Lotus-inspired design to neutralize the coffee-ring effect

Coffee rings occur when the beverage spills over its mug and assumes the shape of the mug’s base. As the water evaporates and the ring dries, the coffee residue does not evenly distribute over the surface but instead concentrates in the outline. This aptly named coffee-ring effect is unwanted in many applications that require uniform deposition of particles, like inkjet printing and producing DNA microarrays.

Inspired by the ultrahydrophobicity of lotus leaves, Wang et al. designed a microwell array that neutralized the coffee-ring effect and enabled uniform particle distribution.

“As the volatile species evaporates, pinning of the contact line gives rise to capillary flows that transport non-volatile solutes to the contact line,” said author Lin Du. “This phenomenon compromises the overall performance of industrially relevant manufacturing processes involving evaporation such as printing, biochemical analysis, and manufacturing of nano-structured materials through colloidal and macromolecular patterning.”

A material’s texture influences its wettability, the measure of how well a liquid can maintain contact with a solid surface. A lotus leaf has an inconsistent, rough surface that water cannot permeate, so instead a droplet rests above the ridges in a near perfect sphere. By using uniform microwells with gradually sloped walls, the researchers took advantage of Wenzel wettability, modifying the lotus design so the droplets easily spread and fill the wells.

In tests, the structure performed well. Initially, internal vortex flow dominated the movement of particles within a droplet, but they eventually settled in an even film as the volatile species evaporated.

“We believe that structural properties originating from animals and plants in nature provide promising directions for studying the evaporation of nonvolatile solute droplets,” said Du.

Source: “Neutralizing coffee-ring effect using gradual structures for uniform particle distribution,” by Jie Wang, Zhenghao Li, Xiaoyi Shang, Zhenqing li, Lin Du, and Dawei Zhang, Physics of Fluids (2024). The article can be accessed at https://doi.org/10.1063/5.0190486 .