In this video, Chris examines the properties of a dental material behaviour known as flow, through the procedure of taking and casting a dental impression. Chris explores the properties of this behaviour using materials you can obtain at home.
We learn that there are different types of flow, which can be summarised simply as:
Dilatant: the material becomes thicker as force is applied (shown using corn flour and water)
Pseudo-plastic: the material will initially be quite thick, but with a sufficiently large force it will suddenly flow more easily (shown using tomato ketchup)
Thixotropic: the material is quite thick, but applying a force will immediately make it flow more easily (shown using Plaster of Paris)
Try it for yourself! The tomato ketchup and Plaster of Paris examples are quite simple (just dispose of your used plaster in the bin and not down the sink). The corn flour mixture is a little more tricky to get right so a recipe is given below.
Demonstration of ‘Dilatancy’
EQUIPMENT NEEDED
Corn Flour (about one cup. It may also be called Corn Starch in some locations)
Water (about ½ cup)
Bowl
Mixing tool (such as a whisk, fork or spoon)
INSTRUCTIONS
1) Add the ingredients into the bowl, starting with the ratio of about 2 Corn Flour to 1 Water.
2) Mix and add flour or water until you achieve a mixture that is about the consistency of PVA glue or tomato ketchup. You should notice it behaving oddly as you mix!
Once you have all the flour mixed in and a good consistency, try:
- Pulling a spoon slowly and quickly through the mixture
- Dropping the spoon into the mixture
- Pressing, and even punching (not too hard) the surface
- Creating a ball. You can either try the pressing method (shown by Ric in the video) or rolling it between your hands like dough. You should be able to make a ball that will dissolve when you stop moving.
WHY IS THIS HAPPENING
You are experience dilatancy in action: the more energy you put into the material, the stiffer it becomes. This behaviour is called Shear Thickening, and occurs due to the inability of particles suspended in the mixture to move past each other quickly. It is a bit like the last day of school when everyone rushes towards the door; too quickly and without order everyone gets jammed, but more slowly and everyone eventually gets through. This material has a name: “oobleck” (from a Dr Seuss book). Other materials that behave this way are instant custard (mixed with water) and Silly Putty.
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