September 26, 2012 | 1
Today I have a piece in Slate’s Food section on what physics has to say about the proper way to crack an egg. Yeah, you heard me: there’s physics in the Food section, which is AS IT SHOULD BE, because physics is everywhere, y’all, and the folks at Slate totally get that. Naturally, there was a lot of stuff that didn’t make it into the final story, but that’s what the blog is for, right? And in addition to the usual outtakes, I thought it might be fun and instructive to give readers a peek into my writing process this time around.
1. The Idea: what piqued my interest? There’s always a bit of serendipity involved. I get regular tip sheets from the American Physical Society alerting me to nifty upcoming papers, but it’s pretty subjective when it comes to which papers appeal to which writers. I’m especially fond of quirky topics with real-world or pop culture tie-ins, and if it’s materials physics, so much the better, because I am very much a Material Girl (TM). In this case, I saw that there was a paper on the geometry of eggshells and how this relates to their rigidity (a different mechanical property from strength, as the Slate piece makes clear).
It’s the latest work from the lab of MIT mechanical engineer Pedro Reis, whose denizens have looked at such colorful topics as how cats lap up water, scratching and fracturing in everything from paraffin wax to steel, and (my favorite) the physics of curly hair. So it’s not surprising that Reis and his co-author, Arnaud Lazarus ended up looking into eggshells — or, more accurately, thin shells of an ovoid shape, building on work this spring on something called a “buckliball.”
Essentially, the MIT model lets scientists predict the thickness of the shell by applying a force to said shell and measuring how much (or how little) it deforms in response. It can also predict the internal pressure provided one knows the thickness and material properties of the shell.
2. The Pitch: Let’s face it: physics is a tough sell at the best of times. How to tailor this rather mathematically intensive piece for a non-physics-y audience? Serendipity! The Time Lord and I had just participated in a mixology class at the Sunset Strip Andaz Hotel, formerly known as the “Riot Hyatt” because of its colorful rock ‘n roll legacy. We had been instructed on how to mix whiskey sours. The recipe calls for egg whites; and there were a few pratfalls as various participants tried to crack their eggs and separate the white from the yolk. The whole experience reminded me of Audrey Hepburn in the original Sabrina, as she struggles to learn to properly crack an egg in her Parisian cooking class.
* something tangible in the every day sphere in which to frame the abstract physics;
* a fun pop culture tie-in with a classic film clip; and
* a news hook, in the form of a new paper about to be published in Physical Review Letters — two papers, actually, since another group of scientists independently arrived at similar conclusions from a complementary angle.
3. The Legwork. Those things, taken together, gave me enough to pitch the story to Slate successfully. And so began the task of researching the story and doing the interviews. It’s my favorite part of the process, because it’s so unpredictable. You know you’ve found a quirky topic when this phase requires you to look up papers from the 1970s in British Poultry Science and the Journal of Textile Studies.
It turns out that eggshells are pretty fascinating objects from a scientific standpoint: think bioceramics. For instance, it’s common to compress chicken eggs between two parallel steel plates to measure the stiffness and strength of the shell. Concluding that this method “is tedious and the required equipment expensive,” one Belgian study opted to create a computer simulation based on a mass-spring model to study the same kind of properties in eggshells, albeit a simplified single-layered version. (Eggshells are rather complicated structures.) Other groups have used acoustical methods to study the mechanical properties of eggshells.
I found a nifty 17th century recipe by one Sir Kenelm Digby, detailing a Chinese method for preparing tea: “Take two yolks of new laid eggs, and beat them very well with as much fine sugar….” The idea was to pour the tea on top of this mixture as a kind of hot toddy for digestive ailments. (Many British folk from Digby’s era were obese, and suffered from gout and kidney stones. That might be because of cake recipes that called for 30 eggs, 15 whites, mixed with tons of cream and butter.) But Digby was also fascinated by the various developmental stages of fertilized eggs: “You may lay severall egges to hatch; and by breaking them at severall ages you may distinctly observe every hourely mutation in them, if you please.”
I also learned about Corentin Kervran, a French scientist who espoused “biological transmutation” — a largely discredited idea, for the record — who first became interested in the topic because he was fascinated by the enigma of eggshell formation. Had he not died in 1983, Kervran could have been gratified by the 2010 work by scientists at the Universities of Warwick and Sheffield on into the metadynamics of eggshell formation. They demonstrated with computer simulations how the calcium carbonate that makes up the bulk of the shell transitions from an amorphous state into calcite crystals. (The key appears to be a protein called ovocledidin-17.)
In 2007, engineers at Ohio State figured out a recipe for using pulverized eggshells to make hydrogen fuel, and earlier this year scientists hatched a plan to recycle eggshells into plastics. Best of all, I got to chat with a Cambridge bioengineer named Michelle Oyen over Skype who has been working on an interdisciplinary project characterizing the fine structure and related material properties of eggshells.
Eggshells are mostly calcite crystals in a protein matrix, although even the mineral portion is laced with organic matter. It’s a pretty ingenious design: tough enough to resist cracking, but breakable enough to allow a chick to hatch once it matures. According to Oyen and her colleagues, “The key to the fracture resistance of these materials is in the mechanism by which these two dissimilar materials, protein and mineral, are organized at the smallest of length scales.”
Oh, and of course, there’s the classic egg drop experiment posed to physics classes, in which students are asked to design the perfect “packaging” for a raw egg, to enable it to survive a drop from the roof of a campus building. And Reis said he and Lazarus were inspired by another popular demonstration: walking on eggshells:
4. The Outtakes. The hardest part about writing a piece is leaving out tons of interesting tidbits for the sake of keeping the piece sharply focused — a.k.a. “killing your darlings.” It’s an essential part of the process. Among the many omitted details: how Reis and Lazarus tested their model experimentally. There is too much natural variation in real eggshells; Reis and Lazarus needed tighter experimental controls for their model.
So they used rapid prototyping techniques and 3D printing to create plastic model shells of various shapes — mostly varying the height, so that the curved ovoid shapes got longer or shorter while the base radius stayed the same.
Reis and Lazarus then measured how the changing shapes changed the rigidity of the shells in response to being poked. It’s the kind of experiment that would have been far too expensive and time-consuming even five years ago, according to Reis, requiring a visit to MIT’s metal machining shop.
They also varied the pressure inside the shells, injecting air with the equivalent of a small bicycle pump to see how this changed the mechanical properties, going from zero air pressure to what Reis calls “balloon mode.” It’s akin to the gassy pressure that builds up inside rotten eggs. And because of that, the model is also applicable to the study of viral capsids. One way biologists study cells (including viruses) is by poking the encasing membrane with an atomic force microscope (AFM) and measuring the push-back from that tiny applied force. This reveals properties like rigidity — precisely what Reis and Lazarus were interested in modeling.
So it’s not just about the best way to crack—or not crack—an egg, even if eggshells were the source of Reis’ inspiration. “Our findings are universal, they are based on geometry, and that makes our [model] relevant over a wide range of scales,” said Reis, from eggshells to architectural domes to biological cells. One of MIT’s signature buildings boasts a thin-shelled dome, for instance. While the model wouldn’t be useful for determining how much snowfall the dome could support before cracking—a distributed load, akin to walking on eggs—it could determine whether or not that structure could support a helicopter landing on the pole—a specific point where the load is focused, akin to cracking an eggshell.
There also wasn’t room in the piece to talk about the second eggshell related paper. As Reis and Lazarus were finalizing their own submission, they discovered a separate team of researchers headed by Dominic Vella, a mathematician at Oxford University, focusing on the entire class of shapes known as ovoids.
Vella et al. approached the problem from a complementary perspective, building an idealized eggshell via computer modeling and running multiple simulations, varying shape, shell materials, and internal pressures. “We tried to base that more mathematically on equations that we know govern elastic shells,” Vella told Inside Science News Service.
There is one last obvious question here: didn’t we already know quite a bit about the mechanical properties of eggshells? Well, sure, particularly given all the work done by the aerospace industry in the 1950s and 1960s, followed by handy computational tools. “You can draw a model airplane design and then use the black box tool to calculate all the stresses,” said Reis. “But each time you want an answer about the particular structure, you have to do the calculation, and that can be extremely costly. With our formula you can go poke the shell and measure the force, and if you know what the material is made of and … the local curvature around the point of poking, we can tell you the thickness of the particular point where you are indenting without having to do any hard calculations.”
We are all about avoiding the hard calculations, and so, apparently, is Reis: “This is what physics is all about, trying to come up with simple models of complicated things.”
Images: (top) Still from 1954′s Sabrina, directed by Billy Wilder. (center) Plastic ovoids of many shapes. Credit: Arnaud Lazarus and Pedro Reis (MIT). (bottom) Eggshells of many colors. User: Hustvedt. Wikimedia Commons.
Armitage, Oliver E., Strange, Daniel G.T., and Oyen, Michelle L. (2012) “Biomimetic Mineral-Protein Composites formed by an Automated Alternate Soaking Process,” MRS Proceedings, 1419 , mrsf11-1419-nn04-09 doi:10.1557/opl.2012.749
Carter, T.S. (1977) “The Hen’s Egg: Relationship of Mean Strain Energy at Shell Fracture to Shell Compression Speed, the Nature of the Compressing Body, and the Location on the Shell of the Point of Contact,” British Poultry Science 18(2).
Coucke, P., et al. (2010) “Comparative Analysis of the Static and Dynamic Mechanical Eggshell Behaviour of a Chicken Egg,” Proceedings of the International Conference on Noise and Vibration Engineering.
Entwistle, K.M. and Reddy, T.Y. (1996) “The Fracture Strength Under Internal Pressure of the Eggshell of the Domestic Fowl,” Proceedings of the Royal Society: Biological Sciences 263(1369), 433-438.
Freeman, Colin L., et al. (2010) “Structural Control of Crystal Nuclei by an Eggshell Protein,” Angewandte Chemie Edition 49(30).
Lazarus, A., Florijn, H.C.B. and Reis, P.M. (2012) “Geometry-Induced Rigidity in Non-Spherical Pressurized Elastic Shells,” Physical Review Letters, forthcoming,
Vella, D. et al. (2012) “Indentation of Ellipsoidal and Cylindrical Elastic Shells,” Physical Review Letters, forthcoming.
Voisey, P.W. and Hunt, J.R. (1974) “Measurement of Eggshell Strength,” Journal of Texture Studies 4, 135-182.