WEBVTT 1 00:00:07.919 --> 00:00:11.039 - Hello, and welcome to the Physics World weekly 2 00:00:11.039 --> 00:00:11.539 - podcast. 3 00:00:12.320 --> 00:00:14.980 - Our guest in this episode is Scott 4 00:00:15.335 --> 00:00:15.835 - Wietekaitis, 5 00:00:16.695 --> 00:00:19.495 - who is a physicist at the Institute of 6 00:00:19.495 --> 00:00:20.875 - Science and Technology 7 00:00:21.414 --> 00:00:21.914 - Austria 8 00:00:22.295 --> 00:00:23.035 - in Vienna. 9 00:00:23.734 --> 00:00:24.635 - Scott studies 10 00:00:24.935 --> 00:00:25.435 - soft 11 00:00:25.815 --> 00:00:27.035 - electrified materials, 12 00:00:27.670 --> 00:00:28.730 - and he's particularly 13 00:00:29.109 --> 00:00:29.609 - interested 14 00:00:29.989 --> 00:00:31.530 - in static electricity, 15 00:00:32.310 --> 00:00:35.510 - which is one of those everyday phenomena that 16 00:00:35.510 --> 00:00:37.609 - turns out to be much more complicated 17 00:00:38.070 --> 00:00:39.210 - than you might think 18 00:00:39.670 --> 00:00:40.489 - for reasons 19 00:00:40.789 --> 00:00:43.125 - that you'll hear about in a moment. 20 00:00:44.085 --> 00:00:47.304 - Physics World's online editor Margaret Harris 21 00:00:47.765 --> 00:00:49.945 - spoke to Scott at the American 22 00:00:50.325 --> 00:00:51.225 - Physical Society's 23 00:00:51.524 --> 00:00:55.545 - Global Physics Summit, which took place in Denver, 24 00:00:55.604 --> 00:00:56.104 - Colorado 25 00:00:56.899 --> 00:00:58.120 - earlier this year. 26 00:00:58.579 --> 00:00:59.960 - Here's that conversation. 27 00:01:08.579 --> 00:01:09.959 - First off, big question. 28 00:01:10.265 --> 00:01:12.424 - What is static electricity, and why is it 29 00:01:12.424 --> 00:01:13.724 - important to study it? 30 00:01:14.584 --> 00:01:16.364 - Static electricity is essentially 31 00:01:16.744 --> 00:01:19.965 - it's the charge that's on objects after they've 32 00:01:20.185 --> 00:01:21.965 - contacted and exchanged charge. 33 00:01:22.504 --> 00:01:24.104 - So you can think of things like rubbing 34 00:01:24.104 --> 00:01:26.480 - a balloon on your hair. The balloon and 35 00:01:26.480 --> 00:01:26.959 - the hair, 36 00:01:27.439 --> 00:01:29.379 - initially start out electrically neutral, 37 00:01:29.760 --> 00:01:32.159 - but when you touch them together, they exchange 38 00:01:32.159 --> 00:01:34.000 - some charge. And when you pull them apart, 39 00:01:34.000 --> 00:01:36.159 - one is now positively charged and one is 40 00:01:36.159 --> 00:01:37.140 - negatively charged. 41 00:01:37.840 --> 00:01:38.819 - Why is it important? 42 00:01:39.120 --> 00:01:41.534 - It's important because it occurs whenever 43 00:01:41.914 --> 00:01:45.134 - any two objects touch. So literally, your 44 00:01:45.674 --> 00:01:48.394 - coat's touching the surface right now, your pants 45 00:01:48.394 --> 00:01:50.734 - touching the chair, your hand touching the phone. 46 00:01:50.875 --> 00:01:52.954 - It's happening all the time. That means it 47 00:01:52.954 --> 00:01:54.575 - happens in nature all the time, 48 00:01:54.920 --> 00:01:57.319 - And therefore, there are tons of physical systems 49 00:01:57.319 --> 00:02:00.359 - in nature whose behavior is highly governed by 50 00:02:00.359 --> 00:02:01.259 - static electricity. 51 00:02:01.879 --> 00:02:03.819 - How did you become interested in this topic? 52 00:02:04.920 --> 00:02:07.239 - I became interested by chance. So during my 53 00:02:07.239 --> 00:02:10.539 - PhD, I worked on problems in granular physics. 54 00:02:10.895 --> 00:02:13.775 - So systems of lots of interacting grains, like, 55 00:02:14.175 --> 00:02:15.474 - sand grains, for example. 56 00:02:16.334 --> 00:02:18.754 - And these are systems where the constituents 57 00:02:19.534 --> 00:02:21.555 - are constantly rubbing, sliding, 58 00:02:21.854 --> 00:02:24.480 - rolling all over each other, and therefore, they 59 00:02:24.480 --> 00:02:25.699 - get electrically charged. 60 00:02:26.319 --> 00:02:28.719 - Most of the time in granular physics, people 61 00:02:28.719 --> 00:02:31.379 - try to ignore this. But during my PhD, 62 00:02:31.919 --> 00:02:34.000 - my advisor decided maybe we should pay attention 63 00:02:34.000 --> 00:02:34.659 - to it. 64 00:02:34.960 --> 00:02:36.180 - And we had no idea 65 00:02:36.480 --> 00:02:38.240 - the kind of can of worms we were 66 00:02:38.240 --> 00:02:39.139 - opening up. 67 00:02:39.735 --> 00:02:41.354 - Why is it such a can of worms? 68 00:02:41.894 --> 00:02:44.155 - Oh, man. It's a can of worms because 69 00:02:44.375 --> 00:02:46.314 - we we don't really know what's going on. 70 00:02:46.694 --> 00:02:48.074 - And if you look at the literature, 71 00:02:48.935 --> 00:02:50.794 - the experiments are extremely difficult 72 00:02:51.495 --> 00:02:53.094 - in the sense that you do one thing 73 00:02:53.094 --> 00:02:53.754 - one day, 74 00:02:54.189 --> 00:02:55.810 - and you can't reproduce it again. 75 00:02:56.269 --> 00:02:58.669 - And there are all sorts of practical reasons 76 00:02:58.669 --> 00:02:59.650 - why the irreproducibility 77 00:03:00.030 --> 00:03:00.689 - is there. 78 00:03:01.310 --> 00:03:03.229 - And as a result of the experiments not 79 00:03:03.229 --> 00:03:04.930 - being able to get things very clean, 80 00:03:05.709 --> 00:03:07.729 - the theorists have basically been unconstrained. 81 00:03:08.215 --> 00:03:10.394 - There's nothing to prevent them from thinking 82 00:03:10.694 --> 00:03:12.854 - all sorts of different fantastical ideas of how 83 00:03:12.854 --> 00:03:13.435 - it works. 84 00:03:14.055 --> 00:03:15.414 - So when you look at the literature, there 85 00:03:15.414 --> 00:03:17.194 - are a bunch of experiments that are inconsistent 86 00:03:17.254 --> 00:03:19.254 - with each other and a bunch of theories 87 00:03:19.254 --> 00:03:19.995 - that are unconstrained. 88 00:03:20.620 --> 00:03:22.300 - And so if you're a graduate student looking 89 00:03:22.300 --> 00:03:23.340 - at it for the first time, it's a 90 00:03:23.340 --> 00:03:25.599 - can of worms. It's just chaotic. 91 00:03:26.379 --> 00:03:28.700 - So what is it we exactly don't understand 92 00:03:28.700 --> 00:03:31.340 - about what happens when two objects contact each 93 00:03:31.340 --> 00:03:33.435 - other? I mean, naively, you might think that 94 00:03:33.514 --> 00:03:35.435 - this surely, there's some sort of microscope these 95 00:03:35.435 --> 00:03:37.534 - days that can show us what's happening there. 96 00:03:38.074 --> 00:03:39.294 - There is no such microscope, 97 00:03:39.594 --> 00:03:40.955 - and I would pay a lot of money 98 00:03:40.955 --> 00:03:41.694 - if there were. 99 00:03:42.235 --> 00:03:45.114 - We don't know I I'm very, conservative, and 100 00:03:45.114 --> 00:03:46.895 - I think we basically don't know anything. 101 00:03:47.469 --> 00:03:49.650 - We don't know what charge is being exchanged. 102 00:03:50.509 --> 00:03:52.849 - We don't know even what the difference is 103 00:03:53.150 --> 00:03:55.469 - between two materials that causes them to exchange 104 00:03:55.469 --> 00:03:55.969 - charge. 105 00:03:56.509 --> 00:03:58.590 - So if I give you balloon and hair, 106 00:03:58.590 --> 00:03:59.250 - for example, 107 00:03:59.724 --> 00:04:01.805 - I don't know if it's the electronic structure 108 00:04:01.805 --> 00:04:02.544 - of the balloon. 109 00:04:02.844 --> 00:04:04.284 - I don't know if it has something to 110 00:04:04.284 --> 00:04:06.205 - do with the chemistry of the balloon, maybe 111 00:04:06.205 --> 00:04:07.504 - the acid base properties. 112 00:04:08.205 --> 00:04:10.044 - Some people have suggested it has to do 113 00:04:10.044 --> 00:04:10.705 - with piezoelectricity 114 00:04:11.245 --> 00:04:11.985 - or flexoelectricity, 115 00:04:13.360 --> 00:04:15.860 - but but we don't actually know. You know? 116 00:04:16.080 --> 00:04:18.399 - Give me a a parameter of the balloon, 117 00:04:18.399 --> 00:04:19.139 - like conductivity, 118 00:04:20.399 --> 00:04:22.319 - electronic structure, work function. We don't know what 119 00:04:22.319 --> 00:04:23.300 - parameter matters. 120 00:04:24.319 --> 00:04:25.680 - So I'd you know, that's why I say 121 00:04:25.680 --> 00:04:26.819 - we're basically clueless. 122 00:04:27.985 --> 00:04:30.144 - So what do the experiments that have been 123 00:04:30.144 --> 00:04:31.845 - done? What do they typically show? 124 00:04:32.224 --> 00:04:34.805 - I think a typical experiment is to 125 00:04:35.185 --> 00:04:37.185 - well, a very classical experiment is to take 126 00:04:37.185 --> 00:04:39.204 - a group of materials, say five materials, 127 00:04:39.985 --> 00:04:41.125 - ideally try 128 00:04:41.490 --> 00:04:41.990 - touching 129 00:04:42.769 --> 00:04:43.750 - every pair together 130 00:04:44.370 --> 00:04:46.050 - and then seeing if there's kind of an 131 00:04:46.050 --> 00:04:46.550 - ordering, 132 00:04:46.850 --> 00:04:48.550 - which one charges the most positively, 133 00:04:48.850 --> 00:04:51.089 - which one charges maybe negative to the first 134 00:04:51.089 --> 00:04:52.790 - one but positive to the rest, etcetera. 135 00:04:53.410 --> 00:04:55.750 - And so this is called a triboelectric series. 136 00:04:56.495 --> 00:04:59.134 - There are lots of instances historically where people 137 00:04:59.134 --> 00:05:01.134 - have found triboelectric series in the data they 138 00:05:01.134 --> 00:05:01.794 - work with. 139 00:05:02.495 --> 00:05:04.014 - And then, of course, the goal is to 140 00:05:04.014 --> 00:05:06.254 - try to correlate that triboelectric series with some 141 00:05:06.254 --> 00:05:08.175 - property of the materials so we can identify 142 00:05:08.175 --> 00:05:09.209 - the property that matters. 143 00:05:09.930 --> 00:05:11.449 - But the thing is when you look at 144 00:05:11.449 --> 00:05:13.069 - those typical kinds of experiments, 145 00:05:14.169 --> 00:05:15.850 - one group will say, hey. Look. We got 146 00:05:15.850 --> 00:05:17.930 - this series. It orders this way. It's because 147 00:05:17.930 --> 00:05:19.789 - of the acid based properties of the materials. 148 00:05:19.849 --> 00:05:21.370 - And the next one will say, hey. Look. 149 00:05:21.370 --> 00:05:23.209 - It has to do with the electronic structure 150 00:05:23.209 --> 00:05:23.949 - of the materials. 151 00:05:24.795 --> 00:05:26.595 - And so you you get these very, you 152 00:05:26.595 --> 00:05:28.254 - know, classical kinds of experiments. 153 00:05:28.555 --> 00:05:30.314 - The series from one lab to the next 154 00:05:30.314 --> 00:05:32.555 - don't agree, and the interpretations from one lab 155 00:05:32.555 --> 00:05:33.694 - to the next don't agree. 156 00:05:34.154 --> 00:05:35.514 - Does that answer your question? I think it 157 00:05:35.514 --> 00:05:37.115 - does. Yeah. Everybody is trying to do their 158 00:05:37.115 --> 00:05:38.955 - best. It's just such a messy system that 159 00:05:38.955 --> 00:05:41.279 - it's hard to see the forest for the 160 00:05:41.279 --> 00:05:43.279 - trees, I guess. Yeah. I might get one 161 00:05:43.279 --> 00:05:44.560 - set of results in my lab, and so 162 00:05:44.560 --> 00:05:46.180 - I think it's the interpretation, 163 00:05:47.279 --> 00:05:48.720 - that makes sense to me. But you do 164 00:05:48.720 --> 00:05:49.759 - it in your lab, and you get a 165 00:05:49.759 --> 00:05:51.759 - different triboelectric series, and you get a different 166 00:05:51.759 --> 00:05:52.259 - interpretation. 167 00:05:53.125 --> 00:05:55.204 - So what specific question have you been trying 168 00:05:55.204 --> 00:05:56.805 - to answer in the latest run of work 169 00:05:56.805 --> 00:05:58.004 - you've been doing for the past couple of 170 00:05:58.004 --> 00:05:59.544 - years? So we've been concentrating 171 00:06:00.404 --> 00:06:01.865 - on a very particular 172 00:06:02.564 --> 00:06:03.384 - and quirky 173 00:06:03.764 --> 00:06:04.980 - aspect of this topic. 174 00:06:05.540 --> 00:06:07.300 - Most of the experiments I just described, people 175 00:06:07.300 --> 00:06:08.839 - rub different materials together. 176 00:06:10.100 --> 00:06:13.139 - We're working with always rubbing the same materials 177 00:06:13.139 --> 00:06:13.639 - together. 178 00:06:14.740 --> 00:06:16.180 - And it's been known for a long time 179 00:06:16.180 --> 00:06:18.285 - that even if you touch, you know, two 180 00:06:18.285 --> 00:06:20.925 - balloons together, so to speak, they always exchange 181 00:06:20.925 --> 00:06:21.425 - charge. 182 00:06:22.045 --> 00:06:24.545 - The rationale for working with identical materials 183 00:06:25.085 --> 00:06:27.185 - is that when you work with different materials, 184 00:06:27.485 --> 00:06:30.125 - you know, they differ in so many possible 185 00:06:30.125 --> 00:06:30.625 - ways, 186 00:06:30.970 --> 00:06:32.569 - and it's hard to nail down the one 187 00:06:32.569 --> 00:06:33.230 - that matters. 188 00:06:33.689 --> 00:06:35.370 - But if you work with materials that should 189 00:06:35.370 --> 00:06:36.589 - be nominally identical, 190 00:06:37.129 --> 00:06:38.730 - it might be easier to find the thing 191 00:06:38.730 --> 00:06:40.810 - about them that's different. And then we can 192 00:06:40.810 --> 00:06:43.389 - actually hone in on what is different faster. 193 00:06:44.144 --> 00:06:46.785 - So, basically, we study why identical things exchange 194 00:06:46.785 --> 00:06:49.185 - charge. And what specific identical things are you 195 00:06:49.185 --> 00:06:51.444 - looking at? Can you describe your experimental setup? 196 00:06:51.664 --> 00:06:54.485 - Sure. In the recent work, we work with 197 00:06:54.544 --> 00:06:55.044 - oxides, 198 00:06:55.824 --> 00:06:58.245 - specifically oxides that are electrical insulators. 199 00:06:59.209 --> 00:07:02.490 - These are materials like silicon dioxide or quartz 200 00:07:02.490 --> 00:07:03.310 - or alumina. 201 00:07:04.009 --> 00:07:05.629 - They're actually very common materials 202 00:07:06.329 --> 00:07:07.769 - on the surface of the Earth or on 203 00:07:07.769 --> 00:07:09.769 - other planets. In fact, they're the most common 204 00:07:09.769 --> 00:07:11.470 - material on the surface of the Earth. 205 00:07:11.834 --> 00:07:13.294 - We're interested in these materials 206 00:07:13.834 --> 00:07:15.754 - because they're so common, so they really matter 207 00:07:15.754 --> 00:07:17.995 - in natural settings, like in the sand of 208 00:07:17.995 --> 00:07:19.454 - a dust storm in the Sahara 209 00:07:19.995 --> 00:07:22.095 - or in the ash cloud of a volcano. 210 00:07:22.875 --> 00:07:24.634 - And what we do in our experiments is 211 00:07:24.634 --> 00:07:25.214 - we take 212 00:07:26.189 --> 00:07:26.930 - a particle 213 00:07:27.550 --> 00:07:30.670 - of one material, so silica, say, and we 214 00:07:30.670 --> 00:07:32.830 - actually make it float in the air with 215 00:07:32.830 --> 00:07:33.730 - acoustic levitation. 216 00:07:34.189 --> 00:07:36.509 - So acoustic levitation is a way to levitate 217 00:07:36.509 --> 00:07:37.650 - objects with sound. 218 00:07:38.245 --> 00:07:40.165 - We can then take that particle and make 219 00:07:40.165 --> 00:07:40.824 - it collide 220 00:07:41.285 --> 00:07:42.185 - with a plate 221 00:07:42.485 --> 00:07:43.785 - made of the same material, 222 00:07:44.404 --> 00:07:46.105 - also using the acoustic levitation. 223 00:07:46.564 --> 00:07:48.324 - So we get a nice clean contact where 224 00:07:48.324 --> 00:07:49.305 - charge is exchanged. 225 00:07:49.764 --> 00:07:51.605 - And then finally, we can measure how much 226 00:07:51.605 --> 00:07:53.729 - charge was exchanged in that contact 227 00:07:54.189 --> 00:07:55.810 - also with the acoustic levitation. 228 00:07:56.509 --> 00:07:58.589 - And the reason we use acoustic levitation is 229 00:07:58.589 --> 00:08:01.810 - because a giant pitfall of these experiments is 230 00:08:02.029 --> 00:08:04.430 - if you touch the thing you wanna measure 231 00:08:04.430 --> 00:08:07.069 - the charge of to manipulate it, you change 232 00:08:07.069 --> 00:08:07.729 - the charge. 233 00:08:08.474 --> 00:08:11.274 - So it's Schrodinger's sphere, basically. You can't, yeah, 234 00:08:11.274 --> 00:08:11.754 - you can't, 235 00:08:12.634 --> 00:08:14.314 - interact with your system without changing it. So 236 00:08:14.314 --> 00:08:15.914 - you need to somehow interact with your system 237 00:08:15.914 --> 00:08:17.134 - in a touch free way. 238 00:08:17.594 --> 00:08:19.194 - And like I said, historically, this is a 239 00:08:19.194 --> 00:08:21.834 - big pitfall is adding charge to objects that 240 00:08:21.834 --> 00:08:24.759 - is not wanted. And so acoustic levitation is 241 00:08:24.759 --> 00:08:25.819 - a wonderful tool, 242 00:08:26.360 --> 00:08:28.459 - because we can hold things without touching them. 243 00:08:28.600 --> 00:08:30.919 - So how do you actually measure how much 244 00:08:30.919 --> 00:08:33.320 - charge is on these little spheres that are 245 00:08:33.320 --> 00:08:36.574 - levitating acoustically above a plate? So imagine I 246 00:08:36.574 --> 00:08:38.495 - have a a little sphere and an acoustic 247 00:08:38.495 --> 00:08:38.995 - trap. 248 00:08:39.375 --> 00:08:41.694 - The acoustic trap is basically a speaker and 249 00:08:41.694 --> 00:08:42.754 - a reflector plate, 250 00:08:43.294 --> 00:08:45.714 - and the particle gets attracted to the node 251 00:08:46.414 --> 00:08:49.840 - of the standing wave in that speaker reflector 252 00:08:49.840 --> 00:08:50.340 - geometry, 253 00:08:50.799 --> 00:08:52.799 - it looks like it's on a spring. Right? 254 00:08:52.799 --> 00:08:54.580 - Because everything that's held at 255 00:08:54.960 --> 00:08:56.720 - a potential energy minimum looks like it's on 256 00:08:56.720 --> 00:08:58.399 - a spring. So what we actually do is 257 00:08:58.399 --> 00:09:01.125 - we apply an external electric field to shake 258 00:09:01.125 --> 00:09:03.445 - it on that acoustic spring. And then by 259 00:09:03.445 --> 00:09:05.285 - looking at how much it shakes in response 260 00:09:05.285 --> 00:09:07.285 - to the external field, we can measure the 261 00:09:07.285 --> 00:09:09.285 - charge. And it's actually it turns out to 262 00:09:09.285 --> 00:09:10.504 - be wonderfully precise. 263 00:09:11.605 --> 00:09:13.559 - So I can show you a grain, 264 00:09:14.019 --> 00:09:16.899 - a particle, a sphere that's, 500 microns in 265 00:09:16.899 --> 00:09:18.339 - diameter. It's big enough to see with your 266 00:09:18.339 --> 00:09:20.579 - eye. And we can tell you how much 267 00:09:20.579 --> 00:09:22.100 - charge it has on it to plus or 268 00:09:22.100 --> 00:09:23.480 - minus 500 electrons. 269 00:09:24.579 --> 00:09:26.004 - So, you know, kind of a countable number 270 00:09:26.004 --> 00:09:28.184 - of electrons on a macroscopic object. 271 00:09:28.485 --> 00:09:28.985 - Wow. 272 00:09:29.445 --> 00:09:31.205 - So I think one thing you've worked out 273 00:09:31.205 --> 00:09:34.004 - is that there are history dependent effects. Like, 274 00:09:34.004 --> 00:09:36.404 - it matters what's happened to this little silicon 275 00:09:36.404 --> 00:09:38.965 - dioxide sphere before you do an experiment on 276 00:09:38.965 --> 00:09:39.465 - it. 277 00:09:40.250 --> 00:09:41.629 - It's extremely frustrating. 278 00:09:42.009 --> 00:09:44.509 - It was extremely frustrating for a long time 279 00:09:44.809 --> 00:09:47.870 - because we could not get consistent results, 280 00:09:48.329 --> 00:09:50.029 - from one experiment to the next. 281 00:09:50.809 --> 00:09:53.129 - In general, the idea that the history of 282 00:09:53.129 --> 00:09:55.154 - an object matters in this effect 283 00:09:56.035 --> 00:09:57.415 - had not really been proposed. 284 00:09:58.115 --> 00:10:00.514 - And for the systems we've been working with, 285 00:10:00.514 --> 00:10:03.075 - we found out that when we really pay 286 00:10:03.075 --> 00:10:05.975 - attention to the history and keep track of, 287 00:10:06.595 --> 00:10:08.134 - in the case with these oxides, 288 00:10:08.470 --> 00:10:09.690 - the environmental history, 289 00:10:10.309 --> 00:10:12.070 - then we can start to see patterns that 290 00:10:12.070 --> 00:10:14.789 - make sense. But, indeed, it was extremely frustrating 291 00:10:14.789 --> 00:10:17.110 - before we understood that the history was the 292 00:10:17.110 --> 00:10:18.009 - the key variable. 293 00:10:18.470 --> 00:10:20.309 - So what is it about the history that 294 00:10:20.309 --> 00:10:21.990 - matters? How did you figure out what was 295 00:10:21.990 --> 00:10:23.370 - going on? Well, 296 00:10:24.294 --> 00:10:27.174 - we had been suspecting, like many people, that 297 00:10:27.174 --> 00:10:30.294 - adsorbed water on the surface of, an oxide 298 00:10:30.294 --> 00:10:32.154 - particle or an oxide plate 299 00:10:32.534 --> 00:10:34.054 - was the thing that mattered. So that was 300 00:10:34.054 --> 00:10:35.495 - a suspicion that a lot of people have 301 00:10:35.495 --> 00:10:36.554 - had for a long time. 302 00:10:37.110 --> 00:10:39.110 - And so we were looking for ways to 303 00:10:39.110 --> 00:10:41.289 - kind of change the water on the surface. 304 00:10:42.309 --> 00:10:44.629 - One of the simplest ways is actually to 305 00:10:44.629 --> 00:10:46.149 - to bake an object, to heat it up 306 00:10:46.149 --> 00:10:48.309 - to an elevated temperature, to get rid of 307 00:10:48.309 --> 00:10:48.889 - the water. 308 00:10:49.535 --> 00:10:51.774 - But when we did that, we saw, number 309 00:10:51.774 --> 00:10:54.195 - one, that the changes we expected 310 00:10:54.654 --> 00:10:56.735 - were not going in the right direction. And 311 00:10:56.735 --> 00:10:57.855 - in fact, they were going kind of in 312 00:10:57.855 --> 00:10:58.915 - random directions. 313 00:10:59.774 --> 00:11:02.190 - And so this suggested that it maybe, you 314 00:11:02.190 --> 00:11:04.670 - know the baking certainly had an effect, but 315 00:11:04.670 --> 00:11:06.129 - it wasn't related to water. 316 00:11:06.590 --> 00:11:08.690 - So what if it wasn't due to water, 317 00:11:08.750 --> 00:11:11.410 - what was it about the history of these 318 00:11:11.550 --> 00:11:14.429 - spheres that was causing this different patterns of 319 00:11:14.429 --> 00:11:14.929 - charge? 320 00:11:15.414 --> 00:11:17.414 - So what we learned is that when an 321 00:11:17.414 --> 00:11:19.975 - object is sitting in some environment, like the 322 00:11:19.975 --> 00:11:21.434 - air we're sitting in right now, 323 00:11:21.894 --> 00:11:23.514 - molecules from the environment 324 00:11:24.134 --> 00:11:25.815 - adsorb to the surface. They get stuck to 325 00:11:25.815 --> 00:11:26.475 - the surface. 326 00:11:26.934 --> 00:11:28.695 - The molecule that everyone thinks about the most 327 00:11:28.695 --> 00:11:30.134 - is water. That's why we were thinking about 328 00:11:30.134 --> 00:11:32.679 - it. But water is not the only molecule 329 00:11:32.679 --> 00:11:33.339 - in town. 330 00:11:33.799 --> 00:11:35.879 - In every environment, there are all sorts of 331 00:11:35.879 --> 00:11:37.579 - small organic molecules, 332 00:11:38.120 --> 00:11:40.839 - organic in the kind of carbon hydrogen chemistry 333 00:11:40.839 --> 00:11:42.919 - sense, not like, you know, pieces of hamburger 334 00:11:42.919 --> 00:11:45.584 - or something, but little molecules that exist naturally 335 00:11:45.584 --> 00:11:47.584 - in our environment, in the environment of Mars, 336 00:11:47.584 --> 00:11:48.964 - in the environment of space 337 00:11:49.504 --> 00:11:51.044 - that also attach to the surface. 338 00:11:51.424 --> 00:11:53.345 - And so while we were thinking so hard 339 00:11:53.345 --> 00:11:54.704 - that it must be the water on the 340 00:11:54.704 --> 00:11:57.264 - surface, it turns out it's these carbon based 341 00:11:57.264 --> 00:11:59.445 - molecules that were the ones that actually mattered. 342 00:11:59.639 --> 00:12:02.200 - And that's why following the water route led 343 00:12:02.200 --> 00:12:03.019 - us off track. 344 00:12:03.480 --> 00:12:04.920 - So is it the case, like, the more 345 00:12:04.920 --> 00:12:07.160 - carbon on the surface means it's more likely 346 00:12:07.160 --> 00:12:09.639 - to charge positively or negatively? Do you know 347 00:12:09.639 --> 00:12:10.299 - the difference? 348 00:12:10.679 --> 00:12:13.160 - That's the basic trend we see. If we 349 00:12:13.160 --> 00:12:14.220 - have one surface 350 00:12:14.794 --> 00:12:17.375 - of one material, say silica, 351 00:12:18.074 --> 00:12:19.514 - that has a lot of carbon on its 352 00:12:19.514 --> 00:12:21.534 - surface because we've left it in an environment 353 00:12:21.595 --> 00:12:22.574 - for a long time 354 00:12:22.875 --> 00:12:24.634 - and has therefore acquired a lot of this 355 00:12:24.634 --> 00:12:27.934 - carbon, and we have another surface that by 356 00:12:28.160 --> 00:12:30.420 - some experimental means, has been stripped of carbon. 357 00:12:30.879 --> 00:12:32.259 - The one that has no carbon 358 00:12:32.720 --> 00:12:35.059 - charges negatively to the one that has carbon. 359 00:12:35.600 --> 00:12:37.220 - And that's the key thing we found. 360 00:12:38.240 --> 00:12:39.920 - So would it make sense to if you're 361 00:12:39.920 --> 00:12:42.019 - wanting to understand these charging relationships 362 00:12:42.684 --> 00:12:44.845 - to put the particle in a in a 363 00:12:44.845 --> 00:12:46.845 - vacuum, an ultra high vacuum, and remove all 364 00:12:46.845 --> 00:12:49.424 - this carbon stuff from the environment? 365 00:12:49.884 --> 00:12:52.125 - That's a funny question because we actually we 366 00:12:52.125 --> 00:12:54.284 - actually were cued into the idea of these 367 00:12:54.284 --> 00:12:55.664 - carbon molecules mattering 368 00:12:56.129 --> 00:12:58.370 - from surface scientists who work in ultra high 369 00:12:58.370 --> 00:12:58.870 - vacuum. 370 00:12:59.889 --> 00:13:02.370 - Whenever they want to study a surface where 371 00:13:02.370 --> 00:13:05.170 - they can, you know, really, for example, use 372 00:13:05.170 --> 00:13:07.330 - a vacuum AFM to see every atom on 373 00:13:07.330 --> 00:13:07.910 - a material 374 00:13:08.605 --> 00:13:10.285 - or to, you know, do some kind of 375 00:13:10.285 --> 00:13:12.465 - spectroscopy or something on a pure material. 376 00:13:12.924 --> 00:13:14.684 - The first step they do after they put 377 00:13:14.684 --> 00:13:16.524 - it in the vacuum chamber is get rid 378 00:13:16.524 --> 00:13:17.504 - of all of the carbon. 379 00:13:17.965 --> 00:13:19.725 - And so when we started working with surface 380 00:13:19.725 --> 00:13:21.730 - scientists, they said, oh, yeah. We put your 381 00:13:21.730 --> 00:13:23.089 - sample in the vacuum chamber and got rid 382 00:13:23.089 --> 00:13:24.629 - of the carbon, and here's your data. 383 00:13:25.009 --> 00:13:26.289 - And we had to do a double take. 384 00:13:26.289 --> 00:13:27.750 - What? You got rid of the carbon? 385 00:13:28.129 --> 00:13:29.029 - What do you mean? 386 00:13:29.490 --> 00:13:31.409 - The funny thing is is that when they 387 00:13:31.409 --> 00:13:32.309 - do their experiments, 388 00:13:32.894 --> 00:13:34.014 - they put it in, they get rid of 389 00:13:34.014 --> 00:13:36.174 - the carbon, they do their thing. If they 390 00:13:36.174 --> 00:13:38.514 - leave the sample in the vacuum chamber overnight, 391 00:13:38.574 --> 00:13:40.894 - the carbon's all back. So there's really not 392 00:13:40.894 --> 00:13:42.495 - an environment where you can get rid of 393 00:13:42.495 --> 00:13:42.995 - this, 394 00:13:43.375 --> 00:13:45.154 - not even in the best ultra high vacuum. 395 00:13:45.679 --> 00:13:48.000 - It's really I mean, every environment we know 396 00:13:48.000 --> 00:13:48.659 - of has 397 00:13:49.120 --> 00:13:50.980 - these adventitious molecules, 398 00:13:51.839 --> 00:13:53.459 - that tend to cover a surface, 399 00:13:53.839 --> 00:13:55.519 - you know, on a kind of hours to 400 00:13:55.519 --> 00:13:57.759 - days time scale. So it just gets all 401 00:13:57.759 --> 00:13:59.875 - schmutzed up Schmutz'd up. Yeah. In fact, that 402 00:13:59.875 --> 00:14:01.235 - was the word we were using when we 403 00:14:01.235 --> 00:14:01.735 - first 404 00:14:02.835 --> 00:14:04.115 - were, you know, thinking what was on the 405 00:14:04.115 --> 00:14:05.315 - surface. The word we had for it was 406 00:14:05.315 --> 00:14:05.815 - schmutz. 407 00:14:06.274 --> 00:14:09.894 - So what's the mechanism behind this schmutz affecting 408 00:14:10.595 --> 00:14:12.115 - charging? Do you know what's going on at 409 00:14:12.115 --> 00:14:13.654 - sort of a nanoscale level? 410 00:14:14.090 --> 00:14:16.009 - We don't know. We don't know what the 411 00:14:16.009 --> 00:14:18.330 - mechanism is that actually causes the schmutz to 412 00:14:18.330 --> 00:14:20.110 - matter. We just know the schmutz matters. 413 00:14:20.490 --> 00:14:22.009 - So the claim, you know, that we're making 414 00:14:22.009 --> 00:14:22.990 - is we've identified 415 00:14:23.370 --> 00:14:26.730 - the symmetry breaking parameter that causes two identical 416 00:14:26.730 --> 00:14:27.950 - materials to be different. 417 00:14:28.325 --> 00:14:29.684 - At least in the case of oxides. I 418 00:14:29.684 --> 00:14:30.985 - wouldn't extend it beyond that. 419 00:14:31.365 --> 00:14:32.804 - But, you know, we don't we don't know 420 00:14:32.804 --> 00:14:34.725 - the mechanism. Now the the great goal is 421 00:14:34.725 --> 00:14:35.544 - to, of course, 422 00:14:35.924 --> 00:14:37.924 - use the fact that we know what matters 423 00:14:37.924 --> 00:14:40.024 - to focus more on what how it matters. 424 00:14:40.240 --> 00:14:40.740 - Yeah. 425 00:14:41.360 --> 00:14:43.120 - You mentioned that it's just for oxides that 426 00:14:43.120 --> 00:14:44.960 - you're saying that this is true. Are there 427 00:14:44.960 --> 00:14:47.920 - other things that are responsible for different symmetry 428 00:14:47.920 --> 00:14:49.540 - breaking and charging for other materials? 429 00:14:50.000 --> 00:14:51.600 - Yeah. So the experiments we've done, 430 00:14:52.160 --> 00:14:54.254 - in this current project are all with, 431 00:14:54.815 --> 00:14:57.394 - oxides and specifically insulating oxides. 432 00:14:57.934 --> 00:15:00.514 - So silica, quartz, alumina, sapphire, 433 00:15:01.455 --> 00:15:01.955 - spinel, 434 00:15:02.495 --> 00:15:03.715 - magnesium oxide, 435 00:15:04.174 --> 00:15:06.754 - they're all materials that are electrically good insulators. 436 00:15:07.519 --> 00:15:09.360 - They are very similar in the sense that 437 00:15:09.360 --> 00:15:11.920 - they they have high surface energies, which means 438 00:15:11.920 --> 00:15:15.059 - they readily absorb these these adsorbates, these molecules. 439 00:15:15.679 --> 00:15:17.440 - And so they're all very similar, and we 440 00:15:17.440 --> 00:15:18.179 - found this, 441 00:15:18.559 --> 00:15:20.960 - you know, this carbon effect exists for all 442 00:15:20.960 --> 00:15:23.174 - of them that we've tried. And that's about 443 00:15:23.714 --> 00:15:25.714 - altogether, you know, probably close to 10 different 444 00:15:25.714 --> 00:15:26.534 - of these oxides. 445 00:15:27.235 --> 00:15:30.134 - This class of materials is it's, inorganic, 446 00:15:30.434 --> 00:15:32.434 - meaning that the material itself does not really 447 00:15:32.434 --> 00:15:34.995 - have a bunch of organic chemistry going on 448 00:15:34.995 --> 00:15:37.950 - inside. There are other materials like polymers, for 449 00:15:37.950 --> 00:15:38.769 - example, plastics, 450 00:15:39.389 --> 00:15:40.929 - where we see very different effects. 451 00:15:41.709 --> 00:15:44.850 - So in polymers, we actually see that the 452 00:15:45.149 --> 00:15:48.110 - contact history of a sample matters. So literally 453 00:15:48.110 --> 00:15:49.250 - a sample that has 454 00:15:49.675 --> 00:15:50.175 - touched 455 00:15:50.555 --> 00:15:53.355 - more things than another sample will charge with 456 00:15:53.355 --> 00:15:55.274 - a sign based on how many things it's 457 00:15:55.274 --> 00:15:55.774 - touched. 458 00:15:56.394 --> 00:15:59.195 - So there are for sure different mechanisms going 459 00:15:59.195 --> 00:16:00.735 - on for different materials classes, 460 00:16:01.355 --> 00:16:03.295 - and this is what's causing us to be 461 00:16:03.514 --> 00:16:04.815 - conservative and cautious 462 00:16:05.169 --> 00:16:08.309 - about extending claims beyond what we have seen. 463 00:16:08.610 --> 00:16:10.529 - I would not be surprised if this carbon 464 00:16:10.529 --> 00:16:12.870 - effect exists pretty broadly for 465 00:16:13.250 --> 00:16:16.950 - inorganic materials that are poor electrical conductors. 466 00:16:17.985 --> 00:16:20.065 - So, you know, an example of that would 467 00:16:20.065 --> 00:16:22.545 - be like calcium fluoride, for example. It's an 468 00:16:22.545 --> 00:16:25.264 - inorganic material. In many ways, it's very similar 469 00:16:25.264 --> 00:16:26.945 - to, say, a piece of silica because it's 470 00:16:26.945 --> 00:16:27.925 - not a good conductor. 471 00:16:28.305 --> 00:16:30.085 - Things readily absorb to the surface. 472 00:16:30.465 --> 00:16:32.245 - But, you know, showing it's 473 00:16:32.639 --> 00:16:34.720 - true for that case would require going and 474 00:16:34.720 --> 00:16:36.320 - doing those experiments, and we just haven't gotten 475 00:16:36.320 --> 00:16:37.139 - to it yet. 476 00:16:37.519 --> 00:16:39.360 - So what's the next step for you, you 477 00:16:39.360 --> 00:16:41.540 - and your team? So what we wanna do 478 00:16:41.759 --> 00:16:42.160 - is, 479 00:16:43.120 --> 00:16:44.500 - start to try to understand, 480 00:16:44.800 --> 00:16:46.820 - like you asked, what is the mechanism 481 00:16:47.485 --> 00:16:50.125 - that makes carbon matter? And we basically have 482 00:16:50.125 --> 00:16:52.764 - two ideas towards doing that. In the experiments 483 00:16:52.764 --> 00:16:54.764 - we did so far, you know, we could 484 00:16:54.764 --> 00:16:56.144 - measure the charge exchange 485 00:16:56.524 --> 00:16:57.745 - of a sphere and a plate 486 00:16:58.365 --> 00:17:00.919 - after they'd gone through certain treatments to change 487 00:17:00.919 --> 00:17:02.679 - the amount of carbon on one surface or 488 00:17:02.679 --> 00:17:03.259 - the other. 489 00:17:03.720 --> 00:17:06.119 - And then independently, in a different setup, we 490 00:17:06.119 --> 00:17:09.559 - could actually probe the amount of carbonaceous molecules 491 00:17:09.559 --> 00:17:10.299 - on the surface. 492 00:17:10.680 --> 00:17:12.039 - But we had to do those at separate 493 00:17:12.039 --> 00:17:13.924 - times. And so the first thing we wanna 494 00:17:13.924 --> 00:17:15.125 - do is build a setup where we can 495 00:17:15.125 --> 00:17:17.045 - do those both at the same time so 496 00:17:17.045 --> 00:17:17.865 - I can watch 497 00:17:18.164 --> 00:17:18.984 - the evolution 498 00:17:19.365 --> 00:17:21.765 - of the exchange of charge with the evolution 499 00:17:21.765 --> 00:17:23.285 - of the growth of this carbon layer on 500 00:17:23.285 --> 00:17:25.845 - the surface. That's one thing. The next thing 501 00:17:25.845 --> 00:17:27.059 - we wanna do is 502 00:17:27.539 --> 00:17:28.680 - kind of remove 503 00:17:28.980 --> 00:17:31.240 - all of the natural molecules from the surface 504 00:17:31.539 --> 00:17:35.000 - and put on artificial ones with specific properties. 505 00:17:35.620 --> 00:17:37.539 - And then by kind of, you know, testing 506 00:17:37.539 --> 00:17:38.680 - all possible combinations 507 00:17:39.299 --> 00:17:41.880 - in a catalog of these artificial molecules, 508 00:17:42.205 --> 00:17:43.965 - maybe that also tells us something about the 509 00:17:43.965 --> 00:17:44.465 - mechanism 510 00:17:44.924 --> 00:17:46.924 - if, for example, we see trends, you know, 511 00:17:46.924 --> 00:17:48.684 - based on kind of the functionality of these 512 00:17:48.684 --> 00:17:49.184 - molecules. 513 00:17:49.805 --> 00:17:52.605 - So TaylorMade Schmutz is a TaylorMade Schmutz. That's 514 00:17:52.605 --> 00:17:53.884 - where that's I should use that in the 515 00:17:53.884 --> 00:17:54.625 - grant proposal. 516 00:17:55.669 --> 00:17:57.829 - Scott Waitakitis, thank you very much for speaking 517 00:17:57.829 --> 00:17:59.429 - with us. It's been a pleasure. Thank you 518 00:17:59.429 --> 00:18:00.250 - very much. 519 00:18:07.909 --> 00:18:09.609 - That was Scott Waitakitis 520 00:18:10.484 --> 00:18:11.384 - in conversation 521 00:18:11.845 --> 00:18:14.265 - with Physics World's Margaret Harris. 522 00:18:14.805 --> 00:18:16.825 - Thanks to both of them for a fascinating 523 00:18:17.125 --> 00:18:17.625 - discussion 524 00:18:18.085 --> 00:18:19.065 - about static 525 00:18:19.365 --> 00:18:19.865 - electricity. 526 00:18:20.964 --> 00:18:22.884 - I'm afraid that's all the time we have 527 00:18:22.884 --> 00:18:24.105 - for this week's podcast. 528 00:18:24.559 --> 00:18:28.480 - I'm Hamish Johnston, and our producer is Fred 529 00:18:28.480 --> 00:18:28.980 - Iles. 530 00:18:29.519 --> 00:18:32.079 - The music for this episode is called one 531 00:18:32.079 --> 00:18:33.220 - three seven, 532 00:18:33.519 --> 00:18:36.240 - and it was composed and performed by the 533 00:18:36.240 --> 00:18:36.740 - physicist 534 00:18:37.279 --> 00:18:38.259 - Philip Moriarty. 535 00:18:39.375 --> 00:18:41.154 - We'll be back again next week.