 Raul, who is our next speaker from the Universidad de Granada. In Spain, he's going to talk about Brownian particles in electro-optical traps. So, go ahead, Raul. Okay, let me see. Can you see my screen? Yes, yes. Hello. Yeah, my point there as well. Okay, good. So, thank you very much. So, I'm Raul Rita, as I told you, from the University of Granada. I just built a new lab that we call the Nanoparticle Trapping Lab. It is in the Department of Applied Physics, or sorry, I put it in Spanish. So, it's in the Department of Applied Physics of the University of Granada. And here, we basically, what we do is to work with Brownian particles in traps. What do I mean with a trap? A trap is basically a parabolic potential, as you see, it looks like you don't see my pointer. You do? I don't see my pointer. I'm sorry. Okay, you see there is a parabolic potential that is described by the potential V that I show here, and it is in principle parabolic in all three dimensions of space. And the origin of this potential can be either, can be due to different types of interactions. In this talk, I will talk about both electrical forces, post-static forces and optical fields. Optical fields are created by gradients of laser light. And here, what you see is a nanoparticle. In this case, it's a nanodiamond, 100 nanometers in diameter that is levitated by an electrostatic potential. And you see it thanks to the interaction with the tightly focused laser. So, the bright spot that you see is a laser that is focused exactly where the nanoparticle is located, and what you see is the scattering from from the particle. Well, we are doing it in the, as I told you, in my new lab, the work started one year ago, as you see on the upper left picture, and the work's finished in back in December. We started to install the equipment in January, but yeah, after the lockdown we've been closed for almost three months, but today we have a lab that is starting to work. So, my talk today will not be about results, because we don't have any results yet, but we are just trying to fix technical problems, technical issues. We are working on the two new experiments, and I will explain a little bit what we want to do with these two experiments and the applications we are looking for. As I said, we work with traps, and in a trap you basically have a parabolic potential that establishes a system that is basically a Brunyan harmonic oscillator. In the first equation you see here is just the equation of the harmonic oscillator, as it is typically written down, you first have inertia and plus the mass times acceleration, then you have a damping term that is proportional to the velocity, and then you have a term that is given by the field that is described as a linear spring, and then on the right we put Brunyan white noise that describes, that in principle incorporates any noise source in the system. We also define the damping rate that is the fraction of the damping and the mass, and in this case we typically write the Brunyan harmonic oscillator equation as you see in the second row. Typically we will work in either water or air, and in these two cases the harmonic oscillators is in the overdamping, this is a classical approximation, and this is valid in many situations, but not always, but in principle you can, for example in optical tweezers, as is the most common experimental setup, it is completely valid in many, many situations, and the good thing with the optical tweezers is that we can not only track the particle, but we can also track the position of the particle with very high position, so what you see here the you see time traces of the position as a function of time, and these are real measurements, you see we have three lines, black, blue, and red, and these are for three different laser powers, and as you increase the laser power you somehow increase the confinement of the particle, and you see that the what happens with the with the black curve. You also see that the distribution of positions that is this Gaussian curves you see on the right also is made thinner. Ro, what's the particle made up of? So in this experiment it's just polystyrene, one micron size polystyrene, but yeah in optical tweezers in liquid you can basically trap dielectric particles or metallic particles. Other types of particles keep travels, but there are also some some ways to fix it, but I will show some so again with other particles give me a second. Okay so we typically also study the the trajectory in terms of the power spectral density that is something like the squared of the Fourier transform of the trajectory, and you see on the top right figure you see three power spectral densities of these three trajectories, and you see that as you increase the laser power you both increase the characteristic frequency of the trap, and you also decrease the the space the particle explodes, so you decrease the value the low value of the the plateau the low frequency plateau of the of the distribution. Okay so this is standard in optical tweezers and with these tools basically the trap and the detection of the motion you can have access to many many to a lot of information about the processes that are going on with the particle. This is standard and there's a lot of bibliography, what we are doing now here is working on two setups, the first one is optical tweezers in suspension in liquid as it is standard, but they will also talk about another kind of traps that is done with electrical forces, and in this case we will talk about levitation because the trapping is not done in liquid but is done in earth. The optical tweezers were invented by Arthur Asking and we had the Nobel Prize awarded for him in 2018, and the the the mechanism behind optical tweezers is well known, and it is it works thanks to the evaluation pressure of light, light also can transfer momentum to particles and if you have as I show here in the first picture, if you have a laser that points from left to right you will push particles along with the direction of propagation of the particles of the laser beam thanks to the radiation pressure that is what is the so-called scattering force and you can use two counter propagating lasers to to to to counteract the scattering force and you get a trapping position at the center or at the focus of both lasers as you see in the second figure in the on the left and if you use a very tightly focused laser beam you can trap with only one one laser but yeah there are many things to to to fix to to do it but basically two forces appear one is the gradient force that points towards the maximum of light intensity you see the f grad is proportional to the gradient of intensity and there is also the scattering force that pushes the particles along the direction of propagation of the particles okay I've been working in optical tweezers for a while already and there are some previous works I did with on stochastic thermodynamics with FEDGAR well we were both working at it for you some of you may know these works but yeah this is all stuff and I will like to show what we are doing now here in Granada we already built up optical tweezers where we can have several traps you see here I have six traps and I am switching them on and off here I put too many particles in suspension and you see that in a single trap you can trap many particles so if you want to do precise measurements you should reduce the concentration of particles but one good thing we achieved is that yeah we have many particles so we can start interactions between particles and we can measure all the trajectory of every individual particle with very high sampling rate with very high sampling rate we are also now working with particles inside droplets so you hear there is a big droplet it is something like 40 microns and I'm here I'm here using two traps with one trap I'm trapping a particle inside the droplet and with the other trap I'm moving around the path the droplet let me show you the video again you see here I'm moving the particle inside the droplet and now I start to move the droplet without moving the particle you see there quite flexibility here we want to somehow study confinement effects inside the droplets but yeah this is only ongoing work so far we are also working with plasmonic particles as I told you at the beginning of my talk you can also trap metallic nanoparticles like gold nanoparticles with very small size and as Ali told us before if you shine gold nanoparticles with the right wavelength these nanoparticles will lead to plasmon resonances where there is a lot of absorption of the radiation and you will have a plasmon resonance as you see here in the far field these effects are responsible for the rightful cause that you you see with colloidal suspensions of metallic nanoparticles but more interestingly in the near field what you get is that you have very strong electric fields that are highly confined way below the diffraction limit and also you see you you have a strong release of heat for some time this heat release has been a problem but this can also be used to do different things for example you can in a micro channel or a nano channel you can use convection thanks to gradients in temperature you can as we saw before also you can try to treat cancers by radiating some parts of body where you have a cancer or you can also create convection around individual particles as you see the two plots on the right hand side of the slide a couple of years ago we demonstrated that the combination of these gradients of temperature with an electric field leads to a very efficient convection because well you may want to heat up your your sample but maybe not too much and in this paper in acs nano that you see up here they demonstrated that only with a gradient of temperature you cannot induce a strong convection in micro channels and this is a problem but we demonstrated that strong convection can be obtained with small gradients of temperature here we were working with something like five degrees about room temperature and these small gradients of temperature combined with an electric field leads to a very high convection that is important many times or in many applications in microfluidics so this is the work we did some time ago but now we are working on these kind of effects but in a more local regime we are going to trap a single gold nanoparticle we will irradiate it with a laser beam to heat up and we will also apply an electric field to induce this very strong convection around it and we want to characterize it and try to use it as a way of creating flow of the micro nano scale this is something that we are working on already and that was all for the optical pieces setup that I told you let me now discuss about the second setup we are building that is a pole trap a pole trap is a trap that is created with an electric field but that is the the trapping here is not that straightforward because because of Laplace equation you cannot build a potential that is stable in all three dimensions of space because as you see here if you consider a potential that is quadratic in all terms Laplace equation imposes that at least in one direction of space the potential will be unstable okay so in principle your trap your potential will be a subtle potential and you cannot trap simultaneously in all three dimensions of space then you have to play a trick do something like this you take your subtle potential and you rotate it and in this case you get to a stable trapping this is a mechanical symbol that we built in our lab for students and this is the idea and then what what you find is that the motion is somehow more complicated the the system is not in complete equilibrium because this is a traveling but we recently demonstrated that the brony dynamics of a particle in a pole trap is very similar to what you would expect for an optical trap for example there are some differences but in the end it works almost the same so it is also a good thing to explore and in fact we have a very built pole trap and you see here we can also track the position of the particle with a high sampling rate we have also a distribution that is Gaussian that is as it is expected for a equilibrium system and the power spectral density also is similar to what is expected there is some noise in the system we still are struggling with the electronics in the system but yeah we we are working on it on the right hand picture you see there is a small spot between two cones that are two electrodes what you see there is a one micron size particle that is shining thanks to a laser beam but what you see here is what you see in the in the experiment if you look with the night eye so these experiments are also quite beautiful because you can really see the bright spot with your night eye and it's also nice students are always happy to see things with night eyes we are also trapping aerosol droplets here I show you there is an fertilizer that is this is from Omron that is an fertilizer that we use you can buy for emulating some drugs and with this we generate droplets as you see here we have the two cones and we generated a lot of droplets and the pole trap allows us to trap a lot of them and here we are now decreasing the potential to try to have only one droplet here you have two now you have only one and now what we do is to defocus the image and here there appear some bright and dark strips as you see here this is the same the good thing here is that there is some interference pattern inside the droplet so there is interference with the light that is incoming to the droplet there is some transmission and there is some reflection so there is a interference with the light that is incoming and the light that is reflected and then you get these bright and dark fringes and these fringes provide an interferometric way of measuring with very high precision the size of the droplet you have trapped so this has very a lot of applications in in aerosol science and we are also interested in these things sorry yeah you have just two minutes more that i mean we can use okay okay then i finish right now i this is just another trap that you built where you can trap a row of particles as you can see we can study interactions and just to finish we are also building hybrid traps where we combine all traps and optical traps and we have these fancy potentials where there's some bystabilities and some improved performance in some cases we also build this and measure the the potential distribution um yeah that's all with this i finish we are interested in many things we will be happy to collaborate with theoreticians and people that do simulations and yeah with this i thank my colleagues and you for your attention thanks a lot for this very interesting talk okay so we have time for one question okay very good one and i know you're working very well but and to talk about this this thing you said about the gold nanoparticle is somehow related to what Ali Rajapul was talking about this morning so how do you plan to measure the heat so you you put in this map so would you be able to measure the temperature in the fluid near the the heated nanoparticle well there are some tricks that could be played to do it one would be as i'll show you here to have one trap that is the one particle of this trap in one trap that is gold and this releases heat and then i have another particle trap with another trap and this particle is dielectric and from the fluctuations of this particle around this one if there is some heat release you will feel these fluctuations with this particle and that's a way of measuring that and there are also other ways that you can use some bollocks use that and so in the second particle and measure the heat out of it yeah nice amazing okay okay so thanks a lot eh i give you an applause in the name of everyone it was a really interesting talk eh now we move to our next speaker of the day eh who is Duma Dick Hartman eh from the université marien eh and in Congo eh and the title of the talk is computational x-ray spectroscopy