Writing a dissertation abstract is a bizarre exercise. I feel like I am writing a news feature story about my own research. Luckily, I like writing feature stories, and I like my research. But sometimes I get carried away in feature mode and end up writing about my favorite building for a while before I get around to telling my audience that I am actually interested in philosophical issues in nanoscience. And that, friends, is where blog posts come from:
One of the ways that we learn about ourselves and others from a young age is by comparing favorite things. From favorite colors and shapes to favorite movies and songs, sharing favorites is one of the ways that people get to know each other. It’s a form of small talk that can give rise to friendly, social-identification-improving teasing. And it is open-ended enough that games of “What’s your favorite [x]?” can go on for hours or months without exhaustion.
When I started college, I learned that architecture was a thing you could go to college to study. I’d never really thought about where buildings came from before. But my undergraduate university has one of the best architecture schools in the country, and so I got to play what’s-your-favorite with a bunch of aspiring architects. I learned from them that one of the categories of favorite things people can have is buildings. Mine is La Sagrada Familia.
La Sagrada Familia cathedral is Antoni Gaudi’s magnum opus. A naturalistic reinterpretation of Gothic architecture, the cathedral teems with arches and flying buttresses, gargoyles and towers. Like many classical Gothic masterpieces, La Sagrada Familia features a rose window: a circular stained-glass window divided by stone into wedge-shaped panes, like the petals of a flower. But unlike the intricate filigree work that fills rose windows in classical Gothic architecture, the panes of Sagrada’s rose window are irregular, geometric blocks of color, inspired by the cubist movement that took root in Spain at the same time as the cathedral.
La Sagrada’s rose window is dominated by blues and greens in a variety of shades and hues. Cubist painters would have tried to depict the window with pigment-based paints, turning especially to ultramarine pigments derived from ground up lapis lazuli and to copper(II) acetoarsenite, or Paris green. They might have tried to overlay these paints with reflective glosses or undercoat their canvases with special primers to capture the otherworldly glow of the colors in the window’s stained glass. But their paintings would never have done the rose window justice, because the colors in the stained glass do not come from pigments or dyes. Where pigments and dyes produce color by selectively absorbing light at certain frequencies, the colors in La Sagrada’s window are produced by localized surface plasmon resonance, that is, the collective, resonant oscillation of electrons on the surface of a nanomaterial in a conducting medium. Localized surface plasmon resonance (LSPR) occurs when a solution of nanoparticles, such as a glass doped with finely divided metals, is stimulated by electrons or photons of a particular frequency.
LSPR is a scale-dependent material behavior, one that can only occur in nanoscale materials. While LSPR has played a role in color technology throughout the ages, it has only come to be understood as a physical phenomenon in the past three decades. Today, scientists are particularly interested in developing nanomaterials with finely-tuned LSPR responses—that is, intense resonances in response to a narrow range of stimulus frequencies. The recent surge of scientific interest of scale-dependent material behaviors like LSPR raises a variety of philosophical questions, from ontological puzzles about color classification and material identity to ethical concerns about the safety of nanomaterials and epistemological and methodological worries about how to reason about a domain of inquiry that is specified by a length scale rather than a set of common material properties or biological functions.
My dissertation answers three interrelated questions of the latter sort. My main goal is to answer the question How are theories and models used to reason about nanomaterials? To answer this question, I demonstrate that there are differences between the ways in which scientists use theories and models to reason about nanomaterials and the uses of theories and models that philosophers of science have identified up to this point. I highlight these differences by answering two further questions: 1) How do scientists respond to new modeling challenges that arise as a consequence of scale-dependent material behaviors? and 2) How do theories and models help scientists synthesize nanomaterials?
These two questions cannot be answered independently of one another. I argue that in order to gain synthetic control over nanomaterials, scientists need to be able to model the behavior of nanomaterials at multiple scales (microscopic, mesoscopic, and macroscopic), because different, interdependent behaviors of interest occur at different scales. I demonstrate that in designing nanomaterials, scientists use models and theories to achieve a balance of desirable features that trades off between behaviors at different scales.
I argue that this balance is best achieved by a particular kind of non-reductive account of relations between models at different scales. I call this the model interaction account of theory and model use. I demonstrate that novel, scale-dependent material behaviors like LSPR are not modeled solely by top-down nor by bottom-up methods. Instead, the two approaches are combined and adapted in order to, e.g., describe LSPR and synthesize materials that exhibit LSPR behaviors. Drawing on Wilson’s “theory facade” account of concepts and theory structure in the physical sciences, I argue that concepts central to nanoscience, such as surface, change meaning between the macroscale and the nanoscale, and that model interaction is required not only to address synthetic challenges but also to develop conceptual understanding of surface and related scale-dependent concepts.