Gold flakes expose the secret forces binding our world together

In one of Chalmers’ physics labs, doctoral student Michaela Hošková demonstrates the setup. She holds a glass container filled with millions of microscopic gold flakes suspended in a salt solution. With a pipette, she places a single drop of this liquid on a gold-coated glass plate positioned under an optical microscope. Almost immediately, the gold flakes are drawn toward the surface, but they stop just short of touching it, leaving behind extremely thin gaps measured in nanometers. These tiny cavities act as miniature light traps, causing light to reflect back and forth and produce vivid colors. When illuminated by the microscope’s halogen lamp and analyzed through a spectrometer, the light separates into different wavelengths. On the connected monitor, flakes shimmer and shift between hues of red, green, and gold as they move across the surface.

Studying ‘nature’s glue’ using light trapped in tiny cavities

“What we are seeing is how fundamental forces in nature interact with each other. Through these tiny cavities, we can now measure and study the forces we call ‘nature’s glue’ — what binds objects together at the smallest scales. We don’t need to intervene in what is happening, we just observe the natural movements of the flakes,” says Michaela Hošková, a doctoral student at the Department of Physics at Chalmers University of Technology and first author of the scientific article in the journal PNAS in which the platform is presented.

The light confined inside these nanoscopic cavities allows scientists to explore a delicate equilibrium between two competing forces: one that pulls the flakes toward the surface and another that pushes them apart. The attractive force, known as the Casimir effect, causes the gold flakes to draw closer together and toward the substrate. The opposing electrostatic force, generated by the charged particles in the salt solution, prevents them from sticking completely. When these forces reach perfect balance, a process called self-assembly occurs, creating the cavities that make this phenomenon visible.

“Forces at the nanoscale affect how different materials or structures are assembled, but we still do not fully understand all the principles that govern this complex self-assembly. If we fully understood them, we could learn to control self-assembly at the nanoscale. At the same time, we can gain insights into how the same principles govern nature on much larger scales, even how galaxies form,” says Michaela Hošková.

Gold flakes become floating sensors

The Chalmers researchers’ new platform is a further development of several years of work in Professor Timur Shegai’s research group at the Department of Physics. From the discovery four years ago that a pair of gold flakes creates a self-assembled resonator, researchers have now developed a method to study various fundamental forces.

The researchers believe that the platform, in which the self-assembled gold flakes act as floating sensors, could be useful in many different scientific fields such as physics, chemistry and materials science.

“The method allows us to study the charge of individual particles and the forces acting between them. Other methods for studying these forces often require sophisticated instruments which cannot provide information down to the particle level,” says research leader Timur Shegai.

Can provide new knowledge on everything from medicines to biosensors

Another way to use the platform, which is important for the development of many technologies, is to gain a better understanding of how individual particles interact in liquids and either remain stable or tend to stick to each other. It can provide new insights into the pathways of medicines through the body, or how to make effective biosensors, or water filters. But it is also important for everyday products that you do not want to clump together, such as cosmetics.

“The fact that the platform allows us to study fundamental forces and material properties shows its potential as a truly promising research platform,” says Timur Shegai.

In the lab, Michaela Hošková opens a box containing a finished sample of the platform. She lifts it with tweezers and shows how easily it can be placed in the microscope. Two thin glass plates hold everything needed to study nature’s invisible glue.

“What I find most exciting is that the measurement itself is so beautiful and easy. The method is simple and fast, based only on the movement of gold flakes and the interaction between light and matter,” says Michaela Hošková, zooming the microscope in on a gold flake, the colors of which immediately reveal the forces at play.

How the researchers study ‘nature’s invisible glue’

Gold flakes approximately 10 micrometers in size are placed in a container filled with a salt solution, i.e. water containing free ions. When a drop of the solution is placed on a glass substrate covered with gold, the flakes are naturally attracted to the substrate and nanometer-sized cavities (100-200 nanometers) appear. Self-assembly occurs as a result of a delicate balance between two forces: the Casimir force, a directly measurable quantum effect that causes objects to be attracted to each other, and the electrostatic force that arises between charged surfaces in a salt solution.

When a simple halogen lamp illuminates the tiny cavities, the light inside is captured as if in a trap. This allows the researchers to study the light more closely using an optical microscope connected to a spectrometer. The spectrometer separates the wavelengths of the light so that different colors can be identified. By varying the salinity of the solution and monitoring how the flakes change their distance to the substrate, it is possible to study and measure the fundamental forces at play. To prevent the saline solution with the gold flakes from evaporating, the drop of gold flakes and saline are sealed and then covered with another glass plate.

The platform was developed at Chalmers’ Nanofabrication Laboratory, Myfab Chalmers, and at the Chalmers Materials Analysis Laboratory (CMAL).

More about the research

The scientific article Casimir self-assembly:A platform for measuring nanoscale surface interactions in liquids has been published in PNAS (Proceedings of the National Academy of Sciences). It was written by Michaela Hošková, Oleg V. Kotov, Betül Küçüköz and Timur Shegai at the Department of Physics, Chalmers University of Technology, Sweden, and Catherine J. Murphy at the Department of Chemistry, University of Illinois, USA.

The research was funded by the Swedish Research Council, the Knut and Alice Wallenberg Foundation, the Vinnova Centre 2D-Tech and Chalmers University of Technology’s Nano Area of Advance.