1. Wetting and Spreading on Patterned Surfaces
Alexandre Dupuis, Halim Kusumaatmaja, Julia Yeomans
University of Oxford

2. The simulations solve the hydrodynamic equations of motion for the drop.
Drop equilibrium is described by a free energy: allowing us to
model surface tension and contact angles.
Input parameters to the simulations are drop and substrate dimensions,
surface tension, fluid density, surface tension and fluid viscosity.

3. Drop spreading on a chemically striped surface: the grey and white stripes have different contact angle (click for movie)

4. Experiments (J. Léopoldès and D. Bucknall) Drops on striped surfaces 1
Experiments (J.Léopoldès and D.Bucknall) Drops on striped surfaces 1. stripes narrow compared to drop radius 4. stripes of width of order drop radius 8. stripes able to contain a drop
64o / 5o

5. Simulations: impact near the centre of the lyophobic stripe click for movie

6. Simulations: impact near a lyophilic stripe click for movie

7. The final drop shape depend on the point of impact.
Quantitative agreement between simulations and experiments
Simulation vs experiments
Evolution of the contact line

8. Effect of the jetting velocity
Same point of impact in both simulations
With an impact velocity
t=0
t=10000
t=20000
t=100000
With no impact velocity

9. Drop pushed gently across a chemically striped surface (click for movie)
60o /110o

10. Mottle: irregular spacing of jetted drops which degrades image quality

11. Drops in a square array, but with small randomness in their points of impact,
lead to mottle (click for movie)

12. A drop can be confined by a hydrophobic grid – the small circle denotes the point of impact and the dark line the final drop position

13. Confining a drop using a hydrophobic grid

14. A hydrophobic grid prevents mottle (click for movie)

15. Experiments (David Bucknall and Julien Leopoldes)
The bottom half of the substrate is patterned by hydrophobic squares

16. Superhydrophobic substrates: patterning with micron-sized posts increases the contact angle of a hydrophobic surface
Bico et al., Euro. Phys. Lett.,
47, 220, 1999.

17. A superhydrophobic surface
Mathilde Callies and David Quere 2006

18. Two droplet states suspended drop: lies on top of the posts
collapsed drop: lies between the posts
He et al., Langmuir, 19, 4999, 2003

19. Substrate geometry
qeq=110o

20. Equilibrium droplets on superhydrophobic substrates
Suspended, q~160o
Collapsed, q~140o
On a homogeneous substrate, qeq=110o

21. A drop undergoes a transition from the suspended to the
collapsed state as it evaporates (click for movie)

23. Drop pushed gently across a superhydrophobic surface
collapsed
suspended