Whether water sticks to a surface or not depends on multiple factors, and friction is a pretty important one. If the chemical composition of a surface is uniform, the expectation is that water will flow away pretty fast. A more complex surface should be doing that less well. New research revealed that the situation is not as clear-cut.
The surface in question was made of silicon and coated with a “self-assembled monolayer” or SAM. These SAMs are molecular layers that can move about like a liquid, but are strongly tethered to the surface below. SAMs act like a lubricant between the water droplets and the surface.
The team was able to control how much of a SAM was forming on the surface, knowing at a nanoscale level the complexity (heterogeneity) of the surface. They found that with too great or too small a SAM, water will flow quickly.
"Our work is the first time that anyone has gone directly to the nanometer-level to create molecularly heterogenous surfaces," lead author and doctoral researcher Sakari Lepikko, from Aalto University, said in a statement. "We found that, instead, water flows freely between the molecules of the SAM at low SAM coverage, sliding off the surface. And when the SAM coverage is high, the water stays on top of the SAM and slides off just as easily. It's only in between these two states that water adheres to the SAMs and sticks to the surface."
Thanks to SAMs, the team was able to create the slipperiest liquid surface in the world, a discovery that the researchers believe might lead to many intriguing applications. Droplet-repellent surfaces are an extremely frequent requirement.
"Things like heat transfer in pipes, de-icing and anti-fogging are potential uses. It will also help with microfluidics, where tiny droplets need to be moved around smoothly, and with creating self-cleaning surfaces. Our counterintuitive mechanism is a new way to increase droplet mobility anywhere it's needed," Lepikko added.
But do not expect these applications to materialize immediately. SAMs have potential but also limitations, and it is important to acknowledge them. But studying them might allow scientists to overcome them or find something similar that lacks the same problems, as Lepikko explained.
"The main issue with a SAM coating is that it's very thin, and so it disperses easily after physical contact. But studying them gives us fundamental scientific knowledge which we can use to create durable practical applications."
The study is published in Nature Chemistry.