Ballonts IX: Blue Sky Thinking (Part 2)

By Abbydon

The previous article discussed the possibility of using soap bubbles to create small lighter than air organisms. A second possibility is more sophisticated and relies on the wonder material, graphene. It was discovered in 2004 by two scientists at the University of Manchester who were subsequently awarded a Nobel prize for their work.

Graphene is made of a flat sheet of carbon atoms, so it is a little bit like 2D diamond. It has many interesting properties but is most well-known for its high strength (100 times stronger than steel) and low density (less than 0.001 grams per square metre). An amusing illustration of this was provided in the Nobel prize paper. Imagine a one square metre graphene hammock tied between two trees. This could hold a 4 kg cat before breaking yet would only weigh as much as one of the cat’s whiskers.

As impressive as this is, recent work suggests that graphene has another property that is extremely relevant to ballonts. Graphene is also impermeable to gases. With such an amazingly low density it effectively produces a matte-black massless membrane even when a few thousand layers are used.

It would however be extremely challenging for life to produce a graphene balloon with absolutely no defects. A more robust approach is to copy the bubble foam concept and use a mass of small graphene “bubbles” instead. Conveniently, a material extremely similar to this called aerographene has been invented by scientists. It consists of a complex 3D network of graphene and carbon nanotubes where most of the volume is air. For this reason, when measured in a vacuum it has a density slightly lower than helium at 0.16 kg/m3.

Click to enlarge; copyright unknown; source here

While aerographene itself is not airtight it is not inconceivable that a material like it could be produced that contains many small airtight compartments in a similar way to a bubble foam. If these compartments contained a lifting gas, then the entire structure would float. When I mention aerographene below I am actually referring to this possibility and not the real material. The chart below shows that both a 1000-layer graphene membrane and a “solid” aerographene balloon would easily enable small (or large) ballonts.

Click to enlarge; copyright Abbydon

This strongly suggests that an “magic” aerographene like material could be used to enable small ballonts to exist. Since they cannot have a membrane then any aerographene produced would be an external structure grown from the bottom up, like hair, and could not be repaired only replaced. Additional graphene would be produced on the lower surface where the organism was hanging while the top surface would slowly degrade.

It is unfortunately unknown whether life could produce graphene in the first place as it does not appear to be produced naturally by life on Earth. Industrial processes for producing graphene based products typically require temperatures beyond the reach of even the most extreme of extremophiles. However, Shewanella oneidensis bacteria can be used to produce graphene from graphene oxide which is at least a start.

I am not a chemist and this is not the blog to delve deeply into the exact chemical process that life could use to produce graphene at ambient temperatures. There is some justification that it would not be implausible though. Inspired by photosynthesis the direct conversion of carbon dioxide into graphene at high temperatures has been demonstrated. A slightly different approach has even been shown to work at room temperature.

As an example of what is possible, an approximately spherical lump of dark grey aerographene with a 9 cm radius can lift about 3 g. This could support something like a praying mantis that is about 7 cm long though could perhaps be longer and thinner. A cup-like abdomen could contain the graphene and hydrogen producing organs while the four long rear legs partially surround the sphere to maintain a grip. Four independent wings could provide mobility. The long forearms would then provide good reach to gather food. Due to the benefits of aerographene this hanging mantis would be able to float even when small and could grow to a size only limited by other factors. 

Click to enlarge; copyright Gert van Dijk. Note that the graphene 'floating body' is narrower at the tope than the bottom, because the animal grew in-between forming the early top and the more recent bottom graphene.  

The soap bubble and aerographene concepts don’t mean that the small ballont problem is solved as they are really just meant to inspire others to come up with their own ballont concepts. In the absence of any lighter-than-air organisms for comparison on Earth there are still many unanswered questions to be considered and that is all part of the fun of speculative evolution.

For example, why would an organism evolve to be lighter-than-air? Is it always lighter-than-air or is it only temporary? How does it control its movement when floating? How does it protect itself from predators? How does it feed? Can it repair or replace its balloon if punctured? How is the lifting gas generated? How fast does the lifting gas leak from the balloon?

If we were only interested in creature design then this would all be sufficient to inspire a range of organisms based on shimmering soap bubbles or black graphene. Speculative evolution requires more than that though. Demonstrating that physics supports the idea and that there is a plausible way for the organism to implement the idea are both important. To be thorough, it is also important to consider whether the proposed organism could have evolved through a series of plausible steps, rather than just spring into being fully formed.

There are various ways that life on Earth already generates hydrogen, such as through fermentation, so that part is not unusual. The formation of chemical laced water to form longer lived bubbles is also fairly common as fish, frogs, snails and insects are already known to do this. A soap bubble based ballont as shown in the previous article therefore seems reasonably plausible.

On the other hand, the formation of graphene is more challenging to justify in a series of steps and I cannot give a solution to this. Graphene does have the ability to absorb light efficiently at all wavelengths, which is why it is black after all. A plausible evolutionary path could involve algae or plants in low light environments developing graphene for photosynthesis related reasons. Since hydrogen can be produced by algae as part of the nitrogen fixing process perhaps forests of lighter-than-air pitch black plants could feasibly evolve. That is however an idea for another time but perhaps it will eventually appear on my recently created blog. It describes the tidally locked world Khthonia, which orbits twin red dwarfs.

Finally, I am very grateful for the opportunity to share my thoughts on this matter in this blog and I hope that they inspire people to develop their own ideas in this area. Please comment to let everyone know what you think about graphene and its possibilities.

Ballonts VIII: Blue Sky Thinking (Part 1)

By Abbydon

Over the past few years, I have discovered an interest in astrobiology as it combines my childhood fascination with both astrophysics and biology. Linked to this I also enjoy speculative biology though sadly my artistic talents are embarrassingly poor (For reference, I did not produce any of the pictures shown below). For this reason, I have followed the various articles on lighter-than-air organisms (also known as ballonts) with interest.

Like many other people, I have found it disheartening that designing a plausible small ballont has proven challenging. Large ballonts appear to be viable as they can generate sufficient lift to allow both the balloon and a body to float but smaller ballonts cannot get off the ground.

This is unfortunate as it means infant “large” ballonts would be unable to float until they had grown sufficiently large. Small ballonts are also interesting as they can form only part of an organism’s lifecycle which may allow the body to have reduced functionality. This could be a mechanism for spore dispersal from an otherwise sessile organism as a more sophisticated version of a dandelion clock. Alternatively, it could be for reproduction only like an adult mayfly which can have a life measured in minutes.

As previously discussed on this blog, the problem for small ballonts is the membrane needed to contain the lighter than air lifting gas. A small volume of lifting gas without a membrane would of course float but it wouldn’t take anything with it. Therefore, clearly a membrane of some form is needed to separate the lifting gas from the rest of the atmosphere. Unfortunately, the mass of the membrane is proportional to the surface area of the ballont whereas lift is proportional to the volume. Since all membranes have mass this means that for small ballonts the mass of membrane is typically greater than the lift produced by the lifting gas.

Solving this problem requires reconsidering what could be used as a membrane in an attempt to get closer to the “magic massless membrane” than Mylar. Mylar is used for long lived helium party balloons and has a thickness of 0.1 mm with a density 1.2 times water, which is perfectly adequate for large ballonts but insufficient for small ones.

Two possible ideas sprang to mind which might achieve this, the first of which will be discussed in this article, with the second reserved for a following article. The first possibility to be considered is whether soap bubbles could be used. There are many videos online showing lighter-than-air hydrogen filled bubbles being produced, though they do normally come to fiery end which is not what we want to happen to our poor ballonts.

Soap bubbles may not seem the ideal form for a ballont but a bubble film is typically around a thousandth of a millimetre thick and has a density of approximately water. This is much closer to a massless membrane than Mylar so perhaps it will enable smaller ballonts. A ballont would not literally use soap to produce bubbles but would instead use some alternative organic chemical.

In previous articles a hypothetical Mylar based ballont was shown to float only once its radius was above 30 cm but the chart below shows that a hydrogen filled soap bubble could provide lift at much smaller sizes. For reference,

LIFT = AIR MASS - BUBBLE FILM MASS - HYDROGEN MASS.

Click to enlarge; copyright Abbydon

However, while the Guinness world record for bubble size is just under 100 cubic metres we don’t expect plausible ballonts to use single bubbles that large. The next chart shows smaller hydrogen filled bubbles can still lift a few milligrams if they have a radius larger than 3 mm.

Click to enlarge; copyright Abbydon

Scientists are known to do strange things in the name of Science but perhaps one of the weirdest papers I have read involves weighing 90 leaf cutter ants to determine their load carrying performance. Apparently they ranged from 1.2 to 36.8 mg with an average of 9.3 mg. The extreme low end of this is light enough that it could be lifted by a single bubble with about a 7 mm radius. Perhaps an alien leafcutter ant could generate a bubble to get to the top of a tree, harvest a leaf and use the ballast to descend once again. Such an “antballont” might look like a honeypot ant with a bubble for an abdomen as shown in the sketch below.

Click to enlarge; copyright Gert van Dijk. The squares are 1 mm in size. The panel on the left shows a circle with a 7 mm radius and a small leafcutter ant. the panel on the right shows an evolved ant holding a bubble between its hind legs.  

A single bubble could be viable for tiny ballonts such as the ant but it would be too fragile for large ones. A better solution for larger ballonts would be to produce a foam of many small bubbles. Each of these bubbles would produce lift on its own and a foam mass would produce more. This approach has been used to generate floating helium filled foam letters for advertising purposes!

The chart below shows that such a foam mass produces less lift than a single bubble of the same volume but a 5 cm radius spherical foam mass of individual 0.5 cm radius bubbles could still carry just over a quarter of a gram. This again sounds light but it could certainly carry a bundle of seeds or an insect several times larger than an ant. It is probably even enough to carry a vertebrate such as the 7.7 mm long frog, Paedophryne amanuensis with its legs spread out to stay in contact with the bubbles.

Click to enlarge; copyright Abbydon

This all suggests that the soap bubble idea is valid for producing a somewhat plausible small ballont without too much hand waving. This is possible because of the thin water-based membrane but there is one important disadvantage, a short lifespan. The bubble will pop eventually unlike a solid membrane. This can be managed if the ballont does not need to remain aloft indefinitely, perhaps because it is only part of a lifecycle (e.g. a seed or a mayfly) or perhaps because it only creates the foam to float at night. Alternatively, the organism could regenerate the bubbles constantly while in flight to maintain and even adjust lift.

How long can a bubble last though? About a hundred years ago, in a sealed container it is claimed that Scottish scientist James Dewar managed to get a 19 cm diameter bubble to last for over three years and a 32 cm diameter bubble to last for 108 days. This is not likely under realistic conditions but it shows what is possible.

Bubbles burst when the membrane becomes too thin as the water in the membrane flows to the bottom of the bubble or it simply evaporates. While low temperature and high humidity conditions may enable bubbles based ballonts to last longer, a more feasible approach is to add chemicals to the bubble mixture to make the membrane more resilient. For fun at home the Soap Bubble Wiki has several recipes for this.

It remains an open question as to how long a bubble foam could last but several minutes is possible with these home-made bubble mixes. Some species of frogs and fish make bubble nests that last for days, however, these require maintenance and might be too heavy to float. It is therefore conceivable that a biologically possible option in between these two extremes could produce a bubble foam that would be light enough to float but had a longer duration than normal. This would be ideal for seed, spore or larva distribution where a 5 cm radius foam containing a few hundred bubbles could be generated by the parent before being sent on its way to pastures new. This is shown below with the small Furahan “brochos” larvae suspended in a floating foam to enable longer range travel than would otherwise be possible.

Click to enlarge; copyright Gert van Dijk. This is a sketch for a painting that has already been finished and will appear in The Book.

Click to enlarge; copyright Gert van Dijk. The Book will contain many secondary illustrations; this one will accompany the one above.

This article will be concluded with a discussion of the alternative option to soap bubbles for small ballonts. Please let us know what you think of soap bubble ballonts by commenting below.

Since people seem interested in my thoughts on these topics I thought I should perhaps produce my own blog. Since I am a physicist and no artist this will be quite different to the Planet Furaha blog. I have therefore started Exocosm which will discuss the possibilities of planets around other stars (i.e. exoplanets). Time will time whether I can manage to continually produce worthwhile articles though…

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This was the first ever guest post on this blog, and I hope you liked it. Abbydon and I decided to split it into two parts; the second one has already been written, but I need to produce an additional illustration before it can be posted. 

If anyone else is interested in writing a guest post, write to me about the idea and do not send a complete text yet!

Sigmund Nastrazzurro 
(nastrazzurro  AT gmail DOT com)