One of the first things you learn when working with nanomaterials is that scale isn’t neutral. Bodies change their properties dramatically as they shift from macroscopic to microscopic, from molar to molecular. While part of these effects are due to the proverbial queerness of the quantum world, most of them depend on a much more trivial property of bodies. When objects are scaled down, the ratio between their surface and their volume increases exponentially. Although it may seem counterintuitive, small things have much more surface than larger ones.
This phenomenon becomes strikingly apparent within the realm of nanomaterials, where objects exist on the scale of billionths of a meter. In this essentially superficial world, the effects of surfaces become much more significant than the inherent properties of materials themselves. One of the most compelling examples of this scalar shift in properties comes from the science of colloids: suspensions of nanoparticles in a fluid medium.
The ruby red liquid in the photo above was prepared by Michael Faraday in the mid-1800s, while he was researching the interaction of matter with light. As he was preparing thin films of gold, Faraday noticed that the solution he used for the chemical etching of the metal exhibited a bright red hue. He also noted that the color of the liquid varied depending on the reaction conditions, shifting from red to purple and eventually to deep blue. Faraday deduced that the liquid’s color originated from minuscule gold particles suspended in the solution — so small that they remained invisible to all observation instruments available at his time.
Today, we know that the striking colors of gold colloids are the result of a physical phenomenon called surface plasmon resonance, in which the electrons on the surface of metal particles absorb specific frequencies of visible light. Because this is a superficial effect, the color of the colloid changes depending on the size and shape of the constituent particles, despite the material itself remaining the same.
The chameleonic properties of gold have been known since ancient times. Skilled artisans exploited them in glassmaking, crafting precious cups, vases, and exquisite stained glass windows. Today, gold nanoparticles play a foundational role in advanced biomedical technologies. You may not know that the pink lines that show up in pregnancy tests (or COVID-19 self-tests) are actually the result of gold nanoparticles, precisely engineered to attach to specific biochemical compounds. If you ever took one of these tests, you have directly witnessed the power of gold colloids in detecting and revealing information about your body.
In a short essay I recently published in collaboration with my friends at Aksioma — the Institute of Contemporary Art in Ljubljana — I explored the idea of colloids as the model for a new ontology of the body. One of the reasons why gold colloids are so fascinating to me is how they demonstrate that surfaces matter much more than we typically give them credit for. At the nanoscale, surfaces can profoundly influence the properties of bodies, challenging the common perception of them as mere abstract boundaries of things.
What might be the significance of an ontological shift from depth to surface? And how could this shift impact the way we perceive our bodies and identities? Surfaces, after all, are the interface through which we connect with the outside world, bridging the gap between ourselves and other human and non-human bodies. Maybe, not unlike minuscule crystals of gold, our own reality doesn’t go much deeper than the electrons resonating on the surface of our skin.
References
David Thompson, Michael Faraday's Recognition of Ruby Gold: the Birth of Modern Nanotechnology. Gold Bulletin 40.4 (2007): 267–269.
Laura Tripaldi, Colloidal Ontologies. The Gendered Body at the Interface of Matter. PostScriptUM, Aksioma, Ljubljana, 2023.