Yesterday, while working on a paper, I spilled a cup of coffee. It went everywhere! While rushing around trying to find paper towels, I wondered – wouldn’t it be great if the coffee didn’t spill everywhere. So, much time would be saved in cleaning! This made me wonder about properties of liquids that make them flow.  

What different liquids have you used, consumed or come across during the past week? What was different about them? Did they have different colors? Were they fizzy or still? Did they feel thick or thin? What about pouring? How did each one flow?

Liquid has many properties –  vaporization, condensation, boiling point, critical temperature and pressure, surface tension and viscosity

Defining viscosity

Viscosity of a fluid is “a measurement of its resistance to flow.” It is measured in poise (P) or centipoise (1cP = 10-3 P). Think about different fluids. What if you poured each one into a container? How do you think they will flow? Fast? Medium? Slow? This variation is viscosity. A substance like honey, when poured, flows slowly. This means it has high resistance to flow and is therefore highly viscous. Whereas, a substance like water, when poured, flows faster than honey, and is therefore less viscous. 

Science behind viscosity

A variety of factors can affect viscosity. One factor is the bonds that bind a molecule within a liquid. The stronger the bonds are within a molecule, the harder it is for the molecule to move and therefore the more viscous it is. Also the bigger a molecule is in size, the more viscous it becomes.

Temperature also plays a role in changing fluid viscosity. An increase in the temperature will lower the fluid’s viscosity. Let us go back to our honey example. What if you heat the honey? Think back to when you were baking and used honey. I bet you found that heated honey flows more easily than colder honey.

Whereas, an increase in pressure, will lead to an increase in a liquid’s viscosity. What happens when honey is poured into a syringe? What if a plunger is placed within the syringe? The pressure in the syringe increases and in turn increases the viscosity of the honey.

Importance of viscosity 

Liquids are not just things we drink or find in the world. There is also liquid in our body. Think about blood. Think about blood, an integral part of our body. It is responsible for transporting things like nutrients, oxygen and immune cells. Our blood moves and keeps us warm or helps us cool down. But what if our body temperature drops? This will make the blood more viscous, which means it will flow more slowly. 

Wait a minute, that’s important.  

Think about winter time. The body’s core temperature is maintained by the blood moving throughout. But have you ever forgotten to wear gloves or a coat in cold weather? If your fingers or body grew cold, that means the temperature of your blood became cooler too. Colder blood is more viscous. More viscous fluids move more slowly. If blood moves slower it doesn’t move through the body as quickly; the blood is less able to do its job keeping us warm. As a scientist I might say: an increase in the viscosity of blood can severely decrease the blood flow to the fingers, thus my fingers are cold. Wow! 

After learning about the viscosity of liquids, I know why my coffee spilled everywhere! Since, it contains water, it follows the properties of water. I think adding honey will help thicken it up a bit. Or maybe I will move on to drinking smoothies that won’t spill as much as coffee.

What is your favorite liquid? How does it flow if there is a change in the environment? 

About the Author: Riddhi Sood is a graduate student at Indiana University, Bloomington studying molecular biology. She is a passionate lover of orchestras, a keen sci-fi book reader and a traveller at heart. On her last visit to Bordeaux, France, she visited a World War II submarine base converted to an art gallery. She was captivated by a digital water flowing exhibit and began wondering about viscosity.