The coffee-ring effect has implications for the manufacture of high tech materials such as fuel cells, displays, and sensors. That’s because a lot of these devices are made by depositing a coating onto a surface, and one of the most popular ways of depositing a coating is by drop-casting – that is depositing lots of tiny droplets onto the surface.
Drop-casting will be more familiar to most of us as ink-jet printing – these days a rather low-cost, low-tech method of depositing liquid onto a surface in defined patterns. If the drop-cast method can be perfected, it could lead to efficient, cheap solutions for the widespread production of a range of useful devices.
Unfortunately the drop-casting method is subject to the coffee-ring effect. Each tiny droplet dries in a ring, making the surface uneven and affecting the performance of the device.
Now, Nicolas Plumeré and his team at Ruhr-Universität Bochum, Germany, have come up with a solution. "Our work is aimed at bypassing the coffee ring effect so that even deposition of materials is achieved", he says. They aren’t trying to change the drying properties of a water droplet, however. "Drying still takes place at the three phase boundary”, he explains. “Instead our strategy consists of locking the materials to be deposited in the volume of the droplet so that they are not moving with the water flow within the droplet."
They achieve this by using small molecules that polymerise when mixed together in the solution. This reaction means that the droplet turns into a gel. The gel in turn stops particles from flowing to the edges of the droplet, enabling them to be deposited evenly.
This article is free to read in our open access, flagship journal Chemical Science: Huaiguang Li et al., Chem. Sci., 2018, Advance Article. DOI: 10.1039/C8SC03302A. You can access all of our ChemSci Picks in this article collection.