In this lesson, we're going to talk about the concept of viscosity and how key the idea of viscosity is to the glass behavior of materials. Let's begin by thinking about the difference between a material, which is a rigid material like a glass or alternatively a rigid material like a crystalline solid. What we're going to do is we're going to start out with a cube and then we're going to deform it by a shear process. And so we look at our solid which can be crystalline or non crystalline, we apply a shear force and it's operating across an area. And what we do then is along the axis, the vertical axis, what I can look at is what I'm going to call dx. And then what I can do is I can look along the axis y dy. And what I have now here is the beginning of discussing what we mean by strain. So the material has been deformed and depending upon what level you are with respect to x, you're going to have a different amount of strain. So if I look at my shear stress, that's the force divided by the cross sectional area. And now I can define the shear strain as being the change in distance with respect to position, so I now have defined my shear strain. And what I can also do is come back to the relationship that we have already learned regarding elastic behavior of materials because what we're assuming here is the material is behaving elastically. So the amount of deformation that we're putting into the material, the material is behaving in a linear way. So what we would say then is that the shear stress is going to be proportional to the shear strain just like we said, the stress is going to be proportional to the strain when we're talking about a tensile test. Now the proportionality constant here is another form of modulus and this time the modulus is referred to as the sheer modulus. So tau is equal to the sheer modulus times the strain. Now let's say rather than having a material that behaves as a solid, we now have a material that's behaving as a liquid. So once again, along the vertical line we have the change in position, but now with respect to our location along the wide direction what we're now looking at is a gradient with the respect to the velocity or the time dependence of the shear strain. So that's our velocity, and what we can do then is again define the shear stress and we would have the shear strain rate, which is d gamma dt. And what we find is that a an expression comes about would says that the shear strain is going to be equal to this new term eta times the strain rate gamma. And this is for material which behaves according to Newtonian flow. Now when we look at this behavior, what we're seeing is this term eta and that eta is what we define as the viscosity. Now the question is for our fluid, how do we actually go about measuring the viscosity? The viscosity of the material is in a sense equal to or related to the fluidity or the ability of the material to flow. The easier it is for the material to flow, the higher will be the fluidity but at the same time the viscosity will be lower and as the viscosity of the material increases it's going to be more difficult in order for the material to flow. So we can actually measure this behavior in a very easy experiment. Where what we have is a motor, it's coupled to through a gear. It comes down into a paddle wheel, and what we do is we turn it at a constant rate and we measure the amount of current necessary to cause the motor to continue to move at a constant rate. And that current that is responsible can be related directly to the viscosity of the material. And typically what we have in terms of our units of measure is Pascal seconds or alternatively the term poise. And what we see is the relationship between the units of Pascal seconds and poise on the diagram. Now, we can get a feel for this whole idea of viscosity, a number of years ago back in 1927, Professor Thomas Parnell did an experiment where, what he did was to take pitch which is tar and he poured it into a vessel which is basically a funnel. And he allowed that hot pitch to settle and it took about three years or he wanted to wait at least three years until the pitch achieved equilibrium and then what he did was to cut off the bottom of the funnel. So if the pitch started to move then he would be able to watch that process by watching the formation of a drop of pitch. Now this began in 1930, and it's been continuously monitored over the years, it's been taken over by another professor at the institute and as it turns out interestingly enough both of these fellas received the Ig-Nobel Prize meaning that they received an award for. Their scientific presentation, except that it turns out that this is awarded to pieces of science research that are good, they provide information but certainly they're not groundbreaking. And one of the things that you find interesting is, that there was a series of drops and when the drops were fell, they were recorded. So this was all done with a camera and what you can see is that the number of years for the drops of the pitch to go down it took somewhere on the order of eight or so years for that to happen. As you go down the column and you get to the lower times or the longer times and say the year 2000 what you find is that we see an increase in the amount of time. Well it turns out is indicated here that what's happened is the room was air conditioned and so now, you're seeing an increase in the time between the drops, which means that the viscosity has gotten higher as a result of the air conditioning. Now, if you take a look at this data and you extract the viscosity out of this data, what you find is that the viscosity of this pitch is 230 billion times the viscosity of water. And that's an extraordinarily large number as indicated by the amount of time it takes for the drop will pitch the form and eventually drop off the end of the funnel. Now it turns out that there was a very interesting urban legend that went around. And that legend was that if you look at glass windows, the thickness of the glass windows at one side of the glass was an indication that the material had been creeping with time, that is flowing with time. Well as it turns out, when you look at the rate at which the glass structure moves in terms of viscosity it would essentially take to the beginning of the origins of the universe in order for that to happen. So this was a myth that was busted and consequently there are other reasons for why there is a variation in the thickness of the glass. Thank you.