This colastic silicon rubber is a remarkable new material that has a stiffness that depends on time. When things happen fast, a bump, a bounce, a jump, a stride, then this colastic silicon rubber appears stiff and resilient. But when life slows down and the visco-elastic silicon rubber has more time, it appears soft and compliant. If you stand on visco-elastic silicon rubber, it gradually conforms to your foot. And if you put visco-elastic silicon rubber under the leg of a rocking table, it adapts to fill the gap. This colastic silicon rubber is stiff and resilient if you strike it fast. But it's soft and compliant if you push on it slowly. If you're in a hurry, it's like hard rubber. If you're patient, it's much softer. Yet, despite this remarkable hard and soft behavior, visco-elastic silicon rubber always returns to its original shape when you leave it alone. This colastic silicon rubber is bouncy and even when you deform it slowly into a new shape, it's still bouncy as it slowly returns to its original shape. One way to measure the stiffness of rubber is with a derometer. This device pushes on the rubber and sees how much that rubber resists dinting. The more it resists dinting, the higher the reading of the derometer and the heart of the rubber. This ordinary silicon rubber resists dinting pretty strongly. It's pretty hard and we get a reading of 55 units and that reading doesn't change with the time of the measurement. It's always the same value. Here's a softer rubber that resists dinting less effectively and so it obtains a lower reading of hardness and that reading doesn't change with time. But when we go to a visco-elastic silicon rubber, now we get a reading that varies with time. This material is harder on short time scales and short measurements that it is on long time scales and long measurements. So you see the resistance to dinting, which is being indicated by that dial, gradually decreasing as the material relaxes as time goes on and it shows a softer and softer behavior. The time scale on which a particular visco-elastic silicon rubber shifts from hard to soft depends on its formulation. The red visco-elastic silicon rubber takes a very long time to show its softer side. It's slow to adopt the curled shape and it's slow to return to its original shape. The mint green visco-elastic silicon rubber on their hand has a much faster response. It adopts the curled shape quickly and it returns to its original shape also quickly. The average stiffness of a particular visco-elastic silicon rubber also depends on its formulation. This piece is so stiff on average that it takes almost my entire weight to deform it even when I'm patient. Once I leave it alone, it slowly returns to its original shape. But this pink visco-elastic silicon rubber is much softer and I can deform it with only my thumb. Once I leave it alone, it returns, in this case, fairly quickly, to its original shape. Visco-elastic silicon rubber could also be formulated as a foam, but despite its reduced density, this material still has all the features of an ordinary visco-elastic silicon rubber. It's bouncy and resilient on impact, but it deforms gradually under sustained pressure. When you leave it alone, it gradually returns to its original shape. You can form visco-elastic silicon rubber in much the same way you form ordinary silicon rubber. And you can use the same three curing techniques, the addition cure, the proxide cure, and the condensation cure. This is the addition cure. The visco-elastic silicon rubber starts out as a soft putty. And we put it in its molds. We put a top on the molds. And we press the material into place. Then we take it off to cure it. After a short while at elevated temperature, the materials become a solid. It dents if you push on it slowly, but it's bouncy and resilient on impact. Here are those same materials, those same discs. We've taken them out of the mold, and you can see that if you push on them for a long period of time, they gradually deform to take on a new shape. And then when you leave them alone, they gradually return to their old shape. This material is a very slow material, and so you have to speed up the motion to let you see it return to its original shape. But sure enough, it goes back perfectly. Now for the proxide cure. This visco-elastic silicon rubber starts out as a soft putty. We put it in its molds. We put a lid on the molds. And we press the material into place. Then we take it off to cure it. After a brief period at high temperature, the material becomes a solid. It's soft if you're patient, but it's resilient and bouncy on impact. Here are those same discs out of their molds. And we push on it and let it change shape slowly. And then we leave it alone. And as you can see, in real time, it gradually returns to its original shape. And it does so perfectly. Here is the condensation cure. In this example, the visco-elastic silicon rubber starts out as a liquid. Although it's also possible to start the condensation cure from a putty-like precursor material. We pour the liquid into the mold, let it settle, let the air bubbles rise out of it and pop, and then it's ready to cure. The next morning, it's no longer liquid. It's now a bouncy visco-elastic solid. While the visco-elastic silicon rubber shown here is relatively firm, each of the three curing techniques offers a wide range of stiffnesses, from very soft to very hard. It also offers a wide range of response times, from very fast to very slow. Here's that same piece of visco-elastic silicon rubber removed from its mold. It's bouncing resilient on impact, but it gradually deforms under steady pressure. When you stop pushing, it gradually returns to its original shape. So there you have it, a brief introduction to the world of visco-elastic silicon rubber. It can be made so that it responds quickly and it can be made so that it responds slowly. It can be made so that it's hard on average and it can be made so that it's soft on average. But in every case, you have a material that adapts gradually to its situation and then provides firm support.