Sessile Drop Evaporation

Evaporation on soft surfaces

Contact person: M.Sc. Marcus Lopes

Evaporation is one of the many process related to the interface research. During evaporation mass transport through a liquid-gas interface will take place, which is connected to great energy transport and therefore one of the biggest research fields in thermodynamics. The research on sessile drop evaporation allows the development of technical applications for power generation (cooling), medical application (DNA analysis methods and lab-on-a-chip devices) and mass production (Ink-Jet printing, printed electronics and rapid prototyping).

We investigate small droplets of liquids (pure, mixtures, suspensions and emulsions) on substrates (rigid, deformable and microstructured) using video microscopes, laser scanning confocal microscopy, tensiometers, contact angle measurement devices and self-written analysis code. We are active working on topics such as: evaporation control using viscoelastic surfaces, coffee-stain effect and microdroplets.

Using tunable viscoelastic surfaces we are able to influence the deformation of the substrate underneath an evaporating droplet and switch from fast evaporation mode to slow evaporation mode. This relative simple method enables the control of the sessile droplet evaporation.

Left: Schematic Scheme of a sessile drop on a deformable substrate showing the contact radius a, contact angle θ, radius of curvature k, and the directions of the z and r coordinates. The inset shows the deformation due to the balance between liquid surface tension ᵞ and yield stress σ, which we can simulate and measure. Right: Evaporation time of sessile droplets on polydimethylsiloxane surfaces (PDMS) with different young modulus E in dependence of the receding contact angle for an initial droplet volume of 0.12µl.
Left: Schematic Scheme of a sessile drop on a deformable substrate showing the contact radius a, contact angle θ, radius of curvature k, and the directions of the z and r coordinates. The inset shows the deformation due to the balance between liquid surface tension ᵞ and yield stress σ, which we can simulate and measure. Right: Evaporation time of sessile droplets on polydimethylsiloxane surfaces (PDMS) with different young modulus E in dependence of the receding contact angle for an initial droplet volume of 0.12µl.
  • Lopes, M.C., Bonaccurso, E.: Evaporation control of sessile water drops by soft viscoelastic surfaces, Soft Matter, 8, 7875-7881 (2012)

Evaporation of water/ethanol mixtures

Contact person: PD Dr. rer. nat. Elmar Bonaccurso

The evaporation of sessile drops on solid surfaces has attracted renewed interest in the last decade. In particular, drops with diameters much smaller than 1 mm are in the focus of fundamental and applied research, due to the development of microfluidics and inkjet technology. Phenomena that are negligible at the macroscopic scale start to be relevant or even dominate at the small scale. This holds for drops of pure liquids, as well as for mixtures, for emulsions, and for dispersions.

For example, when a drop of a binary water/ethanol mixture evaporates at a fixed relative humidity (RH), a fraction of ethanol remains in the drop until the end of evaporation, although the vapor pressure (VP) of ethanol is higher than that of water. This can be concluded from increased evaporation rates of mixtures as compared to pure water drops. The evaporation of this residual ethanol seems to be controlled by its diffusion from inside the drop to its surface and less by convective flow inside the drop. Further, with a high initial ethanol concentration and at high RH, water vapor condenses onto the drop at the beginning of the evaporation. This would never happen on a pure water drop, because the VP of water in the drop is always larger than the pressure of saturated water vapor. Ethanol, instead, decreases the VP of water upon mixing, and thus allows the condensation of water. A high vapor pressure of ethanol in the background also decreases the total evaporation time of water drops, an effect which could improve the efficiency of spray cooling.

When studying evaporation or condensation of vapor from/to water drops on solid hydrophobic surfaces two cases can take place: (i) the volume of drops, V, is found to increase linearly with time, Vœ t , if drops are part of larger arrays and have close neighbours; (ii) their volume changes according to V œ t3|2 if they are single drops. Thus, the spacing between drops affects their growth or shrinking, which can be shown experimentally, theoretically and via simulations.

(a) Schematic of the evolution of the contact angle of a water/ethanol drop during evaporation. (b) Different evolution of the volume of a water drop evaporating or condensing in an array or as a single drop.

  • Liu C.J., Bonaccurso E.: Microcantilever sensors for monitoring the evaporation of microdrops of pure liquids and mixtures, Review of Scientific Instruments, 81, 013702 (2010)
  • Liu C.J., Bonaccurso E., Butt H.-J., Evaporation of sessile water/ethanol drops in a controlled environment, Phys. Chem. Chem. Phys., 10, 7150 (2008)
  • Butt H.-J., Golovko D.S., Bonaccurso E.: On the derivation of Young's equation for sessile drops: Nonequilibrium effects due to evaporation, Journal of Physical Chemistry B, 111, 5277 (2007)