Mysterious powder

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

• Take a blue powder and red powder. Put the blue powder and red powder into the container, in that order. The red powder will be on top, and the blue powder will be on the bottom.
• Cover the mouth of the container and shake it vigorously. The two types of powder are mixed together.
• Next, if you shake it up and down quickly , the top and bottom will be reversed. The blue powder will be on top and the red powder will be at the bottom.
• Next, fill the container with water and shake it well. If you leave it alone for some time, the order will be reversed once again, with the red powder on top and the blue powder at the bottom.
• What happened?
• The trick is that the two types of powder were mineral pigments. Mineral pigments are pigments used in Japanese paintings, and they are finely crushed rocks that are mixed with glue (for adhesion). Particle sizes are number (usually 5–13 and white), and a larger number refers to a smaller particle size.
• In this experiment, extra-coarse Ultramarine powder (equivalent to No. 5) was used for the blue powder, and Rock Red No. 10 was used for the red powder.
• When shaking it up and down in air, the smaller red pigment gets into the gaps of the blue powder and gradually falls downwards. This results in the larger blue powder being on top and the smaller red powder being at the bottom.
• Next, we add water and shake it well, and wait for the powder to settle. Then, the red powder appears on top, and the blue powder appears on the bottom.
• The amount of water resistance acting on the powder particles is approximately proportional to the square of the particle size (length) (e.g., if the size is doubled, the water resistance is approximately four times greater).
• The magnitude of the gravitational force acting on the powder particles is proportional to the cube of the particle size (length) (e.g., if the size is doubled, then the gravitational force is eight times greater). In other words, a larger particle experiences a significantly greater gravitational force. As the gravitational force becomes much larger than the water resistance, a larger grain (blue in this experiment) sinks faster compared to smaller grains of similar density.

[Keywords]	Drag, water resistance
[Related items]	Large ball and small ball 2 (same density)
[Reference]	“Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, p. 72–75.

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