My office mate, who is teaching for the first time this semester asks, “How can I explain the concept of specific surface area (SSA) to my students and the difference between sands and clays?”
For those uninitiated to soil mechanics, the SSA of a soil is the ratio of particle surface area to mass. Sand particles are roughly spherical and have a small amount of surface area relative to their mass. Clay particles on the other hand are extremely thin plates with many times more surface are than those in a sand. For example, a 100 g (4 oz) of some clay may have enough surface area to cover a football field.
We discussed the matter for a few minutes and devised the following demonstration, which my office mate could perform off-the-cuff during class:
- Take two pieces of plain paper. Same paper = same mass.
- Crumple one up into the smallest ball possible. Ask the class to imagine there is no air left inside the ball.
- Compare the surface area of the balled up paper (think – sand particle) and the flat sheet of paper (think – clay particle). The clay obviously has much greater surface area because of its platy shape. The class could quickly calculate the surface area of each, if desired.
- Optional: Throw balled up paper into the class. Try not to hit anyone in the head.
My office mate’s post-class assessment of the demonstration was positive. The class was jolted out of “take notes” mode into “something different is happening” mode. They appeared to pay more attention and really grasp the concept more fully than if a blackboard description had been used. Score one for the Crumpled Paper.
- Hold the ball and sheet the same distance above the ground.
- Drop both at the same time. The ball will fall to the ground while the sheet floats back and forth, impeded by drag forces that are large in comparison to particle weight.
- Ask what is different about this demonstration and the assumptions of Stokes’ Law.
- Repeat Step 4 above.