Viscosity — Definition & Overview

(Video) What Is Viscosity?

by Amanda Polley

In this video, Amanda teaches you about viscosity of liquids. You'll get to see this in action as she tests out how viscous milk and honey are. She also shows how heat affects a liquid's ability to flow. Finally, Amanda talks about kinematic viscosity, which is a fluid's resistance to flow under gravity.

This morning, I made a delightful pancake breakfast. My pancakes were so fluffy. I always drench them in maple syrup. I love maple syrup! I also love coffee. So, of course, I had my daily pour-over. But that made me wonder, "why is syrup so thick compared to coffee?" Chemistry teaches us that the answer is viscosity.

Viscosity is a liquid's resistance to flow. The units for viscosity can be a little hard to understand. They are $\frac{N\cdot s}{m^{2}}$ or $Pa\cdot s$. That's newton-seconds over square meters or Pascal-seconds. Pascals is the SI unit for pressure.

How do we measure viscosity? In a laboratory, we use a viscometer. There are two main types of viscometers. A rotational viscometer spins a small spindle in the fluid. It measures the torque required to spin the spindle at a constant speed. Each spindle comes with a conversion for converting torque to viscosity. A capillary viscometer is a U-shaped glass that has two bulbs. Between the bulbs is a very thin capillary. We know the volume of the upper bulb. We time how long it takes the liquid to travel through the capillary into the lower bulb. Each tube comes with a conversion factor for converting to viscosity.

What makes something Viscous?

A liquid with a higher viscosity has more resistance to flow. So, my pancake syrup has a higher viscidity than my coffee. In colloquial terms, we say that syrup is thicker than coffee. But, what causes the differences? Like all physical properties of liquids, the answer is intermolecular forces.

Recall that there are four different Van der Waal interactions.

• London dispersion forces happen because of the random movement of electrons. Sometimes more electrons end up on one side of an atom or molecule. When this happens, the electrons generate a temporary dipole.

• Dipole-dipole forces occur between polar covalent molecules.

• Hydrogen bonding is a unique case of dipole-dipole interactions. Hydrogen bonding most often results from a hydrogen-oxygen bond. Oxygen is exceptionally electronegative. So, the electronegativity difference between the hydrogen-oxygen bond creates a strong dipole.

• Ion-dipole interactions occur in solutions. As the solution concentration increases, so does the viscosity. Why? The solute and solvent are attracted to each other. The attraction lowers the energy and increases stability. So, they don't want to move as the fluid flows. They resist the flow.

So, let's return to my syrup and coffee. A cup of filter coffee is about 98% water. We know water hydrogen bonds four times. Twice from each hydrogen-oxygen bond. Twice for each oxygen lone pair. The main sugars in syrup are glucose and fructose. They are allotropes. Both have five hydrogen-oxygen bonds. They also have one oxygen that can contribute its lone pairs. Both coffee and syrup have significant amounts of hydrogen bonding. But, syrup has more. So, it is more viscous.

Viscocity and Temperature

Getting to the end of the syrup is so frustrating! You can't get the last little bit out of the bottle. Even though the bottle says not to microwave, I do. After microwaving for ten to fifteen seconds, the syrup flows much more easily. What we're seeing is that viscosity is temperature-dependent. But, why?

We learned that viscosity depends on the strength of the intermolecular forces. When we heat liquids, we increase their molecule's kinetic energy. More movement produces weaker intermolecular interactions. So, the higher a liquid's temperature, the lower its resistance to flow. To standardize viscosity, we often use standard temperature and pressure.

Newtonian vs. Non-Newtonian Fluids

You might hear someone call a liquid Newtonian or non-Newtonian. A Newtonian liquid has a constant viscosity. In non-Newtonian substances, the viscosity changes when we apply a force. For example, when you mix cornstarch and water, it forms a liquid mixture. But, when you hit it, it acts like a solid. Something that's fun to do is to place plastic wrap over a cone speaker. Pour a mixture of cornstarch and water into the cone. Now drop the bass. The bass vibration will cause the mixture to act as a solid. Slowly an amorphous blob rises and bounces around the speaker. There are a wide variety of non-Newtonian fluids. Some thicken, others thin, and many will act like solids.

Two Types of Viscosity

So far, we have used the word viscosity rather generally. But, there are two types of viscosity. We have been talking about dynamic viscosity. Dynamic viscosity ($\eta$) is a liquid's resistance to flow because of electrostatic interactions.

The dynamic viscosity comes from cohesion and adhesion. Imagine water moving through a pipe. The adhesive force is the interaction between the pipe and the water. Adhesion resists flow. The water molecules just inside the surface experience cohesion. They are hydrogen bonding to the molecules on the surface. But, the surface molecules are resisting the flow. So, the inner molecules also experience some resistance. The effect continues for each layer of water. But, each layer encounters less resistance. The viscosity is the total of all the resistances.

We use dynamic viscosity with non-Newtonian fluids. Their viscosities change depending on the applied force. So, we need to consider total internal resistance under different forces.

The other type of viscosity is kinematic viscosity ($\nu$). It is a fluid's resistance to flow under gravity. To derive kinematic viscosity, we divide dynamic viscosity by density ($\nu=\frac{\eta}{\rho}$). For moving many liquids, we only use gravity. There are no extra forces. So, it is easiest to use kinematic viscosity. We also use kinematic viscosity for Newtonian fluids. Remember, their viscosity is not force-dependent.