Séminaire

Mercredi 2 Octobre 2024 à 11h00.

Dew formation on soft substrate

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When moist air encounters a cold surface, it condenses into a collection of droplets, forming what are commonly known as "breath figures". Dew formation has been extensively studied on a variety of surfaces, including rough, chemically heterogeneous, patterned, polymeric, liquid-infused and pure liquid substrates. Despite the sensitivity to the substrate nature, common features for condensation include the primary and usually continued nucleation of nanodroplets, their diffusive growth and their coalescence to a more advanced stage. The collected droplets typically evolve towards scale-free polydisperse size distributions, ranging from freshly nucleated nanodroplets to millimetre sizes (resulting from continued nucleation and coalesce events). Remarkably, on atomically smooth liquid substrates, dew droplets exhibit a 2D crystalline order with high monodispersity and a well-defined density.

In this seminar, I will discuss the condensation dynamics on soft, smooth, crosslinked polymer gels. We conduct condensation experiments in a chamber with controlled humidity onto PDMS gels, whose elastic modulus is varied between 10 and 106Pa, on uncross-linked PDMS liquid as well as on stiff PDMS brushes. Although elasticity should be marginal at the nanometric scale at which drops form, we report that the nuclei density is highly sensitive to the substrate softness, which suggests that nucleation follows a low-energy pathway sensitive to the degree of cross-linking.

A central question is how the water vapor flux allocates between the generation of new droplets and the enlargement of existing ones. Once droplets enter the visible range (R ~ 1 µm), the number of droplets remains constant (until they start to touch and coalesce). The absence of additional nucleation events and therefore the droplet density selection is explained by the formation of a saturated boundary layer at the substrate vicinity. This not only affects the growth of droplets but also dramatically elevates the energy barrier to nucleate new droplets. Hence, monodispersity of the breath figures on soft PDMS gels persists for long.

Later, when neighboring drops get closer, they attract each other due to the so-called Cheerios interactions mediated by substrate deformations. Drops then gather into clusters that exhibit a reluctance to coalesce. The delay in coarsening is found to be an decreasing function of the substrate elastic modulus. This ultimately results in the formation of a persistent bidimensional quasi-crystal of droplets, that coarsens according to a universal law akin to Oswald ripening.

Breath figures offer a macroscopic approach to probe the molecular characteristics of the polymer interface, challenge the elastocapillarity theory at small scales and raise the question of multiple Cheerios interactions between a large set of drops. ''