Viscoelasticity

Location:

Concepts Shown:

viscoelastic behavior as demonstrated by polymers

Equipment:

Slime and silly putty. The slime must be made at least a day and a half before the presentation. 1.5 cups of polyvinyl alcohol powder should be mixed with about 1 gallon of water. Polyvinyl alcohol is not very easy to dissolve. Mixing from time to time will facilitate the process. Also prepare a saturated solution of borax in water. MIx the polyvinyl alcohol solution with the borax solution ina 5:1 ratio (poly:borax) and stir the mixture until there is a significant increase in viscosity.

Procedure:

The hands on experience is the most educational. Viscoelasticity can be effectively demonstrated using the two materials mentioned above. If the slime is rolled into a ball and left to sit on the table, one can actually watch it flow quite like q liquid, until it has flattened itself on the surface. This viscous flow is the result of the small stress exerted by the table on the slime, acting over a relatively long period of time. However, if the slime is rolled into the same ball and dropped from a height onto the same table, it will bounce, just as an elastic solid. This behavior is the result of the large stress of the table applied over a relatively short period of time.

If either the silly putt or the slime is pulled apart quickly with a large force it demonstrates brittle fracturing, a phenomenon demonstrated by brittle solids. However, if the materials are gently pulled over a longer period of time, they will elongate with flow. Both the silly putty and the slime work as well to demonstrate the point. The slime however, is translucent and has a mysterious quality to it. If the slime is not too sticky, one can actually shatter a ball off it, while bouncing. Such brittle behavior is graphic, as is the almost instantaneous commencement of the flattening process of the ball of slime left on a table.

Science:

Viscoelasticity refers to time dependent elastic deformation that is observed in metallic and polymeric materials. There are basically two extremes of deformation: a) Elastic Deformation where the amount of deformation is proportional to the stress applied. This kind of deformation is recoverable. i.e. the material that has deformed snaps back to its original dimensions once the load is removed. Thus, all the energy is also recoverable. This sort of deformation takes place in materials like elastomers. b) Viscous Flow which is irrecoverable deformation. This sort of deformation demonstrated by liquids, dissipates all the energy and makes it irrecoverable. An amorphous polymer undergoes a complex combination of elastic deformation and slow irrecoverable viscous flow which is called viscoelastic behavior. A distinctive feature of the mechanical behavior of polymers is the way in which their response to an applied stress or strain depends upon the rate or time period of loading. The dependence upon rte and time contrasts the behavior of elastic solids such as metals, at low strains, which obey Hooke's Law and are independent of the loading rate. On the other hand the mechanical properties of liquids are totally time independent. Thus the behavior of polymers can be thought of as being somewhat between that of an elastic solid and a liquid. At low temperatures and high rates of strain they display elastic behavior, while at high temperatures and low rates of strain they behave in a viscous manner.

Remarks:

This is a fairly inexpensive demonstration. However, the slime is predominantly water and is bound to dry up. Slime supply, and thus polyvinyl alcohol and borax solution supplies must be replenished on a regular basis. Since the slime is cheap and harmless, every person in a discussion class could ideally have a piece.

References:

Author:
Rahul Pinto
Credits:


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