SupermaterialsPlenary lecture by Prof. Pugno at "FALLING WALLS CONFERENCE. FUTURE BREAKTHROUGHS IN SCIENCE AND SOCIETY" (9 November 2012, Berlin, Germany).
Good morning, ladies and gentlemen. It is a great privilege for me to be here and I am very glad to be speaking in this occasion. Today, I would like to talk about my most recent research activity on the mechanics of super materials. I prepared four examples, and in particular, I would like to talk about the strongest material: graphene, the toughest: the spider silk, the most adhesive: the gecko foot, and the most anti-adhesive: the lotus leaf.
Let us start with the first material, graphene. Graphene is a single layer of carbon atoms arranged in hexagonal shape. The peculiarity here is that it is the strongest material that we have. You can imagine that the strength is of the order of 100 GPa— this is an enormous number. You have to compare this with what we call high- strength steel, which has a strength of 1 GPa. So, graphene is very, very strong indeed. This is the ideal value, considering a defect-free material. But what happens in reality?
If you roll a graphene layer into a cylindrical shape, you obtain what we call a nanotube. If you perform a tensile test of a single nanotube, you can measure a strength—at least in the pioneering paper by Ruoff—of the order of 60 GPa. This is again a huge number, but, of course, it is not the 100 GPa that I mentioned before as the ideal material strength. So, why does this happen?
We already saw—the first speaker mentioned something about defects—that defects play a fundamental role. It is sufficient to say that just a single vacancy can reduce the strength of a graphene layer by 20%. This of course has a tremendous impact in real applications. One application that I would like to show you is the space elevator. At the beginning, the role of defects here was completely neglected, and this—of course— is a mistake. There was an analogy (and I will use this analogy again): if you plan a marriage and assume that your wife or your husband will have no defects, of course, you can have a problem. In materials science, you can have problems like this if you neglect the role of defects.
So, you have to take into account defects, and you have to make a flaw-tolerant design. This is what we did in the past.
The concept of the space elevator is very simple. You can imagine a cable attached to the earth’s surface. If the cable is long enough, the centrifugal force will exceed the gravity of the cable; and it will work under tension. Then you can move climbers and mass at low cost. A problem here is that for the cable you need a material with a huge strength - density ratio, and in particular, even considering the low density of carbon, you need a material with a strength of the order of 60 GPa; That is something that is perhaps feasible with graphene and nanotubes. I would say, “Never say never”. But, you have to take into account the role of defects—for another reason, too, because larger is weaker. Simply for statistical reasons: the larger the structure, the larger the probability to find critical defects that lower the strength. So, you have to take this into account.
With graphene we can also build other systems. For example, consider a graphene nanoscroll. If you twist the nanoscroll in a circular shape, you have the self-rolling of the system (as you can see on the slide). There is a competition between bending energy and surface energy. Basically at the end, you reach equilibrium, in particular an equilibrium of core size. And you can create a lot of configurations, if you manage to tune the surface energy of the system, which you can do with an electrical field—you can create nanochannels, smart porous membranes, nano-oscillators, and even nanomotors, if you deposit the nanoscroll onto a substrate and control the system. There are a lot of applications. You can open the nanoscroll, for example, setting it into resonance, and you can imagine applications even for drug delivery in nanomedicine.
So, now let us talk about the second so-called super material. This is spider silk. The peculiarity here is its toughness. Toughness is the ability of a material to dissipate energy before fracture. The value is quite incredible: 500 J/g. That means it is even tougher than Kevlar, for example. Spider silk is amazing. Spiders are amazing, because they can produce up to 7 different types of silk for different purposes; for example, very strong, very tough, very stiff, sticky at the same time. The final result is that spider webs are very robust, multi-functional in nature, etc. You have to imagine that if you scale up a spider web to the size of this hall, we can basically have a web that can stop a Boeing 747, because the toughness is tremendous.
So, we are trying to carry out some analytical calculations and some simple experiments. For example, this is a tensile test of a bundle of spider silk. There is a huge elongation. We are trying to understand what the role is of the materials in the final mechanical behaviour, thanks to atomistic simulations, for example, in collaboration with MIT. But we are also especially trying to understand the synergy between material and structure. This is the novelty in this field. For example, we are investigating the anchorages of the spider web. Imagine a spider web; you see the anchorage there. At the given point, you basically have the detachment of the system. If the spider web is loaded by say a distributed load, wind for example; at a given point you have failure. But, what we are observing now is that there is an optimal geometry of the anchorage that is a function of the constitutive law of the material. So there is an optimisation, a synergy between material and structure that renders the anchorage as strong as possible. Of course, this has a lot of different applications, in particular in engineering, because anchorages are, of course, of great importance.
Next I would like to discuss another aspect. If you imagine having a localised load rather than a distributed load, for example an impact of an insect in a spider web, if the kinetic energy is sufficient, you destroy the web. But we observe a counterintuitive result when considering what we call a ductile— a tough material. In reality, a spider web—a hypothetical spider web—composed by a linear elastic material, will not be robust. On the other hand, if we consider empirical spider silk, "real" spider silk with its own constitutive law , the robustness of the spider web is maximised. This was quite a counterintuitive result, of interest per se, but also of interest from an engineering point of view. You have to imagine that if we were able to design and realize a spider web-inspired building, in the case of an impact of an airplane, a hole would be created, and the collapse of the entire structure would be avoided.
Now let us talk about the third super material. It is gecko. By the way, this is Ernesto; he is the mascot of our lab. He has a huge ability, his adhesion is extremely strong. It is based on van der Waals and capillary forces. It is not based on suction cups. To give you an idea, the adhesion strength of a gecko is of the order of 1 MPa. That means that it is ten times larger than an ideal suction cup. So, it is very strong. It is fully dry and it is smart, because a gecko can detach in a very easy way.
The result is that if you try to detach a gecko from a surface, you need a force that is of the order of ten times its body weight. So, ten perhaps is the best number that we have in order to have a strong attachment, but at the same time easy detachment. Let me say here, that we observe something in geckos that we also observe in human beings, in the sense that females are smarter. We were unable to make this test with a female, and only the male cooperated. Perhaps this is another good reason to promote science among women.
Let me stress also another aspect. If you look at the gecko foot, you observe a hierarchical, let’s say, to simplify, tree-like geometry of the setae, terminating in two- dimensional contacts that are called spatulae. Considering different animals, you observe the same solution. But for geckos, the spatulae are the smallest. So, why does this happen? It’s very simple. You can write equations and make experiments. If you try to detach a tape from a substrate, you can verify that the force to detach the tape is proportional to the width but not to the thickness. This implies that if you divide one tape into one hundred sub-tapes, the adhesion force is increased by a factor ten. This is the reason why we measure billions of contact in gecko. Or, in other words, the gecko is the animal with the longest total peeling line, because the final force is proportional to the total peeling line. By the way, if you look at the numbers, you can observe that when a gecko is walking there is a crack propagation with a characteristic size of the order of those that we observe in earthquakes. It is a huge number. This is also quite amazing.
So, why can’t we imagine a scaling up of this material, and why can’t we design a Spiderman suit? It is not impossible. You have to take into account that considering ideal “Van der Waals-gloves”, you can support 500 men. Of course, you have to take into account defects, roughness of the surface, dust particles, but still you have a factor of 100 to play with.
Now I would like to show something very rapidly, because my time is flying: this is a simple material based on the principle of gecko adhesion. Of course, you need a kind of pre-compression in order to maximise the contact surface, but then you see the force, the adhesion, at least the shear strength, is quite high. Of course, we have to improve this material, e.g. it must be self-cleaning, etc. On the topic of self- cleaning, I would like to introduce now the last material: it is the lotus leaf. Lotus is extremely water-repellent. The contact angle is of the order of 150 degrees. That means that if you create small liquid droplets, they would be nearly spherical. You then basically have the droplet rolling on the surface. The key for this behaviour is again hierarchical topology of the surface of the leaf. We know 200 plants that are superhydrophobic.
They are again superhydrophobic thanks to these concepts; the picture in this slide is very simple: the so-called “lotus effect”. If you have a surface that is flat and a liquid droplet that is not spherical, you have sliding rather than rolling, and you are unable to remove the dust particle from the surface. But on the other hand, if you have a proper topology on the surfacethe liquid droplet is more spherical, giving rolling rather than sliding, and you can remove the dust particle from the surface.
So, the concept with this is very simple: we are trying to replicate it. Here, for example, you can see we made a physical copy of a lotus leaf. You see in the first column we have a different magnification of a lotus leaf. Then we make a copy with silicone. You see that there is a negative of the lotus leaf. Then we make the copy of the copy with a material of interest that in this case was polystyrene. At the end, we obtain a lotus- inspired surface; the initial contact angle was lower than 90 degrees; so the surface was hydrophilic, but at the end it became superhydrophobic—thanks to the mere topology modification of the surface.
I have finished, and these are my conclusions. I really hope to see the advent of a new era of super materials. I think the final goal is to improve the quality of our life. I have to thank the European Research Council for support, the Falling Walls Conference for the invitation, and, of course, last but not least, you for your attention. Thank you very much.
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