`Surface’ Tension

After an extended vacation from writing on the internet for public consumption, and from writing period (back surgery’s a bitch, friends), I thought it would be nice to put some things I’ve been thinking out for you all to read. So here is an excerpt from the dissertation chapter I am currently working on. It’s the introduction to a chapter about the role of surfaces in chemistry and nanoscience:

 

Surfaces are everywhere. They are the only parts of bodies with which we ever actually interact. They are the boundaries between objects—and they are the access points to objects. Surfaces are the subject, and the limit, of tactile perception. The concept of surface is so important to our understanding of the world around us that the question “What would this table/house/book/hand look like, or how would it behave, if it didn’t have a surface?” is nearly incoherent: a table cannot be a table without a surface; nor can a house or a book or a hand. Surfaces are essential parts of the objects that populate our world.

Surfaces are thus, unsurprisingly, essential parts of the objects of scientific study. In chemistry, surfaces are the sites of reactions. Because surfaces are, by definition, the boundaries between different chemical systems or environments, they are the places where chemical reactions happen. They are where bonds are broken and formed.

To understand the impact of surfaces on the way chemical change occurs, consider the following middle-school chemistry experiment: I have two glasses of water. Into glass A I drop a large crystal of table salt (sodium chloride, NaCl) of the same size and shape as a grape. Into glass B I drop a heaping tablespoon of granulated salt, of the same mass as the grape-sized crystal. If the glasses have the same amount of water and are kept at the same temperature and pressure, the salt in glass B will dissolve significantly faster than the salt in glass A.

The reason for this difference is that while the mass of the salt is the same, there is more surface area on the smaller crystals, which means there is a larger area where negatively-charged oxygen from the water can interact with positively-charged sodium from the salt, and where positively-charged hydrogen from the water can interact with negatively-charged chloride from the salt. With more reaction sites, the reaction proceeds more quickly.

Conversely, in glass A, while it is thermodynamically favorable for the sodium and chlorine atoms that make up the salt crystal to dissociate and dissolve, no oxygen or hydrogen can physically reach the atoms on the interior of the crystal without going through the surface first. Every interior layer of the salt crystal must first be exposed as a surface layer before it can dissolve—in the same way that you can’t peel an onion from the inside out, it is impossible to induce a reaction without first exposing a reactive surface. Whole areas of study within chemistry are devoted to the principle that increasing surface area increases chemical reactivity: this is one of the founding principles of heterogeneous catalysis.*

Thus, surfaces are integral components of chemical reactions. Surfaces are also integral components of material objects: No solid exists without a surface. And the behavior of material surfaces often influences the behavior of the object as a whole: the physical and chemical structure of an object’s surface dictates whether two materials will attract or repel, slide along one another like well-lubricated ball bearings or stick to one another like velcro. Bricks could not hold up bridges or buildings if there was less friction between the surfaces; steel couldn’t cut through metal or food if there was more friction—anyone who has wielded a dull knife against a day-old bagel can recognize that much.

Atoms on the surface of a material behave differently than atoms on the inside of a material. Compared to interior atoms, surface atoms are in a different relationship with other atoms in the material and with atoms in the surrounding environment. Surface atoms have fewer bonds with other atoms in the material, and they have more opportunities to interact with the surrounding chemical environment. Simply put, surfaces are a different kind of system than the interiors.

Curiously enough, many models of materials ignore the physical and chemical behavior of surfaces, rather than attempting to provide detailed, blow-by-blow descriptions of surface behavior. Mark Wilson has recently pointed out the lack of consideration for the physics and chemistry of surfaces in point-mass models of physical systems:

In point of fact, real life blocks and planes will bind very tightly together if their surfaces are appropriately cleansed and polished; the only reason we normally see behaviors that approximate more closely to frictionless sliding is because very complicated physical processes are active within the region of interfacial contact. For example, normal surfaces are quite rough on a microscopic scale and contact each other only at widely separated asperities. These contact points interact with one another through softening at quite elevated local temperatures and through other chemical alterations that remain completely invisible upon a macroscopic scale. Furthermore, the strengths of the bonding at the asperities are usually greatly diminished through atmospheric contaminants serving as a thin interfacial lubricant. And so on, through many unexpected complications. Accordingly, a very elaborate form of interfacial modeling is needed before one can realistically expect that “blocks” and “planes” assembled from mass point conglomerates will interact with one another in a manner that remotely resembles the “block and plane” behaviors invoked in our text book exercises. 

Mark Wilson, “Mixed-Level Explanation,” p. 3

Wilson goes on to point out that despite the lack of consideration of surfaces, point-mass modeling strategies are nonetheless often successful methods of modeling material behavior. He tells a rich and detailed story about the interaction of models to produce what he calls “mixed-level explanations” of how materials respond to stresses and strains from their environments, and how these mixed-level explanations carry changes in the structure of an explanation of a material system. He argues that as the scale at which a material is studied changes, the behaviors of interest change—and, consequently, so do the explanatory structures needed to model that behavior.

Throughout his career, Wilson has demonstrated the existence of changes in explanatory structure that follow changes in the kind of model being used to support explanations of a physical system. In Wandering Significance, he describes how physical concepts such as redness and hardness change with the context of application, and how changes in the concepts affect our abilities to carry out both scientific and everyday projects that require an understanding of what it means to be red, or hard. More recently, he has argued that the field of classical mechanics contains a wide variety of subtle discontinuities in the ways that concepts are applied across point-mass, rigid-body and flexible-body modeling methods, and that attention to these discontinuities provides further insight into the behavior of systems across a variety of scales, as well as insight into the structure of effective explanations of classical-mechanical systems.

Wilson is not the only author to point out the tendency of scientific concepts to slip and shift as they move from one theoretical context, or one scale, to another. Bob Batterman’s theory of asymptotic explanation relies heavily on attention to the mathematical behavior of models of materials as the models interact to produce descriptions of critical, inter-phase phenomena. Rob Phillips has written an entire materials-science textbook addressing the challenge of “modeling across scales.” All three of these authors have recognized the importance of attention to context, and especially the kind of context defined by the time- and length-scales that define the system of study, in forging descriptions and explanations of physical and material concepts and phenomena.

Surfaces, though, are not just physical. Wilson recognizes this in the description of the mechanics of surfaces quoted above, making mention of “chemical alterations” and “the strengths of the bonding.” Physics and chemistry are both needed to adequately describe the behavior of surfaces, and it should come as no surprise that scientists and philosophers looking to better understand the concept surface need to pay attention to the interaction of physical and chemical models of surfaces in order to answer questions and accomplish projects.

Nowhere is this more apparent than in models of nanoscale surfaces. At the nanoscale, the percentage of atoms on the surface of a material becomes statistically significant: rather than making up an infinitesimal fraction of the material’s mass and structure, surfaces make up 10, 20, or up to 80-90 percent of the material. Consequently, the role of the surface in influencing the physical and chemical behavior of the material changes. Additionally, novel physical and chemical phenomena arise in the surfaces of nanomaterials. At the nanoscale, surfaces simply cannot be ignored as they often are in models of macroscopic materials.

The shifting role of surfaces in shaping the behavior of these materials is important for a variety of reasons. First, as the role of surfaces changes, the concept surface itself changes, providing a further example of the kind of conceptual change that authors like Wilson, Batterman and Phillips have addressed. In order to effectively model nanoscale systems, then, it is important to pay attention to the ways in which surface changes at the nanoscale.

Second, it is a change in the scale of the system being studied that induces this conceptual change. Wilson, Batterman and Phillips, as well as many other authors in the philosophy of physics, biology and other sciences, have all recognized the importance of modeling systems at a variety of levels or scales. Batterman in particular has emphasized the role of scale in shaping explanations of physical systems. But none of the examples considered by these authors address the curious position of nanoscience as an entire discipline framed around the study of a length scale, or the implications for modeling of uniting modeling strategies from multiple disciplinary traditions around the study of this length scale.

Third, and consequently, the variety of modeling strategies that are applied to the project of understanding of nanoscale surfaces are more diverse than the strategies used to model strictly physical, chemical, or biological systems. Contemporary neuroscience may prove the closest cousin of nanoscience here in its attempts to marry physical, chemical and biological methods in order to better understand the electrochemical mechanisms of the biological brain. As the modeling strategies used to understand nanoscale surfaces are laid out, it will be important to pay attention to the interaction of models from different scientific backgrounds in shaping the concept nanoscale surface. What a chemist means by surface is not necessarily what a materials scientist means by surface, and for many purposes one need not address what hinges on the difference in their meanings. But when the two come together to tackle the question of how nanoscale surfaces behave, the differences between their concepts of surface, and how those differences play out in the way they model surfaces, must be scrutinized.

Finally, changes in the role of surfaces at the nanoscale are crucially important for understanding the potential applications of nanomaterials in a variety of technologies. The ability to manipulate and control nanoscale surface phenomena such as localized surface plasmon resonance and the semiconductor behavior of carbon nanomaterials is precisely why nanoscience is worth doing: nearly every application of nanomaterials to solving the energy crisis, curing cancer, improving computing and otherwise making the world a better place is an application of some nanoscale surface phenomenon. Modeling and understanding nanoscale surfaces are central projects of nanoscience, and if the science is going to grow up to fulfill the promises it has made, those studying nanoscience need to understand how the surfaces of their materials work. In other words, clearing up the meaning of the concept surface at the nanoscale is a necessary step in developing the field of nanoscience.

 

*Catalysis is the use of an additional chemical agent, the catalyst, to increase reactivity or rate of reaction. Heterogeneous catalysis uses a catalyst that is in a different phase of matter than the reactants, either a solid in the presence of liquids and/or gases, or a liquid in the presence of gases. The heterogeneous catalyst provides extra reaction sites at which the reactants can interact, and the number of sites available is directly proportional to the amount of catalytic surface with which the reactants are in contact.

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About burstenj

Assistant Professor of Philosophy
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