![]() ![]() Honey, being thicker than water, requires higher and higher stresses to achieve the same strain rates that you would get using lower stresses with water. Viscosity is formally defined as the slope of that line. The slope of the line for honey would be greater than the slope of the line for water. shear strain with honey, we would get a linear relationship, just like we did with water, but the value of shear stress would always be higher than for water. That would be true for every possible shear rate. We will have to apply a greater stress in order to achieve the same shear rate that we measured with water. What happens if we do the experiment with honey instead of water? What would happen, for example, if we exerted a certain stress to get a certain shear rate with the water, and we wanted to get the same shear rate using honey? Well, the honey is thicker than the water. That's how Sir Isaac Newton described the behavior of liquids, so when we see that linear relationship between stress and strain, we describe the liquid as a Newtonian liquid. So, if we did this experiment using water as the liquid between the plates, we would get a linear relationship between shear stress and shear rate. When we increase the shear rate, the shear stress also increases proportionally. The stress is called "shear stress" because of the direction of the force parallel to the liquid, causing shear strain. The graph says that shear stress and shear rate are directly related. The result is a graph that looks like this: And what would we have to do to get that faster shear rate? We would have to push the top plate a little harder. The top layer would be moving even faster, and the bottom layer would still be stationary. What if we wanted to slide the plate even faster? What would happen to the liquid? Well, the shear rate would go up. In rheology the force is given per unit area (like pressure) this force per unit area is described as the stress (given as a symbol the Greek letter sigma, σ). What does this picture have to do with viscosity? Well, in order to get that top plate to move, we have to apply some sort of force to it. There is an important quantity, called the strain rate or shear rate (given as a symbol the Greek letter gamma, γ, with a dot on top) that describes how the speed of the liquid moving along the x-axis (left to right) changes, layer by layer, in the y-direction. The arrow beside the layer is meant to convey its relative speed: the top layer is moving the fastest, the next layer is a little slower, and so on the bottom layer isn't moving at all. In the diagram, the symbol, u, stands for the speed of the layer of liquid. If we imagine that the liquid in between these two extremes is divided into very thin layers, then each layer must be moving at a slightly different speed than the next. So at one extreme, the liquid is moving along with the sliding plate and at the other extreme the liquid is perfectly still. There also ought to be some friction between the moving plate and the liquid that will make the liquid move along at the same speed as that plate. What happens to the liquid between the plates? There ought to be some friction between the stationary plate and the liquid that will keep the liquid still. ![]() One plate says still and the other one moves. One of the common ways of assessing properties in rheology is to place a sample between two parallel plates and move one plate with respect to the other. There's an element of force or pressure that comes into play here, too. Another very simple definition, attributed to chemical engineer Chris Macosko at University of Minnesota, is the study of "what happens when you squish stuff". Rheology is, literally, the study of flow. ![]() Viscosity measurements are the realm of a field of science called rheology. ![]()
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