 Here I will be discussing about the pore structure determination. You must have realized that most of the mechanisms that occur in geomaterial largely depend upon its pore structure or in other words pore size distribution. Without knowing the pore structure of geomaterials nothing can be done much. And this is a very intricate work or subject I would say. So, normally we use two types of techniques, one is the scanning electron microscopy which is not a very quantitative way of finding out the pore structure of geomaterials. Incidentally, the MIP is done, MIP is the Mercury Intrusion Porosinitri, well there is a school of thought which keeps on you know evaluating these techniques with each other. But the point is that there is nothing better than these two techniques which is known to the human beings at this stage and which are affordable and feasible. So, I like to give you some feedback, some ideas about you know what the SEM and MIP is all about. Some of you would get a chance to use these techniques in your research career and some of you who do not get a chance to use them right now might get maybe after sometime or you can appreciate by seeing the YouTube videos and so on. So scanning electron microscopy what is known as SEM is normally obtained is normally done to obtain the images of the samples at very very high magnifications. So, we can go up to 1 lakh times of the magnification with the small microscopes curtsy to the recent day electronics. Basically this technique is used to study the surface structure of the bulk materials and surface features could be the texture of the soil mass, the pores, the cavities which are present, the orientation of the grains of the soils. So, I will show you today how easily the disperse and flocculated structure in the fine grain materials can be looked upon by using SEM technique and then how to quantify that. At the same time the biological processes which are occurring in the geomaterials I think I had shown you some of the pictures when we were discussing about the biological processes, biological characterization of geomaterials where people are interested in seeing what type of EPS is getting formed, what type of bacteria is present in the system, what type of deformations are happening in the geomaterials. SEM is used in the field of medical engineering, material sciences or sciences quite a lot. The image is created by the beam of electrons that is a simple thing and one good thing is that we can cover a greater depth of the field and resolution than optical microscopes and you can study the fractures and the cleavages which are appearing on the grains of the soil. There is a new version of SEM which is known as EDS and we call this as energy dispersive spectrometer and this also gives you the elemental analysis of the material alright. So, you can find out what type of elements which are present in the system and then by using simple mathematical calculations you can obtain their percentages also in the oxide form. We can also see how the distribution of the elements has is occurring on the material particularly when you talk about the mineralogical composition, another question would be suppose if I consider a grain of soil, let it be quartz or let it be modern night or kaolin. I would like to see how the mineralogical composition of the grains is changing on its surface. So, Dr. Srinivas Gadali, one of my PhD scholar who has done very interesting studies by using EDS and another is Dr. Bhagwan Jijha who has done synthesis of zeolites and then he wanted to understand what is the mineralogical composition, elemental composition of the zeolites on the surface. All micro-analysis which has been done and these two students have been pioneered in this area and by using the methodologies which they have developed we have done X-ray mapping of the elements which are present in the material and show a process which happens particularly the alteration of minerals over a period of time when the minerals come in contact with contaminants this could be acidic or basic or biological. Incidentally biological features have been studied by Dr. Shashank. This is the working principle of SEM, a beam of high energy electrons is focused on the sample and then interaction of electrons is transformed into a three-dimensional image to obtain the topography, morphology and composition and crystallography of the grains. So, topography, morphology and compositional characteristics I have discussed and crystallography is basically how the crystal structure is changing. If you go Google it you will realize that the working principle of SEM is quite simple. You take the sample and then bombard it with the electron beams and whatever the beams scatter out of this if I take out the X-rays beam, auger electrons, primary backscattered electron, secondary electrons then I can analyze these beams and I can filter them to obtain the information. So, using this concept people have been doing the fabric structure of fine grained soils. What we have done is we have compacted the soil sample, let it be a triaxial sample for that matter and after the sample has been tested for its shear strength properties you can take out some small element from the within the sample is about 1 centimeter cube and then this 1 centimeter cube specimen is looked at from the two sides perpendicular sides to see the material heterogeneity in the perpendicular and parallel to the compaction plane. So, when you are making this sample by compaction you will realize that you compact the sample in the vertical direction. So, I would like to see how the grain structure is changing along the compaction and this is the lateral direction. So, we would like to see the features of the sample perpendicular to the plane of compaction and parallel to the plane of compaction and incidentally the ratio of the two is the formation factor which we talked about. So, it appears to be very simple, but truly speaking is a very complicated way of doing the studies because I am sure next time when you go to the lab you take out a 1 centimeter cube specimen of clays from the triaxial sample and then make it a good specimen which can be utilized to study the micro features, it is not a very simple task. And why it is so difficult I have listed some of it over here. So, when you remove the pore fluids from the specimen you know the structure gets changed because ultimately the fluid is packed between the grains. So, when you are taking out the sample the chances are that you are disturbing the whole synergy of the specimen with the surrounding sample. So, if microstructure gets changed it might be by holding 1 centimeter cube specimen by hand or by force it because you are applying some pressure on this. So, the microstructure gets changed it is not a easy task as I said. The second thing is that most of the CEMs they work on the frozen samples or the samples we do not have water. So, this is where we use the freeze drying technique alright. This becomes very difficult when the minerals are swelling and shrinking type because the moment you take it out from the sample and keep it in the atmosphere the evaporation takes place and the sample cracks. So, once the sample cracks you are not really going to get the real picture of what the sample used to be when it was in the triaxial sample. Air drying technique cannot be utilized here. So, one has to be very careful and there are different types of setups which are available to do vacuum drying or the freeze drying of this material you freeze the sample. Now, the question is when you are applying the vacuum then the sample should be able to withstand the pressure which is coming from the vacuum also. So, from all sides there are issues. Now, another thing is that soils are mostly non-conducting materials for electrons and what we are doing is we want to study how the charge gets spread on the sample that means I need a surface of the material which is conducting. So, this is another problem. So, what is done is that we normally apply a thin layer of gold and or carbon this is what is known as a sputtering material. So, to make the sample conductor of the Tron beam, so this is more of an art rather than an engineering making a specimen itself and people have to spend a lot of time. If your materials are non-conducting then also there is a problem and in short the making of the sample requires lot of good hands and training. But once you have made the samples and the samples which are useful for studying the microstructure these type of information can be deciphered very easily. So, all of you must have studied how the face-to-face interaction looks like but I am sure not many of you have seen it ever. So, this is how it looks like in the SCM images. Now, I hope you can see that this is one clay platelet and these are the other clay platelets which are sitting one over the other. Look at this picture, can you differentiate between the layers of the clay platelets one over the other? No, you cannot make out much. You can. So, these type of investigations are to be done to realize how much is the dispersed state of the material is and then one of my PhD scholar Dr. Suchat Gumasthe he quantified the degree of dispersitivity and degree of you know flocculation of the material by using impedance spectroscopy and SCM is this is was based on this very interesting work he has done and we have simulated how sedimentation occurs in nature in the lakes and the oceans and reservoirs. Now, this is how it looks like. I hope now you can realize the beauty of the kaolin plates the way they are stacked. Later on, we have utilized these SCM images for deciphering lot of information. I hope you can realize that the type of cavities which you are seeing over here are very interesting parking places for any fluid. This could be fertilizer, this could be enzymes, this could be bioenzymes, this could be any type of a medicine, pesticide, whatever. So, I think this is the whole art of you know playing with the minerals and making them more worthful. So, the more and more you zoom into the system, you realize the beauty and you know how much the nature is intricate. So, one of my PhD scholars Dr. Sushmita Sharma she utilized SCM to realize what type of sediments are existing in the sewage and wastewater treatment plants. Because those type of sediments have been ignored until now, but for us the sediments which are occurring in these sewage and water treatment plants are also sediments. And the whole idea was that if I decant these ponds, what I will be doing with these materials. So, to our surprise, we realized a lot of peculiar formations which occur in these sediments including the pathogenic as well as microbial and bacterial activity. So, this thesis of Dr. Sharma was dealing with as I have said earlier also. This was dealing with the socio economically generated sediments, sex we have termed this. So, lot of information can be deciphered from this simple analysis. Now, I am sure that will be intrigued to see how the phase to edge and edge to edge interaction of the clay particles appears to be. Can you make out from this figure that how edge to edge platelets are sitting with each other. This is the edge and some another edge is coming and sitting over here. So, this is the peculiar system of edge to edge contact. If you see here, this is phase to edge combination. So, this is how the microstructure analysis is being done by using SEM. Now, many times people ask a question whether SEM can be utilized to obtain the porosity or not. The answer is not really because SEM as you are seeing is a qualitative technique. It is a pictorial way of demonstrating what exists in the system. Though there are cavities or the voids which can be quantified, but it will not be very easy to look into the third dimension that is a perpendicular to the picture. So, what people do is they do tomography of the pores and tomography of the pores is known as porosimetry. So, porosimetry is becoming very, very important in the realm of environmental geomechanics because of the obvious reasons that pores are the ones which play an important role in any mechanism which occurs in the geomaterials. So, this is the result of mass flux, alright. So, this flux could be thermal, electrical, chemical, biological, radiological, magnetic and so on. So, until now we have been talking about only the particle size distribution in conventional geomechanics. But truly speaking particle size distribution does not give much idea about the geomaterial characteristics. Unless you really talk about the pore structure and the shapes, so there are a lot of people who are doing fundamental research in modeling the pores, why? Because we can decipher the microstructure of the soil and when I say microstructure basically this is the grain size and their lattice or the fabric we call it, how the grains are you know located in the matrix of the soil. Now this is a simple model if you consider a grain of the soil, now what you will realize is that there are different types of pores which are present in the grains. There could be a sort of a pore which is sitting on the surface like a crater and this is what is known as a closed pore, there is nothing which is going to migrate through and through this pore, this sort of a crater on the surface of the grain, alright. But yes, I think when we discussed about these option and adsorption and you know adsorption and absorption phenomena, I think this is where I had talked about the first thing to happen is that the contaminant has to come in contact with the geomaterial which is a physical process. Now once this contaminant stays over here, the chemisorption will start, this will penetrate into the matrix of the grains, alright. Now in this case we have dead end pores, the pore is well defined but then ultimately the fluid flow across the mineral cannot occur, so this becomes a dead end. We have a interconnected pores, you know, these pores are interconnected forming a sort of a network within the particle itself. Then there could be through and through passing of the pores, you know, look at this, so there is a pore which passes through the grain itself. So these type of pore arrangement and the ones which I might not have shown over here or the combination of these type of pores exist in the porous materials. So this becomes a slightly complicated situation where you would like to find out what the pore structure is, what the pore size distribution is and once you know these two things, you would like to understand what the porosity of the system is. And this porosity is going to be the absolute porosity which cannot be obtained by soaking the geomaterials in water which normally is done, you know, they take the rock samples and the soil samples, they soak it in water for 72 hours and then find out the weight and then they say that this is equivalent to the porosity. But the simple logic is as we have discussed earlier also, water molecules cannot enter into the finest of the pores which are present in the grains and hence the porosity which you obtain by soaking of a material in water is not going to be the true well, true porosity. So with this premise and the background, people have gone into understanding and they have developed a lot of techniques for doing porous structure modeling or what is known as the porosimetry. So when we talk about the porosimetry, the MIP becomes important or this is the most important techniques, technique which is used for determining the pore structure and the name is mercury intrusion porosimetry. The beauty of mercury is that surface tension is very high or very small. It is a wetting fluid or non-wetting fluid, very good. So it is non-wetting fluid, so surface tension is going to be extremely high. That means the smallest ball, clear, or the drop of the mercury can still remain in this spherical form and the beauty is the more and more pressure you apply, the drops of the mercury will start becoming finer and finer, clear, better than water. Because water is incompressible, so is mercury also but surface tension and the wetting properties of the water make it not fit for doing porosimetry. Now the question is if I really want to know the finest of finest, finer force in the material, what I should be doing? I can use gases, nitrogen helium gas which we have talked about when we were doing helium gas pycnometry. So you can use helium gas to pass through the sample and capture the total porous structure because size of the helium gas, nitrogen gas is going to be much smaller as compared to the mercury, alright. So these are the facts which are normally kept in mind and one other interesting thing is I hope you should realize that porosity is something which could be in any of these forms. This is a non-porous solid, alright, for that matter a perfect quartz grain of the sand. Now this has extremely low surface area because there is no cation exchange capacity, quartz is a very dull material, it has no affinity towards the external environment, alright. So truly speaking non-porous solids are made up of quartz, quadsetic material, they have extremely low surface area. This is the porous solid and why porosity because of the interstices which have got created under the grain itself and they have slightly high surface area and they have some amount of pore volume and their dimension. Now this is a particulate system, the dispersed particles in the matrix of the soil mass. So when you have a particulate system like this, the particle size and surface area varies and we are more interested in seeing what type of particle size in the surface area the system will give. Incidentally when we do porosimetry, we have two intentions. One is to obtain the pore size and second is to obtain the surface area also. Ultimately when you are intruding something into the pore space, you would like to know what is surface area also. So modern day porosimetry gives you advantage of obtaining the particle sizes also, surface area also by mathematical modeling and the total volume of the pores which are present in the system. These are the catalysts. So catalysts are the one which have active surfaces and they give a chance for the species, chemical species to come and get parked on them. So these are mostly activated particles, monomeronite itself is the activated particle, all right? Or I can create a sort of a zeolite by treating quartz with sodium hydroxide at elevated temperature. So this might get converted into a material of higher cation exchange capacity. These are the different shapes of the pores which are normally used in the analysis. So there could be the pores which could be modeled as cylinders of certain diameter and then slits like the dispersed structure. So these are also the pores in the slit form, we could have conical pores, you know, both sides the diameter of the pore might not be same funnel sort of a thing. This could be the connected to the outside environment and this would be inside the material or vice versa. This could be inside the material and this could be outside the environment. So I hope you understand the consequences of this type of arrangement in the soil mass. Similarly we have ink bottle effects also or ink bottle types of pores also. So there is a small cylindrical pore which is connected to a voluminous pore and this becomes a typical ink bottle. We have interstices also, the pores which are connected with each other. So these are the types of pores which are normally modeled in porosimetry. When we talk about the pore size in the geomaterials, we use these classification scheme micro, meso and macro pores. So as the name suggests, the micro pores are going to be the smallest ones. Then we have in between and the macro pores are the big pores. So up to the 50 nanometers and more than that these are the macro pores and the smallest pores are less than 2 nanometers. So most of the zeolites, carbon, silica fumes which you have studied would fall under the category of micro pores. Different types of alumina, polymers, catalysts which industrialists are using, they fall under the category of meso pores and the macro pores would be different types of soil, cements and rocks. So when we do pore size classification and characterization, we normally talk about the bulk, apparent and the real density of the geomaterials. I hope you can realize I am using 3 terms in the form of the density, bulk, apparent and real densities. So you have to use different techniques to differentiate the types of pores which are present in the system and I can hope you can realize that bulk is going to be the macroscopic in nature and apparent is the one which is due to the presence of the voids which have lot of air into them and the real would be something which could be skeletal. So you have to differentiate between these types of densities of the geomaterials to do complete modeling. When we talk about the percentage porosity, we talk about the pore volume, pore size distribution analysis, total pore volume, average pore size, specific surface area and particle size distribution. Fortunately, in today's world, the type of software which we have, these things can be done in a fraction of time very quickly and easily and most of these things are statistical in nature and you can do a very comprehensive analysis of the sample which you are studying for its pore structure.