Particles Separate When Flowing Downhill

When particle-laden fluids like a mudslide flow downhill, even well-mixed particles can wind up separating. To explore how this works, researchers put glass spheres–of two different sizes but equal density–into silicone oil and let it flow down an incline. Their initially well-mixed oil soon turned red as the larger red particles overtook the smaller blue particles near the front. Looking at the flow from the side, the team observed a Brazil-nut-effect-like behavior where the larger particles move toward the top of the flow. That’s where the flow speed is fastest, and the particles are congregating there despite being denser than the oil carrying them! (Video and image credit: Y. Ba et al.)