 Okay, so thanks. Thank you all for having me. I should say that this talk today is not a modeling talk per se but I'm hoping to illustrate some challenges in modeling based on a couple observations from our work in our lab. And what I think about yes it's very very small particles over short time scales and I'm really a fluid mechanics person interested in small things. But I'll give a little bit of background so we're interested in plastic pollution and we know plastic is everywhere in the environment it's found in the open ocean the deep sea sea I said events atmosphere. Remote Mountain Lake in Mongolia. Everywhere you look we find a plastic it's in humans and our beer. And one way to characterize this is that plastic is a poorly reversible pollutant. It gets out into the environment and it breaks up into smaller and smaller pieces which lets its disperse and lets it disperse widely throughout the environment. It degrades extremely slowly so it accumulates in the environment. We know, we know that plastic is accumulating in the environment and lots of places, and there's a lot of work done on how it affects the environment and ecosystems and biology. That's a very active area of research. But what I'm really more interested in is what its fate and transport is once it's in the environment. And, okay, so what I think about a lot are plastic in the ocean and in aquatic environments. And those plastics that we find out in the, in the environment are considered, we consider micro plastics. So these are small particles from micro to Miller scale but people also consider larger plastic pieces, and their shape and density vary a lot depending on their source material. They're very, they're very varied in their characteristics. And what's really interesting about plastic here to other environmental particles that we study is that they're, they kind of fall outside. They're quite different. So I'm sure many people in this room think a lot about sediment which we know is very heavy relative to water. So we have a lot of work on bubbles which are very light relative to water but micro plastics are interesting the fact that they're very close to neutral buoyancy and water. And so a lot of the assumptions we make for these really high and low density particles we can't always make the same way, or not necessarily which is what I think about. And plastic, while it's close to neutrally boy it can be both positively and negatively boy. Okay, and what I really think about is not just plastic particles but particles in general and how the characteristics of the particles affect their transport. And I think about what happens when we consider the fact that particles do not just go with the flow, but they have either some inertia relative to the flow. They are buoyant or settling so they're crossing streamlines or because these, because they're finite particles they don't have to be uniformly mixed, but they can preferentially sample parts of the flow to so I think about how these manifest in flows like waves and turbulence that we find in the environment. And so why is this important maybe these are all really small effects that you can often neglect. If we are your, if you have your small particle and you want to know where it goes in a flow. You can, your easiest assumptions just assume it goes with the flow right and you got some answer, but even if they're small deviations instantaneously because there's lift or drag on these small particles, you're interested in transport those small deviations can add up so that's what I think about when that's important. Okay, so we're thinking about plastic. Most of the work that's been done so far is really thinking about plastic transport in the ocean. And this comes from this really awesome review paper on the physical oceanography of plastic. What I just wanted to illustrate here is that there's tons of scales of in tons of processes we have to think about if we want to know where plastic goes where accumulates in the ocean at least. And how it is transported from rivers to the ocean how it washes up and is eroded from shorelines. It can move from that the ocean to the atmosphere and back again out in the processes out in the deep ocean are very different than the coastal processes, etc. So there's a lot to think about. And I think about a couple of these different processes, but what I'm going to talk to you today is just one. And we're going to talk about the vertical distribution of my graphics. So imagine we're at the surface of the ocean or another body of water, and we have some buoyant particles, selecting near the surface. So why might we care about this. So if we take measurements of the surface, we want most measurements are taken at the surface so we need to know their vertical distribution to extrapolate. Light decays with depth and plastic does photo degrade with light. So their vertical distribution controls their feet. And it also controls their transport. Many environmental flows we know are shared. This is from a study at the surface of the ocean where they measured the velocity. Oh, I can use the pointer right. So imagine the horizontal velocity is a function of depth and this is a log scale so you can see is just going up and down a couple centimeters here changes your horizontal velocity a time so just moving up and down a little bit in the water column right can totally change your transport. What we're going to think about is what happens in a really idealized scenario where wind blows on the free surface. It generates turbulence and mixing, but these particles are buoyant and they want to rise. Notice that this is maybe similar to sediment in a bottom boundary layer where you have here from the bottom boundary layer, generating turbulence, which can suspend heavy particles which can then settle back down. And there's a lot we can learn from seven transport but there's also some key differences which is what I think about. So right right now we're going to start by doing is taking a really idealized look and assuming we're in equilibrium, where the mixing from turbulence is an equilibrium with the rate at which particles are rising to buoyancy, we're going to make some assume that the particles rise with some constant rise velocity, and that there's some constant mixing we can parameterize with some diffusivity. If we do that we get this really nice result where our concentration of our plastic particles will decay with depth exponentially with our concentration peak at the free surface. So this makes sense this is, you know, more buoyant particles will if they have a higher rise velocity will be closer to the surface if there's more mixing, they'll be mixed lower down. And even with this really simple model, there are still three important parameters we need, we need to know something about the surface boundary condition, which is now different than if we're thinking about a bottom boundary layer of sediment, we have processes like wave breaking their surface tension we need to think about there's wind. And we need to know the local concentration, we need to know something about them mixing so we need to. We can think about that in this talk but it's not necessarily obvious what this diffusivity is all always, we need to know the rise velocity and for small particles, we can you know take their soaks rise, soaks rising velocity. But that's not necessarily what the particles are actually rising with in an unsteady flow. And so that's another thing we think about. And in waves, which are strong at the ocean surface. We showed some work that waves can actually enhance the rise velocity of particles, but we know that turbulence can also reduce or other work by others has shown that turbulence can reduce the rise velocity so that's also not an obvious parameter and so that's something else that we think about, but not that's also beyond the scope of this talk okay so let's get to some observations. So, I'm going to show you some work of microplastics collected from the field because we have that model and it's, but we want to see how well it works. And so this is from measurements taken, not by me but by my collaborators SA where they can take measurements of microplastics at different depths over the ocean. Okay, and so what does this look like. So here we have concentration over depth and so this, these black dots here are by number density. So the number of microplastics per volume goes down with depth that's what we expect. This other line here is mass density, and it's going down, but it's going down more rapidly with depth, which means that they're not only fewer particles at depth but they're smaller. Okay, so we can take that away. And then we can also look at the type of particles because these particles are very varied some of them are these like long filamentous fibers and some are more flat fragments, so we can divide the concentration profile by these lines these fibers and by the fragments and we see that we get different behavior, depending on the type of particle where the lines are more well mixed with depth and the fragments to cave with depth. Okay, so but we want to do something a little bit more than this. So what we can do is we can think about what we expect to be important right buoyancy and turbulence and we can construct a non dimensional number. This is really just very similar to a Rouse number but now, these are positively boring particles. And so this, what do we can think about the limits of this parameter so when this Rouse number is really small we expect the particles to be well mixed. And when it's, when it's close to unity, we expect maybe some partially mixed profile and when it's much, much greater than one all the particles should be trapped at the surface. So this makes sense, but it hasn't really fully been tested so what was really cool about this data set is not only did we collect micro plastics over depth but we measured the rise velocity of each individual particle because that's what we need to remember these particles are really varied and they can't all be all necessarily the same way. So, what we can do is we can estimate the mixing, and we can now segregate our concentration profiles as a function of this Rouse number. So this I've just plotted the total concentration again for reference. So this is really low Rouse number we get, we see that we do see this well mixed profile, which was great to see. If we look at the more intermediate Rouse number then we see this partially mixed that does follow this exponential decay with depth, rather. Okay, and if we look at the really high rest number we see that the particles are service trapped. So this was great to see this in observations in the field. And it shows that there's a lot we can learn right from sediment models and that these from Simon transport and that this kind of formulation works pretty well. Okay, so just to kind of go over what we did in the field was that this did confirm that this Rouse number scaling can apply to a free service boundary layer as well rather than just the bottom boundary layer. So what we can do for these different regimes but what's also really nice is you don't always know the rise velocity or the, you know, all, it's very time consuming to take all these measurements of all the individual particles. So what we also saw is that just sorting by particle type is also really helpful in getting at the concentration. I haven't said anything about transport yet and that's what I'm really interested in so what I really am is an experimentalist in the lab. So now I'm going to show you some laboratory work, which is where we can blow wind over a free surface so we can kind of generate similar conditions that we see in the ocean in the lab so we're going to get some turbulence, there will be waves. This is work done by a postdoc in my group. So what we can do is we can generate a flow and put some particles in and then track them. And so what I'm showing you here is observations so the pop is kind of like a okay reconstruction of the free surface that we now have a better version of during the experiment and here are particle trajectories and they're colored by speed so the darker ones are when they're going faster. So what I'm showing you here is the particles are positively buoyant they're mostly near the free surface and they have these wave orbital motions you get under waves. But everyone's while there's a bunch of particles being mixed deeper and that's because there's this intermittent wave breaking which is generating a lot of turbulence, which is what we often get when you have you get kind of intermittent breaking and mixing. You see particles get mixed down and then they get mixed up. So we have these types of experiments and we can vary the particles and the waves and test different models and measure things like transport now because we have their, we have their velocity. Okay, so what we're going to look at here what we're looking at here is just the equilibrium concentration of three particles under the same conditions. So here's their kind of Rouse number we have a low Rouse number and a higher Rouse number and a medium Rouse number scenario, small medium large particles. And we get again results we might expect so we see that the smallest particles the concentration profile is rather well mixed those are the small dots and the largest ones, you show this kind of exponential decay. These funny points of this free service because we have a wave. So you can't measure the measurement of where you start is a little distorted that this dash line kind of marks the lowest wave trough. Okay, so we see the concentration kind of makes sense but what about transport so what we can also do is we can measure their average horizontal velocity. And in this flow the velocity is sheared so things move faster at the surface than at the depth. And what we see is that all of the particles no matter their location their depth at the bottom all seem to follow this same kind of fluid, or they're moving at the fluid pretty well this is pretty well tracks the fluid, but at the surface we see that the larger particles are actually moving faster than the smallest particles. And this. What I said earlier in the talk is that just though your vertical position should control to the most part your horizontal transport right. The thing here is that at the same vertical location, there is different horizontal transport depending on the type of particle. So why is this happening well let's look a little closer first, so we can normalize the this area by the smallest particles. And what we see is that the largest particles can move on average up to 40% faster than the smallest particles at the same depth. This was confusing to us and we weren't exactly sure where this was coming from. But we. So first we thought maybe it could be do the fact that these particles are inertial they're large they have different dynamics, but we went through and work that out not going to go through and show that that's only really small effect about 1%. What we think is happening is that these larger particles are more buoyant and they're more concentrated near the free surface. So I said that the wave breaking is occurring at the free surface which is coupled to these larger particles are probably only being entrained by the wave breaking events. And we, there's been other work that has shown that wave breaking increases horizontal transport so if you're the same part in a wave. That's not breaking versus the breaking part. You're going to go, you're going to have a stronger horizontal transport under the breaking part than the non breaking part. Okay, so what does this mean. Well, so what we're saying is that the buoyant particles can preferentially sample wave breaking in a way that enhances their overall transport. And what I was trying to illustrate with this is that just knowing sometimes the vertical concentration is not enough in some flows because there can be this preferential sampling effect where some particles spend more time in faster moving parts of the flow than others, which is complicated and hard. So what we're working on is trying to kind of formalize formalize this take more observations that we can get it in a way that can be used in a model. To summarize, microplastics are this new type of environmental relatively new type of environmental particle that fall outside the regimes other environmental particles. The part of the properties of the particles will dictate their behavior. And while there's a lot of similarity between microplastics at a free surface and sediment and a bottom boundary layer. So there are some differences that we need to think about as we apply these models like breaking waves, your neutral buoyancy and that these particles are also in really low concentrations. With that I'm happy to get a question. Thanks. So we have time for one question, which I see here in the front. Oh, that's the high density polyethylene that's the type of plastic. Yeah, so that was 970 kilograms per meter. Yeah, in freshwater. Yeah. So the density difference to see what is going to be like 70 kilograms per meter cubes. Yeah, do people look at the acoustic impedance contrasts. They're very hard. I've talked to people about measuring them acoustically and they're very it's, it seems like the density difference is small. It's, yeah, I'm a headline structure. Yeah, it's a difference smaller than that. Yeah, but they're also that they're very low concentrations, like they're very like there's. There's a lot of plastic but it's typically because the ocean is very large, there's a lot, but it's a very low concentration. Like yeah locally. Yeah. So can you know, like do a comparison between I can't remember what it's called like the very shallow acoustic sampling and the dredging stuff. So I'm not an expert in acoustics but I have talked with people and it seems not trivial is what but definitely and people are definitely interested. That was a great talk. Thank you.