Tiantian Song,Zechao Jiang,Xingkun Man,Weichao Shi
Although past experimental and theoretical research has made substantial progress in understanding evaporation behaviors in various suspensions, the fundamental mechanism for polymer sessile droplets is still lacking. One critical effect is the molecular weight on the evaporation behaviors. Here, systematic experiments are carried out to investigate the evaporation behavior of polymer droplets under the effects of polymer concentration, evaporation rate, and especially molecular weight. We obtain polymer films with various morphologies with molecular weights ranging from 2 orders of magnitude to 4 orders of magnitude and polymer concentration across 4 orders of magnitude. We further develop a theoretical model based on the Onsager principle to explain the evaporation mechanism from a dynamic perspective. Analysis indicates that increasing molecular weight or polymer concentration enhances the contact angle hysteresis and slows down the evaporation, resulting in the transition from multiring to coffee ring and eventually to uniform films. The findings offer a guideline for achieving the desired deposition patterns via droplet processing techniques.
Wei Li,Rong Chen,Xun Zhu,Qiang Liao,Dingding Ye,Yang Yang,Dongliang Li
Photothermal effect has shown great promise in manipulating the droplet evaporation. In this work, the evaporation of a propylene glycol–water binary droplet on the hydrophobic surface induced by the photothermal effect of a focused infrared laser with a wavelength of 1550 nm was experimentally investigated. Dynamic evaporation behaviors were discussed through the visualization method and image processing technique. Experimental results showed that the photothermally induced evaporation of such a binary droplet could be divided into two stages: the first stage dominated by the evaporation of water with more volatility represented by rapid variations of the droplet volume and geometric parameters and the second stage dominated by the evaporation of propylene glycol with less volatility represented by a smaller slope. Correspondingly, the binary droplet evaporation first experienced the mixed mode greatly related to the more volatile component, and then the constant contact radius mode greatly related to the less volatile component. In addition, the effects of the initial composition and input laser power were also discussed. This work provides a deep understanding of the photothermal effect of a focused light-caused evaporation behaviors of the propylene glycol–water binary droplets and a potential route to accommodate various applications using the evaporation of the binary droplet.
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 μm, the pillar-arrayed PDMS surfaces are Cassie-Baxter-wetted; (ii) when the pillar-to-pillar spacing is 20 or 50 μm, part of each pillar-arrayed surface is Cassie-Baxter-wetted while the other part Wenzel-wetted; (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 μm, while it is not found at high ethanol concentration for surfaces with the hole-to-hole spacings of 20 μm and 50 μm, 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.
Yahui Li,Xiaofei Zhang,Yilin Wang,Junfeng Wang,Daoguang Wang
Feasibility of the multi-effect distillation (MED) process was investigated experimentally and theoretically in present work to deal with the high-salinity organic reverse osmosis concentrates (ROCs) generated by the double membrane process for the treatment of wastewater from the refining and chemical enterprises. Two key problems involving the behaviors of organic and inorganic impurities in the ROCs during evaporation and process optimization of MED thermally and economically were highlighted. The experimental results indicated that about 6% and 8% of organic impurities volatilized during evaporation and entered into the produced water and the tail gas, respectively. The produced water and the tail gas which met the related China National standards could be reused or vented directly without further treatment. However, scaling of calcium sulfate was drastically occurred in the evaporator when about 30% water was recovered, indicating the necessity of removal of the hardness in ROCs before evaporation. A forward flow MED model established on Aspen Plus platform was employed to perform the thermal and economic analysis for the ROCs treatment. Performance analysis including the Gained Output Ratio (GOR), fresh steam flow and specific heat transfer area as functions of the effect number and the heating steam temperature were conducted. The results showed that the performance of the MED system was significantly influenced by effect number over the heating steam temperature. An increase in the effect number would improve the thermodynamic performance of MED system but increased the capital costs. The model established in this work could provide a theoretical guidance for the development of MED process dealing with ROCs.
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