Sessile drop evaporation on (super)hydrophobic surfaces: Effect of low pressure on the contact line dynamics

• Hydrophobic and superhydrophobic surfaces were produced using aluminum substrates.
• The evaporation kinetics of water droplets were studied under low surrounding pressures.
• The evaporation process is driven by the contact line dynamics (pinned/moving).
• The superhydrophobicity property of micro-structured surfaces is lost under low pressures (P < 20 kPa).
• The water contact angle decreases with pressure.

In this article, we experimentally study the influence of the surrounding pressure ranging between 1 kPa and 101 kPa on the evaporation dynamics of drops on hydrophobic and superhydrophobic substrates. These surfaces were, initially, constructed by nanocoating of etched and/or nonetched aluminum substrates. The evaporation of droplet is observed to occur in two distinct regimes: pinned contact line (PCL) and moving contact line (MCL). Their contribution to the full spectrum of the drop evaporation is modulated by both wetting properties and pressure values. It is shown that the triple line and its interfaces exhibit a remarkable dependence on the surrounding pressure through a significant decrease in the water contact angle. The influence of the pressure on the stability of the superhydrophobic state is also evidenced as well. A critical pressure value is experimentally determined (Ps ≈ 20 kPa). Under lower pressures a Cassie-Baxter to Wenzel transition occurs, which is shown to be quasi-instantaneous for the lowest pressures (P ≤ 5 kPa). The superhydrophocity of micro-structured surfaces is lost in such conditions.

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