Why do temperature and concentration differences push liquid around?
Surface tension is not uniform when a liquid surface has patches at different temperatures or concentrations. Evaporation cools the liquid surface unevenly, and in a mixture the solvent leaving faster than the solute leaves behind a more concentrated solution in some places than others. Both effects change surface tension from point to point. The liquid at the surface is then pulled toward regions of higher surface tension, dragging underlying liquid with it. This surface-tension-driven motion is called Marangoni flow [2][5].
In a drying droplet, the same evaporation that drives capillary flow toward the pinned contact line also sets up these surface-tension gradients. The capillary flow and the Marangoni flow can point in different directions, so the internal motion is a competition between them rather than a single current [2][5].
What does the competition look like inside a droplet or a micropillar evaporator?
In a sessile droplet, evaporation induces a capillary flow that carries liquid outward toward the contact line, and this flow drags suspended particles with it. Evaporation-induced temperature or chemical gradients at the liquid–gas interface set up surface-tension differences that drive an additional internal flow. The two flows interact, and the resulting pattern helps determine where material ends up in the dried stain [5].
In a micropillar evaporator, a model that couples liquid flow with heat transfer in both liquid and solid predicts that when the Marangoni number is high enough, thermocapillary convection creates circulations beneath the liquid–vapor interface. These circulations produce periodic reverse surface flows on the interface, and the model links them to a sharp drop in evaporator temperature that does not appear when thermocapillarity is switched off [1]. The Marangoni number is a dimensionless measure of how strongly surface-tension differences drive flow compared with how strongly viscosity resists it; a high value means the surface-tension effect is strong.
How do dissolved substances change the picture?
When the evaporating liquid is a solution rather than a pure liquid, solvent loss changes the solute concentration, and that concentration gradient adds its own Marangoni contribution. A model of capillary flow in open rectangular microchannels connected to reservoirs accounts for solvent evaporation, Marangoni flows from gradients in solute concentration and temperature, and finite reservoir size. It predicts significant differences between pure liquids and solutions, and finds that Marangoni flows promote more uniform solute deposition after the solvent evaporates [4].
The same model shows that in the experiments it compares with, the main way evaporation shifts the meniscus position is by raising viscosity as solute concentrates, not by the Marangoni flow itself [4]. That is a useful reminder that a flow can be present and still not be the dominant effect on a particular measured quantity.
When does capillary flow still dominate?
In very small pores, capillary forces are strong, and drying can proceed in regimes described as capillary-driven or evaporation-driven. Simulations of drying in nanoporous media that include evaporation, condensation, vapor diffusion, and the Kelvin effect—the change in vapor pressure over curved liquid surfaces in tiny pores—explore these regimes and how the Kelvin effect influences desaturation and creates preferential paths for gas flow [3]. This work focuses on capillary imbibition against evaporation rather than on density differences from multicomponent evaporation or cooling, so it does not directly measure the Marangoni competition described above.
The overall picture is that the balance depends on scale and conditions: strong surface-tension gradients can create internal circulations that reverse or reshape capillary flow [1][2], while in other settings capillary pressure and viscosity changes dominate the observed motion [3][4]. The studies here identify the mechanisms and some conditions under which each matters, but they do not settle a single threshold that applies across droplets, micropillars, channels, and nanoporous media.
Sources used in this answer
Interplay of capillary and Marangoni flows in micropillar evaporation
A coupled model of micropillar evaporation predicts that high Marangoni-number thermocapillary convection creates circulations and periodic reverse surface flows that compete with capillary flow and cause a sharp temperature drop absent when thermocapillarity is disabled.
Evaporation Dynamics of Sessile Droplets: The Intricate Coupling of Capillary, Evaporation, and Marangoni Flow
A theory of sessile droplet evaporation explains the pseudostable apparent contact angle by accounting for liquid evaporation and internal flow induced by both capillary and Marangoni effects.
Drying in nanoporous media with Kelvin effect: Capillary imbibition against evaporation by smoothed particle hydrodynamics method
Pore-scale simulations of drying in nanoporous media explore capillary-driven and evaporation-driven regimes, including the Kelvin effect's influence on desaturation and gas-flow paths.
Capillary flow of evaporating liquid solutions in open rectangular microchannels
A model of capillary flow of evaporating solutions in open microchannels predicts that Marangoni flows from solute-concentration and temperature gradients promote more uniform solute deposition, while viscosity increase with solute concentration is the principal way evaporation shifts the meniscus in the compared experiments.
Evaporation-driven liquid flow in sessile droplets
A review of sessile droplet evaporation describes how evaporation-induced temperature or chemical gradients at the liquid–gas interface drive internal flows that drag suspended material and help control deposition patterns.