# What determines the lifetime of a soap bubble?

I'm wondering why soap bubbles only last some seconds in average. There is almost no wind in the closed environment I create the bubbles in. Regardless of whether they adhere to a surface (e.g. the floor), they usually last between a second and roughly a minute. What makes them fail eventually? Why is the characteristic time before their failure of the order of the second?

I thought it might have to do with the thickness of bubbles. But transparent bubbles seem to last less time than colored bubbles. And colored bubbles are usually thinner, right? So why would a thinner bubble last longer than a thicker one?

Edit: In a comment, @freecharly mentioned that a very thin bubble would lack interference pattern a thicker bubble displays. So transparent bubbles may be in fact thinner than colored bubbles. That certainly explains why transparent bubbles can last less time than a colored bubble. We still have to figure out the main reason of why they fail, and why it takes a time of the order of the second.

Another edit: If evaporation is the main reason a bubble eventually fails, what does it say about the size of the bubble? In other words, are larger bubbles expected to last longer than smaller bubbles? Here are my thoughts : evaporation rate is proportional to the surface of the bubble, which is proportional to the square of the radius, namely $r^2$. However the quantity of soap is also proportional to $r^2$ or greater than that. This means I would guess that larger bubbles should fail at equal or longer times than smaller bubbles, in average.

Let's do some math. The quantity of soap occupies a volume of $\frac{4\pi }{3}(r_\text{outer}^3-r_\text{inner}^3)$ where we can assume that initially (before evaporation has a noticeable effect on the thikness of the bubble) $r_\text{outer}-r_\text{inner}=d$ regardless of the bubble size. It turns out that this volume is greater for larger bubbles. I think it goes like the square of the radius. This means larger bubbles should last more time than smaller bubbles, in average. If it indeed grows as the square of the radius, a bubble with the double size than another one should last 4 times as much, in average, before failing. That's something I didn't notice in practice, but maybe I should pay more attention.

Also, if evaporation is the main reason a bubble fails, I wonder why colored bubbles stay colored before they pop. They should turn transparent, indicating that the thickness is reducing, before poping.

• A source of alpha particles will make a hole in a foam of bubbles. There are also cosmic rays. – Pieter Apr 8 '18 at 19:35

the chief determinant of a bubble's lifetime is the rate at which the water in the bubble film evaporates. this is why bubbles last longer in humid conditions (slower evaporation). Note also that it is possible to add compounds called humectants to the bubble mix which slow down the water loss rate and make the bubbles last much longer. A good humectant is glycerin, which is effective and nontoxic. You can find DIY soap bubble solution recipes on line which will give you guidance on how much glycerin to add. Have fun!

• Does that mean that in an environment with 100% relative humidity, a bubble is expected to last several order of magnitude more than 1 s? In such a case, what is the culprit of the failure of the bubble and why does it take say $10^3 s$ or $10^4 s$ before failure? In other words, I ask the same question, once you get rid of the evaporation problem/culprit. – thermomagnetic condensed boson Apr 8 '18 at 20:01
• @no_choice99, even in the absence of evaporation there is always the chance that a random perturbation of the bubble film thickness will grow into a thin spot, which the internal pressure of the bubble will then burst. this process is very complex, which means for a good explanation you should look up "stability of thin bubble films". – niels nielsen Apr 8 '18 at 20:22
• I've edited my question to seek whether the size of the bubble would determine its average lifetime if evaporation was the main responsible for the poping out. – thermomagnetic condensed boson Apr 8 '18 at 21:17

I guess evaporation may be important.

• In my personal opinion, this answer (v1) is too brief to be useful. A useful edit would outline a way to turn an estimate of evaporation rate into an estimate of bubble lifetime. – rob Apr 8 '18 at 19:39
• Then why would a transparent bubble last less time than a colored one, in average? – thermomagnetic condensed boson Apr 8 '18 at 19:40
• @rob - "...outline a way to turn an estimate of evaporation rate into an estimate of bubble lifetime". This sounds like a major research project. I wonder if you have some simple way to do this in your mind. – freecharly Apr 8 '18 at 19:51
• @no_choice99 - The "transparent" bubble film is already so thin that you don't see the interference colors that are still seen in the thicker film bubbles. Thus colored bubbles have thicker films that take longer to evaporate. – freecharly Apr 8 '18 at 19:53
• I see @freecharly I didn't realize that a very thin bubble would lack interference pattern. Very good point. Now about evaporation, I will try (in the next days hopefully) to see how long the bubbles last in a very humid room (bathroom with hot shower water turned on). – thermomagnetic condensed boson Apr 8 '18 at 19:56