Oxford Physics: Soft and Biological Matter

Julia Yeomans FRS

Professor of Physics
Pauline Chan Fellow in Physics, St Hilda's College

Drop Video


Theory and simulations: Lisa Moevius and Julia Yeomans
Experiments: Yahua Liu and Zuankai Wang (City University of Hong Kong)
(Nature Physics 10 515 (2014))

Pancake bouncing occurs because fluid is pushed between the posts by the inertia of the falling drop. It is then rectified and pushed back out by the superhydrophobic posts. The drop bounces in a pancake shape if

  1. There is enough energy to lift the drop
  2. The fluid gets back to the surface approximately when the drop reaches its maximum extension.

Pancake bouncing occurs for many different post geometries. If the posts are tapered (fatter at the bottom) it works particularly well. This is because the horizontal fluid motion and the vertical fluid motion can both be modelled as harmonic springs. Therefore the time scales for spreading and retraction are independent of velocity and the right time scale balance is achieved over a wide range of velocities.