The ring is a default, not a law
In an ordinary drying droplet, liquid evaporates fastest near the pinned outer edge, and liquid flowing outward to replace it drags suspended particles along. Those particles pile up at the rim, leaving the dark ring we call a coffee stain [4][5]. The pattern is therefore a consequence of a specific internal flow, not an inevitable outcome of drying.
That is why the effect can be reversed: if you can change the flow field inside the droplet, you change where the particles finish. Studies of polymer solutions show that the final deposit depends on both the flow field and the solute concentration field, two factors that together determine the stain [5]. Interventions that alter either one can move the deposit away from the edge.
A second flow can push particles back inward
One route to redistribution is to set up a competing flow near the droplet surface. Drying aqueous droplets in a saturated water-vapor environment produced Marangoni flow, a surface-driven current, that homogenized nanoparticle deposition and removed the coffee-stain pattern [2]. In plain terms, the added flow acted as a counter-current to the usual outward transport, spreading particles more evenly rather than letting them accumulate at the rim.
This is a redistribution rather than a simple suppression: the particles are still carried by the liquid, but the dominant direction of travel changes. The result is a more uniform deposit instead of a concentrated ring [2].
Making the liquid harder to push changes the pattern
Another lever is the liquid's own resistance to flow. In inkjet printing of ceramics, researchers manipulated rheology, the way the suspension flows and thickens, along with drying behavior, to control particle migration during evaporation [3]. The same study notes that under some conditions the coffee-stain effect can even be enhanced, which shows the outcome is tunable in both directions rather than fixed [3].
Electrowetting offers a further handle: applying electrical control to a drying colloidal drop was examined as a way to manage particle self-assembly and suppress the unwanted inhomogeneous residue [1]. The aim in that work was control over where solids end up, which is the same goal as reversing or redistributing the stain [1].
What a redistributed deposit actually looks like
The change is visible in the final film. Instead of a single dark rim, a droplet dried under Marangoni-assisted conditions leaves nanoparticles distributed more evenly across the deposit [2]. In the cellulose nanocrystal case, the ordinary outcome is a ring, and the crystals are transported to the outer edge; the resulting ring can even be iridescent and three-dimensional because of how the particles pack [4]. That example shows how strongly the edge-collecting flow shapes the final structure, and therefore how much a change in flow can alter it.
Across these studies, the practical answer is consistent: the coffee-stain effect reverses or redistributes when an added flow, a changed vapor environment, or a changed flow resistance redirects particles before the liquid fully dries [2][3]. The exact new pattern, whether uniform, inward-shifted, or otherwise, depends on which of these levers is used and how strongly it competes with the outward edge flow.
Sources used in this answer
Suppressing the coffee stain effect: how to control colloidal self-assembly in evaporating drops using electrowetting
Electrowetting was examined as a way to control colloidal self-assembly in evaporating drops and suppress the inhomogeneous residue known as the coffee-stain effect.
Overcoming the “coffee-stain” effect by compositional Marangoni-flow-assisted drop-drying
Drying aqueous nanoparticle droplets in saturated water vapor generated Marangoni flow that homogenized deposition and overcame the coffee-stain pattern.
Rheological control of the coffee stain effect for inkjet printing of ceramics
In ceramic inkjet printing, manipulating suspension rheology and drying behavior controlled particle migration, and some conditions could even enhance the coffee-stain effect.
Droplets of cellulose nanocrystal suspensions on drying give iridescent 3-D “coffee-stain” rings
Cellulose nanocrystal droplets dried into iridescent three-dimensional rings, with the crystals transported to the outer edge in a coffee-stain effect.
Dynamical visualization of “coffee stain phenomenon” in droplets of polymer solution via fluorescent microscopy
Fluorescent microscopy visualized the flow field and solute concentration field in drying polymer droplets, identifying both as critical to the coffee-stain effect.