Manisha Tripathy, PhD

Adapted from the introduction to my master's thesis, Effect of Adhesion in Line Contact Deformation of Rough Surfaces (Lamar University, December 2017). The enthusiasm is original and unedited.

The 'dark' philosopher Heraclitus of Ephesus said, "Life is flux," more commonly known as "change is the only constant." The validity of this statement has withstood the test of time. Humans have progressed a long way on the path of evolution, from the discovery of fire and the invention of the wheel, to airplanes and trains travelling faster than sound, to major organ transplants, to landing on the moon. (On a humorous note, it can be said that most of these inventions speak more of the laziness of humanity than of its intellectual abilities.)

My thesis presented my small contribution to one of the frontiers behind all that progress: contact mechanics, or tribology, which has always been one of the esoteric ideas hiding in plain sight in day-to-day life. The most fundamental discovery made by Homo sapiens, fire, was probably made by the rubbing of sticks and flints: contact between two substances. The oldest documented tribological research was done by Leonardo da Vinci in 1495, when he laid down the laws of friction and gave us the coefficient of friction. But the path of today's tribological research was first laid by Heinrich Hertz, who opened the gates of research on contact stresses, deformation, and the real area of contact when he formulated the equations describing contact between two elastic bodies with curved surfaces. Of such formulas, Hertz said:

"One cannot escape the feeling that these mathematical formulas have an independent existence and an intelligence of their own, that they are wiser than we are, wiser even than their discoverers, that we get more out of them than was originally put into them."

Time has proved the vision of Hertz to be right, for the equations he gave the world while researching optics have created a world of their own.

I will be honest about how I ended up in this world: not through the equations, at first, but through the questions: the kind that sound childish until you try to answer them.

Just as Galileo was the spearhead of research on contact between surfaces, in the case of adhesion it was Newton. It was his preoccupation with attractive forces between bodies that eventually led him to discover gravity. Adhesive forces, in layman's terms, are attractive forces between two bodies; by a playful extension of that definition, gravity is the grandest adhesion of all, the attraction of the Earth so strong that everything adheres to its surface. All of this leads to a very impertinent question: how big are these attractive forces? If everything attracts everything, why don't all objects get drawn to each other and cluster in a group? Humans, though 'sticking' to the surface of the earth, are still able to walk around and fly inside the atmosphere. Why is that? And my favorite: why don't the pieces of a broken cup just stick back together when you press them against each other? After all, it is the same material and the same molecules the cup had before it broke. What is it about the broken bonds between molecules that they refuse to reform when brought into contact again? These are the questions that hooked me, and they have not let go since.

To answer them, it is necessary to talk about two things: the true area of contact, and before that, surface roughness. All surfaces are rough. Even surfaces like mica, which look perfectly smooth to the naked eye and to the touch, are rough at the microscopic level, made up of millions of asperities of varying height and radius. The true area of contact was probably described best by Bowden: "Putting two solids together is rather like turning Switzerland upside down and standing it on Austria: the area of intimate contact will be small." In simpler words, when two surfaces come into contact, the real contact area is much smaller than the apparent one. Nothing you own is fully touching anything else; contact is a handful of microscopic mountain peaks meeting, while the valleys below never learn of each other.

The lie your eyes tell you: what looks like full contact (a) is really a few microscopic peaks touching (b), each behaving as its own tiny contact (c). Figure from my thesis.

Contact mechanics is thoroughly embedded in every aspect of modern life: tires, bearings, micro- and nano-electronic devices. With chemists, biologists, and engineers all exploring the world of adhesion, it is only sensible to predict that the field will grow by leaps and bounds, and there is no way to be sure where the exploration will culminate. Maybe, in the future, a technology will be invented where nano-manufacturing occurs by the forces of adhesion alone, where atoms just 'stick' together into the desired form and shape until they reach the macroscopic level. My thesis aimed at something far more modest: a mathematical model to predict the behavior and effect of adhesion between curved surfaces, with provision for roughness.

The geometry my thesis modeled: a smooth curved surface pressed against a rough one, every little peak negotiating on behalf of the whole. Figure from my thesis.

That was 2017. I have spent the years since, through a PhD, a stint in industry, and now a postdoc, getting instrumentally closer and closer to those few microns where surfaces actually meet. The broken cup still does not stick back together. I am still asking it why.

Between my PhD and my current postdoc, I spent about a year and a half as a Tribology Application Scientist at Bruker, running tribological characterization for clients across academia and industry. A single lab project teaches you depth. This job taught me breadth of a kind no dissertation can. The same wear problem would arrive week after week wearing a different costume: new industry, new hardware, new vocabulary. Underneath it all, the same few microns of surface, quietly misbehaving. (There is a strange comfort in learning that everyone's surfaces fail. There is a career in learning why.)

The breadth was not only technical; it was human. Some weeks the person across the table was an academic chasing a mechanism, other weeks an engineer whose production line had developed an expensive new habit. They did not speak the same language, but they were asking versions of the same question: why is this surface not doing what I need it to do? My job was to design the measurement that could answer it. Just as often, my job was to gently point out that the question they had brought was not quite the question they had. Listening, it turns out, is a technical skill. Nobody lists it on an instrument's specification sheet, but no instrument works well without it.

What stayed with me, though, was not the instrument time. It was the people on the other side of the data. Behind every worn part was someone with real hardware, a real deadline and a real budget, hoping the measurement would tell them something they could act on by Friday. You learn to translate, to hold the physics in one hand and the person's actual problem in the other. You also learn a harder lesson: that a beautiful measurement which answers no one's question is just an expensive picture.

Industry also taught me a discipline that graduate school, for all its rigor, rarely does: the discipline of enough. In a PhD there is always one more condition to run, one more variable to isolate, and the luxury of time to chase them all. On a client's clock, you learn to ask what level of certainty the decision actually requires, deliver it, and stand behind it. I arrived believing thoroughness was the highest virtue. I left understanding that judgment, knowing which measurement matters and when to stop, is the rarer one.

From my perspective, that is the lasting gift of my time in industry, a test that every research problem must now pass before it earns my attention. Coming back into a research role, I care as much about whether a problem matters to someone holding a failing part as I do about whether it is interesting on paper. The two are not rivals. The best problems, I keep finding, are the ones that are both; and the people I met in those eighteen months, tackling the same stubborn microns from a dozen different angles, remain the audience I quietly write for.

Where does a material actually begin to fail? Nearly every hardware failure I have examined closely, in a lab, on a production line, out in the field, has given the same answer: at a surface, not in the bulk. That is what pulled me toward tribology and characterization in the first place. The bulk is where a material lives; the surface is where it meets the world, and like most of us, it is at its meeting places that it is truly tested. The most interesting physics plays out in the first few microns, at an interface, under conditions that are stubbornly hard to see directly. To a person with my particular weaknesses, that reads less like an obstacle and more like an invitation.

Consider what we ask of those few microns. In an aircraft engine, on a ship, inside a reactor, two surfaces are pressed together, heated, vibrated, and expected to behave for decades. The material's answer to that abuse is decided in a layer thinner than a human hair. The moment of decision is nearly impossible to watch, too; it happens at a buried interface, between moving parts, at temperatures where most instruments politely excuse themselves. So we are left reading the aftermath, the way detectives read a room after the crime. Much of my work, electron microscopy and atom probe tomography, is an attempt to arrive at the scene earlier, to catch a surface in the act of changing rather than reconstructing it from the wreckage.

New ways of making materials keep the question fresh. When a part is built layer by layer instead of cast or forged, its surfaces inherit a different history: different roughness, different residual stresses, a different microstructure waiting a few microns down. Old intuitions about how a surface will wear do not automatically transfer. That is inconvenient for engineering and wonderful for curiosity.But the throughline across my PhD, my time in industry, and my postdoc has never wavered: get as close as the instruments will allow to the place where a material first begins to change, and treat that as the main event rather than a side effect. For a long time surfaces were studied as the annoying part of materials science, the place where the beautiful theory of the bulk stopped working. I have come to believe the opposite. The surface is where a material tells you the truth about itself, under pressure, in real time. Surfaces are where materials meet the world. Someone ought to be watching closely, and I am glad it gets to be me.