The Poke

The Poke

Reader Edition  ·  Companion essay to “Advanced Glass Technology for Handheld and Industrial Devices”  ·  J.P. McFadden

Welcome. I’m Joseph McFadden. This is a companion piece to the main series a step to the side, and a step deeper. The main series is about glass how it breaks, how we make it strong, how it ages in a pocket. This essay is about the thing underneath all of that. It’s about what actually happens physically, mechanically, right down at the bonds when you poke a material and it answers.

I want to tell you up front where this came from, because it did not start with glass. It started with a worry, and it ended at a door I did not expect. So give me a few minutes at the beginning that will feel a long way from fracture mechanics. I promise you the road comes back. In fact, the whole point is that all the roads come back to the same place.

Part One. Why we poke.

Watch anyone meet something unfamiliar, and you’ll see the same ancient gesture. A child meets a new object and pushes it. A mechanic taps a casting and listens. A geologist strikes a rock. A materials engineer drops a device onto concrete, or pulls a specimen in a load frame until it complains. Before we measure anything, before we theorize we poke. And then we watch, and we listen, for the answer.

It’s worth asking why this is so deeply in our nature. Because the answer, it turns out, is the same answer that governs the poke itself.

This didn’t begin with materials for me. It began with a worry. Over the years, I kept noticing something a decline in our collective critical thinking. A fading of the habit of asking why. And being who I am, I could not leave the why of that alone. That question pulled me into neuroscience. Into the brain. Into the electron transport chains the machinery of how we actually run.

And there, I noticed something odd. The authors kept invoking energy the way we all do casually, confidently, as if we had long since settled what the essence of the thing was. But we haven’t. That nagged at me. It pulled me deeper into the mitochondria, and then into the physics of us. And the further down I went, the clearer it became: we can account for energy to staggering precision track it, convert it, balance the books to the last decimal while having no real answer for what it is.

We have mastered the bookkeeping of something we cannot define. I find that thrilling, rather than discouraging. It’s a good reminder to hold our certainties loosely. Which, circling right back to where I started, is exactly the muscle I worry we are losing.

So let me say plainly what took me those couple of years to arrive at. We are, in the most literal thermodynamic sense, dissipative systems. A living thing is a pocket of order that sustains itself by taking in high-quality energy, using it, and shedding low-quality energy heat to its surroundings. We do not persist in spite of the second law of thermodynamics. We persist by serving it sliding the universe toward disorder more efficiently than the bare matter we’re made of could manage alone. A candle flame, a hurricane, and a human being are cousins in this respect. Each is a structure that exists because it dissipates a flow of energy. And each would vanish the moment that flow stopped.

There’s a beautiful idea here, from Jeremy England’s work on dissipation-driven adaptation, building on Prigogine before him. Matter driven hard enough by an energy source tends to fall into arrangements that dissipate that source better. Order emerges not against the current, but because of it. And if that view is even partly right, then curiosity itself the drive to probe, to disturb, to learn how the world will respond is a phenotype of energy dissipation. We poke because we are the kind of structure that energy flow builds.

I won’t wander any further into that fog, because the rest of this is concrete. But I raise it deliberately, because it reframes the whole exercise. When we poke a piece of glass, we are one energy-processing system interrogating another. The material’s response the wave that races out, the heat that blooms, the crack that runs is that material managing an energy flow of its own. The poke is not us acting on a passive object. It is two dissipative systems, briefly, in conversation. And the language they speak is energy.

Part Two. The poke is a question.

Press a fingertip against any material, and you have asked it a question at a single point. Energy and momentum enter one small region. And yet the response almost never stays there. The atoms nearby don’t, as a rule, travel with the disturbance. Instead, they shove their neighbors who shove theirs and a disturbance propagates outward, even though no single atom crosses the room.

Picture a crowded hall, and one person is pushed. That person stumbles into the next, who leans into the next, and a wave of jostling crosses the room while every individual stays roughly where they began. A mechanical wave in matter behaves much the same way. What travels is not the stuff. It’s the state a change in pressure, density, displacement, stress, strain, temperature, or damage handed from neighbor to neighbor.

How fast, and how faithfully, that message travels depends on the material’s stiffness, its density, its internal architecture and, crucially, the timescale of the poke. But speed is only the beginning. As the message travels, it can spread, disperse, reflect, change form, and bleed its organized energy into disorganized molecular motion heat. The message that arrives at a distant point is rarely a faithful copy of the poke. The material edits it in transit. And learning to read those edits is most of what materials characterization actually is.

Part Three. Gases.  Consensus by collision.

Let’s begin with the most dilute crowd, where molecules spend nearly all their time in free flight, meeting only in fleeting collisions. Poke a gas clap your hands, drive a piston and you crowd molecules into a smaller space. Their collision rate rises. That local pressure bump shoves the molecules just beyond it, and the compression travels outward as a longitudinal pressure wave particle motion running parallel to the direction of travel.

Because there’s no permanent structure to carry the news, it can travel no faster than the messengers themselves. This is why the speed of sound in air about three hundred forty-three meters per second at room temperature sits just below the average thermal speed of the molecules. Sound in a gas is a statistical consensus propagating through a randomized, three-dimensional Newton’s cradle always a step behind the gossip that carries it.

Two consequences follow and both echo all the way up to glass. First: a gas transmits only compression. Push molecules together, and they push back. But try to shear a gas slide one layer across another and nothing restoring happens, because nothing connects the layers. A gas has one communication channel. Second: the relay leaks. Every collision randomizes a little organized push into disordered heat. So the pulse fades and high frequencies, which demand many faithful relays per second, fade first. A gentle poke gives a soft, nearly linear wave. A violent one gives a different conversation entirely. The compressed regions travel faster than the rarefied ones, the front steepens, and a shock wave forms the message sharpened from a whisper into an abrupt announcement.

Part Four. Liquids.  The crowded room.

Condense the crowd until molecules touch continuously, and you have a liquid. Now a push transmits not by flight and collision, but by direct contact a shove passed hand to hand through a packed room, rather than shouted across a field. The relay is far stiffer, and faster. Sound in water travels about fourteen hundred eighty meters per second roughly four times its speed in air because the medium resists compression through contact, not through the statistics of collisions.

But a liquid still holds its neighbors loosely. Shear it slowly, and the molecules simply rearrange and forget. The disturbance dissipates as viscosity, rather than propagating as a wave. But shear it fast enough faster than the molecules can rearrange and a liquid briefly answers like a solid, transmitting shear before it can relax. And here is a principle worth carrying with you: whether matter behaves as a liquid or a solid can depend on how quickly you ask. Give the molecules time, and they flow. Deny them time, and they answer as if caged. A liquid is a solid that forgets quickly. And a glass as we’re about to see is a liquid that forgets on a timescale longer than civilizations.

A liquid’s surface adds behaviors its bulk never shows. Because a liquid tolerates compression far better than tension, a compressive pulse reflecting from a free surface can return inverted as tension. Pull hard enough that way, and the liquid tears. Vapor cavities open where none existed. This is cavitation and it’s worth dwelling on, because a bubble is an internal surface that did not exist a moment earlier. Once born, it rewrites the local conversation scattering incoming waves, and, when it collapses near a solid wall, focusing energy into a needle-fine jet that can pit hardened steel. Even in a liquid, the creation of a surface can turn a smooth traveling message into a concentrated local attack. Remember that. It is the whole story of fracture rehearsed in water.

Part Five. Solids.  Communication acquires a grammar.

Bond the crowd permanently, and everything changes. In a solid, every atom sits in a well built by its neighbors, tied to them by bonds that behave, for small displacements, like springs. A poke no longer pushes messengers around. It stretches the network and the network answers collectively. And because a solid resists both compression and shear, it possesses two body-wave channels at last. Longitudinal waves also called P-waves with atoms oscillating along the direction of travel. And transverse, or shear, waves the S-waves with atoms oscillating across it. The longitudinal wave is always the faster of the two. And in stiff solids, both are very fast roughly five to six thousand meters per second for the compressional wave in silicate glass, or in steel.

The existence of that shear channel is the dividing line. A gas, or an ordinary liquid, lacking a static shear stiffness, cannot carry a low-frequency shear wave at all. A solid can. And with it, the solid gains a second language.

And a real poke never launches just one clean wave. It sets off a whole family longitudinal, shear, bending, contact, and surface waves which then reflect from every boundary, return, and interfere into a rich history. The solid doesn’t merely register how hard it was struck. It registers the duration, the rise time, the frequency content, the direction, the contact area, and the location. Two pokes with identical peak force can deliver utterly different messages. Which is exactly why, in a device, loading rate and temperature must be judged for the whole load path never for a single part in isolation.

Part Six. Rayleigh’s wave.  The surface carries its own message.

There’s a third messenger and it belongs to Lord Rayleigh John William Strutt who showed, in eighteen eighty-five, that a free surface supports a wave all its own. A Rayleigh wave rolls along the surface in an elliptical, rolling motion a hybrid of compression and shear its amplitude decaying with depth, so that most of its energy travels within about one wavelength of the surface. It moves a touch slower than the bulk shear wave typically around ninety percent of that speed. Earthquakes deliver much of their damage through Rayleigh waves for exactly this reason. The energy refuses to spread into the depth. It runs concentrated along the surface where everything we build sits.

Now hold that geometry. Because in a sheet of cover glass, everything we care about also lives at the surface. Scratches. Handling marks. Contact edges. And above all, the flaws left by cutting. All of them occupy the exact skin where Rayleigh energy concentrates. A blow that looks mild when averaged over the whole part can be severe in that near-surface zone. The material and the shape always speak together.

Part Seven. Phonons.  The atomic language of vibration.

At engineering scale, we describe all this with smooth fields stress, strain, density, displacement, varying continuously through the body. At the atomic scale, the same communication has a different name. Displace one atom, and you change the forces on its neighbors. The disturbance spreads as a coordinated lattice vibration. And when you count those vibrations in their smallest allowed packets, you have phonons.

A phonon is to a lattice vibration what a photon is to light. The smallest unit of the wave. A quantum of collective vibrational energy, carrying a frequency and a direction. It is not a tiny bead rattling between atoms. It is an organized excitation that the whole lattice shares. Every mechanical message a solid sends itself sound, heat, the stress field racing ahead of a crack tip is written in phonons. And phonons interact with each other, with electrons, with defects, with grain boundaries, with free surfaces. Every interaction is another chance for the message to be redirected, attenuated, or converted into another form of energy. Every one is a road. And every road runs toward the same destination energy, redistributed.

Part Eight. Glass.  A network with no address book.

Glass is the most revealing solid of all. Because it is rigid without being periodic. A silicate network frozen into permanent disorder a solid by every mechanical test, and a liquid by ancestry. Its bonds are as stiff as any crystal’s, so the speed of communication stays high. But the network has no repeating structure. And this transforms the fidelity of communication.

In a perfect crystal, a phonon is a citizen with a passport. The lattice periodicity guarantees it a clear direction and a long, ballistic path. In a glass, no periodicity guarantees anything. Long-wavelength vibrations, which average over the disorder, travel well which is why glass rings, and carries sound. But the short-wavelength, high-frequency vibrations meet a different fate. And here’s the consequence that deserves far more attention than it gets. The violent vibrational energy released when a bond snaps cannot cleanly leave. It scatters within nanometers, smeared into heat almost where it was born. This is why glass conducts heat so poorly. And at the crack tip, it means the tip runs hot, and stays hot vibrationally excited. The stress-corrosion reaction that ages every screen is a thermally activated process. So in glass, the messenger loiters at the scene and the loitering does chemistry.

The disorder also sets fracture’s speed limit. A crack is a disturbance that must continuously announce itself to the material ahead. The stress field at its tip is built and rebuilt by elastic waves. So a crack cannot outrun its own announcement. The ceiling is the Rayleigh speed fitting, since the crack is itself creating new free surface. But long before that ceiling, the channel saturates. Energy arrives at the tip faster than a single clean front can shed it. The front goes unstable. And the crack begins to roughen, and branch. In silicate glass, the terminal velocity lands near thirty to sixty percent of the Rayleigh speed. And the fracture surface keeps the minutes of the whole negotiation. The smooth mirror zone is where the channel still had headroom. The mist is the onset of congestion. And the coarse hackle is the channel overwhelmed the message fragmenting into every available sideband. To read mirror, mist, and hackle is to read a communication log. The record of how a material tried and finally failed to carry a message its own structure could not hold.

And glass never receives a poke as a blank slate. It carries the memory of its melting, its forming, its cutting, its handling, its ion exchange, and every prior load. The poke meets a history not a material.

Part Nine. Plastics.  A message filtered through molecular time.

Polymers carry the liquid’s rate-dependence into permanent solidity. A plastic is a tangle of long chains stiff along each backbone, weakly coupled between chains, with segments that can rotate, straighten, slide, and disentangle on timescales running from microseconds to years. Poke it slowly, and the chains have time to slither and rearrange. Much of the message is absorbed, dissipated as internal friction. Poke it fast faster than the segments can respond and the tangle has no time to be clever. It answers stiffly, even glassily, and passes the pulse nearly intact. This is viscoelasticity. A plastic responds not only to how hard it’s poked but to how quickly and for how long.

And this is not a laboratory curiosity. It is the mechanical heart of every bonded display. The adhesive bead holding a cover glass is a communication component. On a warm day, under a slow push, it whispers absorbing the message before it reaches the glass. On a cold morning or in the tenth of a millisecond of a drop the same bead stiffens into a rigid conductor and delivers the blow verbatim. The identical joint is a different medium at different rates and temperatures. Which is exactly why the glass must be understood as living inside a system not standing alone.

And the environment enters here too. Moisture, oils, fuels, and cleaners can change molecular mobility, or lower the energetic cost of making new surface. In the language of energy-mediated fluid cracking, the fluid does not simply attack the polymer. It alters the conditions under which existing surfaces extend, and new ones form. The mechanical poke, the fluid, the morphology, and the stored stress all speak at once. And failure emerges from their conversation not from any one voice.

Part Ten. Metals.  Communication with shock absorbers.

A metal crystal is the aristocrat of mechanical communication. Periodic order gives phonons long, ballistic paths. And a sea of free electrons adds a second, faster courier for energy. Which is why a metal feels cold to the touch, and glass does not the metal is conducting your hand’s heat away efficiently. Struck within its elastic limit, a metal transmits with crystalline fidelity.

But a real engineering metal is a crystal full of imperfections grains of differing orientation, grain boundaries, dislocations, inclusions, residual stresses. And each one edits the passing message. But then the capability no glass possesses. When the message grows too loud, a metal can reroute it into motion. Dislocations glide, letting planes of atoms slip past one another. So at a stress concentration where glass has no option but to pour all the arriving energy into a crack tip a metal spends that energy moving dislocations instead. The sharp tip blunts. The stress redistributes. The message is absorbed into permanent shape change rather than new surface. That single difference is worth two to three orders of magnitude in fracture toughness.

Glass is brittle not because its bonds are weak but because it is monolingual. It has only one language for stress and that language ends in a crack. Ductility is not the opposite of strength. It is the possession of a second language.

Part Eleven. Every surface changes the rules.

Everything so far quietly assumed a continuous medium. But real matter is full of interruptions. And every interruption is a place where the conditions change a boundary where the wave must split into reflected and transmitted parts, often converting between longitudinal and shear as it does. A wave meeting a boundary may reflect, and transmit. It may refract, bending as its speed changes. It may convert mode, changing language at the border. It may scatter, smearing a coherent pulse into a diffuse field. It may disperse. It may attenuate, its energy converted into heat, or plastic work, or crack growth. And it may focus so that a notch, a corner, a hole, or a crack receives a far stronger local message than the average response of the whole structure would ever predict.

And here is the crucial idea. These internal and external surfaces are the material’s memory. Grain boundaries record solidification. Residual stress records uneven deformation. Scratches record handling. A crack records an earlier moment when local energy exceeded local resistance. So when a fresh poke arrives, it interacts with that memory. And the response is path-dependent. Two parts of nominally identical material answer the same poke differently because their internal maps of surface and stored energy differ. This exactly this is why a datasheet number cannot fully predict a real part. The datasheet describes the medium in general. The poke interrogates this component with its geometry, its flaws, its constraints, its environment, and its history. It is the whole argument for treating a glass component as a phenotype rather than a specification. The datasheet is the genome. But what breaks, or survives, is the material as expressed in a particular body, with a particular past.

And there’s one more thing the external surface does the cruelest thing. It is simultaneously the guided channel and the flaw archive. Surfaces collect handling damage. And edges where two surfaces meet, and where cutting left its worst damage collect the most severe flaws of all. So wave mechanics routes the loudest, most persistent part of every disturbance straight through the one region where the largest stress concentrators already wait. The strongest message meets the weakest listener. That coincidence not bad luck is why most field failures are born at edges. And why edge quality buys more reliability, per unit effort, than any other single variable in the process.

Part Twelve. Turning the tables.  Engineering the conversation.

If a disturbance is a message shaped by structure, then structure can be designed to shape the message on purpose. This is the leap from studying wave propagation to engineering it. And it has a name phononic crystals.

A phononic crystal is a material made periodic on purpose a repeating arrangement of contrasting stiffness or density built so that its internal interfaces interfere with waves in a controlled way. Just as a crystal’s atomic periodicity opens electronic band gaps, a phononic crystal opens phononic band gaps ranges of frequency in which mechanical waves simply cannot propagate. Inside a gap, a wave becomes evanescent, and dies away. The medium becomes, for those frequencies, deaf.

And the point, for a working engineer, is direct. Everything we described as something that happens to a material reflection, scattering, the trapping and channeling of energy can instead become something a material is built to do. A laminated windshield’s polymer interlayer, a rugged handheld’s foam gasket these are early, blunt instances of the same instinct. Deliberate internal surfaces, placed in the wave’s path, to edit the message before it reaches the glass. We began by learning to listen to what a material says when poked. Phononic crystals are where we start deciding what it is allowed to say.

Part Thirteen. What the system heard.

Come back, now, to the bench. A force transducer tells us what we applied. An accelerometer tells us what moved. A strain gauge tells us what deformed. A microphone, or an acoustic-emission sensor, tells us what waves escaped. A thermal camera shows where organized energy became heat. A crack shows where the local conversation went unstable. Not one of these is the whole truth. Each is a single listener, at a single place, tuned to a single part of the message.

The poke may begin as a simple exchange of momentum. But the answer divides among translation, rotation, vibration, stored elastic strain, viscoelastic loss, heat, plastic work, the making of new surface, and outright fracture. All roads lead back to energy. But energy travels by many roads. And a measurement that watches only one of them reports only one province of a continental event. This is the deepest reason the datasheet is not the part and the peak force is not the story.

Part Fourteen. The material is the conversation.

Let’s run the whole progression back. A gas relays a poke by collision slowly, in one channel, forgetting as it goes. A liquid relays by contact faster, forgetting shear unless asked quickly and capable, at a torn surface, of turning a smooth wave into a focused attack. A solid installs permanent wiring, and invents the phonon. The crystal transmits with fidelity and, if metallic, keeps shock absorbers on staff. The polymer edits its answers by rate and temperature. And the glass transmits fast, but scatters the fine detail, hoards energy at hot spots, and, owning no second language for stress, must finally answer overload with a crack. At a speed capped by its own surface-wave courier. And recorded forever in mirror, mist, and hackle.

Which returns us, one last time, to why we poke at all. We are dissipative structures. Built and driven by a flow of energy that we can measure exactly and cannot truly name. And it is in our nature to probe the gradients around us because probing is how such structures find and manage their world. When we poke a material, one energy-processing system questions another. And the material’s whole response elastic and plastic, acoustic and thermal, reversible and final is that second system managing an energy flow on its own terms, according to its own structure, and its own history. Stiffness, sound, heat, damping, and fracture are not five separate properties. They are five dialects of the single thing the conversation is always about.

So here is where I’ll leave you. A material is not a silent, uniform object waiting to be loaded. It is an interconnected energetic system already carrying the story of its making, and its use answering every disturbance with the whole of that story at once. Neighbors speak to neighbors. Structures speak to surfaces. Waves speak to defects. And the present load speaks to the material’s past. What we call material behavior is the sum of that conversation. And if all roads lead back to energy then to listen carefully to a single poke is to catch, for a moment, the one theme that runs through everything connecting the crowded room to the crack tip, the candle flame to the curious hand.

Every failure has a story to tell. Our work is only to learn how to listen.

Thank you for listening. I’m Joseph McFadden. This has been a companion essay to Advanced Glass Technology for Handheld and Industrial Devices. Go poke something. And this time listen to what it tells you.