Wetting and Spreading
With the advancement of current technologies, the question of how micron scale liquid droplet moves across the surface has become more and more important. This is especially true, for example, in ink-jet or microfluidics technologies. Most studies in the past have focussed on the static behaviour of this problem (on homogeneous substrate, this is given by the well-known Young's law) while the dynamics are relatively unknown, perhaps due to the fact that it is almost impossible to solve the droplet equations of motion analytically. In our group, we solve the droplet equations of motion by using a mesoscale simulation based on a lattice Botzmann algorithm developed earlier by Swift et al. [1]. This approach allows us to link the hydrodynamic and the thermodynamic aspects of the problem.
The pictures above show the two equilibrium shapes [2] that the droplet can possess on certain chemically patterned substrates (that is if the stripes are comparable to the droplet radius). The butterfly shape is preferred if the droplet impact point falls close to the centre of the hydrophobic stripe (light grey), while the lozenge shape is preferred otherwise. This is the static behaviour of the system. If we now put the system under Poiseuille flow, we observe a very interesting dynamics in which the droplet transforms from one shape to another as it moves across the patterned substrate (click the images for animations).
Topological heterogenities were also
considered [3] and we found that
topological patterning could be used to produce superhydrophobic
substrate. We also have a recent patent (no ) that shows how spreading
drops can be confined by hydrophobic barriers.
[1] M.R. Swift, E. Orlandini, W.R. Osborn and J.M. Yeomans, Phys. Rev. E 54, 5, pp.5041-5052 (1996) [www]
[2] J. Leopoldes, A. Dupuis, D. Bucknall and J.M. Yeomans, Langmuir 19 9818 (2003) [www]
[3] A. Dupuis and J.M. Yeomans, cond-mat/0401150 [www]