What Makes Something Slippery Or Sticky?
Physicist Laurie Winkless dives into the realm of surface science to figure out how its shaped our manufactured and natural worlds.
The following is an excerpt from Sticky: The Secret Science of Surfaces by Laurie Winkless.
Sticky: The Secret Science of Surfaces
There’s a flowchart lurking around corners of the internet. It is familiar to anyone who enjoys fixing and making things. At the top, it asks “Does It Move?” and at the bottom, it offers two solutions: duct tape for when you want to hold something in place, and WD-40® for when you want to get things moving. These two products have long been seen as must-haves. Essential tools for any workshop; versatile enough to find frequent use. I’m a fan of both.
A few years ago, as the initial idea for this book was rattling around in my head, I realized something about these products. Because one sticks firmly on to surfaces, while the other slips between objects in order to loosen them, they’re often viewed as opposites; as if they each occupy an end point of a stickiness-to-slipperiness scale. In reality, no such scale exists—not in our everyday lives, nor in the controlled environment of a research lab. That’s because the words “sticky” and “slippery” are ambiguous, and certainly not precise enough to exist in opposition to one another. Though widely used, they mean different things to different people on different days. Depending on the situation, they might conjure up images of chewing gum, duct tape and sugary syrup on the one hand, or an icy road, WD-40 and wet tiles on the other. The words “sticky” and “slippery” are also not true materials properties in the way that, say, hardness and thermal conductivity are. They have no agreed-upon scientific definitions, and no specific metrics that can be used to quantify or compare them. That contrast—between the presence of these words in daily life, and their absence from the scientific literature—is one of the reasons I decided to call this book Sticky.
As I see it, this familiar term can be repurposed and applied to a vast array of interesting interactions: specifically, any of the weird and wonderful things that happen on and between surfaces. So much science happens where two things meet; whether that’s air flowing over a curved surface, two pieces of metal sliding along one another, or glue applied to a plank of wood. And while stickiness isn’t something that can be measured or defined, there are lots of other related properties that are measurable, and whole areas of research dedicated to defining them.
Tribology is one of these areas. Sometimes described as the science of “rubbing and scrubbing”, its focus is on how moving surfaces interact with one another. While at first glance that might seem a bit niche, as we’ll discover, such interactions are all around us, defining the movement of glaciers on rocky landscapes and the whizzing of a hard-disk drive in your computer. Regardless of the sector they work in, something all tribologists are obsessed with is friction, the resistive force that acts parallel to surfaces, either to hold stationary objects in place (static friction) or to slow down the motion between those that move (kinetic friction).
By measuring the friction forces between materials, and incorporating them into mathematical models that have been developed and updated over decades, tribologists can glean a deep and sophisticated understanding of surfaces. In doing so, they can find ways to control the friction that acts on them. Every system with connected parts, be it engineered or biological, has been designed with friction in mind. Sometimes the aim is to maximize it; to provide grip or traction between components even in extreme conditions. other times friction is the enemy, causing things to literally grind to a halt. Either way, we can’t ignore it, which is why friction is at the heart of this book. It is the thread that runs through the fabric of every chapter.
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In many ways, tribology is not a new science. Humanity has been exploring and manipulating surface interactions for millennia, far longer than we’ve had the equations or the tools needed to describe them. A famous example of this can be found in the burial tomb of Djehutihotep, a powerful provincial governor who lived in Upper Egypt 4,000 years ago. on the tomb’s richly decorated walls is a mural now dubbed Transport of the Colossus. It depicts a huge monument atop a wooden sled, dragged by a team of hauliers. A lone figure standing at the foot of the monument can be seen pouring liquid directly in front of the sled, in what was initially interpreted as a purely ceremonial act. Engineers who later saw the image wondered if there was more to it. Could this liquid also be an example of an early lubricant; a way to make it easier to slide the heavy sled along the sand?
In 2014, a team led by Professor Daniel Bonn set out to answer that question. The experimental design was pretty simple—they’d load a small sled with weights, pull it along samples of sand that had been mixed with varying quantities of water, and measure the forces involved. The metric they were most interested in was the coefficient of friction, μ (pronounced “mu”). This ratio is used a lot in tribology studies (and in engineering and science in general) because it gives you a clue as to how strongly two material surfaces interact with each other. The closer its value is to zero, the more easily the surfaces can start to slide. So steel-on-ice has a slightly lower μ than wood- on-ice (μ = 0.03 versus 0.05), whereas the frictional interaction of rubber-on-dry-asphalt is 18–30 times higher than either of them (μ = 0.9). This partly explains why tires help vehicles to stay on the road; we’ll cover much more on this in Chapter 5. By measuring the coefficient of friction of the sled being pulled along increasingly wet sand, Bonn could directly determine the effect that adding water had on the sand’s “slipperiness.”
Friction was high for all of the dry sand samples, with a typical μ of 0.55. Bonn attributed this to the “heap of sand [that] forms in front of the sled before it can really start to move”. As he increased the water content, the size of that sand heap decreased, as did the value of μ. In some cases, friction between the sled and the sand dropped by 40%, solely through the addition of water. But once the sand contained anything beyond about 5% water, friction began to climb again, making the sled harder to pull. The researchers concluded that for transporting objects along desert sand, there is an optimal amount of water that can aid in sliding. The mechanism behind it will be familiar to anyone who has ever filled and flipped over a bucket to make a sandcastle. If the sand inside it is dry, it will flow and spread out freely. In contrast, wet sand can retain its shape, thanks to the formation of water bridges between the sand grains. If you get the mix just right, the water holds the material together, providing a smooth, stiff surface on which to slide heavy objects. Speaking to the Washington Post back in 2014, Bonn said that if this lubricating mechanism were scaled up to the projected size of the giant stone monument, it would mean “that the Egyptians needed only half the men to pull over wet sand as compared to dry… the Egyptians were probably aware of this handy trick.”
The world of lubrication has largely moved on from using water. Today, there are thousands of lubricants available commercially, the majority of which are based on mineral oils (aka petroleum). What they all have in common is their aim: to reduce friction between moving surfaces, whether they’re inside a cheap lawnmower or a high-tech Martian Rover. The global market for these friction- reducing compounds is enormous, worth in excess of US$150 billion (£107 billion) in 2020. We’ll talk about some state-of-the art solid lubricants in Chapter 9. Water does still occasionally influence lubrication, especially in geological processes like landslides, and in the earthquakes and ice of Chapters 6 and 7. But more often than not, water, like many other fluids, exerts a friction force on surfaces. It drags on objects, slowing them down as they move through it. These particular resistive forces can be understood through fluid dynamics—the science of liquids and gases in motion—and their implications are widespread. As we’ll discover in Chapter 4, the flight of every ball and every aircraft is controlled by the air around it. For the swimmers among you, Chapter 3 will uncover what it takes to slice through water, and you’ll meet some underwater technologies that reduce water’s influence by pushing it away from surfaces.
There are, however, lots of things that for various reasons didn’t make it into this book. For example, something I’d originally planned to include was a chapter on the medical uses of surface science, from targeted drug delivery via engineered particles to designing implants that encourage cell adhesion and growth. Given that as I write these words (January 2021), the COVID-19 pandemic continues to impact the daily lives of everyone on the planet with a virus that can be transmitted by air and on surfaces, this omission is regrettable. But the truth is, I ran out of both time and space, for a topic that requires plenty of both. other chapters have merely changed focus. Chapter 2 was going to explore the many ways that animals use surface science to navigate and control their surroundings. Spiders, sea-urchins and sharks were all on the list of possibilities. Instead, the chapter now focuses on just one animal—the gecko. In researching this lizard I became captivated by it: the astonishing mechanisms behind its climbing ability, and the many technologies it has inspired. There are other examples from the natural world scattered throughout the rest of the book. In Chapter 8, I’ve taken a physicist’s perspective on our sense of touch, and of its role in human society. And finally, or perhaps, “firstly”, Chapter 1 is an introduction to all things adhesion, including descriptions of how some of the sticky and slippery products that I’m frequently asked about actually work.
At its heart, Sticky is a book about materials, and the forces at play on their surfaces. In one way or another, I’ve been professionally interested in this topic since 2007. That’s when I first got involved in a research project into the use of patterned surfaces to control both friction and fluid flow, which led to work on water-repellent materials, among other things. later, when I was writing Science and the City, these surface interactions just kept popping up, from the slipperiness of leaves on the railway line to the grip of tires to the road. The importance of friction to the modern world seemed laughably outsized compared to our knowledge or appreciation of it. That’s really when the idea for Sticky first took hold. once I started seeing things from the point of view of stuff-that-happens-on-surfaces, I couldn’t stop. This book is the result.
Sticky is not intended to be an exhaustive exploration of all known surface interactions. Nor is it trying to be a physics textbook, a mathematical treatise on friction, or a deep-dive into the best glues on the market. If that’s the level of knowledge you’re looking for, there are lots of other references that I will happily point you to. Instead, what you’ll find within these pages are my favorite examples of how the forces that act on the outer skin of materials can literally and figuratively shape the world around us. The implications of these forces cut across scientific disciplines, and as a result, our journey will take some surprising twists and turns. I think (hope?) that there’s something in here for everyone.
In researching these topics, I’ve been privileged to speak with an array of fascinating people from across science and society; all experts in their respective fields, they generously gave their time to talk to me and share their knowledge. To say “I owe them” would be an understatement. I’m excited for you to meet each of them.
So why not slip into something comfortable, stick on the kettle, and let me tell you some stories.
Excerpted from Sticky: The Secret Science of Surfaces by Laurie Winkless, published by Bloomsbury Sigma.