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Empire State College ​you know as you mentioned my wife and I we met in Berkeley and we're both scientists and one of the kind of occupational hazards of being the child of two scientists like me and my wife is that your parents like to perform little science experiments on you from time to time and I'll show you a couple of those today if you'll let me and I'll demonstrate one of them first this one I did a few days ago with my son they involved these two cubes shown here you can see that they're similar size similar shape similar colors to them right and I handed the first one to my son the sleep fine into his hand he was like okay dad what are you doing and I handed the second one to him that sleep and he went well geez and to prove it to you we took a little photo of him dick using a little teeter totter with them what he realized was that the one on the right there even though identical to the other one is tremendously more heavy than the first it's almost ten times heavier if any of y'all don't believe me after the show tonight you're welcome to come up and lift these for yourself it's not the kind of thing that makes a good visual demo but trust me and so he sense that and in looking at these cubes he realized that a whole new window and opened up on the world tour in some way that you could have two objects that look so similar to each other in every describable way and yet when you feel them when you pick them up you can tell something's wrong something's very different about this one versus this one what is that thing what is this like secret invisible world of objects that we've never really seen before this point I loved watching him have that kind of little Epiphany because it was really a lot like what I had about five or six years earlier I was a postdoc in MIT at the time and I was in a lab where we were trying to measure single cells and in particular trying to see is there any way you could tell the difference between a healthy cell and a six cell based on these measurements and you know just like these cubes often these cells would look very similar to each other in terms of their size and their shape and their appearance and we needed to find ways properties of these cells that you could measure and use to distinguish a healthy cell from a six cell what could that be well turns out in the story I'm here to tell you tonight is that the very property of these two cubes that makes one so ridiculously having the other one feels so light the very property of these cubes is the same property we found of living cells that lets us tell whether a cell is sick or healthy and that property its density do you remember density right if you're like I was five or six years ago you've learned it in high school it hadn't thought much about it since then right well hopefully by the end of tonight I can show you why it's the most awesome physical property and you should really appreciate and love density so you might remember that density is the function of two other physical properties of an object namely their mass how much in object weighs and its volume how much space a little object occupies and you'll agree that both of those measurements mass and volume they're just ways of telling you how much of an object you have how big is it but when you divide the two by each other when you take the mass and divide it by the volume you get the density and that's a whole other thing it tells you so much more than either of mass or volume can tell you on their own I'll try to give you a few examples of that today so first we're gonna talk about density for a few minutes we might as well acquaint ourselves with the range of densities that we encounter here on earth they range from 0 grams per milliliter all the way up to about 22 or 23 for the most dense elements osmium the nuridium and so we can look at a few spots on this little number line and see what the densities of a few things are on the far left hand side here we see the density of gases like helium point zero zero zero two grams per milliliter and air is point zero zero one three grams per milliliter these are all very low densities but you notice helium is just a little less dense than air and that is why when we fill a balloon with helium and let go of it it goes up so sort of the first example of density giving us something meaningful in our interpretation of the world around us in this example what determines when I let something go whether it goes down or up its density if it's more dense than what's around it it'll go down if it's less dense than what's around it like the balloon it'll go up so a little example of density telling us something about the world around us let's go up the scale a little bit further around 2.7 we encounter aluminum nice metal and then way up at nineteen point two on the density scale we encountered tungsten and that is what these two blocks are made of and that difference the difference in density between two point seven aluminum aluminum can very light and tungsten one of the densest elements that's known is the reason why my son Rhee did to these and being so different and so there's another insight that density

can give us I could walk up on those on these blocks and by looking at them I couldn't tell you which is tungsten which is aluminum but if you let me pick them up and if I know these density numbers then it's very easy for me to tell that this is tungsten and that's aluminum so here density a physical property is giving us a way to identify chemically what material and objects made of pretty neat that idea of using density to identify a substance it's not an OL and not a new one in fact over 2,000 years ago Archimedes was asked in the story by the King to determine whether one of the Kings crowns was made of pure gold or not and so Archimedes was a smart man he realized if he could measure the density of the crown and compare that to the known density of pure gold which is 19.3 very dense if they equaled then it proved that the crown was made of pure gold but if they were different it meant that the King been swindled by the crown maker I guess and so by measuring density he knew he could easily tell the difference between pure gold in 19.3 and something like iron pyrite Fool's Gold at 5.0 but Archimedes faced a challenge how do you measure the density of an object like a crown right I told you density is mass divided by volume now getting the mass of the crown is easy enough you just weigh it but how do you measure the volume of the crown without destroying it or melting it down in a way he needed a way to measure the density of the crown without destroying it what he came up with was really one of the most I think brilliant experimental ideas in the history of science and this is what it is he realized that he could weigh the crown under water if you've ever been in a pool you know you weigh a little less under water than you do standing on dry land right well how much less tea away well the way the math works out the amount that different that you weigh in the water is a function of two things the density of the water which is 1 we know that and the density of you the object that we're weighing or the crown so if we weigh an object like a crown in two different fluids in this example a water and air from those two weight measurements we can calculate the density of the crown that's exactly what Archimedes did he was able to determine what the crown was made on so now amazingly 2,000 years later we still do exactly what Archimedes described so long ago in this example this athlete here is being weighed underneath the water and by combining that measurement with this and measurement of her weight on on dry land we'd be able to calculate her density why do you need to know a person's density well it turns out along with telling you what a cube is and all these different things it can tell you if you're fat or thin you've heard the expression fat floats and it does if you have a lot of fat in your body that reduces your net density down to about 1.0 1 or so if you have a lot less fat bone and muscle and things are more dense so that makes your body more dense and you reach about 1.07 or 1.08 so here's our first example of density remember a physical property telling us something very biological very health in nature in this case how fat or how thin a person could be all from density so back five or six years ago as a postdoc we looked at this and saw all the things density could tell you and we had a thought what if it worked just as well on single cells what if just like I could measure the density of a human body and tell things about the health of the body could I possibly measure single cells and infer things like the difference between a healthy cell and a six cell or a cancerous cell based on this density measurement so we set out to try to do that and the first challenge was what we need to measure the density of a cell which hadn't really been done before and we realized we could use the method that Archimedes described the idea of weighing it in two fluids a different density but how do you weigh a cell you might first turn to this if it's in your kitchen but I can tell you cells are a whole lot too small to weigh on this and tiny living cells don't take too kindly to being plunked down onto a little kitchen scale so we needed a way to measure the extremely tiny mass of cells in order to figure out what their density was how do we do that well luckily the lab I joined as a as a postdoc already had a little math sensor that they worked on that was perfect for this job and to explain to you how it works I'll show you another experiment I performed on one of my children this this is Jack he's a little jerky up there but I feel a lot smoother than that in real life jack is it looks like he's on a baby bouncer and he is on a baby bouncer but it's also a sophisticated mass sensor believe it or not you see the frequency at which he's bouncing up and down as he kicks his legs there he can't just control that frequency at will that's actually a fixed frequency it's a function of a few things that determine it but crucially it's a function of his mass so if he were to say sit there and eat a hamburger that frequency would slow down because his mass would have gone up how it works so my wife and I again and this in the spirit of experimenting on our children realized we had an unexpected mass since they're here and we decided to calibrate it first so we put some weights of known weights on it from our weight set ranging from this perfectly normal waging from five pounds which had the highest frequency of oscillation 149 beats per minute all the way to the highest weight 20 pounds on the bouncer which had the slowest frequency of oscillation only 88 beats per minute now that we calibrated it like a good scientist if you're if you're preparing lab notebooks for any of your lab courses remember got a good calibration curve show my data etc you've done you calibrated our mass sensor now we're able to put jack back in it and measure his frequency and determine what his weight is 20 pounds and then do the same for his identical twin brother Henry and see that he has a slightly lower frequency which means he has a slightly higher mass and indeed that's exactly true so here you see this since our so fantastic we have a

baby bouncer this capable of distinguishing two identical twins from each other even I can't do that most days and I'm their father so this measurement principle is really quite good as a way to weigh something it turns out you'll never look at a baby bouncer again the same way I hope so here's the here's the point if you take that baby bouncer and scale it down I mean way way down so that the bouncing part is about the size of a human hair at that point you're able to put a cell on it instead of Jack or Henry and do the same measurement and weigh them and that's what this tool here does this was developed at MIT in the lab before I joined it they call it a suspended micro channel resonator and at the heart of it is a tiny little bouncer a little diving board shaped thing that you can see here and it oscillates like that if we look inside of it we don't actually put the cells on the outside of it we slide the cells on the inside and this little blue channel shown there if you zoom in on it that's the sensor it's oscillating and when we send a cell like this little red blood cell through we weigh the cell basically so by measuring two cell masses and two fluids of different density we can calculate the density of this cell long story short so there that's how we do it what is the density of a cell after all that is this even interesting and what we found we've measured a bunch of cells and we found that most have a density between 1.0 and 1.2 grams per milliliter and on one hand that's not too surprising because water has a density of 1.0 and cells are mostly water so it makes sense that we would see values around that number but not all cells have the same density and I'll give you just a few examples tonight that's how the density of a cell can be used to determine something very interesting and get some insights about the cell here's the first example this plot here shows each black dot is the measurement of a single red blood cell from someone's blood on the bottom you can see what the density of those cells are ranging around one point one oh one point one two on the side here you can see the weight of each of those cells in picograms if you don't know picograms or wonder how much it cell weighs a millionth of a millionth of a gram very very small weights and so in healthy blood cells we can see all of those measurements kind of pile up into that little oval region and that's where we expect them to go but if you do the same measurement Unseld x' that part that some of them are infected with malaria parasite we see a few cells over there to the left that are a little less dense than the healthy cells those are the cells that are infected with malaria it's been known for a while that the little parasite that is malaria eats up the hemoglobin in your red blood cells turns it into a form that's less dense and the result of that is an infected cell becomes less dense and we can see that and we can identify the infected cells compared to the healthy cells based on their density what else here's data showing the mass and volume of cancer cells these are leukemia cells from a mouse and you're seeing in the black points the measurements before we treat them with a drug and in the red points the measurements after we treat them with an anti-cancer drug and if you look just at the mass and volume of the cells before and after black and red you don't see much of a change and in fact if this data was all you had to go by you might not have thought that drug was any good but like I said density is mass divided by volume if you do that math for each one of these points you get this story over here a very much clearer picture we see that after treatment just minutes after we dosed these cells with a drug we see that a large number of the cells has increased in their density by a large statistically significant amount what this means is that just minutes after treating these cells with a drug we can see that yeah those cancer cells are reacting to that drug we hope that eventually this could become a drug screening tool you might be able to use density change as a way to find new cancer drugs based on whether or not the cells density changes when they encounter the drug and then I can finish with just a little recent data from my lab here at UC Riverside this is showing the a buoyant mass which is a function of density of single zebra of a single zebrafish embryo why we're studying zebrafish well a few reasons zebrafish embryos are an aquatic organism and so if something hurts the zebrafish embryo like a chemical then there's reason to think that it would hurt a lot of other fish and it's probably the kind of chemical you'd want to not have in your waterways and to a zebrafish embryo looks a lot like a human embryo so if something hurts the zebrafish embryo and is a good reason to think that you wouldn't want to have it around any of your embryos either so this is what the mass buoyant mass or density of a healthy zebrafish looks like over time and then if we do the same measurement on a sick zebrafish that we've exposed to some chemical that makes it ill we see a very different reaction we see little decreases in it's buoyant mass it means its density is changing we don't know what's causing this yet but what we do know is that we've tried the same experiment on a whole bunch of different sorts of sicknesses different sorts of toxicants for these zebrafish and we still see this same signature it's as if this change is somehow indicative of a sick zebrafish a regardless of exactly what we did to make it ill and so where we want to see this going in the future is to familiar with the idea of a canary in a coalmine right the the bird that the coal miners brought down to see if the air was safe to breathe we hope that the density measurements that we're making right now will in the future they come like a sort of super canary in a coal mine that by measuring the density of these organisms of these embryos or other microorganisms we could have an instrument that will live and sample the groundwater and tell you if there's something dangerous entering into it or an instrument that can live in the air and sample the air around us and tell us if there's something dangerous all based on

how the density of these organisms react to exposure to whatever's in the environment that's one direction we're taking it so hopefully today I showed you and shared with you why I think density is just the most awesome physical property that's out there it's something that every object around here from crowns to cells to fish to you and me we all have a density we can't see it but we can feel it and we can measure it and if you can measure it accurately enough you can use it to sort of open up whole new views on the world around you thank you [Applause] LIU Post (formerly C.W. Post), Brookville.