When some bubbles collapse, especially giant bubbles, there is often a bubble "ghost" left behind momentarily. Oftentimes, the ghost is a gauzy tissue-like residue that pulls into itself, seemingly in slow-motion.
Observers often find them as interesting as the bubbles themselves though performers sometimes feel that they make for a messy appearance.
The "ghost" phenomenon is the result of complex interactions between the bubble juice's chemistry and environmental factors. Some bubble juice is ghostless. The ghostless bubbles made with such juice may seem to spontaneously disappear into a cloud of fine vapor or into nothingness.
Whether a particular bubble juice will create ghosts is largely a function of the juice's surfactants. The nature of the ghosts is heavily influenced by environmental factors such as temperature and humidity. The same bubble juice which may have faint ghosting in warm dry weather may exhibit showy slow-mo ghosts in cold weather (probably due to the increased viscosity of the bubble juice in low temperatures).
The character of the ghosts (but not the overall tendency to ghost) is also influenced by the polymers and other ingredients present in the bubble juice.
What Are They?
The bubble ghost is usually simply a film of bubble juice which, for complex reasons, arranges itself into tissuey-film bits. When the ghosts hit the ground it will revert to its normal liquid form or appear as foam.
Non-ghost ghost-like apparitions. There are unusual situations in which something like a ghost remains if a bubble completely evaporates while in the air. This occasionally happens on hot dry days. When this happens, the bubble will fade to transparency and lose its gloss and become almost invisible. When this happens, a web of extremely fine threads may be visible and float to the ground when the bubble finishes evaporating. These are the solids from the bubble juice. This occurrence is quite rare as it requires the whole bubble film to thin and evaporate without bursting or being popped even in its fragile state. Edward notes that he rarely sees this more than once per summer.
What Causes Them?
The tendency towards ghosting is largely a function of the surfactants used. It appears that bubble juice based on detergents that use Sodium X Sulfate (such as Sodium Lauryl Sulfate and/or Sodium Laureth Sulfate) all have a tendency to ghost. A few home chemists brewing surfactant systems using pure surfactants have noted that the tendency to ghost seems to be a function of the surfactants themselves and not the co-polymers found in commercial dishwashing liquid. These ghosting surfactants are the base of a large percentage of the dishwashing liquids available in the USA and Europe.
The Procter and Gamble dishwashing liquids (such as Dawn, Fairy, Dreft, etc.) are all Sodium X Sulfate based and create bubbles that pop with ghosts -- even with no extra ingredients or polymers.
It is possible to reduce the ghosting tendency of these detergents (see below), but these efforts may also reduce positive attributes such as strength, longevity, and bubble-friendliness.
Influences. For those systems with a tendency to ghost, the amount of ghosting is heavily influenced by environmental conditions such as temperature and humidity. It can also be influenced by factors such as the pH and alkalinity of the bubble juice. The tendency to ghost seems to become amplified at cold temperatures. High alkalinity may decrease the tendency to ghost though it may also make the juice less friendly.
Variability. Even with the same juice in the session, one may notice some variations in ghosting depending on when the bubble breaks. And the same juice may appear to ghost differently under different environmental conditions.
Ghostless detergents. Some detergents such as Japan's Charmy or the version of Sunlight detergent found in Malayasia (but not the version found in the U.S.) work well for creating bubbles and are notable for their ghost-free popping. Charmy has been used to demonstrate that ghosting is a function of the surfactant system rather than the polymers and other additives (such as baking powder).
Using Charmy instead of Dawn in the Wiki's Guar-based recipe creates bubbles with ghost-free vapor-pops as you can see below. The pops are ghostless even when using much larger amounts of solids (detergent or guar gum or baking powder) than are used in heavy-ghosting Dawn-based mixes. Clearly, the ghosts are not simply residue of a solution's solids.
Charmy/Guar Ghostless Pop
The same recipe using Dawn Pro:
What Do They Mean?
If your surfactant is one that has a tendency to ghost (i.e. you are using a detergent that makes use of Sodium X Sulfate), heavy ghosting seems to indicate that the surfactant is at its most effective. With some juice, faint ghosting is a sign that the juice's pH is not in the optimal range for that detergent.
There are ways of disrupting the ghosts without hurting the juice, but some methods achieve ghostlessness at the expense of bubble quality or the bubble-friendliness of the juice.
Truly ghost-free vapor-popping like you see with Charmy or Uncle Bubble Ultra Concentrate or the Malaysian Sunlight dishwashing liquid can only be achieved if the detergent does not contain Sodium X Sulfate. However, even if you are using a detergent based on Sodium X Sulfate (such as all Procter & Gamble products), there are steps that you can take to disrupt ghosting. Some of these techniques reduce the quality of the bubble juice. But you can still achieve quite respectable results.
If ghosting really bothers you and you don't have access to Charmy, Japanese dishwashing liquid, you might want to stick with a commercial bubble juice such as Uncle Bubble Ultra Concentrate which is, as of this writing in December 2013, highly-regarded by many professional bubble artists.
The main techniques for minimizing ghosting when using a Sodium X Sulfate-based detergent are:
- The addition of a surfactant that is non-ghosting. You can add some bubble juice such as Pustefix or Mr. Bubbles or a ghost-disrupting detergent such as Sun Sunsational Scents (available in the U.S.) to the mix. Ghost-busting bubble juice as an additive tends not to hurt the quality of the bubble juice while some ghost-busting detergents may compromise the juice somewhat. Sun Sunsational Scents is inexpensive and quite effective at reducing ghosting but it does result in a somewhat less effective bubble juice.
- Increasing the alkalinity of the bubble juice. Boosting alkalinity but not making the juice dangerously basic reduces (but does not eliminate) ghosting. You can increase the alkalinity of juice by adding enough baking soda (not baking powder). Baking soda won't raise the pH higher than about 8.2 even when increasing the pH significantly. The drawback is that this may also make the bubble juice itself a bit less friendly.
Dawn/Sun Sunsational Scents Note. An equal mix of Dawn Pro (or equivalent detergent) and Sun Sunsational Scents makes a quite friendly surfactant base for bubble juice though it won't be as friendly as straight Dawn Pro. Because Sun is much weaker than Dawn, you should use about 1.25 times as much Dawn/Sun as Dawn Pro when substituting. Also, it is critical that you use baking power or baking soda+citric acid or other pH adjustment to make the solution friendly. The bubble juice will not be quite as effective as a mix based solely on Dawn but the vapor pops are quite nice. See the related blog entry.
Search for 'ghost' in the wiki's index or on SBF, the Soap Bubble Fanciers Yahoo Group (RIP) to read discussions about ghosts and defeating them. Some commercial bubble juices (such as Mr. Bubbles and Pustefix possibly) have surfactants that when added to Dawn (or other Procter & Gamble detergents) based recipes have a tendency to minimize the ghosting without harming the juice. But this requires a fair amount of added bubble juice.
Lack of ghosting when it is expected
Edward has noted that when Dawn-based recipes do not ghost (and have not had a ghost-disrupting surfactant added) that it indicates that the solution is probably not performing optimally and may need some help (with baking powder or baking soda/citric acid) probably because the pH/alkalinity has risen to a level that is suboptimal for the Procter&Gamble detergents. More study is needed to determine how generally this is true.
Here are some quick thoughts while we work on the article:
- Temperature influences ghosting significantly. Even ghostless solutions may ghost when it is cold.
- With Sodium X Sulfate-based surfactants, the character of the ghosting may be influenced by the polymer and amount (probably related to the flow-rate/viscosity)
Dawn/Guar Gauzy Ghost
Dawn-based bubble juice has a strong tendency to ghost. When temperatures are cool (below 60F), the results can be fairly spectacular gauzy ghosts that seem to move in ultra slow-mo. The stills below are from a guar-based bubble juice at 47F.
Charmy/Guar "Ghostless" Pop
Charmy, a Japanese dishwashing liquid favored by those that can get their hands on it, is famous for its ghostless "vapor pops". The bubbles seem to disappear into thin air.