Water Ripples: Everything About Them

By: 이세린

Imagine walking across a river, enjoying a warm, soft autumn breeze. You sit beside it , and feeling bored, you throw a small rock into the water. What do you see? This time, imagine a rainy day. Look at your footsteps. What do you see now? What do these have in common? You may not know the exact name, but if you drew a picture of circular waves in your mind, you’re correct. These patterns and shapes that we see or create in water bodies are commonly referred to as ripples, but why do they form and what affects how long they last?

What Is A Ripple?

No matter how still the water seems or however calm the weather seems, there is still a mere movement of air along the surface of the water, which eventually causes tiny ripples. Wind energy is transferred to the microscopic water molecules, which begin to move up and down, forming minuscule “waves”, as gravitation holds the molecules beside each molecule together. However, these “waves” don’t last forever, for the surface tension of water is quite strong, and this tension will stop the ripples from maintaining their shape. Furthermore, the energy transferred into the water is quickly used up in moving those molecules up and down, so the ripples fade.

Let’s go back to the first example of water ripples. When you throw a rock into the pool of water, the rock will push water out of its way as it enters, causing ripples to move away as they create a ring shape. Water will then rush back in to fill the empty space, which can often cause a splash, resulting in more ripples forming. The larger the rock, the greater the effect and the stronger the ripple, meaning that it will last longer before disappearing.

To form a water ripple, you don’t necessarily have to drop an object into a body of water. Ripples may also form when water moves in a particular direction and encounters an obstacle, such as a fishing boat, a break wall, or even your feet as they dangle off a dock! Water molecules will encounter an object and move upwards against it, before being pulled back down by the neighboring molecules. Unlike the circular ripple as mentioned earlier, this interaction causes a ripple that moves in the opposite direction of the water’s original motion.

Solidified Ripples?

Now, looking back at what water ripples are and the examples of them, you may think that ripples only appear in liquid water. Well, that’s not true. One most commonly found solid ripples are, in fact, icicles. Icicles we see in everyday life all have ripples; none of them have a perfectly smooth surface. However, icicles made from pure water don’t form any ripples. What could be the difference between natural icicles and pure icicles? Here is why:

Liquid ripples form due to an external force caused to the surface of the water, leading to water molecules gently moving up and down, creating waves. But for ice, that is not the case. In nature, most icicles are made from water with a hint of salt. Natural icicles tend to look like skinny cones with rippled surfaces — the result of a thin film of water that coats the ice, researchers think. As icicles grow, the film freezes. Any preexisting small bumps in the icicle get magnified into large ripples because the water layer is thinner above the bumps and can freeze more readily. 

We all know that adding salt to ice lowers the freezing point of water, letting the ice melt much more easily. That’s when scientists experimented if salt would affect the form of ripples on icicles, too. When the scientists added a pinch of sodium chloride, or table salt, regularly spaced ripples formed. When they added more, the ripples became wildly irregular. The saltier the water, the more irregular the ripples become. Physicist Menno Demmenie analyzed another experiment regarding this matter. Demmenie and colleagues grew icicles in the lab, adding a blue dye that was visible only when the water was liquid. Salted icicles not only had ripples but they also were covered in a thin, blue film. Icicles made from pure water had no such film—only small droplets of blue appeared on those icicles.

In icicles with salt, the temperature at which the water on the surface freezes is lowered, allowing a liquid layer to coat the entire icicle which soon becomes ripples once they’re frozen. Without the salt, icicles must build up drop by drop, leading to a much more regular surface.

Sources: Water Ripple: What Causes Ripples In Water? (scienceabc.com)

   Ripple effect (sciencenews.org)

   Here’s why icicles made from pure water don’t form ripples (sciencenews.org)