Controlling the Coffee-Ring Effect

Interestingly, by employing photo-sensitive surfactants, the deposition patterns from drying drops could be tuned on-demand using light [3]. A wide range of patterns from rings to homogeneous disks was achieved by varying the irradiation time. Patterning from single drops is photoreversible upon changing the wavelength while spatial control in multi-drop arrays was achieved using simple structured light patterns.

Particle deposition from drying drops of colloidal dispersions phenomenologically seems to be a rather simple problem. However, our everyday experience is surprising: ring-shaped deposits are observed after the evaporation of spilled coffee drops.

The so-called Coffee-Ring Effect (CRE) was explained based on the evaporation-driven capillary flow of liquid toward the pinned contact line of the evaporating drop, transporting dispersed particles to its edge [1]. Motivated either by its scientific challenge or by its technological relevance, researchers explored ways of controlling the CRE.

The generic character of the reported effects could possibly offer a versatile and straightforward way to   direct particle patterning at solid or liquid surfaces off different kinds. This possibility is currently under exploration.

Since the CRE is directly linked to the hydrodynamics within the drying drop, a common method to suppress the effect is the modification of the flow patterns using various additives such as polymers, co-solvents etc. In particular surfactants, typically at concentrations higher than the critical micellar concentrations (CMC), have been frequently employed.

On the other hand, surfactants, at concentrations lower than the CMC, modify the surface properties of the suspended particles. We have recently discovered that these modifications affect in turn the interactions between the colloids and the liquid/gas and liquid/solid interfaces, which essentially dictate the dry deposition patterns.

For like-charged particle/surfactant mixtures, coffee rings were always observed. For oppositely charged systems, three pattern regions were observed. For low and high surfactant concentrations, deposits were ring-shaped. For intermediate concentrations, homogeneous disk patterns were obtained, resulting from surfactant-induced changes in the electrostatic and hydrophobic particle properties [2]. The observed effects are robust and hold for various surfactant/particle mixtures.​