Condensation

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Drop condensation on soft surfaces

Contact Person: Dr. Julien Petit

The phenomenon of condensation is of great interest in a lot of practical applications such as dew harvesting, heat transfer, self-cleaning surface and pattern formation… In the EIP group we focus on the condensation process onto soft polymeric surfaces. These soft substrates are made of Sylgard 184 (Dow Corning©) that is a commercial two component silicone elastomer. The base (monomer) is mixed with a curing agent (cross-linker) whose the ratio determine the softness of the sample. Preliminary results have demonstrated that a sessile drop can deform the polymer film, due to the surface tension that pulls up the film and the Laplace pressure that compresses the film underneath the droplet [1] (figure 1-a). It results the formation of a wetting ridge at the rim of the droplet. First experimental results show that the softer the substrate is, the higher the nucleation density and the condensation rate are [2] (Figure 1-b). We suggest that the high nucleation density is the result of the soft surface deformation by the condensation of water droplets. Indeed, since the soft surface is deformed by the presence of condensed drops, each droplet reduces its surface free energy because the water-air interfacial area is reduced. Therefore, the activation barrier for nucleation is reduced and nucleation is promoted on soft substrates. Furthermore, the surface deformation leads to a delay or a suppression of droplets merging due to the presence of polymer lamellae between two drops. Otherwise, if merging of two adjacent drops occurs on a soft substrate, the relaxation time to come back to a spherical shape is longer than that on rigid surfaces as is depicted by the Figure 2-a. This is due to energy dissipation during the motion of the TPCL. These characteristics lead to higher and faster surface coverage on soft substrates (Figure 2-b).

Figure 1 – a) Scheme of the deformation of a soft substrate due to the surface tension (γL) and the Laplace pressure (ΔP). b) Four PDMS surfaces with cross-linking density decreasing from left to right, 20s after cooling from 21 to 1ºC at 90% humidity (5x magnification). From left to right, the PDMS surface is respectively predominantly elastic (|G| = 50 kPa, δ = 2º), viscoelastic (|G| = 10 kPa, δ = 12º) and increasingly viscous (|G| = 1 kPa, δ = 30º and |G| = 0.1 kPa, δ = 82º). (Pictures from 2)
Figure 1 – a) Scheme of the deformation of a soft substrate due to the surface tension (γL) and the Laplace pressure (ΔP). b) Four PDMS surfaces with cross-linking density decreasing from left to right, 20s after cooling from 21 to 1ºC at 90% humidity (5x magnification). From left to right, the PDMS surface is respectively predominantly elastic (|G| = 50 kPa, δ = 2º), viscoelastic (|G| = 10 kPa, δ = 12º) and increasingly viscous (|G| = 1 kPa, δ = 30º and |G| = 0.1 kPa, δ = 82º). (Pictures from 2)
Figure 2 – a) Two surfaces cooled simultaneously under the same experimental conditions. The left part corresponds to a rigid PDMS surface (|G| = 180 kPa, δ = 2°) and the right part is related to a soft PDMS substrate (|G| = 30 kPa, δ = 5°). The photography was taken 10s after initial merging was observed on both surfaces (i.e. about 240s after initial cooling). b) Surface coverage as a function of time for two PDMS surfaces with different mechanical properties (under the same experimental conditions), in 36% of humidity. (Pictures from 2)
Figure 2 – a) Two surfaces cooled simultaneously under the same experimental conditions. The left part corresponds to a rigid PDMS surface (|G| = 180 kPa, δ = 2°) and the right part is related to a soft PDMS substrate (|G| = 30 kPa, δ = 5°). The photography was taken 10s after initial merging was observed on both surfaces (i.e. about 240s after initial cooling). b) Surface coverage as a function of time for two PDMS surfaces with different mechanical properties (under the same experimental conditions), in 36% of humidity. (Pictures from 2)

Dropwise condensation

Contact Person: Dr. Julien Petit

Condensation of vapor (dew formation) leads to complex patterns on cold surfaces (breath figures). This phenomenon has been studied for a long time but some important features are still under debate and this apparently simple situation poses many open questions. We are especially interested in the growth of droplets just after the surface has been cooled below the dew point.

When drops condense close to each other their growth rate becomes a function of the drop spacing. If the distance between them is large, the drops grow independently. The closer the droplet packing on the surface, the more the droplet growth resembles that of a continuous wetting film on the surface.

Drops growing on soft (deformable) surfaces interact with the substrate via the Laplace pressure and the surface tension. Compared to rigid surfaces, condensation on soft surfaces turns out to be more efficient. Similarly, the possibility of neighboring drops to merge is decreased with increasing softness of the surface.

  • Sokuler M., Auernhammer G.K., Liu C.J., Bonaccurso E., Butt H.-J.:Symmetry of condensing and evaporating drops, EPL, 89, 36004 (2010)
  • Sokuler M., Auernhammer G.K., Roth M., Liu C.J., Bonaccurso E., Butt H.-J., The softer the better: fast condensation on soft surfaces, Langmuir, 26, 1544 (2010)
  • Pericet-Camara R., Best A., Butt H.-J., Bonaccurso E.: Effect of capillary pressure and surface tension on the deformation of elastic surfaces by sessile liquid microdrops: an experimental investigation, Langmuir, 24, 10565 (2008)