different mechanisms going
- on for different materials classes,
- and this is what's causing us to be
- conservative and cautious
- about extending claims beyond what we have seen.
- I would not be surprised if this carbon
- effect exists pretty broadly for
- inorganic materials that are poor electrical conductors.
- So, you know, an example of that would
- be like calcium fluoride, for example. It's an
- inorganic material. In many ways, it's very similar
- to, say, a piece of silica because it's
- not a good conductor.
- Things readily absorb to the surface.
- But, you know, showing it's
- true for that case would require going and
- doing those experiments, and we just haven't gotten
- to it yet.
- So what's the next step for you, you
- and your team? So what we wanna do
- is,
- start to try to understand,
- like you asked, what is the mechanism
- that makes carbon matter? And we basically have
- two ideas towards doing that. In the experiments
- we did so far, you know, we could
- measure the charge exchange
- of a sphere and a plate
- after they'd gone through certain treatments to change
- the amount of carbon on one surface or
- the other.
- And then independently, in a different setup, we
- could actually probe the amount of carbonaceous molecules
- on the surface.
- But we had to do those at separate
- times. And so the first thing we wanna
- do is build a setup where we can
- do those both at the same time so
- I can watch
- the evolution
- of the exchange of charge with the evolution
- of the growth of this carbon layer on
- the surface. That's one thing. The next thing
- we wanna do is
- kind of remove
- all of the natural molecules from the surface
- and put on artificial ones with specific properties.
- And then by kind of, you know, testing
- all possible combinations
- in a catalog of these artificial molecules,
- maybe that also tells us something about the
- mechanism
- if, for example, we see trends, you know,
- based on kind of the functionality of these
- molecules.
- So TaylorMade Schmutz is a TaylorMade Schmutz. That's
- where that's I should use that in the
- grant proposal.
- Scott Waitakitis, thank you very much for speaking
- with us. It's been a pleasure. Thank you
- very much.
- That was Scott Waitakitis
- in conversation
- with Physics World's Margaret Harris.
- Thanks to both of them for a fascinating
- discussion
- about static
- electricity.
- I'm afraid that's all the time we have
- for this week's podcast.
- I'm Hamish Johnston, and our producer is Fred
- Iles.
- The music for this episode is called one
- three seven,
- and it was composed and performed by the
- physicist
- Philip Moriarty.
- We'll be back again next week.