Evaporation Kinetics of Sessile Water Droplets on Micropillared Superhydrophobic Surfaces

Abstract

Evaporation modes and kinetics of sessile droplets of water on micropillared superhydrophobic surfaces are experimentally investigated. The results show that a constant contact radius (CCR) mode and a constant contact angle (CCA) mode are two dominating evaporation modes during droplet evaporation on the superhydrophobic surfaces. With the decrease in the solid fraction of the superhydrophobic surfaces, the duration of a CCR mode is reduced and that of a CCA mode is increased. Compared to Rowan's kinetic model, which is based on the vapor diffusion across the droplet boundary, the change in a contact angle in a CCR (pinned) mode shows a remarkable deviation, decreasing at a slower rate on the superhydrophobic surfaces with less-solid fractions. In a CCA (receding) mode, the change in a contact radius agrees well with the theoretical expectation, and the receding speed is slower on the superhydrophobic surfaces with lower solid fractions. The discrepancy between experimental results and Rowan's model is attributed to the initial large contact angle of a droplet on superhydrophobic surfaces. The droplet geometry with a large contact angle results in a narrow wedge region of air along the contact boundary, where the liquid-vapor diffusion is significantly restricted. Such an effect becomes minor as the evaporation proceeds with the decrease in a contact angle. In both the CCR and CCA modes, the evaporative mass transfer shows the linear relationship between mass(2/3) and evaporation time. However, the evaporation rate is slower on the superhydrophobic surfaces, which is more significant on the surfaces with lower solid fractions. As a result, the superhydrophobic surfaces slow down the drying process of a sessile droplet on them.

Effect of superhydrophobic surface morphology on evaporative deposition patterns

“…9 6 3.4 . This stick-slip phenomenon 26 is characteristic of the sparser pillar geometry in the CCA mode. Near the end of evaporation (s % 0.90), the droplet returns to the mixed mode, and at s % 0.96, the contact radius experiences a sudden increase, signifying that the droplet has been impaled by the pillars and displaced the air gaps, entering the Wenzel state.…”

“…In recent studies, [23][24][25] the droplet evaporation rate was reported to be reduced on superhydrophobic surfaces due to increased influence of evaporative cooling at the droplet interface. Three modes of droplet evaporation on superhydrophobic surfaces have been reported 20,26,27 as well: a constant contact radius (CCR) mode, a constant contact angle (CCA) mode, and a mixed mode. In the CCR mode, the contact line remains pinned while the contact angle decreases, whereas in the CCA mode, the contact angle remains fixed as the contact line recedes.…”

“…The Cassie-to-Wenzel transition is driven by the competition between the Laplace and capillary pressures, while a droplet is evaporating on the textured surface. 1,26,37 The Laplace pressure can be expressed as P L ¼ 2c=R, where c is the liquid surface tension and R is the droplet curvature radius; P L increases as the droplet size decreases. The capillary pressure is defined as P C ¼ À4cðcos h Y Þ½u=ðW 1 À u ð ÞÞ, where h Y is the Young's contact angle (120 for a water droplet on a smooth, Teflon-coated surface) and u ¼ W 2 =P 2 is the surface solid fraction.…”

Evaporation of sessile droplets

“…For simple liquids this stage is very fast and evaporation can be neglected during it, therefore in most studies it is out of consideration and only three consequent stages of evaporation are taken into account [31*-33*]. It is noteworthy that the mentioned stages of evaporation were observed on substrates of various wetting properties including superhydrophobic micropillar-patterned ones [34]. During the first stage of evaporation the drop base radius remains constant, whereas the contact angle decreases to the value of receding contact angle.…”

Role of surface oxygen-to-metal ratio on the wettability of rare-earth oxides

“…[1][2][3][4][5] There has been considerable focus on understanding their wettability 6,7 and developing them for a wide range of applications including droplet impact resistance, [8][9][10][11][12][13] anti-icing, [14][15][16][17][18][19] dropwise condensation, [20][21][22][23][24] electro-wetting, 25,26 drag reduction, [27][28][29][30][31][32] evaporation, 33,34 and anti-corrosion. [35][36][37][38] An important challenge for broad applicability of these hydrophobic materials is their limited robustness.…”