The non-equilibrium physics of colloidal sedimentation

Albert Einstein, the father of relativity, was also the hero of another great scientific discovery: Brownian motion. The latter is the disordered motion of small particles - even smaller than a hundredth of the diameter of a hair - inside a liquid such as water. Thanks to Einstein and his contemporaries, we now know that these particles are bumped incessantly by the molecules of the liquid, and it is because of these molecular bumps that they move in a seemingly random way on a larger scale. One of the crucial experiments for the confirmation of this molecular hypothesis was carried out by Jean Perrin. He understood that these molecular collisions would be so strong to avoid the accumulation of the particles all at the bottom of a container and would rather cause a more continuous distribution: more particles at the bottom and a decreasing yet non negligible number when moving up in the rest of the container. This is the so-called sedimentation profile, for the measurement of which Perrin was awarded the Nobel Prize for Physics in 1926. During the formation of the profile, each particle moves in a mesmerizing, and seemingly random way, not only because of Brownian motion, but also because it feels the motion of the surrounding particles, in the same way as each particle in a suddenly flipped snow globe wanders and changes its direction. Despite the extensive investigation performed over the last century, we still do not know how to explain and control the fluctuations occurring during the formation of the profile. Gaining this knowledge would be important for fundamental reasons, as well as for applications, sedimentation being a very common process spontaneously occurring in the natural environment and widely exploited in many industrial applications, such as water purification or wine production. In this project, by walking in the footsteps of Einstein and Perrin, we will combine novel quantitative microscopy approaches and advanced computer simulations to capture in full detail the richness of the sedimentation process in model samples that will be suitably produced in our laboratory. The key idea of our experiments is to combine the classical lateral observations of the sample with observations from above. Our expectation, based on previous theoretical work, is that this new observation geometry will provide the missing piece to the understanding of the fluctuations occurring during sedimentation. Beyond answering fundamental outstanding questions, our project will make available novel optical tools for academic research and industry. Some of our experiments will also be performed onboard the International Space Station, where we will be able to perform our studies in microgravity as well as in controlled artificial gravity conditions. Dr. Enrico Lattuada, Univ.-Prof. Dr. Roberto Cerbino,

Imagine dropping a handful of sand into a glass of water. At first, the sand swirls around, but eventually, it settles at the bottom. This simple act reveals a fascinating world of physics that has intrigued scientists for over a century. Albert Einstein, famous for his theory of relativity, also helped explain why tiny particles in liquids do not just sink straight to the bottom. It turns out that these particles are constantly jostled by even tinier water molecules, causing them to incessantly dance around in what we call Brownian motion. This microscopic dance creates a delicate balance with gravity, resulting in more particles near the bottom but some managing to stay suspended higher up. Now, picture shaking a snow globe. The swirling flakes eventually settle, but not all at once. They get far and close, sometimes bumping into each other, creating intricate patterns as they fall. Similarly, as particles in a liquid settle, they interact in complex ways, influencing each other's paths. While we have known about this process for a long time, there is still much to understand about how exactly these particles move and settle. Understanding this is not just about satisfying scientific curiosity; it has real-world applications in everything from purifying drinking water to making the perfect wine. Our team set out to learn more about this open problem. We used a special microscope to watch particles after they had settled, almost like spying on them from above, and we studied the complex collective motion of the particles. To this aim, we created a powerful computer program, capable of crunching long microscopy videos in seconds rather than taking hours. We have made this tool freely available, hoping it will help other scientists and industries unlock discoveries about how particles behave in liquids.