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

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.

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

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

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

Simulations: impact near a lyophilic stripe click for movie

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

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

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

Mottle: irregular spacing of jetted drops which degrades image quality

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

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

Confining a drop using a hydrophobic grid

A hydrophobic grid prevents mottle (click for movie)

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

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

A superhydrophobic surface Mathilde Callies and David Quere 2006

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

Substrate geometry qeq=110o

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

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

Drop pushed gently across a superhydrophobic surface collapsed suspended