等离子体纳米织构化聚合物表面的液滴蒸发机制

Evaporation Mechanism of Water Droplets on Polymer Surfaces by Plasma Nanotexturing

超疏水性表面的微纳结构改变极大影响了液体蒸发行为,在超疏水性材料工程应用方面具有重要研究价值。采用氧等离子体处理(OPT)和八氟环丁烷等离子体聚合沉积(FPD)的两步等离子体纳米织构化法在聚丙烯表面制备纳米线和纳米锥结构,研究具有不同纳米织构聚丙烯超疏水性表面的去离子水滴在30℃和60℃温度下的蒸发行为,并对其蒸发机制进行讨论和分析。结果表明:液滴在超疏水性表面总蒸发时间随FPD时间的增加变短。液滴蒸发初期,液滴在聚丙烯表面处于Cassie态,此时主要传热方式为聚丙烯表面通过气体与液滴间接传热,液滴均匀蒸发,蒸发模式为恒定接触角(CCA)模式;随蒸发时间增加,液滴在表面的浸润状态依次转变为Marmur态和Wenzel态,主要传热方式变为聚丙烯表面与液滴直接传热,液滴蒸发加快,蒸发模式转变为混合(Mixed)模式。聚丙烯表面纳米织构的尺寸增大和团簇增加导致液滴与超疏水性表面之间的气相占比减少,造成聚丙烯表面与液滴直接传热加强,促进了液滴从CCA到Mixed的蒸发模式转变。

The influence of surface textures on the droplet evaporation behaviors is important in the applications of the superhydrophobic surfaces. The plasma nanotexturing methods including the oxygen plasma treatment( OPT) and the following octafluorocyclobutane(C;F;) plasma polymerization deposition( FPD) were utilized to fabricate the superhydrophobic surfaces on the polypropylene substrates. The surface nanotexture transition from the nanowires into the nanocones was achieved by the plasma polymerization of octafluorocyclobutane monomer with the duration time. The evaporation behaviors and mechanisms of water droplets at the temperature of 30 ℃ and 60 ℃ were studied on the superhydrophobic polypropylene surface with the different nanotextures. The total evaporation time of water droplets decreased on the superhydrophobic surfaces fabricated by the plasma polymerization with the increase of duration time. During the initial evaporation process,the surfaces with the different nanotextures showed the droplet evaporation in constant contact angle( CCA) model due to the prominent heat transfer of the interfaces between the vapor and the droplets in the superhydrophobicity with Cassie state. The evaporation of water droplets which transited into Marmur and Wenzel state became the mixed model due to the enhanced heat transfer between the interfaces of the nanotexturs and the droplets with the increase of evaporation time. In addition,the increased width and aggregation of nanocone in FPD for the increased duration time led to the reduced proportion of vapor between the droplets and the nanotextured surfaces and the enhanced heat transfer between the interfaces of the nanotexturs and the droplets,which caused the increase of the area-average evaporation flux over the droplet surfaces. The transformation from CCA evaporation model to mixed evaporation mode of water droplets became easy on the superhydrophobic surfaces with nanocones.