Why is Water Slippery?

Have you ever slipped on the floor? You probably have slipped at least once in your life. That’s why there’s a rule about no running on the pool deck. You might have slipped because there was liquid covering the surface. Have you ever wondered: why do liquids make surfaces slippery? In this article, we’ll focus on how water can turn a dry surface into a safety hazard.

Before we get into that, let’s talk about friction. Friction is a resistive force that acts on an object when it slides against or rolls on another surface. The rougher the surface, the more resistance there will be, which means that there will be a greater frictional force between the two surfaces.

You may be wondering: if friction is a resistive force, does that mean it’s a bad force? Actually, most of the time, friction helps us! If there was no friction, you wouldn’t be able to walk from one place to another. Instead, you would be walking in place. In addition, without friction, objects wouldn’t be able to roll; that would mean that cars and bicycles wouldn’t be able to move forward or backward—wheels wouldn’t work the way they do now.

The “roughness” of a surface dictates a constant called the coefficient of friction, which differs from surface to surface. It is one of the two factors on which friction is dependent. The other is a force called normal force. Normal force is the force that counters weight. Weight is the force from the Earth’s gravity. Imagine a book on a table. If there was no normal force, the book would keep falling down to the center of the Earth. The normal force is essentially the reaction force against gravity that is keeping the book up; the normal force, in this case, is from the table.

There are two types of frictional forces: static friction and kinetic friction. When an object is stationary, the frictional force, if there is any at all, is static friction. For instance, if an object is resting on a sloped surface, there is a static frictional force preventing it from sliding down. Kinetic friction exists when the object is in motion. For example, if you slide (not roll) an object on the floor, kinetic friction will act on the object, slowing it down until it stops. Rolling is a slightly different case, but we won’t go into that in this article.

What does friction have to do with slippery surfaces? The slipperiness of a surface depends on the frictional force. The less frictional force there is, the easier it would be to slide on the surface. Essentially, a slippery surface is one with little friction. A prime example of a slippery surface is ice; in fact, ice is slippery because of a thin layer of liquid water around it, most likely melted from the warmer temperatures surrounding it.

If you think about it, slippery surfaces are surfaces that were already smooth before the water covered them, such as ice, tiled floors, and waxed hardwood floors. Usually, if you walk on a wet sidewalk or on a wet carpet, it doesn’t feel too slippery. That’s because water reduces the existing frictional force rather than set the frictional force to a certain value. The roughness of a sidewalk causes the frictional force to be higher than a smooth hardwood floor, and when water covers both surfaces, the sidewalk will still have a higher frictional force than the hardwood floor.

If we take a closer look at friction, we find that that it is best observed at the atomic level. When two surfaces are touching, their atoms interact with each other. The more atom interactions, the stronger the frictional force will be. This relates back to the normal force; the harder you push down, the more atom interactions there will be since the atoms are closer together.

Here’s where water comes into play. Although I did my research for everything before this section, this part is a guess based on what I have researched since I couldn’t find a definitive answer. When water is between two surfaces, its molecules limit the atom interactions between the two surfaces, reducing the frictional force between the two surfaces. Instead of the atoms of the bottom of a shoe interacting with the atoms of the floor, both groups of atoms are interacting with the water atoms.

If you have a different speculation, feel free to comment below! Since water is considered a substance with rather strange properties, such as its liquid state being denser than its solid state, it continues to mystify people. Keep in mind that even though water was our main topic in this article, other liquids can also make surfaces slippery, including oil and soap. I hope you learned something new! Keep a lookout for the next post!