Viscosity and surface tension are properties that are often intuitively linked to one another. Because these properties are of primary importance in cleaning it will be worth the while to understand them a bit better.
Surface tension is the property of a liquid that exists at the interface of the liquid with another media (usually air) to hold the surface intact much like the “skin” of a balloon. Surface tension is created as molecules or atoms at the surface of the liquid have a stronger attraction for one another than they do for the particles or atoms of the adjoining media. The higher the surface tension, the more stable the barrier between the two that prevents their mixing. As you have probably heard, a blob of liquid in an environment with no gravity will form a perfectly round ball as it seeks the lowest energy state. “Lowest energy” meaning the least stretching of the “skin” formed by surface tension. The surface tension of a liquid nearly always decreases as the temperature of the liquid is increased. Soaps and other chemical additives also reduce surface tension. In fact, surface tension is a good measure of the effectiveness of many cleaning agents. One means of measuring surface tension is by determining the contact angle where a drop of a liquid meets a surface. If the activity of the surface is known, the contact angle measures surface tension. If the surface tension of the liquid is known, the contact angle indicates the activity of the surface.
Viscosity is a measure of a liquid’s resistance to motion. Viscosity is weird, taking on one of five general characteristics. Some materials increase in viscosity as motion is increased while others decrease in viscosity as motion is increased. These are known as non-Newtonian liquids and include things like quicksand and a mixture of corn starch and water. It is sufficient to know for our purpose here that viscosity is the resistance to motion of a liquid. The viscosity of tooth paste is greater than the viscosity of motor oil while the viscosity of motor oil is greater that that of water. In most cases except for a few “engineered” fluids like particular kinds of motor oil, viscosity decreases as temperature is increased – – sort of the “molasses in January” thing.
So why the confusion? In general, we think of very viscous liquids as having high surface tension and intuitively link the two. In fact, a very viscous liquid like wood glue has very low surface tension to be able to penetrate and bond to the irregular wood surface. Liquid mercury, on the other hand has relatively low viscosity and extremely high surface tension. The table below shows the viscosity and surface tension of some familiar liquids.
The important “take away” here is that one can not assume that just because a liquid has low viscosity that it also has low surface tension or vice versa. The two properties are different and not related in the way we would intuitively think. Liquids with high surface tension and/or high viscosity can be difficult to remove from a surface. Liquids with low surface tension and low viscosity are good cleaners. Increasing temperature moves both properties in the direction in favor of successful cleaning.
Things to ponder – Is the viscosity of “silly putty” high or low? What about its surface tension? How about mayonnaise? (answers are available on the internet)
– FJF –