Hi, I’m Doctor Kiki Sanford and today on Food Science, we’re going to be learning about fluids. Kitchens are full of fluids. Try and identify all of them and I assure you you’ll be hard pressed to find them all. Water is the most obvious but take a careful look and you’ll find yourself surrounded by fluids you never knew were there. Why is that? Psychics.
A fluid can be a liquid, a gas, plasma and even a solid in some situations. Solids can be described by two properties -- plasticity, their ability to be deformed, or elasticity, their ability to return to their original shape. Fluids can be described by their viscosity, or resistance to flow. Some fluids, like water, flow at the same rate regardless of the amount of force that’s applied to them. Water has a low viscosity while olive oil has a higher viscosity and is perceived as being thicker. These are known as Newtonian fluids after Sir Isaac Newton who gave us the three laws of motion, which form the basis for classical mechanics and enabled the description of fluid flow as a linear relationship between force and velocity. In Newtonian fluids, the viscosity remains constant regardless of force. However the viscosity of some fluids changes with changes in force. These fluids do not follow the linear relationship described for Newtonian fluids and they’re therefore called non-Newtonian fluids. Lots of common kitchen fluids are non-Newtonian like ketchup, yogurt, custard and gravy for example.
So let’s take a look at what it means to be Newtonian or non-Newtonian. If you stir water, it acts the same no matter how fast or how slow it stirred. The same goes for olive oil or syrup even though they’re really thick. Ketchup is non-Newtonian. One of the reasons it’s so hard to get out of the bottle is the way that it reacts to force. Ketchup is thicksatropic. Its viscosity decreases as force gets applied so smacking the bottle does actually make it flow more easily.
Other non-Newtonian fluids experience increases in their viscosity when forces act on them. They’re called dilatants. Gravy is a great example of this because it thickens as you stir it. We can make a really fun non-Newtonian fluid with just cornstarch and water. You add the cornstarch in small amounts and then slowly stir in water. You use about one and a half to two parts starch to each part water. As the mixture thickens, you’ll notice a change in the way that it behaves. When you stab it, it turns into a solid rather than remaining a liquid. If you treat it more gently, it remains in its liquid state. However if you try pulling the spoon back out, it turns out into a solid again.
Dilatants are an area of interest for researchers developing body armor. The material needs to be flexible enough for the movement of the wearer yet rigid enough to provide protection when under attack from a bullet or a knife.
A greater understanding of kitchen thickeners like cornstarch may end up saving lives. But remember, it’s not just food, it’s science.
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