 Okay. So let's call the next session to order. I'm glad you all are here today. Before we begin the talk, Mike has a few things to tell us. Okay. So as I mentioned in the opening, we have poster presentations that we'll do on Friday. So I want to call particular attention to folks who are listed up here. You see your name here. You're in the first group that will present on Friday. That means you need to have your poster printed. Right? That means the poster has to be prepared in advance, which means your poster is due this time tomorrow. Okay? How you submit, basically you have a workspace on Google Drive. You put on that workspace, you put the PDF which has the poster, listed as final, and that's then the poster will be basically taken from there and then sent to the printers. You don't have to do anything. Just make sure that your poster is ready. If you don't see your name here, you will have your return later. Okay? But people whose name is listed here, make sure you have your poster due tomorrow on the Google Drive in the final form. If it is not in final form but is in some form, that poster will be taken as is and printed. Okay? So make sure you get that prepared. Any questions about that? Does everybody know the size that they need? So the size poster, I hope in the poster tutorials, this is something that's covered. About A0, the standard poster size, it can be either landscape or portrait, but it has to be in the standard A04 map for posters. Okay? Questions about that? Yes? I present this. Who presents and then presents already? So what will happen is there will be you, the end, the poster session, so let me describe what that is, how that poster session works. So the poster session is Friday, starts at 3 o'clock in this room. It starts with presenters listed on that last page, first presenting one after another, an oral presentation, a snapshot. That does not have to be ready until Friday. So you will have, those of you who are listed here, on Thursday afternoon you will be in a session where you present the snapshot. You will prepare your snapshot, right? So that comes after you've prepared and submitted your poster. You present in this session on Friday the two-minute snapshot, one after another. It's an advertisement for your poster. After that, you will then, if you're a presenter, go out, your posters will already be posted outside on the poster boards outside. You stand by your poster, the rest of us will walk around and look at the posters and have the poster sessions. There will be refreshments served to you during this time. Now, just remind you, I said there are prizes. The competition, the grand prize is 100 Euros cash, and we'll have other prizes at different levels. Okay, so this is, you know, in addition to wanting to present your science and, you know, the opportunity to practice communication, there's an opportunity to win some cash too, okay? So that is, all I have to say about that, any questions. But again, if you see your name here, then you need to have your poster submitted by this time tomorrow on the Google Drive in your workspace, and the rest of you, if you don't see your name, you will have an opportunity to turn your posters in later. Yes. What is the value of what you value from posters? Okay, so when you attend the poster session, you will have this discussed by the poster tutors. Am I on the same page as the poster tutors? There's a rubric that tells you the criteria and the scoring. Poster presenter, our poster tutors, Lorette. Yes, you're going to, Luigi and myself, we go over that in the poster tutorial session. And I'll say a little more about the details of the judging exactly how it's judged before the first session. But that's, that's that. Okay? Okay. Any other announcements that we need before we begin the talk? Okay, well then it's my pleasure to introduce Luigi Cristofolini. Cristofolini? Thank you. It's a long name. I apologize. Take it away. Thank you. Okay, so it works. It's my first time here, so I hope I get the spirit of this gathering and give you some hint of what can be interesting in surface physics and also take the opportunity to imitate about the experiment that you can do in the hands-on session. And finally I will also talk about something not really tabletop experiment. So why should one focus on soft interfaces? Why soft interfaces are important, at least interesting to me? Because of beer. The pointer is, this is not the pointer. How can I get the pointer? Should be laser pointer as well. Here. Sorry. Okay. So if you look at the beer flow, for instance, it's a complex system with a hierarchy structure from different scales. Starting from the large scale have a foam which consists of gas bubbles which are separated by this called plateau, which are made of bilayers of surfactants and water in between. And if you look on the, this is on the centimeter scale or millimeter scale, and if you go on the smaller scale if you go to the plateau, if you look into the details of this plateau you discover that they are formed by surfactant molecules which have a size of nanometer scale. And what is happening at this scale actually is determining what is happening on the other scales. So the stability of a foam, as an mention of many systems is determined by the dynamics, the structure and dynamics that you have in the local scale on the surface and the surface made of soft material. Just another example where I was preparing this talk, this Nature Cover was showing that basically the surfactants, the organic surfactants have a role in determining formation of a nucleation of water droplets in clouds. Is it too loud? And so I have also an effect in climate. So from one extreme to the other, I mean you have soft interfaces that are determining the behavior of your system from your beer to climate change. So the outlook of this kind of lesson or talk, I will start from simple surfaces focusing on long-wave monolayer that are very simple and then just a brief overview of self-assembled and layer-by-layer structures. And then focusing on more complex systems like foams and emulsion and then this will give me the opportunity to talk about the diffusing ways of heteroscopic experiment that some of you will do in the hands-on session in the next days. And then as I was saying, I think that it could be of interest for at least for some of you to know about x-ray techniques which are not tabletop, it's large scale facilities, but if you have a good sample, a good idea you can submit your proposal and get beam time to do maybe good experiments and do good science at a little cost for you. That's why I think it's not really tabletop but it can make sense in this audience. So just a bit of history, the studies of soft surfaces perhaps started with Benjamin Franklin with a drop of oil in a pond and discovered that this drop of oil makes the surface smoother but he didn't go much farther. It was Lord Rayleigh who quantified the thickness of this layer and it is astonishing if you think at that time it was the time at which the size of the molecule was not clear. Apogadronambe was not so well established and by measuring the size of the drop oil spread you can extract an estimate, a rough estimate of the thickness of the layer and this is the thickness of your molecule which comes out to be at that time 1.6 nanometers I mean it was not nanometer at that time it was measured in inches but converted to modern units and it is close to our estimate of molecular size and then after that people started to work on this in a more systematic way and Apogadronambe was working in her kitchen doing layers and it was with LAMWIR and BLOGET that this thing started to be studied in a more modern way in laboratory and so this is more or less the people that mostly contributed to the development of LAMWIR monolayer studies so first of all when you have an interface between two phases you have a surface tension why do you have a surface tension? a surface tension I mean in the simple case of air water you can understand it it's due to the fact that molecules in the water are happily bound to other molecules all around molecules on the surface are not so happily bound because they don't have the upper half of the world to bind with so there's a kind of additional cost in energy to have this surface you can also model this in this way you have molecules here and I mean less molecules here so this is the interface and there's some energy associated with this interface we'll return on this later on you can also think of this surface energy as a line tension a force per unit of length dimension they are the same and you can use this virtual work argument to show that if you move this black line to extend the darker area to unlock the darker area by the amount of days time cell you're creating an extra surface and you're making a work and this work is related to this force so the surface tension is equivalent to a force per unit of length so you measure it in new tones per meter or million new tones per meter and depending on the nature of your liquid you may have different values of this surface tension for water it's particularly high because of the strength of the hydrogen bonds between water molecules and it is lower I mean if you put ethanol or other hydrogen forming hydrogen bond forming system you have a lower surface tension just for curiosity you have a very high one of the magnitude largest surface tension on the surface of mercury because you have the sea of electrons that are keeping the mercury together this is forming a stronger bond but okay in the past people were working also on the water on the mercury surface but it is kind of unhealthy so it's not so common nowadays I mean you can do it but it's just a niche not many things happen now so what are the effects of this surface pressure the first effect that comes to my mind is Laplace pressure inside of a bubble you have a pressure that is larger than the pressure that you have outside the same bubble and the reason is that you have all this surface that is kind of pulling together this guy would go to zero would like to shrink but at the same time you have some gas inside that is equilibrating and the difference in pressure you can calculate out is two times gamma which is this surface tension we are measuring over the radius of the bubble remember this radial dependence one of the radius because it is important we were talking about forms we will talk again about forms later on and this Laplace pressure this is called Laplace pressure and the difference of Laplace pressure that you have between large and smaller bubbles makes an effect in terms of migration of gas from small droplets from small bubbles to large bubbles and this phenomenon is called Oswald lightning I mean so this is important for the evolution of a foam if you go to a soap bubble a double, soap bubble is a double layer you have an outer layer and an inner layer so you have twice this Laplace pressure and so it is doubled in a gas bubble so it is single interface, double interface another phenomenon in which you see surface tension is wine tears if you like red wine or cognac maybe you have already observed that if you let it rest in a glass it will form this weird tear why does it happen? it happens because the mixture of ethanol and water plus more interesting things that make the wine this mixture has some surface tension it will climb on the hydrophilic surface of glass hydrophilic means it likes I will come to it later on but it likes the water it will climb on here and then as the liquid is climbing ethanol is evaporating and as the mixture gets poorer in ethanol surface tension grows and if the surface tension is high you form droplets to reduce the surface and once these droplets are formed they get down so this is the mechanism for formation of wine tears next time you are drinking with your friends you can bore or impress them depending on their attitude with this story about wine tears another effect of surface tension everybody knows is capillarity I mean you probably study this in primary schools when you have trees, feed, water in this way and ok I won't spend much on this I mean I think that you already know this maybe you don't know the some defects of capillarity one effect that I happen to study is the role that capillarity has on chocolate blooming you know what is chocolate blooming when you have chocolate, old chocolate it starts to develop a whitish appearance on the surface and the seller says it's a proof it's genuine actually it's not a proof it's genuine it's proof that it's old and badly stored so why is chocolate blooming? bloom is originated by migration of fat through pores if you look again as I was saying for the foam it's true for many many systems you have a hierarchy of structures in chocolate you start from the molecule if you start from the molecular level you have triglycerides that form that aggregate into crystals or crystals I mean it's maybe not very proper to call them polycrystalline amorphous solid matrix and pores in between I mean in those pores a liquid fraction of fat can migrate and then you have pores between particles and on a larger scale you have particles that are made of sugar of cocoa butter and other things and if you look at the time evolution of the whitish appearance this red line is the L value I mean it's just a matter of a way of parametrizing the colors the L is the whitish appearance of the color and you see this object stays constant flat and at some point it starts rising if you look at the surface roughness like with a microscope maybe atomic force microscope is better because you want to go in details but you see the surface roughness starts to develop early and this is due to the migration and migration and recrystallization of the surface and when the crystals are very small they don't scatter light effectively very little scattering so the surface is still shiny but as they grow the surface starts to become rough and scatters light and the blooming appears so in conclusion surface microscopy is an early predictor for the blooming I mean this is something that is related to the fact that you can anticipate what is happening and perhaps vary conditions so that is not happening so much I mean just an example of where you wouldn't expect surface forces to act and actually they are acting and dominating the phenomena I was mentioning contact angle I was mentioning actually hydrophilicity which is measured by the contact angle that is the angle that a droplet of water say would form on the surface if you have a solid surface this is the case here if you have a liquid surface that could deform and form a lens there are three parameters in any case Young's law would I mean just a way of stating equilibrium forces the tension due to each interface interface 1, 2, 1, 3 and 2, 3 the vectorial sum of this tension has to be 0 at equilibrium and you may also define a spreading coefficient by this difference and if this spreading coefficient is larger than 0 sorry I translated my Italian lesson but I didn't translate all of them you have total wetting so the liquid would spread completely if it is below 0 you have partial wetting so the contact angle is between something in between 0 and 90 degrees there's a lot of interest in developing supra-hydrophobic surfaces amphiphobic as well in surfaces because imagine if you can make a surface that is really repelling water completely that is kind of self-cleaning so it can be self-cleaning you can imagine that this surface would be kept clean from dust and pollution this could be useful for membrane for water purification filters and the automotive section the state of the art in this field is that we are able of doing very nice supra-hydrophobic and supra-anthophobic that is hydrophobic and also repellent for grease but in the laboratory and these very nice surfaces are very dedicated, won't stand in real life applications so all this is made by making a rough surface with nanostructures on many scales and these very nice nanostructures won't stay there forever that would be probably taken away just wiping the surface so this is the state of the art coming to interfaces to interfacial layers we have to introduce surface pressure surface pressure is the reduction of the surface tension we are talking about by the action of something that I put at the interface imagine that I put molecules that like to stay on the surface that would reduce the cost of the surface and typically this molecule could be amphiphilic molecules that have the double nature of liking water on one side being polar and liking water on one side and being apolar on the other side liking the fat or the grease so this molecule would reduce the cost of the surface examples are surfactants like long surfactant we will talk about in a brief after soap and emulsifiers let's start with long surfactant you know when you breathe the lungs expand so you have an increase of surface which is what you need for the oxygen to be absorbed by your body by the blood but this surface increase has an energy cost as we saw and in order for this to happen with the low cost you have surfactants otherwise if you had no surfactants basically the lungs would collapse because the minimum energy is when you have all the air out and all the liquid in and to prevent these collapse you have these lung surfactants which are a mixture of phospholipids and other components which are some proteins and there are some problems related to this when you have premature babies which are born too early they don't have this natural surfactant so they wouldn't survive they wouldn't breathe in air and you have to provide them with some drugs inhalation drugs that substitute the natural surfactant that premature baby can start breathing so there's some research going on on this to model this system that is far too complex for many experimental techniques to model this system we can build a very simplified model that is really tabletop if you make a single layer at the air water interface what is called a Langmuir and with the Langmuir software we mean something in which you can study the field that you have the interface as a function of the area that you are allowing the molecules so remember the act of breathing, expanding and reducing we can play this game by moving the barriers and at the same time maybe we measure the surface tension by measuring the surface energy is also forced by a unit of length so we can put a balance here and if the balance is pulled by the layer if I measure the force that I have here I have a measurement of the surface tension so as a function of the area that I am allowing the molecules to have I can measure the tension so the energy of the surface so if it takes a system that is made with a phospholipid which is maybe this this schematic view of a phospholipid we have this double chain that is hydrophobic and a polar head which will go in water if I make a monolayer of this phospholipid on a Langmuir trough I can measure as a function of the area I am allowing the molecules to have I can measure the energy that they have I can also use other techniques like fluorescence microscopy or other techniques that are more complex I want to talk about the Brewster angle microscopy or ellipsometry to measure thickness and lateral structure of this film and this is the case for the simple phospholipid as a variety of phases from on this side you have a lot of space for the molecules so I have an expanded liquid expanded phase as you compress this convert into a compressed phase and you have a first order transition with a plateau of coexistence of the two phases this analogous to what you have in 3D simply it's made in 2D but the phenomenon is the same as when you have a phase transition in 3D but the good thing in this case that I have all this thing in my laboratory on a small trough and I can apply other techniques to characterize what is happening not only the energy but the lateral size, the thickness other things just an example another example of thing that you can study with this Langmuir technique is the polymers polymers can, some polymers can spread on the water surface, not all of them very hydrophilic polymers would simply see would be disoluted if they polyethylene oxide that would go in the surface but some polymers would stay in the surface and could have different conformations and here I can apply the concepts of the same concept that I have in 3D maybe you know in 3D, I mean in a solution a polymer can have different configuration depending if it is in good solvent it will spread out completely if it is in a bad solvent it will collapse completely and if it is in the intermediate situation in which the solvent is as good as the polymer is the so-called theta condition it will be a run of work and this same thing happens on the surface depending on the behavior of the polymer on the surface you can have a good solvent situation in which the solvent in which the polymer really spreads out and likes to spread out or in the poor solvent regime you have that are kind of rigid and avoiding each other and in either case you have a different dependence of the surface pressure from the area so just by measuring the Lammur isoterm that is the surface pressure of the function of the area see how this changes you can tell which kind of situation you have these are two different polymers polyturbutacrylate this one is a good solvent so it is a very shallow dependence PTBMA is in a poor solvent condition so it is not seeing each other at low concentration and all of a sudden you have a strong interaction between because it is in this regime here there are also cases in which you can revert the same polymer from one situation to the other yesterday there was the azobenzene ok anyhow I have this polymer this polymer has azobenzene side chain and the good thing this azobenzene the interesting thing for me is that azobenzene can be converted from one form to the other isomer they say the chemist from one isomer to the other by light and the equilibrium isomer has very little dipolar moment so it is not polar it does not want to go in water the cis isomer that I obtained by illuminating with UV light has a lot of dipolar moment so it is about 3.5 device so it is hydrophilic and by converting one to the other I can vary the nature of the polymer and you see here the equilibrium trans configuration is the red line very steep the cis with a strong dipolar moment hydrophilic much shallower start seeing each other at large distance but then if I put it in the scaling low plot in a log log plot I can really extract those parameters they are the same data recast as a function of pressure concentration that is the reciprocal of the area and you can really tell one is in bad solvent or theta bad solvent one is in the good solvent condition those language films can be also used to make structures so once you have created the language monolayer you can transfer it onto a solid substrate you can do it with a vertical substrate that is sink into sunk in the liquid and then extract it and this is the so called language blodgett deposition in which you deep and get out and each stroke you get an additional layer you can also I mean this is good in some case in other case you prefer to use the language shaffer deposition in which you have a flat surface and this is faster and better for rigid films this is more accurate for soft films and in this way you can create multi layers or multi structures and this can be of use other techniques to make layers layer by layer layer by layer deposition now I am mentioning another technique to make layers that has not to do with the surface of the air water but directly with the adhesion on the substrate if you have a substrate that bears some charge you can have polymers, anions and polycarthions that adhere on the surface in this example I am starting from a surface that bears negative charges that would be the case of silicon oxide covered by the natural oxide which has the OH- groups and this would attract cations so if I have a poly cation that would adhere on the surface and leave probably an excess of positive charge because of the polymagnature so if I wash this out and sink it into polyion then wash again and I can make a structure a multi layer structure and here are a few examples of polymers that are commonly used for doing this and this can be done on flat objects but this could be interesting but perhaps more interesting you can make capsules with this this was a quite popular ten or fifteen years ago in the group of Mowalt from Max Blanken in Gölm you start with some colloidal particles which can be covered by successive layers of polymers and then the colloidal particle can be dissolved if it is calcium carbonate you dissolve it in acidic environment and you can replace the content so you can get these nice structures that are capsules that can be controlled can be controlled by the pH with the acidity of the environment other techniques just to mention just to give you the flavor of things that you can do with surfaces self-assembly of course in many cases the most famous example perhaps is that of thiol on gold thiol is sulfur that binds some gold and you can form monolayers in this way not only you have self-assembly also in nature so phospholipids for bilayers and vesicles and it's not only confined to interfaces because the self-assembly also for viruses do self-assemble many structures that do self-assembly basically also protein folding can be considered self-assembly but it's not interfaces ok coming back to general problem I talked about insoluble surfactants molecules that will stay only on the water most commonly you have molecules that have a balance between staying in the water and on the surface in this case for instance you have a molecule that like sodium of the sea soil or many detergents if you start with low concentration you have monomers you have single molecules in the water but if you increase the concentration at some point those would aggregate and form micelles because of the amphiphilic nature so the water repelling part of the molecules stay packed together and form a micelle the molecular size for the micelle that is given by the molecular size and as soon as you have enough concentration to form micelles the concentration of micelles will grow while the concentration of monomers will stay constant monomers is not appropriate single molecules and the equilibrium between molecules that you have in the surface that is kind of constant and this concentration that you have this molecules that are in the surface and the molecules that are in the interface is given in most cases by the Gibbs equation or variation but let it keep simple on the Gibbs and this equation tells us that the surface concentration is related to the thermal energy to the surface and how the surface tension depends on the concentration and this is a typical example of dependence on the concentration look on the horizontal scale I have the concentration of surfactant in logarithmic scale when vertically I have the surface tension so how many molecules are reducing how the surface energy is reduced by the surfactant and this value is reached I mean by measuring surface tension as a function of time and there is probably also some kinetics when you add more molecules in the surface they will take some time to migrate to the surface one way of measuring this is doing with the alamutroff and other methods are pendant droplets there are many ways of measuring this so the model that I have in mind that keeps for the model which have concentration in the liquid I have zero concentration in the air in the gas and I have an excess in the interface and this excess is related to what I was saying to the reduction in energy cost you can have also nanoparticles that are stabilizing going to the interface so not only solid molecules but also nanoparticles nanoparticles are kind of similar could be completely hydrophobic then they stay on the surface in any case but could be also partly hydrophilic so you have a balance between nanoparticles that are in the liquid and particles that go on the interface and you can also play with surfactants so that you can vary the hydrophilic or hydrophobic nature of the nanoparticles nanoparticles nanoparticles stabilized interfaces are important for stabilizing forms and emulsions because once a nanoparticle has reached the surface it won't go away while single molecule absorbed can desorb the thermal energies enough to desorb them this is not the case for nanoparticles nanoparticles have such a large energy gain in going in the interface that they stay there forever and this makes much more stable interfaces so this is a way of stabilizing forms and emulsions a picker in emulsion it's called picker in emulsion is an emulsion stabilized by 100 nanometers of particles and they form a kind of armor around each droplets to make it very strong and stable okay and now we are working on this subject and the fact that you can use different particles could allow you to have stimuli responsive systems if you have magnetic particle or light sensitive surfactants then you can make forms or emulsions whose stability is controlled externally by the stimulus and this could be important say for instance in the oil industry you want to extract oil from the water separate and then recycle the same surfactant or many situations in which you would like to have a surfactant whose surface activity is controllable so that you can use and reuse it and recycle it rather than wasting it every time so a few words about emulsion an emulsion as we already mentioned is a combination between two liquids that are not missable and what drives the stability of the emulsion there are basically three phenomena that can happen the first is drainage due to the difference in density if oil is lighter than water it will go on the surface on the upper part of your emulsion separating from the water that is going down drainage is also important in a foam you have simply that water content of the foam would decrease simply because the water is going down and the foam is getting dry and it gets dry and dry it will be less stable so drainage one cautioning and osval writing we mentioned before is the migration of gas from small bubbles to large bubbles because of difference in Laplace pressure this can be controlled by controlling the diffusivity of the gas or of the liquid if you have an emulsion between the two but even if you have immiscible liquids then you could have other methods of migration myself could transfer mass from one droplet to the other so osval writing is happening is ubiquitous also for immiscible liquids and finally coalescence that is the case where you have two droplets that get into one because of breakage there is no border between the two so these are the mechanisms that basically drive the emulsion stability there are many techniques to study emulsion you can look at them on a microscopic scale you can do anything what we brought here for the hands-on session is a diffusing waste petroscopy that is DWS a technique that we used to study complex systems and the technique is based on having a laser light coherent light source monochromatic and coherent that is shining a light on your sample if this sample is static you have a collection of speckles and they are static if something is moving in your sample the speckles would move and this is the case in this animation that I have taken from a vendor site a less instrument and if you measure the intensity as a function of time you have this then you can calculate correlation function for the intensity and then from this correlation function I will show you you can get information about size of the particle that you have or the dynamics and the mechanical modules of your system so all this reduces in having it's a very simple experiment it's really tabletop and low cost it's cost less than 2,000 euros you have just a laser and a detector that can be fast or not fast depending on your system and then you calculate correlation function it's not really difficult to do it good question, thank you thank you, good question you can do it with single point detector you can do it with an array of detectors if you do it with a single point detector probably you are faster if you want to go very fast you could put two photo multipliers in coincidence you go to microseconds your single point this is good but sometimes for instance if your system is out of equilibrium and erratic hypothesis fails then measuring for a long time a single point doesn't it's not equivalent to the ensemble average because your system is stuck in some local minima it's not exploring all the other configurations in this case you really need multispectal analysis and that is done with an array of detectors can be done with an array you can do it in different ways but one way the way that we are doing is using an array of detectors that is simply a camera a CMOS camera you can use a webcam if you do not need to go very fast a webcam would go 30 frames per second that is the speed of a webcam and we can do it or you can go faster cameras depending on your needs and on your budget and in this case the passage that we are kind of suggesting from the time average of the correction function calculated as a time average to a pixel average and then maybe you make a time average what is the difference between the two as you were saying first of all non-negody systems then slow dynamics if a system that is relaxing in one day in this approach I need some days to get the correction function for statistics which is maybe not ideal whereas in this case with a single couple of images probably I can get enough statistics I can also calculate two times correction function which is something more subtle than this I mean you probably know from other techniques two times correction function is the product of intensity in this square I have time 1, time 2 and each point is the product of the intensity I measure time time time 1 times the intensity I measure time 2 so the diagonal is the squares of the intensities what does this graph tell me first of all if I have some age and some evolution the evolution goes along this way if the dynamics is stationary this thing would remain parallel if something is happening some here is slower here is faster here the case is lower I can see it here so two times correction function this can be only calculated if I have pixel average and then I can calculate also higher order correction function but I mean two details so how I understand what is going on the hypothesis is that I have a diffusion path of light don't confuse diffusion of light with the Brownian diffusion of the particles particles can't have their own Brownian diffusion in some case or if I have a phone it's another story so it's not the diffusion of the particles that I'm talking about it's the path of the light that it is a diffusive path and this diffusive path is characterized by a length that is the typical length that the photon would travel before it's captured again and it's losing memory of where it was going and you can do accurate calculations to tell this transport photon mean the path how it goes with the distance between the particles because it can coincide with the distance between the particles very strong scatter or you may need a number of single scattering event in order for the photon to lose memory of its direction anyhow given this idea of a star of a photon the correction function that we measure the correction function of the intensities that we measure G2 is related to the correction function of the electric field that is the thing that I can more easily calculate by the usual Seagate relation in which you have this is the correction you measure this is what you calculate and there's some contrast that is determined in your experimental geometry and G1 is given by the sum of all the contributions so the integral over all the path length that the light could follow weighted by or measured in units of the transport mean free path so this is the number of path of scattering event that the light has undergone during this path S weighted by the probability path S is really covered by the light and contributing a defacing term which is typical of this path so this is the main ingredient of my signal it's not very easy for me it was not very easy to understand at the beginning I found it easier to understand it in a time resolved mode which is a very nice experiment which requires a lot of power pulse laser but I mean it's not what we are going to do but I just show you this because I think it's easier to understand so this is the expression that we are writing and if I can divide each path measure each path on its own then I have an individual contribution to the correction function that is arising from this single path and this is done as we're saying if I have a very narrow path which is impinging on my sample the light coming out of the sample will be spread out in time depending on the length of the path that the light has covered and if I calculate correction function for a given delay for a given time like then I'm selecting the correction function for a particular path length S and the result is that the correction function of this path length is an exponential simple exponential and this simple exponential has a characteristic time that is scaling with the length with the delay with the length and I can also measure the P of S, this is the probability of finding this path simply by the intensity of light so I think that in this way you can understand what is going on in a diffusing wave experiment the only disadvantage of this experiment is that you need a very intense pulse laser which is expensive and not all the samples would stand you don't want to fry your sample because you want a lot of power if you have a foam you put a lot of power simply destroy it so that was just for understanding it's not what we are doing what we are doing we have a continuous wave laser we can work out the calculations discover that in transmission if I look across the sample the correction function we have a shape that is this square root of time of the hyperbolic sinus of this which looks much like an exponential we won't use the exponential because it's not very accurate we use the proper form but just remember it's kind of an exponential I can also work in best scattering which is advisable in many cases because there are two advantages one is that if I'm working in a real environment I want to monitor the mechanical models of something that is coming out in a real situation maybe it's easier to have a single optical access than having two so one advantage that I have a single optical access I can use it in industrial environment to have it the other is that it is lower we will see why in best scattering I have a different shape the shape is that of a stretched exponential and the stretched exponential is due to the fact that in best scattering I have contribution from many different thicknesses from many different depths shorter depth means fast decay deeper depth means slower decay and the sum of them means this stretched exponential shape can also use this extract the mean square displacement if I have particles that are scattered in light independent the interpretation depends on what I have if I have particles that are scattered in light I can extract the mean square displacement of these cutters and from this mean displacement by the generalized Torx Einstein relation I can calculate the mechanical model so the viscosity in the easy case we will do it with the glycerol but in the general case for a jammed system like this you have micro particles that are jammed or close to be jammed because of this high concentration and you have this viscoelastic behavior with a crossover from viscose to elastic or have the concentration of polymer again this is just in the literature to show you that by this technique you can make a measurement of mechanical properties of your sample without touching it just optically and accessing a much wider frequency range that you would access with the mechanical rheometer and this can be also coupled to mechanical rheometer mechanical rheometer would put some strain in your system this is just based on the spontaneous fluctuations so this is always an equilibrium mechanical rheometer is under shear ok just keep some details just an example of application this technique to real life problem making of yogurt yogurt is made out of milk by bacteria that eat sugar and produce acids and the acid environment the proteins get they go to their isolated point and they become larger and they form a jelly structure and engulf each other and form a jelly structure we can follow the formation of this gel by dws as a function of the acidity as the acidity is increasing ph going down the correction function is getting slower and slower because the system is jamming and ok I don't think we can go in all these details classical studies on foam cautioning which is based on basically on size in the particles by comparing if I compare best scattering and transmitted decay they are related by the L star and the L star is related to the droplet size the bubble size so I can get a direct measurement of the average droplet size inside the sample which I couldn't do without tomography perhaps looking it's kind of looking inside the sample not with all the detail that tomography will do because you have just the average but maybe with a faster result you can do it on a system that's not so stable that you can do it in real time you can also follow cautioning and time evolution basically of a foam and also on emulsion and this we'll see in the laboratory if you want we can see it in the laboratory formation on emulsion and the evolution both in terms of the distance between I mean the size of the droplets and the distance between them that is the information contained in the photon mean free path but also in terms of mechanical properties of mean square displacement here I'm showing this as a function of logarithm of time and plotting the logarithm of the mean square displacement if a Brownian motion you have this dashed line and at early times you observe something that is pseudo-diffusive so it's kind of like this but as you go to later times you find sub-diffusive and then caging effects and these effects are more evident in the emulsion that have been stored there for some time so I mean this is something we can look into in the experimenter session so for the DWS I think it's a powerful tool for an easy quick and dirty characterization but you need also other techniques to understand what you are seeing so it's not that it's containing everything there you probably have to combine this with optical microscopy and other characterization but if you know many things on your system then from the diffusing waste pathroscopy correction function you can get the evolution of some important parameters now the last five minutes I guess I've been talking too long on the first part I wanted to kind of advertise a large-scale facility techniques like reflectivity and grazing incident diffraction which are useful for characterization of structures and photocorrelation spectroscopy which is a new technique that is coming out these years for characterizing the dynamics I think I will skip the first part I mean synchrotron radiation is what you need to have enough intensity for these techniques to work well in particular for the photocorrelation spectroscopy you need a coherent beam which is not something that you can easily have in your laboratory you need a very high intensity and this is only available at lasers third generation synchrotrons and maybe X-ray free electron lasers which may have the opposite problem of too much intensity for the experiment that we are used to so there are other experiments that you have to think of for these new sources and how I think that I will skip this part on reflectivity you find in most textbooks it's just a way of characterizing the thickness of your sample and the internal structure you can combine X-rays with neutrons neutrons have the advantage that you can put isotopic substitution you can difference in different parts but on the other hand they don't access as much q-rays as X-rays but I mean that is kind of well established you can also apply fluorescence to get the distribution of your ions and all this is and diffraction but this is I think it's quite well known I want to spend just a few words on this kind of new technique that is X-ray photocorrelation spectroscopy which is covering a time-space region which was not previously accessible directly by other techniques I mean directly yes of course many techniques but it is on the local scale of the atoms because it's all with X-rays and on the slow time scale of dynamic light scattering basically it is like dynamic light scattering but done with X-rays so you see dynamics on the scale of milliseconds to hours on the local scale of the nanostructures for doing this you need a coherent X-ray beam which was not available till a few years ago so this is a really new technique because it relies on something that was not available before nowadays I mean synchrotrons like the SRF synchrotron in Grenoble has a decent flux enough flux for doing these experiments what you expect in a correlation function if you have just something that is moving with a constant velocity you get this kind of shape if you have a powder average you have a kind of compressor exponential if you have Brownian diffusion it's like in dynamic light scattering you have the usual thing as in the Stokes Einstein model in the rest of the system you have this very nice double step this is not actually XPCS from Cipolletti and Franz so you can really address and explore the region in space time and for instance this is just an example of what my friend Beatrice Ruta looking at the dynamics inside the glass and you see some dynamics that you wouldn't expect below TG below the glass transition temperature where everything is arrested and still you have some local dynamics coming to the interfaces and you can apply the same trick in grazing incidence so you see the dynamics in the monolayer you can see out of equilibrium dynamics this is again the two times correlation function I was showing for DWS time 1, time 2 and if the dynamics was stationary this would be just constant and this is broadening meaning that the dynamics is slowing slowing down and so on and so forth you can combine this technique with optical microscopy with epifluorescence microscopy and in this way you cover even a larger range of space or momentum on the same time scale and in this way for instance we could follow the arrest transition in mixed monolayer of phospholipid and nanoparticles which as you increase the density you go from Brownian diffusion which is well understood in the Hughes regime to an arrested regime and ok I mean this is just an example of things that you could do and just to finish I want to leave you with some literature a few references on DWS there is a chapter in this book and the classical book on dynamic light scattering is also very informative something on forms and scaling loads and about x-rays techniques the best books for me are these elements of modern x-ray physics by Ars Nielsen and many others but also this chapter by Grubele and Matz and what kind of founders or fathers of this technique that is and this is all available and thank you for your attention