How to get rid of the coffee stain effect

The coffee stain effect is a well-known effect in physics and daily life in which a dark colored edge remains when a liquid containing particles evaporates. This is caused by an “avalanche” of particles moving toward the outer edge, scientists at the University of Twente showed in an earlier study. In inkjet and 3D printing, this is an unwanted effect. Now researchers have shown that the effect can be suppressed by modifying the surface using an oily coating, according to the results published in the procedures of the National Academy of Sciences (PNAS)

Previous work by the UT research group showed that if a particle-laden drop evaporates, the particles begin to move toward the edge of the drop. At first, this is a slow, steady motion, but as soon as the droplet loses height by evaporation, the particles rush to the edge in a disorderly fashion, like an avalanche. After complete evaporation of the liquid, a dark ring remains. In fact, the diameter of the drop dictates this process very early. The researchers tried to avoid this, because in many cases, a homogeneous distribution is required without a dark, ring-shaped area. An oil-wet surface is the answer, new results show.

Also in this case, the drop has an edge, but is limited by a layer of oil that does not evaporate. It also prevents water at the edge of the drop from evaporating quickly. This in turn prevents the particles from moving to the edge. They even move to the other side, from the edge to the inside of the drop. Once all the water evaporates, the particles uniformly cover the surface instead of forming a ring. The researchers also saw another effect: the oily layer completely covers the drop. In that case, a concentration of particles is formed, a so-called nano-eye or “coffee eye”. This could be a desired effect for some applications, such as the assembly of nanoparticles. By adding surfactant to the drop, the final particle deposition can be manipulated from the concentrated coffee ring to a homogeneous distribution.

Transfer of diseases

The Fluid Physics group has a long-term research relationship with printer manufacturer Océ, now called Canon Production Printing. This company adds a special layer to the substrate (such as paper) before the printer applies drops. The new research results are invaluable in improving processes. New insights and improved control are also valuable for other applications, such as 3D printing and surface design. A better understanding of how liquids evaporate on a surface can also provide more insight into how viruses are transferred from one person to another.

The research was conducted in the Fluid Physics group and at the Max Planck Center for Complex Fluid Dynamics. A week before publication, the paper’s lead author Yaxing Li (Anhui 1991) successfully defended his Ph.D. thesis, “Evaporation of multi-component drops”.