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Kitchen experiments part 1: Dental materials

Kitchen Experiments
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I’m joined in the kitchen by Lynne Bingle, who is a cellular biologist. And we’re going to do a couple of experiments that you can try at home to investigate the properties of dental materials. Now we’ve been discussing the properties of dental materials this week and what we want the material to do in different situations. And I’m hoping to show through the sequence of experiments that the properties that you want can change over time. And in fact, one simple property that you might explore can be quite complicated. So I’m going to take the example of a dental impression as a means of investigating the material flow. So this is the flow of material.
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So the first thing that we want a dental impression to do is we need to fill the tray. So I’ve got a plastic empty one here with an impression material that will be loaded into the mouth. Now just in that simple process, quite a lot is going on. So Lynne, if you can squeeze some of the material into the tray for me, what you will see is it’s quite hard. You have to squeeze the trigger quite hard, because you want the material to flow out of the dispenser and into the tray quite readily. There we go. Enough? That’s wonderful. So that’s our first challenge is getting the material out of whatever we’ve got it in and into the tray.
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And silicon material exhibits a property called dilatancy, which means that as we apply load to the material, it actually gets stiffer and fights you, and doesn’t want to come out. Now we can demonstrate that in the kitchen with some cornstarch and water to make a non-Newtonian liquid or a dilatant liquid. So Lynne, you’ve got a bowl. And what we’ve done, we’ve preloaded the bowl with about two scoops of cornstarch, which a thickening agent for cooking. And we’re going to put about one scoop of water in. So if I put the water in, and you can mix. OK. And as you start mixing that, what will happen is it initially will seem like quite normal material to deal with.
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The more load we’re applying to this material the more it’s becoming thicker, which is causing a challenge with getting the material out of our dispenser and into the impression tray that we want. The material is initially quite liquid. But the faster you try and stir it, you’ll notice that the faster it will fight back in a way. And in fact, if you try and impact it, it will present almost a solid surface. If you get your hands in there and put your fingers in, you can pick it up as a liquid and run it through to your hands.
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But then on the other hand, if you were to try and roll it in your hands, you could probably make a ball and hold it. If you keep it moving it will stay in your hands as a lump. There you go. And then stop, and it will flow through your fingers. It’s gone.
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Once we’ve dispensed the silicone into the tray, and the important property there is it flowing easily into the tray, we now have the opposite thing that we want the material to stay put in the tray. Patients are quite inconvenient in the fact that we need to invert the tray upside down in order to get an impression of the lower teeth. And we don’t want material to go all over the floor, the carpet, you, them, and everywhere else. So we need the material to stay put but also be sufficiently flowable that it will go around the teeth and capture all the detail that we need to make our models.
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So we are lucky that silicone impression material exhibits a property called pseudoplasticity. That means that initially it will resist flow. It’s quite stiff. But if you apply a sufficiently large force, it will flow much more readily. It becomes quite thin. Now you probably won’t have dental silicone at home. But you can see this property in action with tomato ketchup. So Lynne’s got the ketchup and a bowl. So Lynne, if you were to invert the bowl of ketchup, so we’ve just got ketchup in a glass bottle. A squeezy bottle of is, of course, cheating. If you try inverting it, it won’t flow, just like our silicone impression material.
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But if you can apply a sufficiently large force, we should be able to get some ketchup - yay. And in fact, you can see that it’s quite thin, once you give it a whack into the bowl. There we go. So we could be here - there we go. So that is pseudoplasticity in action. You can try that next time you have some chips Right, so the final challenge is that once we have a completed impression, the material must then set. Now we’re lucky that silicone impression materials for dentistry set in a couple of minutes. So this material in your mouth is not too unpleasant. It doesn’t taste horrible.
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And it will only be in your mouth a couple of minutes before it can be removed. It’s also quite flexible, so that it will flex around the teeth so it can be removed without pulling any of your teeth out as you do it. The final challenge and the final demonstration of flow is that we want the material to come into the impression material, adapt to all the surfaces, and then make a nice hard model that we can use to build prostheses on or diagnose problems. We’re going to use plaster of Paris, which is the common dental material for modelling. And that exhibits the final type of flow, which is a thixotropic type of flow.
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And that means that as we apply the load to the material, it gets instantly thinner. Now rather than pouring it in the impression and making a mess everywhere, let’s pour it into a tray so that I can show the property. Now when we’re making dental impressions, normally what we would do is load the plaster into the impression and we would vibrate it, using of vibrating table or banging it on the side of the bowl in order to instigate that flow by applying that force, that vibrating force onto it. So if I just ask you to put some plaster of Paris into the centre for me, just try and make an nice little mound.
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If you’re very gentle with it, you can actually build it up quite tall. So you should be able to put another one on top there now, and sculpt it up. And when we’re making dental models, the base of the model that we build is - we build a mound like this in order to rotate the impression over onto to finish it off. So there’s our material. So you can see that it’s actually quite stiff and holding its shape quite well. If I were to vibrate it, as soon as I apply a vibrating force, that material flows very readily. So that is the desirable property, and the opposite of what we’ve seen already.
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And the material will flow readily into all the spaces that we have in the impression, but will later on set hard. So that’s the thixotropic flow. And that is the type of flow that when we apply load to it, the material will get instantly thinner, as you’ve seen. So there’s thixotropic flow. And then we also have dilatant flow, which is where the material, when loaded, will get stiffer. And that was our cornstarch and water or our dental example was the silicon impression material coming out of the gun. And we also have pseudoplastic behaviour, which in our dental example was the silicone impression material in our tray.
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But in our kitchen example was tomato ketchup, where the material will hold its form quite readily, but with a sufficiently large force will become extremely runny. Those are three experiments showing that one property of flow can be quite complex. And we will want different properties from the material at different stages. And you can try these experiments at home to experience them for yourself. And I think they show quite clearly the challenges we face in designing dental materials to provide oral health care.
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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