Time lapse movie of pattern formation in a drying particulate suspension. A mixture of silicon dioxide particles and water is spread out on a horizontal glass slide, and evaporation generates a flow from the center towards the edge of the droplet. Large particles become pinned to the substrate by the fluid meniscus, and act like obstacles to the flow. Gradually smaller particles become trapped by the meniscus, and deposits grow sideways into irregular stripes. Patterns like these can be seen on the sides of muddy cars on rainy days.