Evaporation of Sessile Water Droplets on Horizontal and Vertical Biphobic Patterned Surfaces

Abstract

ABSTACT: This paper presents an experimental study on thermal transport to single water droplets evaporating on heated bi-phobic surfaces consisting of a superhydrophobic matrix with a circular hydrophobic pattern with strong contact line pinning. A single water droplet of 8 µl volume is placed on a preheated surface and allowed to evaporate in an open laboratory environment. We investigate the influence of substrate orientation (horizontal and vertical) on evaporation dynamics. Using optical and infrared imaging, we report droplet fluid dynamics and heat transfer characteristics of the evaporating droplet. Overall, evaporation is more efficient on the vertical surface, exhibiting higher total heat transfer rates and up to 10% shorter evaporation times. Counterintuitively, on the vertical surface, the substrate-droplet interfacial heat flux was higher near the lower contact line than in the upper region, despite a high contact angle and an expected wedge effect at the bottom. At the same time, the temperature is colder in the lower part of the droplet. We attribute this apparent anomaly to the competition between sensible heating and evaporation, and a modified convective flow signature (both within the droplet and the gas phase) compared to a horizontal surface. We also show that the thermal signature becomes uniform once the contact angles at the upper and lower contact lines become equal towards the end of the evaporation process. Insights from this work can guide the design of spray cooling devices, or be used to alter particle deposition patterns during evaporation-based fabrication techniques and ink-jet printing.

Evaporating droplets on inclined plant leaves and synthetic surfaces: Experiments and mathematical models

“…On the other hand, the effect of inclination angle on evaporation is dependant on surface properties and surfactant concentration, but appears to be less influential compared to the effect of increasing the surfactant concentration. Indeed, for many of our examples, low concentration of surfactant on wheat and Teflon, we observed the constant contact radius mode for the lifetime of the evaporation even on a 90 • incline, which illustrates how our very difficult-to-wet surfaces can strongly pin the contact line in a similar manner to that demonstrated very recently [53] on vertical surfaces (that were biphobic by construction).…”

“…On an incline, θ L − θ R decreases as time progresses as the effect of gravity on the droplet shape diminishes due to its smaller size. The continued CCR mode for the effective lifetime of these evaporating droplets, even on a 90 • incline, demonstrates a strong tendency for the contact line to remain pinned on these very difficult-to-wet surfaces (similar observations have recently been made for evaporating droplets on so-called biphobic surfaces at 90 • inclines [53]). Finally, we note from the top row of Fig.…”

Optical whispering-gallery mode barcodes for high-precision and wide-range temperature measurements

“…Finally, the temperature returns to the initial level (as does the spectrum). These transient thermal dynamics are related to the intrinsic thermal properties of the liquid (such as the evaporation rate, thermal conductivity, and molar specific heat capacity), so the two sensorgrams show distinctive dynamics of the temperature during evaporation [57][58][59] . Acetone has a larger evaporation rate With a larger volume, the droplet would consume more heat from the surroundings and lead to a larger temperature change (maximum shift).…”

One-Step Methods to Fabricate Durable Superhydrophobic Coatings for Flexible Electronic Sensors

“…Due to capillary action between the surface microstructure and water droplet, the adhesion of water droplets on the surface increases, and the SA increases to 90 • . At present, the measurement of surface wettability is mostly based on normal temperature water drops [47]. Hence, it is necessary to measure the wettability of water droplets at different temperatures, and the related results are displayed in Figure 8d.…”