Evaporation of ethanol/water mixture droplets on micro-patterned PDMS surfaces

Abstract

We report evaporation of ethanol/water mixture droplets on both pillar-arrayed and micro-holed polydimethylsiloxane (PDMS) surfaces. Analysis of wettability shows that (i) when the pillar-to-pillar spacing is 5 lm, the pillar-arrayed PDMS surfaces are Cassie-Baxter-wetted; (ii) when the pillar-to-pillar spacing is 20 or 50 lm, part of each pillar-arrayed surface is Cassie-Baxter-wetted while the other part Wenzelwetted; (iii) all micro-holed PDMS surfaces are Cassie-Baxter-wetted; (iv) the advancing and receding contact angles increase with the increase of the pillar-to-pillar spacing, while they decrease with the increase of the hole-to-hole spacing. The investigation on evaporation of mixture droplets demonstrates that (i) evaporation on pillar-arrayed PDMS first proceeds with constant contact radius mode (CCR) due to the strong adhesion between the surface and the droplet; (ii) on micro-holed surfaces CCR stage is also observed for the surface with the hole-to-hole spacing of 5 lm, while it is not found at high ethanol concentration for surfaces with the hole-to-hole spacings of 20 lm and 50 lm, which is attributed to the weak adhesion between the micro-holed surfaces and the droplet. Finally, the evolution of droplet volume and its two-third power are discussed.

Wetting of Superhydrophobic Polylactic Acid Micropillared Patterns

“…All the average θ values measured on the PDMS patterns with micro-pits were larger than that of the values obtained on the pillared SU-8 patterns because of the hydrophobicity differences on flat SU-8 (slightly hydrophilic; apparent θ = ∼80° and θ a = ∼90°) and flat PDMS (hydrophobic; apparent θ = 108 ± 1° and θ a = 119 ± 1°). The apparent θ value of 108 ± 1°, which was measured on the flat PDMS, fits the reported value in the literature where apparent θs of 110 ± 2°, 86 113.5 ± 2°, 87 116.1 ± 0.8°, 88 and 116 ± 2° 85 were given on the flat PDMS. θ a = 119 ± 1° and θ r = 95 ± 1° were measured on the flat PDMS as provided in Table 4 , similar to the literature reports of θ a = 119 and 120° and θ r = 101° on the flat PDMS.…”

“…The increase in the separation distances for the PDMS patterns having a constant pit diameter caused a decrease in most of the contact angles similar to the previously published results on the PDMS patterns made of micropits. 85 …”

“…The increase in the separation distances for the PDMS patterns having a constant pit diameter caused a decrease in most of the contact angles similar to the previously published results on the PDMS patterns made of micropits. 85 The average apparent θ on the PDMS pattern containing micropits was equal to 127 ± 11°, where θ a = 136 ± 14°, θ r = 89 ± 19°, and CAH = 47 ± 20°as given in Table 4. The deviations from the average θ values were found to be small and around 10% for both apparent θ and θ a values.…”

Shape Optimization of a Microhole Surface for Control of Droplet Wettability via the Lattice Boltzmann Method and Response Surface Methodology

“…Choe et al numerically simulated the wetting state of oil droplets. Yu et al studied mixture droplets evaporated on a surface of column arrays and microporous poly­(dimethylsiloxane) (PDMS). Yuan et al experimentally studied the dynamic diffusion on the surface of micropillar array and proposed a theoretical model including the mesoscale viscous resistance and energy dissipation caused by microscale molecular friction.…”

Effect of a Cationic Surfactant on Droplet Wetting on Superhydrophobic Surfaces

“…The presence of a gas layer minimizes the interfacial energy, thereby making a CB state more favorable by reducing the contact area between the liquid and solid phases. However, the long-term stability of the preferred CB state on SH surfaces is still challenging and can be lost through an irreversible wetting transition to Wenzel (W) state, where the liquid fills in the surface cavities, when exposed to chemical, high-temperature environment, , surfactant additives, or evaporation. , To date, the wetting properties of SH surfaces have been extensively investigated, but mostly limited to pure liquids. ,− ,,− In contrast, amphiphilic aqueous surfactant solutions have been investigated to a less extent on SH surfaces, and there are only a few experimental studies on CB–W transition of surfactant-laden drops on SH surfaces. − …”