Wetting describes the ability of a liquid to spread on a solid surface. Depending on the application, one can try to limit or maximize this spreading. We are particularly interested in the case of superhydrophobic surfaces (but not only) which present remarkable anti-adhesive properties but for which the dynamics of wetting remains little explored and poorly understood, regarding in particular the identification of the parameters which determine the relationship between the contact angles and the speed of the contact line as well as the fluid entrainment phenomena.
To make progress on these issues, we have developed different unconventional and new experimental approaches at INPHYNI, to measure the contact angles at both macroscopic and microscopic scales as a function of the contact line velocity. These different experimental devices allow the contact line to be set in motion in a forced (mechanical displacement of the interface) or unforced (evaporation) steady state or in a transient state via vibrations of the substrate. We study both surfaces with homogeneous wetting properties and hybrid hydrophilic-hydrophobic surfaces.

Wetting dynamics of superhydrophobic surfaces.

We have used for the first time the capillary bridge technique to study the wetting properties (advancing and receding contact angles, effects of large wetted area) of transparent, curved, textured and superhydrophobic surfaces. This method consists in following the shape of a capillary bridge formed between the solid surface to be characterized and tested and a liquid bath when the solid is approached or moved away from the bath. To do so, we have developed a new analysis method in order to obtain contact angle value for any position of the substrate. We compare contact angles measurement with the classical side view method, showing that advancing contact angles are systematically higher with capillary bridge method which confirm results from literature that shows underestimation of contact angle with sessile drop measurement for superhydrophobic surfaces. We compare to a few existing models, concluding to a good agreement for receding values but not for advancing angles for which models must be refined. Finally, we show that this method allows to determine contact angle – contact line speed law that is still not identified for superhydrophobic surfaces.

 

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  • COHEN C. et al. (2019) Soft Matter 15(45) 9352-9358, 10.1039/c9sm01402k
  • COHEN C. et al. (2019) Soft Matter 15(14) 2990-2998, 10.1039/c8sm02458h  Collaboration: F. Guittard (Nice)
 
We are currently working on the contact angle – contact line speed law problem, coupling macroscopic measurements provided by the capillary bridge experiments to more microscopic measurements with a sessile drop setup mounted on an inverted microscope.

Collaboration: Jerôme Fresnais (PHENIX, Paris), Etienne Barthel (SIMM, Paris).

Liquid dragging and capillary bridge break up

The pinch-off dynamics of a liquid bridge is relevant to several industrial applications that need to transfer a liquid volume from a surface to another one. Break-up mechanism depend on the size, the geometry of the bridge and the rheology of the liquid, it can also be affected by contact lines motion. All those parameters make of this process a rich source of challenging issues in fundamental fluid mechanics. Most studies consider fixed contact lines and smooth substrates. We currently use the capillary bridge technique to study liquid dragging and break-up with moving contact lines on textured and smooth surfaces for Newtonian and Non Newtonian fluids. Depending on the elongation rate and the surface properties, the volume dragged after the breakage of the capillary bridge varies and eventually remains attached to the surface in a shape of a droplet.

Collaboration: R. Valette (CEMEF)

Impalement transition

When a water drop is placed on a superhydrophobic substrate made of array of pillars, two wetting states can be reached: (i) a Cassie-Baxter state where the water drop stand on the top of the pillars and an air layer remains between the drop and the base of the substrate or (ii) a Wenzel state where the water totally impregnates the substrate. The transition from Cassie-Baxter to Wenzel state is called the impalement transition.
The origin of this impalement is greatly discussed in literature but is still poorly understood, for deformable pillars surfaces in particular. We have developed a sessile drop set-up mounted on an inverted microscope to study the impalement transition at the pillars scale. We vary the volume of the water drop and the nature of the substrate (made of rigid or soft materials).
We aim to better describe the causes of this transition and understand its dynamics.

Condensation, coalescence and vibrated droplet

We study the effect of a mechanical impact on a solid substrate supporting a breath figure (droplet pattern formed when a vapor condenses onto a surface). A falling spherical metallic ball impacts the top of the plate, the droplets being on its bottom part. We have studied the effect of height of the falling projectile on the evolution of the breath figure with time and compared the droplets size distribution before and after impact with the droplet number reduction (DNR). We show that, for a given mean radius of the droplets, when the acceleration of the substrate exceeds a threshold, the final number of droplets starts to decrease and keeps on decreasing as acceleration is increased.

We interpret this result as follows: knowing that droplets vibrate, their contact line unpin above a threshold in acceleration, presenting oscillations of their radius which make them contact and coalescing with neighbors giving birth to liquids networks. The impact accelerates the natural aging of a breath figure.

This could provide a new solution to increase the efficiency of dew recovery processes.