Eroding Candy

When you pop a hard candy in your mouth, you probably don’t give much thought to the fluid dynamics involved in dissolving it. The series above shows a hard candy suspended in water being slowly eaten away. As sugars in the candy dissolve into the water, the fluid becomes denser and falls away. This creates the downward flow visible in the center of the image. As sugar-laden water sinks, fresher water is pulled in alongside the walls of the candy. That flow helps erode the candy, creating a rougher surface. Since rough surfaces have a greater surface area exposed (than a smooth surface), they prompt further and faster dissolution. That strengthens the downward flow, pulls in more ambient water, and keeps the whole process going. (Image credit: M. Wykes)

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