 previous lecture talked about some introductory issues on microfluidics. In particular, what is microfluidics and why is it important, what are its various applications and what are some distinguishing features from the classical micro, classical macro flows. So, this we discussed very briefly, of course we will be elaborating on these concepts throughout this particular course and slowly but steadily we will proceed into the theoretical description of microfluidics. But before that, we will be discussing about certain examples. Now, why we discuss about certain examples is because many times when we are learning a subject, we do not have a big picture, we do not have a motivation. Like for example, like from the first day, if I start deriving Navier-Stokes equation, you will try you will start believing that I mean we are forced to learn the derivation of Navier-Stokes equation or the derivation of some other equations related to microfluidics I am just giving Navier-Stokes equation as an example that we will be going through these things with a purpose. So, not just because we have to learn this and this kind of learning objective is very very important. So, we will try to look into the issue from a somewhat application perspective that what are interesting applications, what are the outstanding problems that remain to be solved in research, what are the outstanding industrial problems that need to be considered. So, by keeping all these in view, we can go for microfluidic studies for scientific level understanding for deep technological development and even for societal issues and we will talk about these as time progresses. So that is why I thought that before getting into the mathematical equations and the mathematical details, let us talk about some examples. So, I will talk about some examples may be possibly in the next couple of lectures where I will not get in get deep into the examples because I will try to get deep into the examples with their mathematical background and all those things for the remaining over the remaining part of the course. Since we have not yet developed the full theoretical basis of going through these examples, we will better sort of appreciate these examples from like from a common man's perception that right what are the interesting facets that these applications have and how can microfluidics solve these problems. So, most of these problems are of like interesting practical oriented nature, I mean these are not hypothetical abstract problems, these are practical problems. Many of these problems are addressed by my own research group but I will also talk about a couple of problems which are not touched upon by our own research group but are addressed by leading researchers worldwide. So we will start with that, so some illustrative examples, as I told you that in microfluidics we have broadly 2 types of applications bio and non bio, so when we say that it means that biological applications are quite significant and interesting. So I will talk about one biological example and I will try to demonstrate that we start with describing a biological problem but how fluid mechanics is coming into the picture, fluid mechanics over small scales to address this problem. So this problem is about DNA hybridization. So what is DNA hybridization, I will try to talk in a very simple terms and when we talk in simple terms there is always a danger that you oversimplify the problem. So please pardon me if I am oversimplifying the problem, this is just for the sake of your own understanding at this level without getting into too much of a depth. So let us say that, well before getting into the DNA hybridization let us get into the structure of a DNA. So DNA as you know has double strand, double stranded structure, so each single strand is associated with a phosphate group which you see here, phosphate group which is responsible for the negative charge of the DNA, a sugar and 4 bases, nitrogen containing bases, A, T, C and G. So if you look into the backbone of DNA strand then you have alternate sugar and phosphate group and with each base binded with the sugar that you see in this second panel. So what it means is that a single stranded DNA is structured like this. Now how a single strand mates with another single strand to form a double strand? That is possible because A wants to get combined with T and G wants to get combined with C and this tendency is there because of hydrogen bonding. So that means that you have a double strand like this where you see this kind of mating is like A mates with T and G mates with C. So this is the structure of a double stranded DNA. Now how can we use this for detection of a disease? How can we use this? So let us say that like some disease, let us say some arbitrary disease I do not want to mention any specific disease has some DNA sequence associated with it. Let us say A, G, T, C, G, G, T, A, C, C whatever some kind of sequence if that is there in the DNA then we will ascertain that yes this particular disease is there in the sample. So to do that to identify whether that sequence is there or not we can put a complement of that sequence on the wall of a channel because if that sequence is there it will mate with its complementary sequence and if this mating is successful then we say that that single stranded DNAs have hybridized to form a double stranded DNA. So this is what is DNA hybridization. Now how can microfluidics come into the picture here? See DNA hybridization typically so typically what you can do? See first see consider all these processes like first you have to break the cell and bring the DNA out of it. This process is called as cell lysis. So you break the cell and bring the DNA out of it this can be done even mechanically okay. So it can be a pure mechanical process that does it. Then when you break the cell and bring out the DNA then what you can do? You can heat the DNA because what you want? You want single stranded DNAs. So the DNA is double stranded so by heating the double stranded DNA can break into two single strands. This is known as DNA melting. This is a very typical name. So why this is called as DNA melting or DNA denaturation but melting is also a common term used. Why it is called as DNA melting is because it is like a phase transition process which resembles the melting process just like melting of ice thermodynamically. So you can see that there is a biological process which really has nothing to do with melting of ice or melting of a substance. But notionally the thermodynamics may be somewhat related. So biological processes are also interesting processes involving good amount of physics, chemistry or other fundamental aspects of science. Now but let us not get into that. So that single stranded DNA is passed through the channel and on the wall of the channel you have the sort of the capturing DNA probes. The capturing DNA probes are those which are complementary of what you really want to capture. What you want to capture is the target DNA. So you can see here that you have a test sample you have target DNA and these are capturing probes. Target DNA if that is the sequence that you want to interrogate then that target DNA will bind with the capturing probes and there will be successful hybridization. But how it will bind? It will slowly move and it will go to the reaction sites. The reaction sites are the walls. So it is a diffusion process and as we all know that diffusion alone if it takes part is a rather slow process. So diffusion is a rather slow process and like it typically like say the diffusion coefficient of the single stranded DNA of course it depends on how many base pairs are there and so on. So I will not quote any particular number but let us say it is typically of the order of 10 to the power-11 meter square per second. So it is very slow. So the DNA the single stranded DNA will take a long time to reach the reaction sites. So that means that essentially means that for disease detection a long time will be taken. So you want to make rapid DNA hybridization. So to make rapid DNA hybridization what you want to do? You want to accelerate the movement of the DNA. So even without applying fluid flow you can do it. How you can do it? Let us say that you apply an electric field. Why electric field is important here? This has something to do with the DNA structure. We have already discussed that DNA has a phosphate group. The phosphate group is responsible for the negative charge in the DNA. So because the phosphate group is responsible for the negative charge in the DNA if you apply an electric field the DNA will move because of the electric field. So like this is called as electrophoresis. So if you have a charged particle see the DNA is not a particle. So see we will study later on electrophoresis in this particular microfluidics course. So I am just giving you a motivation that why study electrophoresis out of so many things. See it will give you we will study electrophoresis of a sphere in a flow field. So when we study electrophoresis of a sphere see DNA is not a sphere but many times DNA is modeled with an equivalent radius of gyration. So you see that this is what is mathematical modeling all about. There is a physical reality you cannot you cannot represent the entire physical reality by a mathematical description because the physical reality is very complex. So what you try to do you try to simplify the problem and represent the essential physics in terms of some mathematical equations boundary conditions and so on that is what is mathematical modeling and simulation all about. So we are not in most of the cases representing the actual problem that is why we call modeling. See it is not the actual problem it is a idealized representation of the physical reality and when we do that we have to keep in mind that the I mean the actual geometry and all those things may not be well captured. So we can apply an electric field and the DNA will move but there are several issues associated with the use of electric field which may not be so advantageous. So it might accelerate the DNA movement but there may be several other features which are not which are not so favorable. I will not elaborate on those features now because we will discuss about these in more details as we progress in the course and as we discuss about biomicrofluidics we will talk about those things that what are the positive and negative consequences of applying electric field on a DNA sample we will talk about that. So but here we try to see that well electric field being not being always the most favorable solution let us consider some alternative mechanical means pure mechanical means of accelerating the DNA transport. So this is a research problem on which we worked in a collaborative project with Professor Mark Madhu's group in the University of California at Irvine and I mean it was funded by one in the US research program and what we proposed and was something like this. So I will just run a movie which will which is showing that there is a flap there is a flap which is oscillating on the wall of a microfluidic channel this is a flap not made of a rigid material but made of a flexible material called as polypural. So this flap is flexible and what it is doing is when it is oscillating it is oscillated by an actuator the actuator may be electrical actuator. So when it is oscillated by an actuator the fluid which contains the DNA sample is being you see that because of its oscillation it is being targeted towards the wall. So this oscillator is acting simultaneously like a mixture and a pump. So it is sort of directing the flow in a certain direction. Now when it is doing that is it satisfying the entire purpose you see that if you look at this this view graph you will see that there may be various DNA test sites not just one single DNA test site because you may be interrogating several diseases with one particular sample. So there may be several target sites so if all the DNA falls on one site then the next site will not get the DNA sample from the solution. So you want to make sure that all the sites get the requisite amount of DNA from the sample. So how do you do that? To do that what you do is instead of using a single flap you use multiple oscillatory flaps and the design problem see you see when you use multiple oscillatory flaps then all the DNA does not fall here I mean the DNA I mean here in this example just 2 flaps are shown but it could be n number of flaps and it is very important as a design problem and this is the pure CFD problem. It is important as a design problem that what is the amplitude of oscillation, what is the frequency of oscillation and see in terms of a CFD problem it is not a very straight forward CFD problem. These kinds of problems in CFD are called as fluid structure interaction problems. So there is a structure which is an oscillatory structure the fluid is interacting with it but the structure is a flexible structure the structure is not a rigid structure. So the flexibility of the structure plays its own role that alters the pressure in the surrounding fluid that pressure alters the flow field and that flow field in turn dictates the oscillation I mean in turn influences some of the oscillatory characteristics of the flap although it is in addition externally controlled. So there may be a 2 way coupling between the flow and the structural motion and this kind of problem has to be approached by considering the mechanics of solids and mechanics of fluids over small scales. So what I want to impress upon you is that to design this kind of setup see we started with when I first started narrating this problem many of you might have been frustrated because we started with phosphate groups, sugar, base all these things I do not think that many of you have come to engineering to study phosphate group or sugar or base or this kind of things. But when we went into the final solution of the problem you see we are talking about the fluid flow equation, the structural mechanics equations, the CFD and all those things and many of you I believe are interested in these areas. So you see this is what is the modern outlook of engineering that you can use engineering skills not just to solve traditional engineering problems but some problems of these kinds and this is one of the very important applications of micro fluid. Let us get into a second example. Let us again prepare a little bit of background of this example before we get into this example. In many scenarios even in classical fluid mechanics we are bothered about that how to drive a flow like as engineers we are very much bothered about how to drive a flow through a system. So when we say that how to drive a flow through a system we essentially mean that right what is the mechanism by which we drive the flow for example in the large scale system we require pumps to drive a flow and of course what is the penalty that we have to pay to get a particular output that means say what is the pressure drop that takes place for a particular flow rate something like these are very common facets which we tend to address when we are designing transport system that not just a classical system even in a micro fluid system these are important just that we may not use classical pumps we may use pumps I mean which are like typically say syringe pump peristaltic pump we will talk about these pumps and I will show you demonstration examples of these devices. So these are like I mean these are not pumps in the very classical sense but these are like pumping devices which can actuate fluid flow. But now many of these devices have their own shortcomings like for example these devices have mechanical components and mechanical interaction like say frictional interaction for example these are associated with losses and in a small scale system these losses are very significant like if you recall I mean we will again get into all these because I can understand that there may be some students here who may not have a comprehensive background in fluid mechanics but just to like reiterate that at least if you have if you if you recall the fully developed laminar flow through circular pipes for example. So the head loss is given by the Hagen Poiseuille's equation. So head loss is inversely proportional to the fourth power of the hydraulic diameter. So that means if you reduce the hydraulic diameter from 1 meter to 1 micron see you see how by how many times the head loss increases. So with that very significant head loss and with also like mechanically moving components working unfavorably over small scales you need to look for alternative strategies of driving the flow and that is where like a big whole lot of activities are going on in the area of alternative flow driving mechanisms in microfluidics like flow driving by electrical field for example and we will discuss in length in great details about how to drive a flow by electric field and entire science and technology behind that it is a big part of this particular course. But we have to understand that the electrical field also has its own limitations. So can we go for alternative techniques? So I will show you one example where we can move water by light by optics. So how it is possible? So let us let us consider an example. Let us look into this T shaped microfluidic channel. So T shaped micro channels are very commonly used for droplet generation because if you are passing some fluid then at the junction because of shear the continuous fluid may get broken into droplets. This is one of the mechanisms of droplet generation but here in this particular example we will not focus on droplet generation but something else. So let us say that we are injecting the fluid say deionized water from this bottom of the channel now it enters the junction. Now we have to decide that whether we want to transmit the fluid towards the right or towards the left. So it is just like a smart logical system. So we have a logic. If the logic says we want to move towards the right it will be moved towards the right. If we say that if we want to move it towards the left we will do that. So how we do that is essentially something like this. We quote this top inner surface of the microfluidic channel with a metal oxide semiconductor. So the metal oxide semiconductor is like zinc oxide, titanium dioxide like that. These are typical examples. Now these semiconductors have energy gap roughly if I remember correctly around 3.2 electron volt. So and that falls within the regime of actuation of ultraviolet light. So if ultraviolet light shines on these semiconductors then electron hole reactions will automatically start. So when electron hole reactions will immediately start then these are competitive reactions. So the surface will eventually have either excess holes or excess electrons. So no matter whatever is there the surface charging state will change. And when the surface charging state will change the surface energy will change. And when the surface energy will change eventually that will be reflected in terms of an altered weightability of the substrate. We will get into the physics of this I will discuss about the details of this later on. So once that happens a surface which was originally hydrophobic may become hydrophilic. So that is what we intend to do. It is a very simple physics. So what we want to do is when the water comes here and we want to move it towards the right we shine ultraviolet light here. When we shine ultraviolet light here this part which was earlier hydrophobic will become hydrophilic and water will move towards that direction. If we want to stop that we will just switch off the light. If we want to move the water towards the left we will shine the ultraviolet light in this part of the channel. So what will essentially happen is that we can have a control over the flow by light. We can have a pattern of the flow. In microfluidics many times creating patterns of flows is very important because I mean as I told in the previous lecture that it is it may be very challenging to achieve a turbulent mixing in microfluidics because of low Reynolds number. So it is if you can create local vortices and patterns in the flow by some clever means even in a low Reynolds number flow that may be important for various applications. So that kind of pattern you can make by making a substrate making a surface which has patches of say this titanium dioxide or zinc oxide. So it will make a surface passed with alternative hydrophobic and hydrophilic zones. So that will create a pattern of flow on the substrate and this flow you can switch on and off by switching on and off the light. So you can have a combined spatial and temporal control on the flow that is a combined position independent and time dependent control just by switching on and off the light in a non-intrusive manner. So this kind of device that is shown here this we call as an optofluidic device. So this is just to show you as I told you in the previous lecture that microfluidics is an interdisciplinary subject because it is an interdisciplinary subject. It interfaces with many other aspects of say physics, chemistry, even biology and so on. So this is just an example to demonstrate that how microfluidics interfaces with say optics for example. Now the next example that I will talk about has a little bit of motivation. As I told you that droplets are important in microfluidics and we are interested to study droplets because I mean with droplets you can achieve certain things. For example with droplets I mean you can have two droplets say droplet A carrying a reactant A, you can have a droplet B carrying a reactant B and droplets A and B when they merge together the A and B rapidly react because of large surface area to volume ratio it is a highly active system the A and B rapidly react to form the product C. So droplets can be thought of as micro-reactors not only that you can bring a droplet to a hot spot and cool the hot spot because the droplet can take the heat from the hot spot and get evaporated. So droplets are very important entities and sometimes droplets can be thought of as carriers of information in information technology I will talk about that. So in a nutshell droplets are very important. Now how does a confinement affect the motion of a droplet or the dynamics of a droplet? Why is it important? See always when we want to solve a problem right if you are say think about a child when you start thinking about writing when you you do not immediately start writing an essay as a child you first learn alphabets then you learn how to make words then you learn how to make sentences and then you learn how to make collection of small sentences to give a meaningful idea. See that is how our education proceeds as we grow up. Similarly in research also we address problems in this way I will try to again like to give you a I will again like to give you a big picture. Let us say that we are interested to solve the problem of dynamics of a cell in a confined environment. I will discuss about why dynamics of a cell in a confined environment is very important and again this will happen to be one important aspect that we will cover in the later part of our course. Now when we are thinking of dynamics of a cell in a confined environment let us say a cell in a blood vessel. Now the cell is not a droplet right. See as a fluid dynamist if I am asked that please make a mathematical model of a cell and I know nothing about it. I will first try to relate that with whatever I know. This is how many times people start doing research and this is please do keep in mind that this is not a very bad way of doing research. That is sometimes you oversimplify a problem but you start with some basis which is comfortable to you. So as a fluid dynamist I am much more comfortable with a droplet than with a cell. So I start with a droplet. So when I start with a droplet I very much appreciate that there is no real comparison between a droplet and a cell. I mean cell is entirely a different entity as compared to a droplet. But a cell like a droplet deforms but there are certain aspects which are different from the deformation different as compared to the deformation of a droplet and deformation of a cell. We will talk about all this but to begin with we can study with the dynamics of droplet. So with of course the motivation of studying the droplet dynamics itself which has important applications in microfluidics as I told you micro-reactors or cooling applications. I mean droplets have their own applications or it is even for a big picture that yes at the end even if we want to study dynamics of cells we may have a basic building block by starting with droplets. So that gives a motivation of studying droplets in a confined system in a microfluidic system. So but how do droplets behave in a small scale system? So to make an understanding of that what we did is we try to make a map of the dynamics of droplet as a function of the contact angle and time. So remember I mean we will define the contact angle formally later on as we study surface tension and all these things. But just to give you a basic idea that a small contact angle means it is a hydrophilic substrate that means the surface likes water and the large contact angle means it is a hydrophobic surface. So the surface does not like water I mean it is the more scientific terminologies are lyophobic, lyophilic and all this because hydro when we say we commit to water but I am just again I am telling that I am not trying to be over technical when I discuss about these examples because these are just to create motivations in your mind that why do we go for studying microfluidic rather than getting into the in depth science. We will get into the in depth science immediately after we complete going through these examples. So like if you see that now if you have small if you have low value of the contact angle what happens the surface likes the drop. So this is a this is an example where we are thinking of a shear flow, quet flow that is you have flow between 2 parallel plates and one plate is moving relative to the other this creates a shear in the flow. So when you have this shear flow then because of shear the droplet will try to be deformed but on the other hand because the surface is liking the droplet because it is a hydrophilic surface the droplet will still try to stick to the surface. So what will happen is something which will happen is called as pinch off. So in the pinch off what is happening is that a part of the droplet will remain on the surface and the part of the droplet so first there will be a ligament formation like this. And this ligament formation because of high stresses in the ligament it will eventually break and it will sort of set apart from the original volume which is sticking to the surface. So this phenomenon is called as pinch off. On the other hand for large contact angles you do not see this pinch off what you see is that because the surface does not like the droplet or the droplet also does not have an affinity to the substrate what will happen is that it will simply detach from the surface. So you see that there is there are 2 important aspects of dynamics one is pinch off which is occurring at typical low contact angles and another is detachment which is occurring at higher contact angles. And there is a critical limit at which there is a transition from one type of dynamics to the other and it depends on so many factors. So I mean the essential objective of the study is to like assess that like how these are affected by various factors including the shear rate or even like the sort of the rheology of the fluid and so many other considerations. Now the previous in the previous case we studied the sandwich droplet. Now here we are studying an adhering droplet. Why we are studying an adhering droplet? Because again I told you that one of the big pictures is to study the dynamics of cells. So when you study the dynamics of cells you have to keep in mind that there are many cells which adhere to a microfluidic substrate or when the cells adhere to a substrate so this is the first point. The other distinguishing feature between the this study and the previous study is that we are considering instead of a linear shear also an oscillatory shear. Oscillatory shear why oscillatory shear? Because again the big picture if you think about the dynamics of human body is the blood flow is oscillatory in nature. It is oscillatory over time. So to mimic that mimic the shear in that flow we can make an engineering device where instead of giving a say motion of one of the plates in one direction instead of giving it is a velocity u equal to u1 towards a particular direction we give it a velocity u u equal to u0 sin omega t or cos omega t so that it is oscillating with a particular amplitude and frequency. So we can see that we can see the difference in the dynamics as we go from linear shear to oscillatory shear. I mean these movies are showing that what are the interesting dynamical features. Now as I told you that it is not just the droplet that we are interested to talk about. We are having a broader interest to model biological cells. So when we have a broader interest of modeling biological cells we have one step which is sort of in between because cells are very complex cells are very complex. So before modeling cells one can go for one step of advancement from a droplet that we can consider something which is called as a vesicle. Vesicles are not droplets but they are not cells. Vesicles are fluid enclosed by semi permeable elastic membranes. So it is less like a balloon filled with water just to give a common sense understanding. It is like a balloon filled with water. So it is not just the water but it has a membrane. So if you consider the elastic membrane this membrane in addition to the properties of the fluid within the membrane it has bending rigidity, it has elasticity, it has membrane incompressibility. So these are some of the additional facets beyond the mechanics of fluids that you have to consider to model vesicles. So that is the second step. But is that all? Now if you consider a cell, a cell is also having some properties which is like the cell has a cell membrane which is having these kinds of properties that is the elasticity, bending rigidity, membrane incompressibility all this. But there is a very important aspect and this is I think the ultimate question that the CFD for biological systems is yet to answer. The cell has life but a balloon filled with water has no life. So if you have a living system, a living system responds to a stimulus in a particular way which depends on so many living activities within the system that cannot be mimicked by a dead system. So obviously when you are considering a CFD model of a living system you have to consider these kinds of aspects of a living system, the scientific features of a living system which include some of its activities and these activities may be best represented by coupling thermodynamics with fluid mechanics. So you can have the free energy of the system. See the cell and inanimate say like a balloon like body filled with water may have in an idealized condition similar mechanical attributes. But whatever has life will have certain additional free energies to sustain its living activities and these free energy considerations need to be properly coupled with the mechanics of fluids and that is where actually the future of CFD for biological systems should lead to. So that is I am many times you will see that during the course I will talk about several things which are still outstanding problems and I do not mind to do that because like I mean typically as a teacher many times I get upset with the education the way the manner in which we impart education to you that as if we know everything and we want to tell you what we know but there are many aspects there are many things which are yet to be solved or maybe there those have been solved by me some people recently but I am not very much aware of that. I think as a teacher it is my duty to let you know that what are the outstanding problems what are the very interesting yet to be solved problems what are the challenging problems. Now it is up to you whether you get interested in that and you take that up as one of your future activities but it is not my responsibility as a teacher just to read what is there in a reported book and tell that to you that that is the end of end of the scientific activity in these areas not like that. So what I am what I have just mentioned is an outstanding area of CFD not many CFD practitioners do that because if you just like start solving the Navier-Stokes equations as they are in their form in their routine form it is not possible to like take these aspects into consideration. So you have to have a nice coupling between thermodynamics and fluid mechanics that has also been addressed but the thermodynamic model still it is a it is an open question that how to make thermodynamic models which mimic all the living all the aspects of a living system which it is not as straightforward as this and one has to get deep into the biological applications of thermodynamics to get a get a clue towards answering these questions. So this is what is an interesting area as I told you that eventually our objective may be to study the dynamics of biological cells I mean we may study droplets independent of this interest for the interest for the sake of interest on droplets or even vesicles but when we are thinking of biological cells I mean I mean these are not just complex droplets. So we have understood that cells are not just like complex droplets cells are complicated by their own by their own merit not you cannot just think of a cell as a very complex droplet. So this is an example where we are thinking of the dynamics of a cell now why I mean again I am trying to give you a big picture that why is this important see like in this in this few introductory lectures my one of the objectives is to tell you that why are these problems important why should we study at all these problems. So if you think of I am giving you one example I mean there are many examples for which we should study dynamics of cells in a microfluidic confinement and again we have a chapter on this later on. So we will discuss about that much more details than what we are doing now but just think of a think of a problem all of us know that the progression of cancer becomes lethal the progression of cancer becomes lethal when the cancer cell from its origin moves to a location within the human body by the bloodstream and creates a new cancerous growth at a new location and how is it possible that the cancer cell moves by bloodstream and this process is known as metastasis. So this is one of the very critical stages of cancer progression. So I think a large number of deaths in cancer can be prevented if this metastasis stage may be arrested. So there are several like medical ways of looking into the problem but we try to see that what is the fluid dynamics way of looking into the problem. This remained an outstanding problem and what I will discuss I mean little bit now and elaborately later on is a work of one of my PhD students. So like when the cancer cell moves through the human body it moves through various blood vessels of various sizes and one of the typical blood vessels is the micro capillary. So in the human body as I told that you have large arteries, large veins, small arteries, small veins, arterios, venous and you have micro capillaries. Micro capillaries are like sort of I mean naturally made micro channels. So in these micro channels what happens when the cancer cell moves or even when a normal cell moves it is a highly stressful environment. In that stressful environment it is expected that a normal cell will die but a cancer cell is able to survive. So what is the physical mechanism that is going on in the cancer cell that is allowing the cancer cell to survive so nicely in a stressful environment. What if we understand this mechanism maybe certain drug can be introduced to combat that mechanism and then progression of cancer can be prevented. This is a pure fluid dynamics outlook of cancer progression. This may not be related to the traditional medical outlook of looking into cancer. So like how we do that is basically so this is a cell which is adhering to microfluidic channel and when this cell is adhering to a microfluidic channel what is happening when the cell is adhering to the microfluidic channel you have various I mean these are the legs of the cell which we call as focal addition points. These are the legs of the cell. Now these legs of the cell just like if you are walking and somebody is kicking you at your back you first feel destabilized on your feet right. When you feel destabilized on the feet that is what happens to the cell also. The cell feels destabilized here so these focal addition points are disturbed or disrupted. Now that is how the cell gets a message that yes I am subjected to a stressful environment. So when that message comes that message then propagates by a very intricate mechanism to the most distant part of the cell membrane which is called as apical cell membrane. So in a cell membrane there are two types of entities one is called as lipid which is like a fluid you understand lipid and then something which is little bit solid type which is called as lipid raft. So when this stressful environment is there one possibility is that how the lipid raft will adjust to the stress it is a mechanism which is very obvious. The lipid raft what it will try to do it will try to go from cell membrane to inside the cell. So many times I tell my students that the analogy is something like this. Say I am residing in an apartment now say suddenly decoys have attacked my apartment. So I am not a very brave person like many of you. So what I will do is I will try my best to save myself first and hide in the bathroom. So what will happen to the lipid raft is also something like this because of the highly stressful environment they try to escape from their original location and go to inside the cell from the cell membrane or otherwise if they still want to remain that maybe with a very high stress they may break. So whatever may be the possibility the eventuality is that the cell membrane becomes more fluid because lipid raft is more solid type and that has that the cell membrane is depleted of that now. So because it is more fluidic it is more malleable it the cell can then reconfigure its shape and by that it can sustain a large amount of stress. This mechanism is there both for cancer cells and normal cells but we have found from our research that this mechanism is more prolific in cancer cells than normal cells and that makes the cancer cells survive in a highly stressful environment as compared to a normal cell. Now we have studied this in a microfluidic system. Why we have studied this in a microfluidic system is because in a microfluidic system you can make micro channels which resemble this micro capillaries but when we make micro channels that resemble this micro capillaries and we talk about the dynamics of a cell we can isolate the dynamics of we can isolate a single cell the length scale of a typical cell in a human body is let us say of the order of 10 microns. So when you are thinking of a microfluidic channel you see the length scale is compatible with that of a biological cell in a in a human body. So that makes it possible that you can address individual cellular length scales. So you can focus or isolate on a single cell and study the dynamics of that and in that process if you can understand that how the dynamical features including some fluid properties like the membrane fluidity. We will discuss about that the fluidity of the cell membrane and how is it changing how is it different from different for a cancer cell as compared to a normal cell. If that can be well understood then it is possible that the mechanism by which the cell adapts to the stressful environment that mechanism can be disrupted and if that mechanism is disrupted that will sort of arrest cancer progression. So this is a pure fluid dynamics way of looking into cancer progression and that can be possible if we are making control studies in microfluidic environment. So these are some of the sort of very interesting and outstanding problems that we may consider like most of the problems that we have considered today are of biological applications. But as I told you that these the applications of microfluidics are not purely confined towards biological application but because biological applications are quite a lot. I thought that I will better give you some very interesting examples and I mean of course we are running out of time in today's lecture to continue with the remaining examples. So we will discuss about the remaining examples and those examples some of those will be biological and some of those will be non biological but I mean we will try to cover various aspects of microfluidics and just like we have covered some biological applications we will cover some more biological applications and some energy related applications because biology I mean the healthcare and energy are two very interesting areas of modern day science and engineering. So I will talk about energy related applications that is where microfluidics comes into the picture. Anyway we stop here today I mean for this particular lecture thank you for your attention and we will continue with these examples as we move as we meet next time thank you.