Séminaire IUSTI – 12 Avr. 2024 – 11h salle 250

Dew formation on soft substrate

Ambre Bouillant – MSC, Paris

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 delve into the intricate vapour 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 100 Pa and 1 MPa, 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 the droplet density selection are 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 obtained on soft PDMS gels persists for long. Later, when neighbouring drops get closer, they attract each other due to the Cheerios interactions mediated by substrate deformations. Drops then gather into clusters that exhibit a reluctance to coalesce. The delay in coarsening can ultimately results in the formation of a persistent 2D quasi-crystal of droplets, that coarsens according to a universal law akin to Oswald ripening. Breath figures thus 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.