Viscosity is a measure of a fluid's resistance to flow (or deformation). While we normally think of a fluid as having a particular viscosity, some fluids' viscosity change with different kinds of stress. Such fluids are called non-Newtonian. Non-newtonian fluids may be quite viscous when at rest but much less viscous when under stress (such as when they are being shaken or rapidly stirred or vice-versa. The water and cornstarch mix sometimes called oobleck is a familiar and extremely non-newtonian example.
Shear-thinning and pseudo-plasticity. Fluids that become less viscous under shear-stress (stress applied parallell to the object) are called shear-thinning or pseudoplastic. Pseudoplasticity may be the key property provided by the polymers used in bubble juice though this has not yet been demonstrated.
Viscosity plays a role in determining the flow rate of bubble juice from the bubble wand or wick, but its role in bubble juice is generally misunderstood. Because commercial bubble juice tends to be noticeably viscous, there is a common misconception that bubble juice generally needs to be viscous ("thick") to work well. Many world-class bubble juice recipes are not very viscous at all.
MUST-READ!: Viscosity's role in bubble juice is widely misunderstood. We recommend reading FAQ: Thick Solutions
See the Wikipedia article about viscosity.
Random Related Facts[]
[EDITOR: This information is currently not organized but may be useful nonetheless.]
Viscosity and film thickness. People often have the sense that "thick" (meaning viscous) fluids create bubbles with thick skins (films). This is not necessarily the case. There is no specific relationship between a bubble juice's viscosity (its "thickness") and the thickness of the resulting soap film. Viscous solutions flow slowly, but that does not mean that the bubbles made from them are thick. Many of the polymers popular in bubble solutions have little or no impact on the soap film thickness in their usable ranges. If you compare, for instance, solutions based on PEO you will find that the soap film is unaffected by the amount of polymer even though the solution viscosity is heavily influenced. The confusion may be due to the fact that viscous fluids are (in English) often called thick. This usage of the word thick seems to lead people to assume that bubbles made from them have thick walls. Some polymers seem to influence the soap film thickness. The amount of guar gum does seem to influence the film thickness. The film thickness of guar-based solutions as measured by the bubble color seems to vary with the amount of guar gum to some degree.
Viscosity and evaporation. It is commonly assumed that "thick" (meaning viscous) solutions evaporate more slowly than "thin" solutions. There does not, however, seem to be a systematic relationship though some viscous solutions may evaporate more slowly. [EDITOR: we need a good reference here.]
Viscosity and temperatures. The viscosity of most solutions changes with temperature usually (but not always) becoming less viscous with higher temperature. The degree to which the viscosity changes is dependent on the ingredients. Some ingredients change more with temperature than others.
Viscosity and bubble-friendliness[]
People (including some chemists) have the impression that there is a specific relationship between viscosity and bubble-friendliness. This widely believed notion seems to be wrong as their are very friendly bubble-juice mixes that can create bubbles from small to whale-sized that are not very viscous at all.
If you take any particular detergent concentration, there is not a specific viscosity range which is best for making bubbles regardless of polymer. It is the case that for any particular polymer that there is a range of viscosities that is best for that particular polymer. What that range is however is particular to the polymer. Even with a polymer family, there may be a difference in the corresponding range among different versions (particularly different molecular weights) of that polymer.
It appears that to a large degree, changes in bubble-friendliness are correlated with the use of our polymers which happen to change viscosity rather than with the viscosity change itself. Changes in the level of the polymer may change the bubble-friendliness. It appears that whatever is causing the change in bubble-friendliness also affects the viscosity. But the change in viscosity itself is not cause of the change in bubble-friendliness (i.e. the ease with which a bubble can be closed). This is quite easy to demonstrate.
If bubble-friendliness were directly correlated with viscosity then the bubble-friendliness of two solutions should be similar if the viscosities are similar. However, this is not the case and is easily observable if you compare different polymers whose effective ranges have different viscosities. For example, PEO and medium-viscosity HEC achieve bubble-friendliness at very different viscosity ranges.
Here are a few examples of how this plays out in the real world.
If you were to take a PolyOx WSR301-based (PEO-based) bubble mix whose viscosity is in the ideal range for that polymer, its viscosity would be quite a bit lower than the viscosity of a solution based on HEC. PEO-based solutions are often only somewhat more viscous than water while some bubble solutions based on other polymers must be quite viscous to work well.
Similarly, it seems that when using two very different molecular weights of HEC that the higher the molecular weight of the HEC the less viscosity that is required to achieve a particular degree of bubble-friendliness. In other words, with high molecular weight HEC (Cellosize QP100HM, for example) you can achieve a very bubble friendly solution with less viscosity than when using a much lighter weight version.
Viscosity and Flow Rate[]
While viscosity may not determine how well bubble juice works, the flow characteristics (the rheology) have a big influence on what you might call the juice's feel. Juice feel seems to be a very personal matter. Juice might feel right to one person but not to another. Juice feeling right to the person making plays an important role in how well the juice works for that person. Since different people have different feel preferences, there is no bubble juice recipe or product that we know of that everyone agrees is best. (See: Best Bubble Juice.)
Flow rate (rheology) is likely to influence how well bubble juice works with certain materials. Though, this may be the case less frequently than many assume. It is a topic that certainly needs better study.
Secondary and Tertiary Polymers[]
Edward Spiegel has come to believe that the secondary and tertiary polymers are often perceived to improve a bubble juice because of the influence on the juice's flow rate and feel rather than due to any change in the more measurable characteristics such as bubble-friendliness, film thickness, and longevity. This would account for the varying opinions about whether certain multi-polymer mixes are better or worse than related single-polymer mixes. (NOTE: some polymer combinations are at worse neutral -- i.e. they have no negative consequence on measurable characteristics -- though some polymer combinations clearly work less well than related single-polymer mixes. See: Multi-Polymer Mixes)
Measuring Viscosity[]
It can be useful or interesting to measure the viscosity of solutions. There are a quite few methods that you can use at home. Here are a few that I have found useful
- Ball-bearing method. A ball-bearing can be dropped into a column of fluid and timed. If you have accurate measurements for the ball-bearing's weight and diameter, the distance traveled and the time required to travel that distance, you can calculate the viscosity. You can also simply compare the time traveled for different fluids to get a sense of their relative viscosities.
- Syringe Test. The time required for a fluid to drain from a syringe can be used to compare relative viscosities. More at Syringe Test.