Abstract:In this paper we show a way that allows for the first time to induce arbitrary humidity of desired value for systems without convective flow. To enable this novelty we utilize a semi-closed environment in which evaporation is not completely suppressed. In this case, the evaporation rate is determined both by the outer (open) humidity and by the inner (semi-closed) geometry including the size/shape of the evaporating medium and the size/shape of the semi-closure. We show how such systems can be used to induce d… Show more
“…The droplet seems to evaporate faster on micro-pillar and hierarchical structured samples than on the nano-grass samples. This effect could originate in the fact that samples patterned with micro-pillars have a larger surface area exposed to air [ 42 ]. Figure A1 The droplet volume change (evaporated volume) with respect to time.…”
Super-hydrophobic natural surfaces usually have multiple levels of structure hierarchy. Here, we report on the effect of surface structure hierarchy for droplet evaporation. The two-level hierarchical structures studied comprise micro-pillars superimposed with nanograss. The surface design is fully scalable as structures used in this study are replicated in polypropylene by a fast roll-to-roll extrusion coating method, which allows effective thermoforming of the surface structures on flexible substrates. As one of the main results, we show that the hierarchical structures can withstand pinning of sessile droplets and remain super-hydrophobic for a longer time than their non-hierarchical counterparts. The effect is documented by recording the water contact angles of sessile droplets during their evaporation from the surfaces. The surface morphology is mapped by atomic force microscopy (AFM) and used together with the theory of Miwa et al. to estimate the degree of water impregnation into the surface structures. Finally, the different behavior during the droplet evaporation is discussed in the light of the obtained water impregnation levels.
“…The droplet seems to evaporate faster on micro-pillar and hierarchical structured samples than on the nano-grass samples. This effect could originate in the fact that samples patterned with micro-pillars have a larger surface area exposed to air [ 42 ]. Figure A1 The droplet volume change (evaporated volume) with respect to time.…”
Super-hydrophobic natural surfaces usually have multiple levels of structure hierarchy. Here, we report on the effect of surface structure hierarchy for droplet evaporation. The two-level hierarchical structures studied comprise micro-pillars superimposed with nanograss. The surface design is fully scalable as structures used in this study are replicated in polypropylene by a fast roll-to-roll extrusion coating method, which allows effective thermoforming of the surface structures on flexible substrates. As one of the main results, we show that the hierarchical structures can withstand pinning of sessile droplets and remain super-hydrophobic for a longer time than their non-hierarchical counterparts. The effect is documented by recording the water contact angles of sessile droplets during their evaporation from the surfaces. The surface morphology is mapped by atomic force microscopy (AFM) and used together with the theory of Miwa et al. to estimate the degree of water impregnation into the surface structures. Finally, the different behavior during the droplet evaporation is discussed in the light of the obtained water impregnation levels.
“…Drop evaporation has many applications in fields such as microfluidics, lab-on-a-chip, ink-jet printing, combustion and pesticide spraying [1][2][3][4][5][6][7]. In 1977, Picknett and Bexon [8] pioneeringly defined three evaporation modes, viz., constant contact radius mode (CCR mode), constant contact angle mode (CCA mode) and mixed mode.…”
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.
“…Note that there should be a typical time to reach a pseudo steady state. For example, a 1 μL water drop covered by a hemisphere of 50 mm diameter takes about 4–5 min to reach a pseudo steady state …”
Section: Evaporation Rate For a Spherical Drop Enclosed In A Leaky Spmentioning
confidence: 99%
“…The technique emanates from an older common practice in our lab to suppress evaporation. This article adds the theoretical component that makes this method mathematically accurate and allows the user to predetermine a desired vapor pressure as well as obtaining an exact real time monitoring of the actual vapor pressure in the a semiclosed environment.…”
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