 In our previous lecture, we were discussing about some examples of applications of microfluidics and we will continue with some more examples in this particular lecture. In previous lectures, we were discussing about some applications related to medical sciences and some applications where we were discussing about DNA cells and all these. Many of these fundamental studies could be directed towards medical diagnostics and we will come into that later on. But in addition to medical diagnostics, in microfluidics, we are also looking for possibilities of delivering medical treatment that how can microfluidics help towards delivering medical treatment. So I will talk about one example which is based on collaborative work between us and the Tokai University at Kanagawa, Japan and this work is related to the development of a painless microarray for blood extraction and drug delivery. So again I want to give you a broad picture before entering into this example. So like I can give you the example of management of diabetes although this particular innovation is not restricted to management of diabetes but I can just give you a perspective that there are many diabetes patients who need regular monitoring of the blood glucose level and who need regular injection of insulin. Now this is something which is for example as you can see that these like injection of insulin requires I mean some pushing of a drug and that many times is considered to be a little bit painful considering the psychology and the physiological aspects together. I mean it is not something which is very comfortable. On the other hand many times the dosage of insulin which is given is based on some average value that is prescribed and it is not consistent with the requirement of the insulin that is necessary at that instant of time. So sometimes maybe the dose is overdose sometimes maybe the dose is under dose. So the objective of making a new device is to design a painless system for testing of the blood and then delivering necessary insulin based on the level of glucose at for existing at that instant of time in the blood sample. So how this is done? Just look into this cartoon that is going on it is just like a wristwatch type of device. So it I mean it gives an elusive appearance that it is a wristwatch but actually there are micro needles at the back of this wristwatch. So this particular wristwatch type of a device it has micro needles in the back and these micro needles can suck blood. So these micro needles can suck blood in a particular using a particular mechanism which I will discuss in details in one of our later lectures but in short we are trying to we have tried to design it in a painless fashion. So how is it possible? Here is an important concept that is used many times not in microfluidics only but in many of the micro and nanotechnology applications that is bio mimetics. That means mimicking the biological world for making a device. So towards that what has been done is something like this. When a female mosquito sucks blood there is a particular mechanism by which the mosquito sucks blood and the mechanism is little bit involved we will talk about that later on but the mechanism is such that the suck the pressure that is created for sucking the blood is a negative pressure or a suction pressure. And therefore if you can emulate that through a engineering device which can create a suction pressure in a miniaturized environment then by suction pressure the blood is sucked into the needle and that is a small amount of blood but that is good enough to make the test. Next question is when the blood is sucked how will it be transported through the needle? Because it is a micro needle there because of the micrometer dimension surface tension is a very important force. So by surface tension driven flow blood is transmitted from one end of the needle to the other and then that blood what is done again let me run the movie of the device again and what happens is that there is a micro electrical pumping system for blood extraction using a piezoelectric micro actuator. So basically there is a blood extraction system and this system is designed based on a female mosquito's blood sampling mechanism then there is a MOSFET based biosensor to detect and evaluate the amount of glucose in the extracted blood. So in the same device there is a biosensor the biosensor detects and evaluates the amount of glucose in an extracted blood using an enzyme such as glucose oxidase and then when the device senses that this is the amount of glucose that is there in the blood sample then there is a delivery of the necessary amount of insulin based on the level of glucose of blood at that particular instant of time. So this entire activity is sort of I mean compacted in the form of a wrist watch type of device and this device is claimed to be painless because of several reasons. I mean when we are thinking of pain, pain has 2 different facets one is the physical aspect of pain I mean which is taken care of by the painless action of the needle of the micro needle because it takes the blood by creating a suction pressure by itself the way in which a mosquito sucks blood. Now we can always argue that when a mosquito is sucking blood we are feeling some irritation but that irritation is not because of the mechanical pain associated or any mechanical indentation pain associated with the penetration of the labia of the mosquito that is just because of a chemical irritant that the mosquito spreads during the biting process. So mechanically it is a painless process on the other hand from a psychological point of view also instead of like somebody extracting blood and delivering a drug using some injection syringe if you feel if you see that the same thing is taking place by virtue of a device which is like a wrist watch and which is operated just by pressing a button then you feel much more comfortable mentally instead of somebody using a reasonably sized injection syringe for taking blood and delivering the necessary drug. So this combination makes it painless device so to say. Now I mean we worked on this project through an Indo-Japan the DSTJSPS collaborative program and as you can understand from the total scope of this project that it is a true interdisciplinary project as one of the points that I highlighted the microfluidics is an interdisciplinary field by itself. So you can see that this entire gamut of activities that has led to the development of this wrist watch type of device it is not something which is alone within the scope of microfluidics. So microfluidics is only bothered about the design and analysis of the micro review and then that has to be integrated with several other technologies which are closely related to microfluidics may be memes, sensors, actuators and all these and then packaging, integration all those things together make a device which I mean sort of tries to propose an alternative diabetes management as compared to what is routinely done in under normal circumstances. So we have talked about some bio examples just for a change let me talk about an example of like thermal management this is not a biological application but it is an application of a different kind. So the background of this is something as follows as we have reduced the sizes of the electronic devices what happens is that the rate of heat dissipation per unit volume has increased because the power rating of the devices have not decreased but the size of the device has decreased. So the volumetric energy dissipation rate because of heating of these devices has gone up. So when that has occurred it is necessary that you manage these devices thermally by devising a cooling strategy. Now how do you cool these devices? Now to cool these devices one possibility is that you use a fan like fan cooled CPUs are very common in desktop computers. Now when you think of using a fan in a miniaturized device you are actually losing the advantage of miniaturization because you have made the device small for certain advantages. Now if you are using a big fan to cool a small device then your entire purpose of miniaturization is lost. So what you want to do is you want to have a system of thermal management which is commensurate with the miniaturized design of the electronic device itself and this is very very important because in many of the electronic devices the devices actually fail not because of the failure in the electronic design but maybe because of the failure in thermal design. So one has to be very very particular about that. So to do that there are several technologies I will talk about one such technology which is called as micro heat pipe. So I will talk about two technologies one is micro heat pipe. So what is the heat pipe? So if you can see here that this is essentially a capillary where in contact with the heated section some fluid will evaporate and then this evaporated fluid is transmitted by capillary action to the other end which is cooler one and there it is condensed and the condensed liquid is again back. So in this way what happens is that it is possible that it is like a circulation loop that goes on evaporation condensation then back evaporation condensation like that. So this type of system this type of arrangement is called as a heat pipe. So if you are using no mechanical pumping but you are using just capillary pumping to drive the evaporated fluid or to drive the condensed fluid then that is something which is called as a micro heat pipe where in a micro scale arrangement you are using the capillary action to drive the transport otherwise you require weeks for transmitting the fluid. So I mean these kinds of devices I mean have already emerged in the practical scenario and these devices have several advantages. So no use of mechanical pumping micro groups enhance capillary pumping no requirement of weak in heat pipes because the capillary action itself takes care of the transport over small scales. Lower volume to surface area ratio leads to efficient control over interfacial transport as I already mentioned that in these kinds of devices because of miniaturization you have large surface area by volume ratio that means that the heat transfer or as a matter of fact even any transport may be heat transfer may be mass transfer the transport coefficients are large and one can control the transport processes further by patterning the weightability and roughness of the substrate. This is something which is very unique to small scale system in a large scale system it is not so easy to control the transport by controlling the roughness and weightability but over a small scale by controlling the roughness and weightability of the substrate it is possible to control the flow in a nice manner. So that is one of the applications and we have worked on certain projects in this particular area and I will talk about another example which is going on in this movie I mean which is pointed here that you are using droplet to cool at electronic gadget. So how is it possible? So if you have a droplet you can move the droplet by use of electrical field. I will show you later on how that is possible I am not going into the science at this stage that how you can move a droplet by using electrical field. But let me tell you let me summarize you that I mean how it is possible. So if you apply a voltage to a droplet then there will be a change in contact angle. Now if you are applying the same voltage to all the corners of the droplet then the change in contact angle will be symmetrical and there will be no resultant force but if you are applying 2 different voltages at across 2 ends of a droplet then the contact angles at the 2 ends will be different and that will give rise to a net surface tension force in a certain direction and that will move the droplet from one location to the other. So if you can so this kind of microfluidics where you handle droplets is called as droplet based microfluidics and sometimes the droplets are considered to be carriers of digital information so it is a particular example of something which is called as digital microfluidics. Not that all examples of droplet based microfluidics are digital microfluidics but if the droplets are considered to be carrying digital information I mean we can perceive that as digital microfluidics. So what is happening that the droplet is moving in a certain path let us say there is a source of the droplet and there is a destination. The destination is a location which is a point which is a hotspot in the device. So we have to understand carefully that first we have to know the temperature distribution in the device. So you require to either have a thermal imaging of the device or better if you have a nice simulation tool then by solving the heat transfer equations the energy equation you can get a distribution of temperature over the device. Then you know where the hotspots are located where there are locations of high temperature. So what you can do you can target your droplet to go and sit on a hotspot. So then you have to design electrodes and you have to design such that design a algorithm such that the droplet moves by an optimal path for going from the source to the destination. This is a problem of computer science. So as I told you that these kinds of problems are interfaces of say fluidics with other branches of engineering, other branches of science. So it is not that just like somebody with traditional knowledge of fluid mechanics will alone be able to solve this problem. So this requires interfacing with electrical and computer sciences. So then the droplet is targeted to move in a certain manner and when the droplet moves in a certain manner so you can see in this movie that like these are electrodes and these electrodes are sequentially switched on and off on and off like that. When the droplet goes from one electrode to the other then the previous electrode is switched off and the next electrode is switched on and then we have to design what should be the voltages for how long this droplet should be kept switched on and then off and so on. So these are all nice design problems. So then finally the droplet goes and sits on the hot spot and takes the heat. Now a good thing about the droplet is that it can take the heat in the form of latent heat so that there is no appreciable rising temperature of the system and when the droplet takes the heat in the form of latent heat it may be locally evaporated also and then you replenish that by a new stream of droplets that goes and sits on the hot spot. So it is possible to selectively address hot spots by targeting droplets to hot spots. So this is a very modern outlook of thermal management of electronic devices and we have worked on a project where we have tried to implement this for laptop computers for cooling of laptop computers. So droplets can have various operations so you can see that this is an example where 2 droplets have merged and I mean these are all examples from our own lab. So these droplets have merged and once these droplets have merged so I have shown this example for chip cooling but the same problem can be used for considering droplets as reactors. So if we run this again let us say that this red droplet has reactant A and the other droplet has reactant B so once they merge together the A and B will mix quickly and they will react to form a product C. So that can be used to achieve biological or biochemical reactions. So the same technology can be used on one hand for thermal management on the other hand the same technology can be used for bio applications or chemical application. So you can see that interesting fundamental science can lead to so many applications in the microfluidics domain. So I mean previously we were discussing about low cost healthcare that how is it how microfluidics can help in achieving the paradigm of low cost healthcare. So as we see in many applications related to medical technology see medical technology is a huge area and it is not that microfluidics alone plays a role in that area there are several aspects of medical technology but I am just trying to address some aspects which are covered by microfluidics. So on one side we have seen that like we can address cells, DNAs through microfluidics. On one other side I have shown you that how you can make say for example micro needles for blood extraction and drug delivery using microfluidic principles but you have to keep in mind that one big part of the domain that remains to be challenging not just in the developing world but also in the developed countries is proper medical diagnostics. So when we say diagnostics again diagnostics is a huge area so I am restricting myself only to those cases where the diagnostics implies the testing of blood urine saliva these types of fluids because I mean essentially when we are handling small volumes of fluids that is where microfluidics comes into the picture. So let us say I am just giving an example from the perspective of say rural India as an example. So let us say that somebody is suffering from a particular ailment let us say fever some kind of unknown fever for a few days then when the person is suffering from fever for a few days and the fever and he or she is not recovering from fever then there is a suspicion that it could be some vector bond disease for example there it could be malaria, dengue or even it could be typhoid I mean whatever I mean sometimes symptoms are so vague that it is very difficult to identify from the symptoms that what is the origin of the fever. So then the patient has to be tested for his or her blood sample. Now this kind of healthcare facility is not available at all remote locations and it is well understandable. I mean there is nobody to be blamed for this because in a huge country I mean which is very densely populated and I mean there are places where there is not so much access to modern healthcare I mean this is something with which we have to live with but we have to get a solution to overcome these constraints. So I will tell you that what kind of solution is possible and how can microfluidics play a role in that. Traditionally what will be done the patient will be taken to a healthcare center which may be is located a few kilometers from his or her own location. Then the blood sample will be taken there not a small amount of blood sample but a significant amount of blood sample which adds to the agony of the patient but anyway we are all habituated and accustomed to this. And then that particular blood sample if it requires a specialized testing then that will be taken to a very specialized lab which again may be located at a far off distance. And then that sample will be tested and the result if it is a specialized test it may take a few days for the result to come and by the time the result has come many times we are lucky if the patient is still surviving. So that is how these kinds of I mean this kind of scenario I mean it is not very uncommon and that is how it traditionally goes on. And now think of an alternative system I am proposing an alternative system which appears to be something I mean which is a sort of a dream system but this is just an alternative microfluidics based approach. So instead of a very expensive lab which is doing this test you what you do is you employ a health worker a health worker who is not very highly trained but just good enough to prick the fingertip and take a single drop of blood from that and not a huge amount of blood just a single drop of blood and that will be loaded in a handheld miniaturized device which is a microfluidic device what kinds of devices these are we will talk in great details of that through in this particular course. But for the time being assume some handheld type of device if it is if the handheld type of device looks like a compact disc look like a CD it is called as lab on a CD that is where the entire activities of a chemical laboratory are sort of integrated in a small disc like device. So then this device is run by a motor and when the motor runs this device and you load the device with a blood sample and in many of the in these devices there are many radial and cross radial channels which are made these are micro channels again I will demonstrate you that how micro channels are made in these devices. So when you make channels in these devices then what will happen and let us say that you sort of load each of the channels with a particular chemical that will test the existence of a certain disease in the blood sample. So the very common principle can be that in contact with the blood sample if so these chemicals are essentially like antigens or antibodies. So if there is a matching antigen or antibody in the blood signifying the disease there will be an antigen antibody reaction and there because of the antigen antibody reaction there will be a change in color. So the change in color may be visible but that is very ideal many times it may not be visible very sharply. So one has to do an image processing and then by image processing one can one can sort of decide whether a particular disease is there or not. So this device is rotated in a motor and the images of the blood sample as the blood is being transmitted in some in in these channels then those images can be taken and then when these images are taken then what happens then the question is how will these images be processed. Now I mean when we first started working in this particular field I mean many of my students were sort of trying to do that image processing using MATLAB. There is nothing wrong with that but in a in a sort of a more convenient paradigm one can nowadays use instead of such complicated image processing capabilities much simpler may be less accurate but more or less giving the solution type of technology that is using just the android based platform of a smartphone to grab the image and then based on that image will be processed and when that image is processed the result of the image processing by an SMS will be sent to a medical doctor who is sitting in a big city and that medical doctor will look into that result of the test and immediately give necessary advice through a return SMS and that starts the treatment. So this entire process has several advantages so what are the advantages see I mean there of course you can see here is that it is a integration of microfluidics technology with mobile technology information technology and all this. I am not bringing the information or the mobile technology part into the picture here because that is not an explicit contribution of microfluidics but what microfluidics is achieving here you see that first of all you do not have to do the tests in a big lab you do it in a miniaturized device and this device can be fabricated in a very low cost manner I will show you that how you can fabricate these devices at a very very low cost and then you use only a small volume of blood and because you use only small volume of blood you require only very small volume of reactants the chemicals which will react with this blood for the blood testing so that will reduce the cost because of high surface area to volume ratio you will see that these devices are highly reactive and you can you can expect rapid reactions and rapid outcome of the test. So you can do low cost rapid diagnostics in a portable manner and this is called as point of care diagnostics that instead of taking the patient to the hospital you sort of bring a mobile hospital to the patient and then do the necessary testing so this is a sort of a paradigm which is not just suited for a country like India but many countries throughout the world for diseases of any for say diagnosing diseases of any generic type and you can appreciate from this discussion that how microfluidics can play a big role towards that. So now these devices you can make more inexpensive by having very simple innovations this is what a point I want to highlight that in microfluidics for the low cost medical diagnostics it is not the complexity of the system that may be an USP of your device it can actually be the simplicity of the system that can be an USP. So this is a device that we innovated in our group this is called as paper and pencil device. So you have a simple paper based microfluidic device paper based microfluidic device is not our innovation it is an innovation from professor Whitesides group in the Harvard University. So but we made certain innovations to make the fabrication of micro channels on paper based devices in a very inexpensive manner I mean which is greatly advantageous as compared to what is done routinely but I will come into that later on when I discuss about fabrication of microfluidic channels. But on the top of that what we tried to do is as I discussed earlier that in many microfluidic systems you can augment the rate of fluid flow by applying electric field. So we try to apply electric field on the paper based device to make the water move faster or for example here the blood sample for diagnostic device it will not be water but a blood sample that we can make it to move faster. So what I what we did is something like this that we had to fabricate electrodes. Now electrode fabrication is a very involved process in MEMS devices. So here what we did instead is that instead of going for expensive or elaborate electrode deposition techniques what we did is we scratched the paper you can see this black region which scratched the paper with HB pencil just HB pencil and the graphite in the pencil helped to act make these scratched locations act as electrodes. So this device is made just by a simple paper with a simple printer used to sort of print channels on these papers and then scratching pencil makes electrodes. So by using a simple printer a paper and pencil we can make a smart microfluidic device which you can use for blood testing this is called as paper and pencil device. So I mean there are several innovations possible I will talk about the scientific aspects of the paper and pencil device how these how do these devices work and so on. But for the time being we will move on to some other examples. So I talked about some examples related to bio and some other examples which are not related to bio but as I hinted that there are some applications in microfluidics which relate or nanofluidics which relate to energy and in energy sector microfluidics is now becoming a very key area I mean which is playing its role. So I will talk about some concept which is called as nanofluidic battery it can also be microfluidic battery but just that these devices are more efficient if we go to the nanodomain than in the micro domain. So that is why I have made like the title as nanofluidic battery but I mean it could also be microfluidic battery. So what is the principle? So again we will discuss about that in details. So you can see there is a pressure driven flow if you follow this movie there is a pressure driven flow that is taking place there is a parabolic velocity profile you see this is a pressure driven flow and there are ions moving with the flow. So how you do you have ions in the flow so you have a device in the device the surface gets spontaneously charged and the surface gets spontaneously charged because of sudden electrochemistry at the interface between the substrate and the fluid I will talk about this later on that how the surface gets spontaneously charged. Let us say that the surface gets negatively charged this is just an example. So if the surface gets negatively charged and the entire fluid is electrically neutral so the bulk of the fluid will have more positive charge than negative charge and when this pressure driven flow is occurring then this excess positive charge will be accumulated in the downstream direction and that means that a voltage is created across the device. So when this voltage in a dynamic condition is created across this device and then if you connect this with an external resistor which is not shown in this cartoon if you connect this with an external resistor then a current will flow through the external resistor and a power will be generated. So you have an input energy which is the hydraulic form of energy and the output energy is an electrical form of energy. So this is hydraulic to electrical form and this is this kind of device is called as nanofluidic battery. So I will later on discuss that how you can you can have enhancement of efficiency of these devices what are the challenges. Now each of these devices can have energy conversion I mean the power generated may be of very low magnitude that is say few milliwatts may be for example but in many devices you can have large number of parallel arrays of micro channels and nano channels. So if each of these devices can generate a few milliwatts of power then it is possible to generate a good amount of power using these devices. You cannot use these for large scale applications but in small scale applications it is very much possible to use these for energy purposes. So one of the big advantages is that it is a clean energy generation technology. So it does not require any combustion of a fuel and all these things so it is direct conversion from hydraulic to electrical form. Now one of the limitations when we try to learn a device we have to also appreciate what are the limitations. Why are these devices not in the market? I mean these devices have very low energy conversion efficiencies. So may be I mean the maximum value that people have realized so far in experiments is at the most I mean not even 10% but I mean it has been shown theoretically that if you go to nano fluidic devices then in the nano fluidic domain you can use the slip boundary condition at the wall to have instead of the no slip boundary condition to have a more rapid rate of transport of these ions and that means more current. So to have more current in the system you should the current is a combination of advection because with the fluid flow the ions are moving and the like something called as electro migration. Electro migration is the movement of electrical charges because of applied field. So here no electric field is applied but an electric field is induced a back electric field is induced and what the back electric field does is it creates a current due to electro migration which is a back current which is shown in this direction and there is a forward current due to advection. At steady state these two currents should balance so that net current through the system is 0 because you have no excess charge in the system. So the net current must be 0 but there is a potential and you can realize a current the a net current by having an external register but not the through the system the net current will be 0. So coming back to the point so the efficiencies of these devices is low. So you can enhance the efficiencies of this device and if you can do that if you go to the nano fluidic domain and if you can enhance the efficiencies of these devices then it may be possible that using this kind of devices you can compact the entire device the entire power generation device and that device can work in a cyclic process just like a power plant you can you can miniaturize that in the form of a chip. So I mean we have undertaken such a project titled plant on a chip. So we are trying to now make a small miniaturized power plant on a chip by using this particular principle. So I can tell you that this is not a technology of the present but I mean this is currently in the lab scale and many people are investigating fundamentals of this but sooner time will come when this will be converted into viable technology. Another example this is not based on work from our group but work from some other research group synthetic lives for power generation. So the reference from which this work is taken is highlighted here. So if you are interested you can read this paper. So this takes an example takes a clue from photosynthesis for pumping fluid. So what it is doing is water is sucked from the main stem of artificial system at 1.5 centimeter per second it moves toward the edges of the leaves and evaporated through the created pores. So this is something what commonly takes place. Now how can you harvest energy from that? So that is done by this innovation. The main stem has metal plates connected to circuits and the charge plates and water within the stem form two conducting layers separated by an insulating layer. So it is a system where you have a dielectric. Now as the liquid is getting converted to vapor during evaporation the permittivity of the system changes because the dielectric constant of the liquid is different from that of the vapor. So the charge stored in the system you know q equal to c into v and c is a function of epsilon like for a parallel plate capacitor is epsilon by d. So c is a function of epsilon the permittivity. So if the permittivity changes with time because of a dynamical evaporation process then the capacitance also changes with time that means q also changes with time because q equal to c into v so you have a dq dt that means you have a current. So that is how in the system you can harvest energy and it is possible that a tiny amount of electricity is generated the output may be 2 to 5 micro volt with a power density of about 2 micro watts per cc. So you can see that this is a very interesting way of harvesting energy. I will discuss about a couple of more applications before we call it a day today. Now I have discussed about biological applications energy related applications. Now an application related to computer science this is called as bubble logic microfluidic bubble logic. Again this is not a work done from our group this work has been done by one of my friends Dr. Manu Prakash I mean when he was a student of the Harvard University. So you can see here like let us say that you have say bubbles or droplets they are arriving at a junction one from the direction A and another from the direction B. Bubble arriving at the junction at the junction always enters the wider channel right it has a option whether it will enter the wider channel or the narrower channel because the wider channel has a less resistance any system remember always has the tendency to go through a least resistance path. So the bubbles will initially go to this junction that is the direction of A plus B increasing the output flow resistance of A plus B and after sometime the bubbles will be diverted to the other direction that is A dot B. So you can make a logic circuit using that and in this movie you will see that how like bubbles arrive at junctions and after sometime you will see that instead of going through the wider channel it will start going through the narrower channel. So these bubbles if they contain digital information then it is possible to make new generation computers using this microfluidic logic and this is called as bubble logic. So these are very fascinating areas of science that can be addressed by microfluidics. So I have talked about many applications but I mean I will try to wind up today's lecture with some little bit of science that may go in some of these applications. Now in many microfluidic channels we will we find that roughness is very critical roughness of the channel is very critical and the reason is straight forward in a microfluidic device or even in a nanofluidic device the roughness length scale is comparable to the system characteristic length scale. So roughness plays a very critical role and poorer the manufacturing technique is rougher is the substrate and we consider that as a negative consequence that is an adverse effect on the device because we expect that friction will increase because of the roughness. Now this was an understanding for a long time and this is something which is very intuitive but in science studies are often non-intuitive that means certain experiments revealed that if you are having a rough micro channel then you are actually getting a very smooth flow instead of a rough behavior instead of a high resistance against the flow. So this is something which we tried to look into from a fundamental theoretical perspective because this is something which is which at that point of time when this was addressed by the research community appeared to be an outstanding problem I mean a problem which is associated with lots of anomalies and then what came out of the investigation to cut the story short that it was revealed that roughness of the surface in conjunction with the weight ability that if you have a coupling of hydrophobicity and roughness. Hydrophobic micro channel surfaces are very common because many of the microfluidic substrates are normally hydrophobic and they are actually treated specially by some oxygen plasma or maybe some other technology to make hydrophilic otherwise these surfaces will be hydrophobic. So when these surfaces are hydrophobic and they have roughness elements then roughness and hydrophobicity combination can give rise to small scale bubbles which are nano bubbles these bubbles are residing on the surface of the micro channel. The bubble formation in microfluidics is a very critical thing if you have bubbles which are having the same laying scale as that of the system then these bubbles will block the flow and bubble clogging is a very serious problem in microfluidics I mean anybody who has done practical work with pressure driven micro flows will know that it is actually a great job to see that the fluid is actually flowing through the micro channel forget about the science part of the work that many times occurs because the bubble is clogging the flow passive. Now here we are not talking about such bubbles we are talking about nano scale bubbles that is bubbles which are say of the laying scale of around 10 nanometers or so these bubbles are very stable and these bubbles are standing on the microfluidic substrate. So when these bubbles are standing on the micro fluidic substrate then what is happening then the liquid which is flowing on the top of these bubbles that liquid is not directly encountering the roughness elements. So that liquid is flowing as if on a cushion of bubbles. So the bubbles are acting like blankets which are not exposing the outer liquid to the with the roughness elements which are there on the surface. So that makes the flow apparently frictionless or with very low friction because instead of the liquid encountering the roughness elements the liquid is smoothly sailing over the bubbles. So the in systems where bubbles are not there then what will happen that instead of bubbles you can have a low density depleted layer which may not be bubble but a low density layer adhering to the solid boundary over that you have the bulk density. So the bulk density fluid is moving on a low density cushion that is being separated by the surface roughness elements and therefore although the surface is rough you can see that how rough it can be this is an image of atomic force microscopy so you can see how rough these are but if this is blanketed with a low density fluid then the high density fluid which is flowing on the top of it is not exposed to these roughness elements and what has given rise to this low density fluid it is the combination of roughness and wettability. So here the roughness acts as the blessing in disguise and we call it the rough makes it smooth. So it is the roughness that gives rise to the smoothness of the flow and this is something which is very non-intuitive and that is why it is a very interesting and fascinating aspect of science as all of us grow up we tend to realize that there are certain things which are very intuitive right even a little child does not put his or her finger in fire because he or she will know that that will burn who has taught this just maybe nature has taught this to the young child so this is from intuition. So most of the learnings that we have in science are primarily out of intuition and there is nothing wrong with that but as we move ahead with our life we understand that there are certain things which do not occur just out of intuition they are actually counter intuitive if we keep our eyes open towards that and do not think that as a mistake and instead of that we try to understand or unveil the science that goes behind that non-intuitive finding to me that is what is learning of science and that is how learning of science progresses. So this is one particular example I mean this kind of example is thought of can be thought of as hyper fluidity in micro or nano channels we have done some work on this but it is very difficult where the challenge remains is that this inception and distribution of nanobubbles is actually a stochastic phenomenon. So it is not possible always to make a reproducible engineering device out of this fascinating science. So that remains still as a challenge that to translate this science into a device and if we can translate this science into a device then you see that it again like it can add value to many systems for example the low cost medical diagnostic devices because they are of low cost they are made of such manufacturing processes which can give rise to serious roughness elements and those roughness elements can be then exploited as a benefit so that you can have rapid transport of the blood sample and you can have rapid reaction. So but this is not already realized in a device I am just trying to give you a futuristic outlook that how can this science be brought into a technology which may be useful for the common people. So in microfluidics we handle or we deal with many such fascinating examples where there are wide ranges of applications and the beauty of the subject is that on one hand you have lots of practical applications on which you can work as an engineer on another hand you are fascinating science which you can bring into the technology to make the technology more effective and one requires experiments and theory combinations of both to address these particular needs. So there could be many more examples which I would have talked about but I think we should be restricted here in terms of the number of examples and it is good time to talk about that we now enter into the theoretical and the experimental foundations of the subject. So in the next lecture what we will do with that we will revisit the classical continuum equations typically the Navier-Stokes equation, the energy equation and the species conservation equation the mass transfer equation these equations we will derive starting from the very basics so that everybody comes to the common platform and then we will use those equations for certain applications related to microfluidics and try to find a contrasting feature with the corresponding macro flows and see and try to also discuss about situations when those equations may break down either those equations or the boundary conditions accompanying those equations. So we will start addressing those from the next lecture onwards thank you very much.