 In this lecture, we will learn about multiscale hierarchy. As we say, there are two terms, multiscale and hierarchy. So, let us see what do they really mean? Multiscale, like if I take a ruler or a scale and then I measure the length and breadth of a paper or normal sheet of a paper. So, I can measure it using a particular ruler. But say, if I want to go to much final scale, such as say, I want to go to a microstructure and I want to measure something more at a micron length scale. So, I cannot take the same ruler or the same scale bar to go and measure a microstructure. So, I need to have a scale, which is at a bit of a different level. So, that is the reason that that brings out to the aspect of multiscale. And similarly, we have different scales going from atomistic or going to even higher microns, molecular scale, nanoscale, and then maybe something like at a millimeter scale, then it can even go to constellations and starts with millions and millions of light years. And coming to the second part of it, the hierarchy part, like if there is a king, he will have certain ministers beneath him, working for him. Then again, all the ministers will have some servants under them. So, they can be second category of hierarchy. And then again, all the servants will have kind of sub-servants under them. So, this tells a hierarchy built within the structure. So, we have multiscale. Scale means differentiating going from one line scale to a different line scale. And then hierarchy, it means what is the level, like a ruler, like a king, working, and the person will have many, many servants under them. Each servant will be, now all the servants will have many, many sub-servants under them. So, we will see that each minister will have certain servants and each king will have many, many ministers. So, overall this basically is telling a hierarchy within a particular system. And how it is useful? Coming back to the lens scale, like we can see that we can go, we can start going from a hydrogen atom. So, it will have certain atom with some electronic structure around it. And we see that the lens scale, it corresponds to around one angstrom. Going to a much higher lens scale of around microns, we will see that all the red blood cells, which are existed in our body, it can have certain red blood cells, which amount, which have a diameter of around 2 to 5 microns. And an ant, an ant basically will be millimeter in length. So, going from an angstrom, angstrom level to micrometer level to millimeter level. And as we know that a human being is approximately so many meters tall. So, we will now come back to the lens scale of a meter. So, we will have human, which will have a kind of lens scale of around meter. So, this tells the overall lens scale and hierarchy comes from the next thing. So, let us see what is multi-scale hierarchy. So, in this particular case, we have a fundamental building block. And that particular block will start repeating itself at certain lens scales. And then that particular block will serve as a building block for a next hierarchical structure. What is the advantage with a nano lens scale building block? That we can make any complex shape out of it. That if we have the fundamental building block itself with order of few molecules of few atoms, we can go atom by atom and then we can construct something very complex, which is not achievable otherwise. If the overall block itself is a very, is it existing at a very higher lens scale such as order of meters, we cannot construct something which is smaller than a meter. So, the overall block has to be much more, much more bigger and much more simpler as compared to that if you have a building block, which is nano meter in lens scale. So, that tells the advantage of nano structures. And how can you utilize that nano structure in terms of enhancing its mechanical properties or enhancing certain specific volume or maybe say something like surface area or any functionality that part we will see later on. But this tells that it gives us a chance to be able to play at that particular level, at the nano level and then form a hierarchical structure. So, this tells that how the structure are, structure are repeating. So, this is our basic unit and then it goes on to form much higher lens scale elements. And then again it goes to forming bigger blocks and again to a very kind of a bulk structure. So, what is so specific about this thing is that each block is again a repetition of what was there earlier. So, we had all these blocks which are now forming a unit in the bigger entity. And now structure elements themselves will have some structure. So, that is one more advantage to it that we can have certain structure which is existent either at macro level or at a micron level or it can be at the atomic level. So, we can play with those couple of entities that we can have a structure which with some hierarchy that a bulk structure will have certain micron level hierarchy. It will have certain structure repeating itself at a micron scale. And then again that microstructure can have something which is repeating at a atomic or a nano scale. So, that is how the hierarchy is defined in structural materials. So, coming back to the way of hierarchy, we can define a certain system like which is existing which is also man made. We can have certain networks. So, we have one very huge stations made up of many computers and they basically do the data exchange. So, we have certain networks with some computers. So, each computer forms a basic unit of a network. So, now we go from a network to a computer and then coming deciphering the computer itself, we will see that each computer will have certain chips introduced in it. Because computer is the one which is doing certain calculation and then it is performing certain functionality of a network. A network is the one which is giving us the overall picture or the overall processing of a particular process which is being assigned to it. So, computer forms the basic unit of the network. Now, computer and computer will have many many integrated circuits or integrated chips which basically do the function they form certain IC IC chips which will have many many circuits devised on to them. And then again the way integrated circuit work is through certain gates. So, we have diodes and all that which will have which will assign a certain direction to the information. So, the gates are the one which will have an input and have an output and depending on on the on the design of the gates will get a path of the for the flow of information. So, that is what is defined by the gates that in which direction the information is to go and how much it has to go. But again all those gates will have certain very fundamental or individual components such as resistors, maybe capacitors, inductors. So, these are the very functional units, the basic fundamental units which are dominating at the individual component level. So, we can see that we need to have resistors, capacitors, inductors and all that to really go and form a gate. And now these gates they decide the flow of information and those that thing is decided by a certain integrated circuits. And many integrated circuits they go and form a computer and many many computers they go in they become a functional part of a network. So, this is how the overall hierarchy occurs how the hierarchy occurs basically in the network systems. So, and this thing the hierarchy and this particular length scale it is not only existing only at a networking system not only on the man made structures, it is also existing in a biological structure. So, how does it occur is like we have a human human will have certain organs and kind of organ systems which work in harmony to give the overall functionality to human. And organ systems such as the overall system which is working it will have many many organs to it like for digestion we need to have our mouth the food has to go there the person will chew the food it are not has to go through stomach get digested out there it has to flow through intestine and it has to follow excretion there on. So, there are many organs which really go and form a organ system just such as for digestion as we just realized. So, there are many organs which are dominant in taking a certain part of this particular function such as chewing chewing or ingesting it getting it processed and all those things in stomach and then again getting it digested in the intestine and so on. So, each organ now each organ will have many many tissues. So, each tissues will it is a kind of there are many many cells which really go and form a tissue and cells are nothing, but more complex features with a nucleus cytoplasm and so on. So, if you and again they cells they service pathways for the information. So, one cell has to interact with another cell in terms of passing certain information that the food has now to be digested the food is now in this particular condition and what is the overall condition of for the for the action and again the pathways are created again by a performing some bio bio bio chemical reactions that they they they generate some sort of potential over its surface and through those potentials or those chemical reactions the information is passed from one cell to the another cell and again if you if you see that how this particular information is generated it is through again by a protein change of the genes which which are again the basic units of all those cells and tissues and those basically go about give providing the information and we can see there is some more similarity between the structure which was which is existing biologically or naturally that all the individual components they correspond more like proteins and genes gets the correspond more to like which were like pathways or they are governed by a certain bio chemical reactions and certain integrated circuits more like organs and tissues and the overall computer like maybe like a organ and then whole network like a organ system. So, that is how we can see the similarity between the multi-scale hierarchy which is existing either like an as engineered or what is existing naturally. So, this is what is all about the multi-scale hierarchy. So, how the functionality changes at each particular stage and again coming to a more engineering problem if we see that what can happen starting from a macro scale to a micro scale. So, we have more more of a macro scale and we see that we have a particular body which is getting fractured. So, we have a crack and then we can see some crack is already propagated. So, this particular part is yield by fracture properties or the bulk properties are being yielded by a macro scale and then again more at just the tip of this particular crack we can see that there can be formation of some yield region. So, just at the crack tip we will see certain elongation of bonds and all that how the how the particular region is getting yielded. So, that that that part of plasticity we will see near the crack tip and then going more to it more at the molecular scale which is comprising of 10 nanometer to 10 microns. So, this called molecular scale we will see how molecules are basically interacting to lead to the debonding of particular species at that molecular level. So, we will see that the bonds which were existing out here the how they are broken and then that is basically leading to a overall debonding between the material and eventually cracking of a particular material and then again we see more or less molecular properties are again dominated by individual molecules. So, we see that individual molecules how they are interacting with one another and then leading to a overall cracking between them. So, just at the crack tip we will see we had atoms, atoms, atoms and then we see this some debonding which is occurring between these two particular atoms. So, that is what is being decided by individual or single molecules. Then again more at the atomistic level we can see how the chemical interactions or atomistic scale how the things are occurring. So, it will go as take us more to the atomistic level of may be of an individual atom and how the electronic structure is being defined to it and how does it is basically overlapping with the electronic structure of another atom and then how do they basically separate out at that particular scale. So, we can see that at atomistic level we only single atom and we are defining its electronic structure at that particular scale and how it is interacting with the another molecule another atom at the similar scale how it is basically overlapping the electronic structure and then it is deciding the mechanical property at that particular level. Then going on to the molecular state we see that there are certain chains certain chains of atoms and how they are interacting with one another it can it is more at the molecular level that we have many, many atoms which are aligned with one another and how they are interacting with the nearby chain to overall cause a cracking and then coming to a more of a higher level molecular scale like 10 nanometer to 10 micrometer we see there is something called intermolecular adhesion. So, we have many molecules how they are adhering to the nearby lying molecules and those basically go on to form a kind of yield region yield region means it is more at a meso scale. So, initially we had bonding at atomistic scale then went on to molecules molecules again they had some between the chains interactions and then we have something called intermolecular adhesion or within less than 10 microns. So, what is happening individually at each molecule how they are responding to it then we have some sort of plasticity within the grain like if you had one grain. So, there are many, many atoms associated with that. So, what is happening within a particular grain and then eventual elasticity of fracture properties or yielding which is more at a bulk scale. So, it is it is it means how the slipping is occurring across many, many grains. So, we can see the overall dimension is around 100 to 1000 microns. So, what is the overall yielding occurring across the grains formation of yield region within particular grains at 10 to 100 microns and then intermolecular adhesions which are which which can access up to 10 microns and then molecular scale and again atomistic scale. So, this is how overall picture looks like in terms of solving an engineering problem. So, solving a cracking how it is occurring at different how the each level is responding to the particular cracking. And again it has been said by Albert Einstein that you can never solve a problem on the level on which it was created. So, basically if you want to solve a problem you have to break it down into smaller entities. So, that is what thing is all about multi-scale hierarchy that we have a hierarchy and we have variety of scales. So, basically break down the problem in terms of its hierarchy we go from one step to a second step lower and then we we solve at a different length scale as well. So, this is how it comes out to be we have a time scale out on the y axis and length scale on the x axis. And we see that more at more at the angstrom level we see the dominant effects are nothing but the density functional theory of quantum mechanics which is much more dominant at atomistic level. And the time required for its modeling it is to the order of picoseconds. Then we have some force field parameters which can incorporate up to say hundreds of atoms and it can go up to a nanometer scale. And then going on eventually on to more number of atoms up to approximately 5000 number of atoms and it can go up to say order of couple of less than a micron, but it tells about the non-reactive molecular dynamics out here. And then again we can go to something called mesoscale. Mesoscale is a kind of inter layer between macro and micro. So, that is how it is defined by the mesoscale. So, what is the problem with with with atomistic modeling is that if we are incorporating only very few number of atoms and few number of atoms they do not really represent what is happening at the bulk scale. And if you go to the bulk scale the computational abilities are not good enough to mimic and to really capture all the millions and millions of atoms 10 to about 23 order of 10 to about 23 atoms which is nothing but an Avogadro number for one mole. And to incorporate many moles of atoms and predict what will happen eventually in bulk. So, that is not really feasible and it is very time consuming very costly as well. So, there we utilize more of a scaling laws or constitutive equations which are nothing but the bulk equations. So, we can test them in those particular manner and it comes to the order of seconds and it is in length scale comes out of the order of few meters which is nothing but the bulk material. So, which is nothing but the representative of the actual thing what will how the material is being really represented as. So, now the continuum theory or the mesoscale kind of bridges between those two and we can see that now here we are considering around 10 to the power 23 atoms. And they can go to continuum theory with 10 to the power 10 atoms or more or again in mesoscale we are thinking about we are considering only 10 around 10000 atoms and more in the non deductive M D or real force field parameters we are taking up to 5000 atoms. And quantum mechanics we consider around 100 atoms. So, this is how the overall thing really go on to and again if you see one more thing the kind of characterization techniques which are much more dominant in those particular location like for to the for deciphering something in the order of angstrom to nanometer we have x rays or NMR. And then if we want to go to more of a micron level or more we have optical or magnetic tweezers or perform nano indentations and so on. And then we have AFM MFS and so on. So, these are the overall things how we can really distinguish and classify and characterize all the nano materials or going to the bulk properties. And bio mimicking bio mimicking is one of the very fascinating fields which are recently emerged because nature is the perfect engineer. And unlike v engineers the nature will tend to optimize everything around it if we want to make something very tough we will go only about its toughness not about its strength not about its ductility not about its say wetting properties not about its shock shocking properties shock absorbing properties. But we will go only about one property of probably increasing its hardness or maybe say strength. So, nature is the perfect engineer and that is the reason we want to learn from it always because nature can handle compression tension shear at the same time it can distribute energy in terms of absorbing the impact it can stick the things together very nicely we see that how the gecko can really stick itself on to the wall it can provide strong foundation in terms of providing a good skeleton or a good scaffold it can regulate temperature humidity and light such as in termite mounts it can also create beautiful colors as in peacock feathers or even a butterfly wings. So, we can see how nature can really go about perfecting everything what it has created. But as engineers we want to mimic it for the sole reason that we want to optimize many many many properties which can be structural which can be related to color related to chemistry or related to anything. So, that is the thing we need to learn from nature and for that it is something called engineering branches called biomimicking. And we have always been learning from nature in the past like we have polar bear it tends to go and go and form some holes in the polar regions. So, basically we get something which is a certain region a kind of a covering with with ice and it keeps the bear pretty warm in even in winters. Similarly, we have in the polar regions humans have certain making igloos which can keep the eschemos warmer even during winters. So, that is the overall insulation effect of the of the of the snow or the ice and that keeps them pretty warm pretty warm even during very high winters or during very chilly winters. Similarly, for the birds we have learnt how to make aeroplanes. So, that is what how we have learnt to make aeroplanes from the flight of birds. So, we have always been mimicking the nature in any way such as starting from aeroplanes or even going to the ships we have we have learnt to how to make it float using veils and all such things. So, we always have been mimicking nature as an engineer we want to always go for perfecting it. And the many many examples of how we can tap the how we can really tap what is happening in nature and bring it back to the bring it back for for for the community. And there are certain properties of biological materials which tend to be very very stronger very very tougher like the bone it has bending strength of around 270, but it is a work of fracture around 1700 joules per meter square. And as we compare to alumina which is a ceramic material it is a sorry it is work of fracture of around 7 joules per meter square. So, it is thousands of times lesser than what is available in nature even dentin enamel the work of fracture is around 20 to 50 times as that of a alumina. And again coming back to nacre which is nothing but a seashell it is again made up of a ceramic material. So, ideally it should have a very low work of fracture, but it it shows a work of fracture around more more than 100 times as that of a alumina. But but seeing to one more thing that tensile modulus is pretty low for all these materials the bending strength is pretty low for all these materials, but for alumina the bending strength is very high, but still the work of fracture is very very poor. This happens just because the way the nature has designed all these materials the architecture of how the bones are basically made or how the dentin enamel or the nacre structure has been made. This is the way the the architecture has been done in terms of having a proper way in which the crystallites and the and the compliant material how they have been merged together. So, that part we will see as we go along, but that is the beauty that the way they have been arranged they can increase their fracture toughness by more than 1000 times of their actual ingredient materials. So, that has the beauty of the multi-scale hierarchy out here. In nacre there are some tablets which are held together with a protein nacre is nothing but a seashell. So, probably what you see in the near the near the beach that you have certain seashell with certain linings over it and then this is nothing but a seashell and then we can see that the this particular material does not break even when you drop it. So, in this case what is happening is there are certain tablets and those are held with a protein layer. So, you have certain tablets which are glued basically with certain protein material and this is what basically absorbs the shock. So, you have ceramic tiling kind of thing and again these are nothing but the laminated and tabular nano composite. So, they are kind of tabulated out here and then they have a protein layer which is binding them together and the protein layer basically plays multiple roles. First of all it serves as a nucleation template for the inorganic phase or the kind of a carbonate phase calcium carbonate phase it also controls the topological features because this is what is actually. So, deciding factor about the overall toughness for this particular material and also serves as a glue. So, basically it is nucleating the template it is controlling the topological features it is also serving as a glue and that basically gives it much more toughening effect as compared to anything else and how this particular layout has been generally generated it is something it has something called a brick structure. So, we have something like this and then brick structure basically emerges out of it and these brick structures are they have a glue or the protein layer between them. So, we have certain protein layer which is kind of holding the all the bricks together and these are around order of 200 to 500 nanometer and you have certain regions of glue which is very very thin couple of nanometers out here, but this architecture gives them very much toughening and how does it do it is basically the protein is nothing but a very compliant material. So, upon any shock it can absorb the shock very easily and all these are glued in the region of around 2 to 5 nanometer or so. So, we have certain bricks as we see in the construction and upon any impact all these bricks can also come out to take the particular shock. So, it is not that if a crack is generated it has to propagate through no the crack may just get restricted at a certain brick one of these bricks and this brick itself may come out in order to comply to the particular shock or an impact and that is the reason that this particular material becomes very very tough and if you see that one of the bricks can just come out like they have architecture like this and then one of the bricks say this particular brick may just kind of slide away. These are nothing but something like platelets these are platelets and this whole platelet can come off and make the structure very very tough. So, that is the beauty of this particular structure that all these are tablets or platelets and upon any impact they can just slide over one another in terms of rendering or absorbing certain energy and then becoming much more tougher. So, these are called nanoportable interfaces because that interface is the one which is allowing the tablets to move over one another or the tiles to move over one another. At the same time this glue or the protein glue which is holding them that also can absorb much shock and it will try to hold the bricks above itself and below itself together. So, that is the beauty of this particular nanoportable interfaces again coming to a second example like in GECO and GECO basically GECO has a very nice ability to work on any of the surface any at any on any surface even when they are walking upside down see on the roof still it is able to still it is able to stick itself and work very easily. But we cannot really do that because we do not have that much we cannot really stick to the wall why because we have so much weight that even if you want to create certain vacuum or something like that we would not be able to stick. And there had been a concept that GECO stick on the wall because they create certain vacuum on their feet but that is not true. They stick because of intermolecular forces which are called vendor wall forces to stick on the wall. So, if we have a roof and we try to stick below it we will basically fall down. But GECO what it can do GECO GECO can really stick can stick on the onto the walls because because of something called vendor wall forces. And how does this vendor wall forces come arise is because that every square millimeter of a GECO's foot pad contains contains about 14,000 here like CTA where we have feet of GECO which have around 14,000 here like CTA. So, they have certain lamellas on their feet and they have and then lamellas will have some CTA's around 14,000 here like CTA and CTA's are very fine they are up to the order of 5 micrometer diameter. And each CTA will have in turn more than 100 to 1000 spatulae. So, it is more like this add a surface which is very flatter in nature then I create some micro roughness. So, if I give it certain perturbations we are seeing that this surface is occurring the surface area has gone up dramatically because this is no more a projection area it is more of a surface area which is providing it the vendor wall forces. So, initially we have flat surface. So, if flat surface is sticking over to a flat surface it is nothing but like 1 to 1 contact, but once it has a CTA then CTA will have some roughness to it because it has a diameter of around 5 microns which is couple of microns longer as well. So, we get some extra roughness on from just because it has some CTA on the surface and again all the CTA they are in turn again tipped with more than 100 to 1000 spatulae. So, what it is doing just by inducing the micron size roughness the surface area is going up by say around 1000 times. And going to a next level one more level of nano because it the spatulas are around 200 nanometer and diameter they are more like nano hairs we are reducing one more level of roughness over this micro roughness. So, we have overall surface area going by order of 10 to the power 6 times. So, 1000 into 1000 that much at least it is going up the surface area and once we have so much surface area that can automatically induce vendor wall forces because there will be atom to atom to atom contact and then that can easily have very high vendor wall forces and it can go up to say around 50 newtons or so around it can go up to that much load it can easily take. And similarly one company 3 m has been able to manufacture the able to mimic what is there in the gecko's feed and it is able to take around 50 newtons load out there. So, we can stick a very huge even a person of around 5 around more than 5 to 10 kgs and we can easily stick on to the wall without letting it drop just within by a 1 centimeter square tape. So, that is how that is how the particular structure can really give out much much very nice adhesive forces to be able to stick it. But Teflon is the only surface on which a gecko cannot stick because Teflon has a very low surface energy and it does not allow anything to stick over it. So, it will not allow anything to stick over it. So, the gecko generally falls while walking on Teflon surface, but it is because it has a very low vendor wall force out there. So, how the gecko foot structure really looks like? First of all the overall chemistry of a gecko foot it is it has a sticky feed because it has some something very viscousy and sticky. So, it can provide a certain anchor to it. So, we have its gecko feed. So, we have we have certain feed and it is itself the chemistry itself is very sticky over that we have some certain lamellas. So, the gecko's feed has certain lamellas which will provide extra roughness to it and then all this lamellas will have C-tae. So, C-tae are nothing but micrometer they have a micrometer diameter. So, lamellas they will have certain C-tae. So, we will see there are certain micron size C-tae on to it which are lined up on each lamellas. So, if this were this were lamellas then we have many C-tae which are couple of microns in diameter with 5 to 10 microns in diameter around 5 micrometers in diameter and couple of microns long and then the tip of each C-tae is it has spatula and spatula is nothing but kind of nano hairs with a diameter of around 200 nano meter. So, the tip of this particular C-tae will have nano hairs on it and as we have nano hairs on it we are increasing the surface area by millions of times. So, from lamellas it is a sticky feed. So, composition itself is very very very sticky then lamellas will have C-tae it gives out it renders some micrometer diameter. So, the so the surface area increases dramatically and the tip of this spatula will have again some tip of C-tae will have spatula and spatula is a nothing but 200 nano meter diameter. So, so that is that is one more advantage that it is providing extra roughness to it and then Gekko can easily stick on to it. But one more quality with the Gekko is it can not only stick it can it destick itself as well very with very very much ease whereas the man-made adhesives we are able to stick them but desticking them or debonding them is very very difficult. So, how Gekko does it? It basically slides itself it basically shears itself and then now it has to remove only one by one by one C-tae from its feet. So, the overall the net force the net Vendor ball force which is dominant on a single C-tae it is not that high. So, the Gekko feet is able to easily destick or debond itself from any wall surface. So, sticking also is very easy desticking also becomes very easy and which is not. So, easy in the man-made structure or the man-made adhesive which are mimicking the Gekko feet. So, that is the overall thing about this multicellular scale structure in the Gekko foot and coming to the bone cells. The bone cells also perform variety of functionality that it has to bear load for certain duration of years like since we since we are born we walk and walk and walk the same time we jump and all that we run we do sports and play a lot and at the same time we walk a lot. So, the bone has to take so many cycles of say any impact or any load at the same time it has to keep the bone alive. So, it has to also provide certain nourishments. So, we see that the bones will have a central havershin canal. So, in the center we have havershin canal which is nothing but which applies certain arteries or the blood to the particular bone and then we have a system of something called lamellage around it. So, each havershin canal will have certain lamellage around it and from there even the arteries will basically extend to certain other location and the outside of it is basically more porous and again if you see there are certain something called lacuna or canaliculi which is which are around it which are nothing but different kind of which are nothing but kind of porous regions which are extending from the central havershin canal and it has basically blood vessels which are flowing through it. So, we have blood vessels and certain lamellage and then we also have canaliculi which is nothing but the central region out here and then it has to basically the bone the bone itself will have to perform under certain load and all that and the bone itself shows a microstructure which has something like this and then again it has certain lacunae and all that to take care of the food supply or the nutrient supply and how do they basically work is that bone is nothing but a structure which is comprising collagen protein and then certain micro crystals of hydroxyapatite. So, stiffness basically comes from a combination of either combination of how these particular collagen and how the crystallites of the mineral part is basically being formed all together. So, this composite structure provides much toughness to it because of the mineral micro crystals hydroxyapatite. Also, at impact it also needs to creep sometimes because to reages itself it also needs some weakness or some cement lines at the weak interfaces to take care of the toughness part. There can be creep as well because with time and all that it might want to slip at we want to slip. So, there will be slip at the cement lines which are between the osteons. At the same time we have some lacunae or the ellipsoid pores which will provide osteocytes because upon impact the bone has to reshuffle, re-change itself. So, it needs to have certain location where osteocytes can really survive and they are the living cells of the bone and bone cells basically they are able to remodel its structure because of the weak interfaces or because of these osteons they are able to reshuffle itself or remodel itself to respond to the prevailing stresses and that basically permits the bone tissue to remodel. There are certain cylindrical pores to allow which will allow the blood vessels to nourish the tissues because all these tissues will need some chemicals and all that some nourishment to be given through the blood vessels and blood vessels can go through really. These haversian canals or the cylindrical pores of the bone which can contain blood vessels and they can provide nutrients and there are certain very fine channels or canaliculi and basically it is help for pumping the nutrients through this particular channels. So, we have canaliculi which will help distributing or pumping the nutrients. We have haversian canals again which will allow the blood vessels to go through it and provide the nourishment. We have certain pores of the lacunae to provide space for the living cells. So, that is what we see here and the pore structure also helps to maintain its viability and adapt to a mechanical stress by allowing a remodeling of the bone. Now, coming to a next example of lotus leaf structure we see that lotus leaf has known for its super hydrophobicity. So, we see a lotus leaf and then we over that if we take it if we take a particular water droplet we will see that it will basically it will not wet the surface it will it will remain sitting on the lotus leaf without wetting it. So, that is the non wetting of lotus leaf, but ideally we can also see that once we have a lotus leaf if you put a water droplet it basically comes and basically without wetting the lotus leaf it basically rolls off on the surface. And that arises because it has certain hierarchy in as a micro protrusions that is some micro protrusions which are to the order of 5 microns as a globules of globules which are sitting on the lotus leaf surface and then they are spread across with a distance of 5 to 10 microns apart and around 5 nano meter 5 microns in diameter and those basically provide extra roughness to it. So, that basically is giving it extra or additional hydrophobicity in terms of to the lotus leaf surface and more than that there are certain nano hairs which have diameter of around 100 to 150 nanometers. So, they basically provide much more order of 100 to 200 nanometers and they provide additional or additional surface. So, the apparent contact angle goes even beyond 160 degrees. So, the transition from hydrophobicity to super hydrophobicity is very very drastic even though the difference might be only couple of couple of theta value of around 30 to 40 degrees, but still the property itself changes from hydrophobicity to super hydrophobicity. And there is a group in Michigan they have basically tried that they burned the surface without damaging the micro protrusions and they just removed the nano hairs and from that they could see that the contact angle has reduced from 165 degree to less than say 140 degrees. So, the additional 14, 15 or 16 degrees which is coming out it is only because of the nano hairs and that is imparting it super hydrophobicity. It means the contact angle has to be in excess of 160 degrees. So, that part is being provided by the lotus leaf surface because of their nano hairs or nano roughness which is inherent in the on the lotus leaf surface. And again there is there are the spider silk is also known to be very strong. It is because there are certain amino acids and which have alternating layers of glycine and NLA and then they have assembled into a beta sheet. Beta sheet is nothing but kind of a zigzag structure with a bulky side group. So, they form certain crystalline regions and those basically comes as a they form a kind of a crystal and all these crystals basically come as certain crystalline regions with a amorphous amino acid amino acids kind of hovering around it. So, basically we have certain crystalline regions and certain non crystalline regions or amorphous regions and these are basically they basically have a beta structure. So, this is a beta structure, beta structure around here we see the beta sheets and they start to form crystals and other segments apart from these crystals they remain amorphous. So, we have a combination of amorphous and a crystalline region that basically gives it much more strength and it is basically the interplay between the hard crystalline segment and the elastic semi amorphous region and that is the reason which gives it additional or extraordinary properties of basically its strength and that is what defines the spider silk and basically one coming to one more example of termite mounds. Termite mounds are basically seen in Africa, Australia and the Amazon. The advantage they have is that they are basically they can stay cool in they can stay cool in summer and they can remain hotter even in they can survive the cold even in even during nights and all that because the because of the energy they have stored during the day time. So, they can stay cooler for longer times and this basically forms a basis for engineers because it can serve as a low energy low energy intensive material. If we have this particular material we can provide passive air conditioning. So, we we are not basically using any energy in terms of providing this particular air conditioning. So, that is the advantage of this particular thing. So, again coming back to the slide and how does it work is that the mounds they collect the warm sun in the morning and evening and the as the center stays cool, but once the night has fallen the heat which has been captured earlier is now transferred back to the interior. So, it is now transferred to the back to the interior and also it uses a stack effect to cool and ventilate the interior of the structure. So, it is basically a stack structure more like this with certain closures and all that around around itself. The overall idea is that the ventilation is dependent on the temperature outside and inside. If if it is very hot then the ventilation will go stronger if it is hot then ventilation will go stronger and the air will keep passing and it will get released from above itself. So, we will have very good ventilation very very huge air flow when the overall climate is little warmer, but when it is very very cold it is very cold outside then then basically what is happening is the heat which is being trapped outside it starts flowing in inside because there is no ventilation anymore. There is no difference between the temperature outside and inside. So, the ventilation overall ventilation is basically slowed in cold during the times of cold. So, the overall ventilation is slowed down. So, heat which has been captured here it cannot go out it basically comes back because of poor ventilation and it starts warming the interior of the mound and now warmer is drawn up to the network of tunnels that are similar to capillaries in the human skin and the warmer gases here is exchange at the structured surface. So, that is what the overall thing is about termite mounds that how the overall ventilation can occur that once the once the temperatures are very very hotter there will be very nice ventilations very nice ventilation which will be available from termite for this particular structure. And also there are certain designs which are admitted to be such as by East gate that they have they have designed structures like this that they have some kind of a stacking effect it is also utilized in either material processing that it can retain most of its most of its heat during the material is being processed or even some structures or the buildings that take care of this particular heating effect. In winters the sun is warming the sides side walls. So, this is basically heating the overall surface and as the heat is very very high the ventilation will go to a good extent and then heat will get released from the top channels. But if the climate is very very cold then what will happen that the heat which is being collected from the walls or from the surface or from the roofs will stay inside and it will and because of poor ventilation it will again warm the chambers of the building inside. So, that is how the overall thing works that it stays warm because of poor ventilation in winter and in summer what happens because of a better ventilation the air basically goes away and this basically brings us to something called peacock feather colors. We know that the color can be given either it can be either pigmented or it can also be through some structures it can either diaphragm it can get transmitted through the through the corners and it can provide certain colors. But the problem with the pigment colors is that they do not stay longer those chemical colors they do not stay longer the lifetime is couple of years hundreds of years but again it basically does not last eternally. Whereas this color which is being provided by the structure it stays forever. So, it does not require a require any dyes or any chemicals to provide itself a new color it just remains there as such. And as we know that peacock feather color and the butterfly feather color they are not pigmented they are basically structured. So, we can see that pigmented colors they basically die with this time whereas structures they remain eternally because that is the structure which is giving out the color. So, we see that a peacock feather will have certain regime like this and then it will have certain colors around it may be very dark blue then will have certain fringe of kind of green and certain fringe of brown. So, all these colors are these all these colors are provided just by the way the structure or the number of melanin rods which are making this particular feather how they are basically structured. So, we see that there are certain number of melanin rods which are on 9 to 12 for green or blue or around 4 for brown and they have certain very nice regular structure. So, these all structures which have which have certain porosity as well of a controlled size or controlled distance between them and these blocks are around couple of nanometers around 2 to 5 nanometers in length and breadth and those particular form a kind of a rectangular lattice structure. And this particular lattice structure is what which is responsible for providing a color which is kind of a structured color to the peacock feather. So, this is the overall thing what we learned in this particular multi scale that initially we need to have a fundamental building block which can go on to forming something at different scales and at different hierarchies and each length scale like say coming to micro it can have a different structure at nano it can have a very different structure. Say in case of a gecko or in case of a lotus leaf we can have a nano here which is predominant at nano scale. But once we come to micron scale in gecko we have more something called c t or spatulae and then those go on to form something lamellae whereas in lotus leaf we have nano hairs and coming to micron scale we have something called micro protrusions which are more like microspheres and then they go on to forming a super hydrophobicity. So, we can see that gecko can stick on to any surface whereas hydrophobicity is coming out via non wetting. So, very drastically different properties how they can be engineered by utilizing this multi scale hierarchy and then we realized how differently what a hierarchy can be classified say in terms of cracking. So, what kind of interactions we see more at a atomistic scale and it basically defines single item and what is the electronic structure around it to give it certain bonding properties. How it can go on to forming a molecule and how it can go on to forming molecular chain which will have certain interactions between them and how it can go on to forming certain localized yield regions and then how it can go on to forming a more plastic regime which can extend to certain grains to many, many grains and what is the overall bulk yielding which can define the bulk property of a particular material. So, we see the how the hierarchy or how the length scale is basically deciding the what is happening differently at each scale. So, we had different hierarchy at each scale and that is what is defining the mechanical properties and also we saw that how the nature has been the master engineer starting from starting either starting from termite mounds or gecko gecko feet or the nacre structure how nicely the platelets of calcium carbonate are arranged with certain protein layer to render fracture toughness which is to the order of 1000 times more than the its ingredient ceramic material which is calcium carbonate. So, how well it can go on to forming something very tough or how it can be really arranged to provide a colouring effect such as in peacock feather or butterfly feathers or even providing warmth and winter and being able to manage the energy of the heat around it such as in termite mounds. So, this gives us the overall feel of the multi-scale hierarchy and if the building block itself is to the order of nanometers or atomic scale we can realize that we can go on to forming something very complex and that will give the advantage that we can control the overall structure much more precisely and that is where basically I will end my lecture and thanks a lot.