Pollen, water, and wind: Chaotic mixing in a puddle of water

This paper talks about how pine pollen grains dispersedin an approximately 1 m wide and 1 cm deep water puddle. The pollen has mixed due to wind blowing across the liquid surface, revealing a strikingly complex flow pattern. The flows revealed by nature’s tracer particles may influence circulation and nutrient distribution in puddles and small ponds.The flow patterns are generated by wind blowing across the puddle surface. This causes a shear stress at the atmospheric interface, which drives a flow in the liquid below. Chaotic mixing can occur if the wind direction changes over time. A fluid patch is repeatedly stretched and folded into itself, resulting in a drastic reduction of the length over which molecules have to diffuse. Pollen from pine trees was unusually abundant in 2012 when the photograph was taken, due to a largely dry spring which brought more than usual. Pine pollen is winged, and typically measures ∼50 μm in diameter. Pollen is transferred from a pollen cone to the ovule by wind or insects, and produces male sperm cells