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Video 1: Scanning Electron Microscopy
Here, I will be discussing about the pore structure determination, you must realizethat most of the mechanisms that occur in geomaterial largely depend upon its pore structureor 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 findingout the pore structure of geomaterials.Incidentally, the MIP is done.MIP is the mercury intrusion porosimetry.Well, there is school of thought which keeps on you know, evaluating these techniques visa vis each other.But the point is that there is nothing better than these techniques which is known to thehuman beings at this stage and which are affordable and feasible.So, I like to give you some feet interact some ideas about, you know what the SEM MIPis all about.Some of you would get a chance to use these techniques in your research career, and someof you who do not get a chance to use them right now might get maybe after some time,or you can appreciate by seeing the YouTube videos and so on.So, it is scanning electron microscopy, what is known as SEM is normally obtained is normallydone to obtain the images of the sample at very very high magnifications.So we can go up to one lakh times of the magnification with the small microscopes curtesy to therecent day electronics basically this technique is used to study the surface structure ofthe 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 dispersed and flocculate structure in the fine grainedmaterials can be looked upon by using SEM technique and then how to quantify them.At the same time the biological processes which are occurring in the geomaterials, Ithink I have shown you some of the pictures when you are discussing about the biologicalprocesses.Biological characterization of geomaterials, where people are interested in seeing whattype 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, earth sciences, quite alot.The image is created by the beam of electrons.There is a simple thing.And one good thing is that we can cover a greater depth of the field and resolutionthan optical microscopes.And you can study the fractures and the cleavages which are appearing on the grains of the soils.There is a new version of SEM which is known as EDS, and we call this as energy dispersivespectrometer and this also gives you the element 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 theoxide form, we can also see how the distribution of the elements has is occurring on the materialparticularly 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 itbe montmorillonite or kaolin.I would like to see how the mineralogical composition of the grains is changing on itssurface.So, Dr. Srinivas Kadali was my PhD scholar who has done very interesting studies by usingEDS and another is Dr. Bhagawanji Jha, who has done synthesis of zeolites and then hewanted to understand, what is the mineralogical composition, elemental composition of thezeolites on the surface.These are all micro analysis, which has been done and these students have been pioneeredin this area.And by using the methodology in which they have developed.We have done X-ray mapping of the elements which are present in the material and showa 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 beenstudied by Dr. Shashank.This is the working principle of SEM, a beam of high electric, high energy electric electronsis focused on the sample and then interaction of electrons is transformed into a 3 dimensionalimage to obtain the topography, morphology and composition and crystallography of thegrains.So, topography, morphology and compositional characteristics I have discussed and crystallographyis 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 beamsscatter out of this if I take out the X rays beam, other electrons, primary, backscatteredelectrons, secondary electrons, then I can analyze these beams and I can filter themto obtain the information.So, using this concept we will 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 itsshear strength properties, you can take out some small element from the within the sampleis about one centimeter cube and then this one centimeter cube specimen is looked atfrom the sides perpendicular sides to see the material heterogeneity in the perpendicularand parallel to the compaction plane.When you are making this sample by compaction, you will realize that you compact the samplein the vertical direction.So, I would like to see how the grain structure is changing along the compaction and thisis the lateral direction.So, we would like to see the features of the sample perpendicular to the plane of compactionand 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 it is a very complicated way of doingthe studies because I am sure when you go to the lab, you take out a one centimetercubic specimen of clays from the triaxial samples 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 back between the grains.So, when you are taking out the sample, the chances are that you are disturbing the wholesynergy of this specimen that the surrounding sample.So, if microstructure gets changed, it might be by holding one centimeter cube, specimenby hand or by forceps.Because you are applying some pressure on this.So, the microstructure gets changed it is not a easy task, as I said the second Is thatmost of these SEMs they work on the frozen samples or the sample 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, theevaporation takes place and the sample cracks.So, once the sample cracks, you are not really going to get the real picture of what thesample used to be, when it was in the triaxial sample, air drying technique cannot be utilizedhere.So, one has to be very careful.And there are different types of setups which are available to do vacuum drying or the freezedrying of this material.You freeze the sample.Now the question is when you are applying the vacuum then the sample should be ableto withstand the pressure which is coming from the vacuum also.So, from all sides there are issues.Another thing is that soils are mostly nonconducting 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 iswhat is known as a sputtering material.So, to make the sample conductor of the beam.So, this is more of an art rather than an engineering making a specimen itself and peoplehave to spend a lot of time.If your materials are non-conducting, then also there is a problem and in short, themaking of the sample requires a lot of good hands and training.But once you have made the samples and sample the samples which are useful for studyingthe 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 surenot many of you have seen it ever.So, this is how it looks like in the SEM images.Now, I hope you can see that this is one clay platelet and these are the other clay plateletswhich are sitting one over the other.Look at this picture, can you differentiate between the layers of the clay platelets oneover the other.Now, you cannot make out much.You can.So, these type of investigations are to be done to realize how much is the dispersedstate of the material is and then one of my PhD scholars Dr. Suchith Gumasthe, he quantifiedthe degree of dispersity and degree of flocculation of the material by using impedance spectroscopyand SEM.His thesis was based on this, a very interesting work he has done and we have simulated howsedimentation 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 stacked.Later on, we have utilized these SEM images for deciphering lot of information, I hopeyou can realize the type of cavities which you are seeing over here are very interestingparking places for any fluid, this could be fertilizer, this could be enzymes, this couldbe bioenzymes, this could be any type of a medicine, pesticide, whatever.So, I think this is a whole lot of you know, playing with the minerals and making themmore worthful.So, the more and more you zoom into the system, you realize the beauty and you know how muchthe nature is intricate.So one of my PhD scholars Dr. Sasmita Sharma, she utilized SEM to realize what type of sedimentsexisting in the sewage and wastewater treatment plants because those type of sentiments havebeen ignored until now, but for us, the sediments which are occurring in these sewage and watertreatment plants are also sediments.And the whole idea was that if I decant these ponds what I'll 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 wasdealing with the social economically generated settlement SEGS, we have termed this.So, a lot of information can be discovered from this simple analysis.Now, I am sure that will be intrigued to see how the face to edge and edge to edge interactionof the clay particles appears to be.Can you make out from this figure that how edge to edge platelets are sittings with eachother, this is the edge and some another I just coming and sitting over here.So, this is a peculiar system of edge to edge contact.If you see here, this is face 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 in the porosityor not the answer is not really because SEM as you are seeing is a qualitative technique.This is a pictorial way of demonstrating what exists in the system though there are cavitiesor the voids which can be quantified, but it will not be very easy to look into thethird dimension that is perpendicular to the picture.

Video 2: Porosimetry
So, porosimetry is becoming very, important in the realm of environmental geomechanicsbecause of the obvious reasons that pores are the ones which play an important rolein any mechanism which occurs in the geomaterials.Transport of mass flux alike.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 conventionalgeomechanics.But truly speaking particle size distribution does not give much idea about the geomaterialcharacteristics and 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 poreswhy because we can decipher the microstructure of the soil and when I say micro structurebasically this is the grain size and their lattice or the fabric we call it how the grainsare, 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 presentin the grains, there could be a sort of a pore which is sitting on the surface likea crater and this is what is known as the closed pore there is nothing which is goingto migrate through and through this pore it is a sort of a crater on the surface of thegrain.But yes, I think when we discussed about the sorption and adsorption, you know adsorptionand absorption phenomena, I think this is where I had talked about, the first thingto happen is that the contaminant has to come in contact with the geomaterial which is aphysical process.Now, once this contaminant stays over here, the chemisorption will start this will penetrateinto the 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 interacted pores you know, these ports are interconnected forming a sort ofa network within the particle itself.Then there could be through and through passing of the pores, you know look at this thereis a pore which passing through the grain itself.So, these type of pore arrangement and the ones which I might not have shown over hereor 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 thepore structure is what the pore size distribution is, and once you know these two things, youwould like to understand what the porosity of the system and this porosity is going tobe the absolute porosity, which cannot be obtained by soaking the geomaterials in waterwhich normally is done, you know, they take the rock samples and the soil sample theysoak it in water for 72 hours and then find out the weight and then they say that thisis equivalent to the porosity.But the simple logic is as we have discussed earlier also water molecules cannot enterinto the finest of the pores which are present in the grains and hence the porosity whichyou obtain by soaking of a material in water is not going to be the true porosity.So, with this premise and the background people have gone into understanding and they havedeveloped a lot of techniques for doing porous structure modeling or what is known as theporosimetry.So, when we talk about the porosimetry, the MIP becomes important or this is the mostimportant techniques technique, which is used for determining the pore structure and thename is mercury intrusion porosimetry.And 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 thespherical form.And the beauty is the more and more pressure you apply the drops of the mercury will startbecoming finer and finer, clear better than water, water is incompressible so is mercuryalso, but surface tension and the wetting properties of the water make it not fit fordoing porosimetry.Now, the question is if I really want to know the finest of finer pores in the materialwhat I should be doing.I can use gases nitrogen, helium gas which we have talked about when we were doing heliumgas pycnometer.Alright.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 tothe mercury.Alright.So, these are the facts which are normally kept in mind.And another interesting thing is, I hope you should realize that porosity is somethingwhich 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, it isa quartz is a very dull material, it has no affinity towards the external environmentAll right.So, truly speaking non-porous solids are made up of quartz.Quarzitic material.They have extremely low surface area, this the porous solid and why porosity becauseof the interstices which have got created under the grain itself and they have slightlyhigh 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, their particle size and surface areavaries and we are more interested in seeing what type of particle size and the surfacearea, the system will give.Incidentally when we do porosimetry, we have intentions.One is to have 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 issurface area also.So, modern day porosimetry gives you advantage of obtaining the particle sizes also surfacearea also by mathematical modeling and the total volume of the pores which are presentin the system.These are the catalysts.So, catalysts are the one which have active surfaces and they give a chance for the specieschemical species to come and get parked on them.So, these are mostly activated particles, montmorillonite itself is a activated particle.Alright.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 andlength, slits like the dispersed structure.So, these are also the pores in this slit form, we could have conical pores, you knowboth sides the diameter of the pore might not be same funnel sort of a thing, this couldbe the connected to the outside environment and this would be inside the material or viceversa, 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 thisbecomes 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 model in porosimetry.When we talk about the pore size the in geomaterials, we use this classification scheme, Micro Mesoand macro pores.So, as the name suggests, the micro pores are going to be the smallest ones, then wehave in between and the macro pores are the big pores.So, up to the 50 nanometers and more than that, these are the micro pores and the smallestpores are less than 2 nanometers.So, most of zeolites carbon silica fumes which you have studied would fall under the categoryof micro pores.And different types of alumina, polymers, catalyst which industrialists are using.They fall under the category of meso pores and the macro pores would be different typesof soils, cements and rocks.So, when we do pore size classification and characterization, we normally talk about thebulk apparent in the real density of the geomaterials.I hope you can realize that I am using 3 terms in the form of the density bulk, apparentand real densities.So, you have to use different techniques to differentiate the types of pores which arepresent in the system.And I can hope you can realize that bulk is going to be the macroscopic in nature andapparent is the one which is due to the presence of the voids which have a lot of air intothem and the real would be something which could be skeleton.So, you have to differentiate between these types of density of the geomaterials to docomplete modeling.Then we talk about the percentage porosity, we talk about the pore volume, pore size distributionanalysis.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 canbe 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 itpore structure.