This image shows Rayleigh-Benard convection cells visualized by taking a long exposure image of flakes of aluminum in a 1mm deep layer of silicone oil! The oil is heated from a smooth bottom surface (whose smoothness gives way to the hexagonal shapes).

Image sourced from the NOAA page: https://psl.noaa.gov/outreach/education/science/convection/RBCells.html, which itself sourced the image from page 83 of Milton Van Dyke’s An Album of Fluid Motion (published by The Parabolic Press).