 Welcome to the 30th lecture in our particle characterization course. In the last lecture, we started discussing nano particles and in particular, there are three aspects of nano particles that we need to be concerned about the synthesis, dispersion and characterization of nano particles. So, in the last lecture, we started to get into some introductory material on what is nano technology, what are the various aspects of nano technology, some applications of nano technology and then we started our discussion of synthesis methods of nano particles and in particular, the bottom up methods. Essentially, nano particle synthesis methods can be classified as bottom up and top down. The bottom up approaches require that we take precursors that are essentially in atomic form and assemble them together to form a nano dimensional material and that is the definition of bottom up. Whereas, in the case of top down, we start with materials that are of micron scale and fragment them down to nano dimensions. So, in terms of bottom up nano particle synthesis methods, we again sub classified them based on the nature of the precursors and we discussed several methods where the precursor is in the form of a solid which must be vaporized to generate the vapor which can then be crystallized to form solid particles. Now, later we also started discussing applications where the precursors are in the form of liquid droplets or vapors. So, we will continue that discussion in this class. There is a method called thermal plasma synthesis which basically requires that you take vapor precursors and introduce them into a thermal plasma. When you do that, the plasma supplies additional energy to break down the precursors completely into atoms. So, the precursors may be in the form of a high molecular weight molecules which using the plasma irradiation can be broken down into their constituent atoms which then can react or condense to form particles and this can be accomplished by providing cooling, the mixing with a cool gas or simply expanding the vapors through a nozzle. This method is used quite widely for the production of silicon carbide and titanium carbide for nano phase hot coatings. Another technique is flame synthesis. Here, the primary difference is that combustion reactions are used to generate the heat that is required. So, particle synthesis here occurs literally within a flame. The flame is used to generate the energy or the heat that is required to drive the reaction to completion. It is a very successful commercial process. In fact, there are millions of tons per year of carbon black and metal oxides being produced using this method. The drawback to this method is that combustion is not an easy process to control. So, it is a complex process but widely used for making oxides and recently we have been able to make various types of oxide nanoparticles as well as sintered agglomerates. In terms of control, it has been found that if you can apply a DC field to the flame that can actually stabilize the combustion reaction and produce a more tightly controlled particle size distribution. Flame spray pyrolysis is a combination of two methods that we have discussed earlier flame pyrolysis and spray pyrolysis. Here, the liquid is sprayed directly into the flame. So, the liquid precursors are injected into the flame. The advantage of this method is that you can use low vapor pressure precursors because the combustion process will supply sufficient heat to vaporize even low vapor pressure precursor materials and this is applied to synthesis of silica particles. Low temperature reactive synthesis is the name given to nanoparticle synthesis methods that do not employ high temperatures. Now, in most of the methods that we talked about earlier energy is being supplied in the form of heat and therefore, there is some limitations to such methods. If you are trying to make materials that are not compatible with high temperatures, then you can actually wind up damaging or changing the composition of the material that you are manufacturing. Here, we try to bring vapor phase precursors to react without adding heat and without producing heat. For example, zinc selenium nanoparticles can be made from dimethyl zinc, trimethylamine and hydrogen selenide using this technique. All we do here is provide intimate mixing. Essentially, we use a counter flow jet reactor that is kept at room temperature, but it provides very intimate mixing of the reactants. The heat of reaction itself is sufficient to allow particles to crystallize. So, it is a very energy efficient process because it can be run at room temperature without external addition of heat and the fact that there is no heat generation also means that it can be essentially done in simple equipment designs where you do not have to worry about heat generation and the heat dissipation and so on. Another method that is been widely used in the bottom up side of manufacturing nanoparticles is sonochemical nanosynthesis. Ultrasound is an interesting technique in the sense that it can be used to synthesize nanoparticles both from the bottom up approach as well as top down approach. Later, we will see how ultrasound can be used to make nanoparticles in a top down kind of application, but when we apply an acoustic field to a liquid, instantaneously extremely high temperatures and pressures can be realized within the liquid as cavitation happens. So, essentially when you apply an oscillating acoustic field, you start forming bubbles in the liquid which collapse and as they do, they release high amounts of energy and according to the hot spot theory, as bubbles implode, temperatures of the order of 5000 to 25000 Kelvin can be realized for a few nanoseconds which is followed by very rapid cooling, but during the time when these high temperatures and high pressures are in effect, chemical reactions can be driven by several orders of magnitude faster than they would proceed otherwise. So, this acoustic cavitation effect can break chemical bonds and can make chemical reactions run very very fast and therefore, it can intensify the process by which nanoparticles are synthesized using chemical reactions. The high cooling rate can be advantageous in some ways because by quenching the reaction which happens naturally in a cavitation field, you prevent crystallization of the product. So, you can essentially make amorphous nanoparticles very easily by using the sonochemical method. Therefore, it is considered a superior process for preparation of amorphous nanoparticles because of this cold quenching mechanism. Insertion of nanomaterials into mesoporous materials also becomes possible because as we have discussed in one of the earlier lectures on surface cleaning, when you couple an acoustic field to a liquid, two things happen cavitation which is a mechanism of bubble implosion and acoustic streaming which is the high velocity unidirectional flow that is induced with very high frequencies when the bubbles are very very fine, but bubble density is very very high. So, the net effect of these bubbles forming and collapsing is that there is no cavitational force that is significant. However, the phenomenon of bubble implosion at very very small size scales leads to this process of acoustic streaming by which the fluid is directed at high velocities in one direction. So, if you can harness this acoustic streaming process, you can take the nanomaterials that you have fabricated and actually make them flow into porosities in materials. So, you can actually insert or incorporate nanomaterials into a porous matrix by taking advantage of acoustic streaming. You can deposit nanoparticles on ceramic surfaces and polymer surfaces and you can form essentially spherical nanoparticles. Sonochemical reactions are particularly suited to making spherical nanoparticles. Indeed, we will see later that when you use sonofragmentation that also has the effect of making spherical nanoparticles. So, there are applications which require sphericity in the nanomaterials and for that, sonochemistry is a good process to use. You can also make very small particles because of this rapid quenching effect. As the particle starts to form, if you can quench the formation process, then you do not provide time for the nanoparticles to grow to larger sizes. So, sonochemistry can be very effectively deployed to control the size. Some examples of materials that are synthesized using sonochemical methods are shown here and other materials are listed here. So, there is a huge variety of materials and compounds that are commercially manufactured using sonochemical nanosynthesis techniques. Now, moving over to the bottom from bottom up to top down methods of nanoparticle synthesis, size reduction is the primary mechanism by which we can make nanoparticles starting from larger sized particulate material. Crushers, grinders, ultrafine grinders, cutting machines, you are familiar with all of these tools from your courses and labs in industrial processing, mechanical operations. They can actually be used quite effectively to do size reduction. However, reaching nano dimensions is very difficult with many of these machines. Crushers, grinders are only good for reducing particle size from say millimeters down to possibly tens or hundreds of microns. A ball mill as well as ultrafine grinders are effective in making particles down to roughly 10 microns. But when you try to get the size below 10 microns to 1 micron or less, there is a huge energy penalty. Some of these size reduction techniques do not work very well and that has somewhat limited their application to synthesis of nanoparticles. A ball mill which I am sure everyone is familiar with is basically a cylindrical shell that slowly turns about a horizontal axis. It is filled to about half the volume with a solid grinding medium and essentially as it rotates, the balls that are in the grinding chamber reach a maximum height and then drop and as they do the impact on the material being size reduced and break it to a smaller size. The advantage of a ball mill is that it can be operated in continuous mode or batch mode. Size reduction happens by a simple impaction process. If you look at the forces acting inside a ball mill, you have centrifugal force which is lifting particles along the sides and gravity force which then causes the ball to drop on the feed and crush it and you can establish a critical speed of the ball mill NC which is related to the acceleration due to gravity, the radius of the shell and the radius of a ball. You want to keep the actual operating speed of the ball mill to no more than 75 to 80 percent of the critical speed otherwise essentially all the balls will become entrained and just keep following the circumferential motion of the grinding chamber rather than dropping on the material to be crushed. So this is a schematic of how the inside of the grinding chamber looks, the yellow popcorn like things or the material that are being crushed and the spherical balls or the grinding media. The ball mill is the workhorse in industry as far as fine grinding is concerned. Applications are limited by high power requirement and low efficiency. I am sure you all recall the lab experiments you have done which demonstrate that as the effective particle size decreases, you have to keep supplying more and more energy to produce new surface area. So it is essentially a self-limiting process as the surface becomes finer, it becomes more and more difficult to reduce its size further. However, it is widely used in many mineral processing industries, cement industry, coal processing and so on. Here are some examples of industrial ball mills. You can see a you know the scale of the equipment. These are huge chambers that are used to make 1000s of tons of material on a weekly basis. So you know it is difficult to think of these machines in the context of nanotechnology where our production rates are much more modest and so many of these industrial ball mills really do not lend themselves to making nanoparticles. However, more recently there have been advances in ball milling technology in particular the high energy ball mill which is an improvement over the conventional ball mill in the sense that it is a much it is a more controlled operating environment. So you can actually control the temperature inside the milling chamber, you can control the pressure, you can even control the chemistry of the ball milling chamber in order to produce product that you need. This is again a schematic of how a high energy ball mill might look. Essentially, the mechanism remains the same, you are still taking material to be size reduced, putting it in a chamber and allowing the grinding media to impact on the material with some force which causes a size reduction to happen. However, in this example you can see that the inside of the chamber is filled with argon and inert material to prevent oxidation. So for example, if you have aluminum or copper that you are trying to produce, if you do this in the ambient environment air itself is sufficient to cause aluminum and copper to oxidize. So if you want to make sure that you synthesize the metal nanoparticle without any oxide being taken up, then you have to provide an inert environment. This table shows the differences between ball milling and high energy ball milling. One of the key differences is in a high energy ball mill, you run the mill for much longer. Essentially, the mean size will keep decreasing as you run the mill longer and longer. So while in a ball mill with a milling time of 1 hour or less, you can achieve particles that are say 10 microns mean size. In a high energy ball mill just by running it much longer, you can achieve much smaller sizes. Also the impact energy is designed to be much higher in a high energy ball mill. The particle size that is produced, well actually if you take a look at it, the output from the high energy ball mill looks not very different from the output from a conventional ball mill. That is because even though you may be size reducing to much smaller sizes, because you are running the process for so long, agglomeration sets in. So the final product that you get from a high energy ball mill is a kind of a powdery substance that has agglomerates or clusters that are micron in size. Now it is possible to take these clusters and then disperse them. So that extra step is required after high energy ball milling to take the material that you have produced and disperse it into the sub micron sized individual particles. In a high energy ball mill structural changes and chemical reactions can be induced. The atmosphere can be vacuum or it can be gas filled in a high energy ball mill and the temperature can be controlled all the way from sub zero temperatures that are achieved using cryogenic fluids to 700 degree centigrade. So the very wide temperature range is possible in a high energy ball mill. The impact energy of a vibrating ball mill can be calculated based on the operating conditions in particular the frequency of the vibrating ball mill. The vibrating ball mill is an alternative to the impact ball mill where the balls drop by the force of gravity and it is another method for making fine particles from coarser feed. However vibrating ball mills are not widely used in industry compared to the impact based ball mills that are more conventionally used. Now why is there a size limit in mechanical grinding? There are really two reasons. One is that as the size decreases particles become monocrystalline in nature and as they do the energy requirement to break the particle keeps increasing. So essentially the particle itself becomes more and more unbreakable as it gets smaller. In addition agglomeration begins to happen. Belting of small particles into clusters begins to happen. And again the longer you run it the stronger are these inter particle cohesive forces and essentially sintering begins to happen after some period of time and these clusters that form over long period of operation can be very very difficult to break down into their component materials. Extended milling also leads to amorphization. This is shown here in a series of graphs that are taken at various time intervals ranging from 23 hours to 73 hours to 165 hours. And you can see that what started out as very crystalline material where the structure can be easily identified soon reaches an amorphous form where it is very difficult to really identify the shape of the individual particles. Essentially you get an amorphous powder which you know if you were to test it using x-ray diffraction would basically tell you that it is a highly non crystalline material. So one of the problems with the extended time high energy ball milling to make nano particles is that it is very difficult to get crystalline materials using this process. There is over time an amorphization that happens which is very difficult to avoid. It is TEM pictures of titanium nitrite nano particles that are produced by high energy ball milling and here you can see the clustering that is happening. This is a good example of characterization of nano particles using tunneling electron microscopy. You can see that the resolution is fantastic. You can actually see particles that are you know nanometers in size and you can actually resolve particles that are nearby which are about a nanometer apart. So the primary advantage of the TEM over scanning electron microscopy is its ability to sensitively detect as well as resolve particles that are in the nano size range and the clustering is very apparent in this picture. Now these are examples of metal oxide nano particles that are produced by milling of various powders titanium, iron, zirconium in an oxygen environment. Again this is done in a high energy ball mill where the interior of the of the mill can be made to be oxygen rich and what you produce is nano titania nano Fe x o y and nano z r o y and here again when you look at the TEM picture at the bottom you can see the extent of clustering that is happening. Clearly nano particles are being produced but by the time you take them out of the mill they have essentially clustered into an agglomerate that is of the order of several hundred nanometers in dimension. So they have to be broken up somehow before you can employ them for further processing. So there is this process that is labeled as Meccano thermal process which involves a two stage procedure. In step one you use high energy ball milling to produce a structure that is a nucleate structure. Step two is to then produce a controlled thermal annealing process which leads to growth. Now thermal annealing is essentially taking a nano material that you have produced and exposing it to high temperatures in a controlled manner in order to obtain a certain structure and size that you are interested in. Typically thermal annealing is done to get a more sintered a harder structure compared to the amorphous structure that a high energy ball mill would normally produce. So if you want to take the output from a high energy ball mill and give it more of a crystalline structure then thermal annealing is a process that is highly recommended. These are examples of nano tubes that are produced by ball milling. Here you can see that ball milling is been able to produce the precursors for nano tubes. So the nucleation structures are there. There are pentagons, microprores as well as nano sized metal particles and you can see that carbon is being incorporated as the source of carbon atoms. Now when you look at this process that combines mechanical milling and thermal annealing it is actually difficult to decide whether you should label it as a top down process or a bottom up process because the mechanical milling itself is a top down process obviously but thermal annealing is where you are essentially taking loose fragments that you have generated by the mechanical milling and bonding them together to produce a crystalline structure. So it is kind of a combination of top down and bottom up methodologies just like the liquid gel technique was that we talked about earlier. So here is some output from this combined process. Boron nitride nano tubes can be produced using this process and you can see the structure of the nano material that results after 10 hours and after 16 hours. In fact high energy ball milling combined with thermal annealing was the first commercial process to be used for the production of boron nitride nano tubes and cost wise actually this is very competitive with bottom up approaches. You can see some prices listed here for grams of material. In comparison if you were trying to make the same materials using a bottom up process it would cost you at least 3 to 4 times as much. There is in literature many applications of ball milling for the synthesis of nano particles and you can see that there is a large variety of materials that are made using the ball milling process particularly popular for making nano tubes. Now these are some results from our laboratory where we have taken carbon and do and try to do size reduction using various techniques. So when we do this in a high energy ball mill and you plot the mean size as a function of time you can see that there is a leveling of effect or an asymptotic effect which in this case is reached at about 6 microns or so and the size reduction stops happening after this limiting size is reached. When you increase the rpm of the ball mill it really does not affect the asymptotic size that is reached what it does affect is the rate at which the size is approached. So by increasing the rpm you can increase the kinetics of the size reduction process but you really cannot do anything about the final size which is more dependent on the dynamics of the mill itself such as the size of the ball the size of the chamber and ultimately the size reduction process itself will stop after a certain minimum size is reached. This is a comparison by size versus energy. So if you look at energy that is being input and the size that is achieved as a result of the energy input you can see that there is a similar amount of energy being consumed at various milling intensities in other words even if you change the rpm as long as the input energy remains the same the final size that is achieved also remains the same and again you can see that the process of size reduction by ball milling essentially becomes energy inefficient after about 5 microns is reached because you can keep increasing the amount of energy that you input without causing any change in the in the mean size after roughly in this case about 50 kilowatt hours per kilogram. Here we compare the ball milling processes with ultrasonication to look at the energy efficiency of these processes. As we have seen in the previous pictures milling at various rpm's starts to show a leveling of tendency around 6 microns whereas if you did the size reduction using sonofragmentation as a technology at in this case at 40 kilo hertz you can keep driving the size much lower than you can with conventional size reduction processes such as ball milling. In fact if you look at this figure more closely you can see that actually milling is a more energy efficient process until a size of roughly 5 microns is reached after which sonication becomes the more energy efficient process. So if you are trying to do nano particle synthesis by size reduction then you are probably better off using a combination of techniques. So to get from an initial size that is let us say around 70 to 100 microns to about 5 microns you should use ball mill to reach that size and after that you start sonicating the particles to do further size reduction. So let us quickly recap what is sonotechnology. We have discussed this in the context of surface cleaning earlier. Sonotechnology refers to intensification of bulk fluid as well as surface and interfacial processes by the combined action of cavitation and acoustic streaming and here we see schematics of what a cavitation bubble looks like just before it implodes. We see a typical acoustic streaming pattern which is essentially uredirectional in nature and at the top we see the typical compression and rarifaction cycles of a sound wave. Here of course the bubbles will be forming during the rarifaction cycle and they will be collapsing during the compression cycle. This is how the cavitation actually happens. There is a bubble growth which leads to a maximum bubble size at which point the bubbles collapse in the compression cycle and releases energy as a shock wave that propagates through the liquid medium and either impacts on particles that are suspended in the liquid or it impacts on a surface that the liquid is adjacent to and the energy that is transferred due to this shock wave impaction causes physical changes or chemical changes to happen and then the cycle keeps repeating itself as new bubbles form and implode. Frequency is a major player in this. As you increase the frequency of the acoustic field the cavitation intensity drops quickly. Actually there is a 1 over f cubed dependence. Cavitation strength decreases with frequency as 1 over the cube of the frequency. So you can see here that as you approach 400 kilohertz, 800 kilohertz or close to 1 megahertz cavitation is virtually absent and it is at its most significant levels for ultrasonic frequencies that are less than 100 microns. The 25 kilohertz to 100 kilohertz range is where you get the most intense cavitation. Some of the advantages of sonofragmentation for synthesizing nano particles are high purity because essentially you are taking the base material. If you are trying to make nano alumina, you are taking micron sized alumina. So there is no additional chemicals involved and all you use is high purity water typically. So you immerse the micron sized particles in high purity water, turn on the acoustic field and you start fragmenting the particles. So it is a very simple process, energy efficient, easy to scale up and also it does some interesting things to the crystalline structure of the material. The sonication introduces what is known as lattice strain. So the crystallinity of the particle is maintained. It does not become amorphous with extended exposure like high energy ball milling does, but there are subtle changes in the crystallinity. Effectively from a physical view point, what you see is a rounding off of the corners so that a material or a particle that is subjected to long term sonication becomes increasingly spherical with the passage of time. It also lowers the sintering temperature. Now that is an advantage when you are using these particles in an application that requires them to be sintered and the ability to use a lower temperature is advantageous from an energy efficiency view point. Of course the reason that the sintering temperatures are lower is because the process of sonication increases the surface energy of the particle. So the natural adhesiveness or cohesiveness of the particle is greatly increased by the process of sonication. This is actually true for all top down methods, but in particular ultrasound because it imparts such high energy to the particles to fragment them, leaves behind a residue of the energy which is much higher compared to for example high energy ball milling. So the particles, nano particles that are synthesized using sonofragmentation are essentially much more reactive, much more adhesive and much more conformable to your process compared to nano materials that are made by other techniques. Here are some examples of particles that are produced using sonofragmentation and again from a particle characterization view point what you see here are data that are produced using a particle size analyzer. In this chart you can see that compared to the untreated alumina particles as you expose them to sonication for various periods of time and this is using fairly low power ultrasound 88 watts you can see that the mean size keeps shifting to a smaller size range. This is an illustration of a silica particle or silica powder that has been exposed to ultrasound and you can see here that a very characteristic bimodal distribution develops over time and here again what you see is a reduction in the mean size of particles and also an increase in the volume fraction of particles in the finer size range. So you can see that there is a significant enrichment of particles in the sub 1 micron size range as you subject the powder to sonication over an extended period of time. This is a sample of zirconia powder that has been subjected to ultrasound and again you show you see the same trend the volume fraction of particles in the larger size range decreases with increasing sonication time and the volume fraction of particles in the sub micron size range increase with increase in sonication time. So the mean size decreases as well. So if you actually took all the data that was in literature and plotted it as mean particle diameter versus sonication power in watts, you see a very interesting trend that with increasing sonication power you can drive the mean particle diameter after 30 minutes of sonication down to 1 micron. However if you look at the literature data there is not much work reported for particles that are smaller than 1 micron. It is reasonable to expect that this line cannot simply be extrapolated to sub micron sizes. There will be a leveling of effect even with sonofragmentation. However based on this it is reasonable to expect that sonication will remain a viable technology even for particles that are smaller than a micron. If you look at the fragmentation rate expression reported in literature the fragmentation rate in sonofragmentation is related to the ultrasonic power input with a exponent of 0.38, particle volume with an exponent of one third and total suspension volume which is in the denominator. This is an interesting expression what it implies is that as the total suspension volume increases the fragmentation rate will decrease reciprocally. The reason for that is that sonofragmentation is a volumetric effect. You are supplying a constant power if you increase the volume of the liquid then the power per unit volume decreases and that results in reduced fragmentation. It is reasonable to expect that power input will be in the numerator but an interesting finding is that particle volume is in the numerator. What that implies is as the concentration of particles in the suspension increases sonofragmentation efficiency will increase. That is somewhat counter intuitive because what that says is that if you process more material the process will be more efficient. That is usually the reverse of what happens with most chemical processes as you try to pump in more material your efficiency will decrease. The fact that in this particular case the particle volume or concentration is in the numerator gives us a lot of hope that scale up would not be that difficult because in scaling up what you are trying to do is essentially process more material. At least this expression would indicate that fragmentation rate will not decrease when you do that if anything it will increase as the particle volume to the power one third. Now, if you look at the mechanism of sonofragmentation there are actually two mechanisms by which particle fragmentation happens. There is direct interaction of the bubble with the particle and also interaction between two adjacent particles that are being accelerated by the bubble explosion. So, in this particular case the bubble collapse due to implosion is one of the mechanisms but the other mechanism is simple particle to particle impact. So, particle fragmentation here occurs due to the violent bubble collapse as well as inter particle collisions and the resulting attrition in the particles. So, this is basically what happens the bubbles impact on the particle particles impact on each other and the net result is a fragmented population of particles. So, in a laboratory setting this can be easily simulated by taking a beaker with distilled water put some micron sized feed particles into it and subjected to ultrasound which can be in the form of a tank with bottom mounted transducers or it can be in the form of a horn or nozzle through which the ultrasonic energy is transmitted. And if you look at what happens in the beaker now you will see that you have essentially three layers that show up. The bottom most layer will contain the unfragmented particles mostly the feed particles that have escaped the process of sonofragmentation. The top most layer will show the fragmented particles that have been reduced from micron sized to nano sized because of the sonofragmentation mechanism. Then there will be a middle layer which is a mix of both it will have a fraction of the particles being the size reduced nano dimensional particles and it will have a fraction of the particles being the original micron sized particles. So, this gives us a clue that if you were to combine this sono process with a kind of beaker decantation process you can not only make nano particles but you can also quickly collect them by a process that is based upon the sedimentation characteristics of the various sized fragments. If you have a beaker decantation type of setup you can simply take the top layer off and drain it into a collector and that will be enriched in nano particles whereas the bottom most layer can be recycled so that it can be size reduced further. What you can also do is take this sample which is stratified you know with three different size distributions and you can actually mix them up again. Now, that may be important in some applications where you need a range of particle sizes rather than a particle size in a particular size range. So, it is very easy to take this stratified layer of sub micron, micron and nano sized particles and remix them to produce a homogeneous suspension. So, in this particular case it is possible to take particles that have been produced by sono blending as this is called and essentially make a uniform composition of nano sized, sub micron sized and micron sized particles which can then be used to make composites. One of the major applications of sono technology is not only to synthesize nano particles but also to incorporate them into polymers and other materials to make composite materials. So, the way that works is you take this blend of various sized particles dry them out and keep the blended sample ready to make into composites. The polymer precursor itself has to be prepared by dissolving it in a solvent and then you take the nano materials that you have prepared by your nano your sono fragmentation process and blend them into the polymer that you have prepared by dissolving in a solvent and then you can again use sonication to uniformly mix the particles into the polymer and the polymer matrix that you have prepared in this fashion will essentially look like this. You will have the polymer material within which the nano particles are incorporated. Now, there are two important things here. The particles should be non-agglomerated. So, they should be dispersed very well. They should also be distributed. We will see later on when we talk about nano particle dispersion that these are two different characteristics dispersion and distribution. They are both very important in preparing a high quality polymer composite material. The strength of these nano composites can also be tested by subjecting them to cavitation erosion. For example, you can take this composite material that you have prepared and expose it to an ultrasonic field and look for weight loss and as the material becomes stronger, there will be less weight loss due to cavitation erosion. So, ultrasonic processing can be used very effectively to synthesize nano particles, to prepare nano composites and also to test the cavitation resistance or strength of the nano composites. In our laboratory, we have some state of the ultrasonic facilities that we use to make nano particles as well as the nano composites. You can see our 20 kilohertz probe type sonicator here on the left hand side and the 58 kilohertz tank type sonicator on the right hand side. On the right hand side picture, if you look down on the water, you can see that the ultrasonic field is very uniform over the entire surface. That is in fact, the primary advantage of the tank type sonofragmentation process that you produce very uniform cavitation patterns over the entire liquid, whereas in the probe type sonicator, the acoustic field is highly localized. So, you get very high intensities in the immediate vicinity of the probe, but as you move farther away from the probe, the energy intensity drops off very rapidly. Some of the analyzers that are used for testing particle compositions and sizes and counts are shown here. The left hand side top picture shows a turbidimeter. You may recall that we discussed this in the context of measuring particle concentrations in suspensions. Turbidities are measure of the total scattering intensity of particles in suspension. So, it can be used as a qualitative measure of particle size as well as particle concentration in liquids. The top picture on the right hand side shows a laser particle counter, which works by the principle of laser scattering, which we also discussed in one of the earlier lectures. This particular laser particle counter can count particles in liquids down to 1 micron. And at the bottom, you have a particle size analyzer, which works by the principle of acoustic attenuation spectroscopy, which we also discussed earlier in one of the lectures. So, these are the three instruments that are very widely used in our laboratory as well as many other laboratories to characterize particle sizes and particle counts in liquid suspensions. The data that we will be reviewing here are taken from one or the other of these instruments. So, here we see what happens to feed particles of alumina that are subjected to sonofragmentation. So, the feed size that is shown on the left hand side graph is 74 to 80 microns. When you expose it to 20 kilohertz ultrasonics at 500 watts for 30 minutes, you can see that there is a substantial reduction in size and the particle mean size is now close to 1 micron and you can see that a substantial fraction of the particles are less than a micron. So, even a fairly short duration sonication is sufficient to make a sizable number of submicron particles. Another indicator of size reduction is the color change. You will find that for the same material in this case white fused alumina, which is being size reduced, the color of the suspension changes as particle size decreases, which actually gives us an idea that if you can monitor the turbidity or the color of the suspension quantitatively, that can actually be used to track size reduction without having to use more sophisticated techniques such as a particle size analyzer. So, the graph shows sonication time in minutes and on the left hand side vertical axis, we see the particle size that results after sonofragmentation and you can see that that decreases to a very low value. On the right hand side vertical axis, the data are plotted in terms of turbidity, which is a measure of particle suspension in the liquid and also the color of the suspension and you can see that with sonication time the color essentially becomes darker and darker, which is indication that we have many more particles now in suspension but of smaller sizes. So, this is an interesting plot that says that color change can actually be correlated to size reduction when you subject a suspension to sonication in order to achieve size reduction. Here we are showing the effect of feed particle concentration on size reduction. The interesting thing here is that it is actually not a monotonic relationship. The fragmentation rate expression that I showed earlier seemed to suggest that as particle concentration increased, the size reduction efficiency will keep increasing. However, you will see from this figure that that is not necessarily the case. There is actually an optimum value of particle concentration at which you have the best size reduction efficiency but if the concentration increases beyond that the size reduction efficiency actually starts to drop and the reasons for that are interesting and we will pick up the discussion from this point in the next lecture and we will highlight the influence of feed particle concentration on size reduction efficiency. Are there any questions on what we have talked about today? So, see you at the next lecture.