Drops on slip-pery surfaces

Drops on slip-pery surfaces

A recurrent theme on this blog has been posts describing talks presented at conferences. In keeping with that tradition, I’ll use this post to briefly introduce two talks from a recent-ish conference that I had the privilege of attending. Both talks relied on the use of what are known as slippery-liquid-infused-porous (SLIP) surfaces, or lubricant-impregnated surfaces. According to the folks who make them, SLIP surfaces ‘combine a lubricated film on a porous solid material to create low-cost surfaces that exhibit ultra-liquid repellency, self-healing, optical transparency, pressure stability, and self-cleaning’. While this sounds impressive and worthy of further unpacking, I’ll leave the discussion on the science behind such specialised surfaces for a future blogpost, and keep this one focused on the fluid dyamics instead, i.e. the drops.

Making a SLIP surface. Image credit: Wyss institute (Original here)

The first talk, by Zuzana Brabcova, discussed her recent results published in these two papers (one, two). Basically, there exist two popular techniques to electrically manipulate drops on surfaces. The first is electrowetting, where the ions in a conducting liquid droplet are transported by the electric field, resulting in an electrophoretic interfacial force. The other one is dielectrowetting, where the electric field acts on dipoles (in an effectively dielectric liquid droplet) at the solid-liquid interface, resulting in a dielectric interfacial force. One of the problems when using either mechanism is the observed contact-line hysteresis, which prevents a smooth transition from wetting to dewetting. This talk demonstrated that, by using a SLIP surface combined with custom spiral electrodes, the hysteresis for electrowetting and dielectrowetting could be completely removed. By modifying the applied signal, near-axisymmetric wetting was also realised, resulting in the formation of a circular thin film on demand.

The second talk, by Jian Guan, looked at translating drops on SLIP surfaces. To induce controlled drop motion on a SLIP surface, a patterned surface (before the lubricant layer is applied) was used, resulting in an uneven lubricant distribution on the surface. For a V-shaped channel, droplets moved towards the regions of higher lubricant deposition to minimise surface energy; these regions were invariably near the edges of the channel. The size of the droplet determined its net motion. For small droplets, either side of the channel was the energetically-preferred destination, while for larger droplets which straddled both ends of the channel, motion into along the channel was observed. Thus the motion and final equilibrium position of the droplet could be determined in advance using patterned SLIP surfaces. These results are discussed in further detail in the author’s recent paper on the topic (link).