Capillarity and Wetting

On the effect of insoluble hydrophobic heterogeneities on the wetting dynamics of soluble thin filmsRecent studies have highlighted the complex mechanisms governing the spreading of a solvent onto a homogeneous soluble film, such as water on soluble …

Melides_et_al_2025_CES_video4.mp4

Effect of Macroscopic Surface Heterogeneities on an Advancing Contact LineThe shape of a liquid–air interface advancing on a heterogeneous surface was studied experimentally, together with the force induced by the pinning of the contact line to surface defects. Different surfaces were considered with circular defects introduced as arrays of cocoa butter patches or small circular holes. These heterogeneous surfaces were submerged in aqueous ethanol solutions while measuring the additional force arising from the deformation of the advancing contact line and characterizing the interface shape and its pinning on the defects. Initially, the submersion force is linear with submerged depth, suggesting a constant defect-induced stiffness. This regime ends when the contact line depins from the defects. A simple scaling is proposed to describe the depinning force and the depinning energy. As the defect separation increases, the interface stiffness is found to increase too, with a weak dependency on the defect radius. This interaction between defects cannot be captured by simple scaling but can be well predicted by a theory considering the interface deformation in the presence of a periodic arrays of holes. Creating a four-phase contact line by including solid defects (cocoa butter) reduced pinning forces. The radius of the defect had a nonlinear effect on the depinning depth. The four-phase contact line resulted in depinning before the defects were fully submerged. These experimental results and the associated theory help to understand quantitatively the extent to which surface heterogeneities can slow down wetting. This in turn paves the way to tailoring the design of heterogeneous surfaces toward desired wetting performances.

70_ShortAttachment_labelled.avi

Encapsulation of a Bubble in a Droplet

Glass Transition Accelerates the Spreading of Polar Solvents on a Soluble PolymerWe study the wetting of polymer layers by polar solvents. As previously observed, when a droplet of solvent spreads, both its contact angle and velocity decrease with time as a result of solvent transfers from the droplet to the substrate. We show that, when the polymer is initially glassy, the angle decreases steeply for a given value of the velocity, ${U}_{g}$. We demonstrate that those variations result from a plasticization, i.e., a glass transition, undergone by the polymer layer during spreading, owing to the increase of its solvent content. By analyzing previous predictions on the wetting of rigid and soft viscoelastic substrates, we relate ${U}_{g}$ to the viscosity of the polymer gel close to the glass transition. Finally, we derive an analytical prediction for ${U}_{g}$ based on existing predictions for the water transfer from the droplet to the substrate. Using polar solvents of different natures, we show that the experimental data compare well to the predicted expression for ${U}_{g}$.

Page updated

Google Sites

Report abuse