Nair Hrudya, Staat Hendrik J J, Tran Tuan, van Houselt Arie, Prosperetti Andrea, Lohse Detlef, Sun Chao
Physics of Fluids, University of Twente, P. O. Box 217, 7500 AE Enschede, The Netherlands.
Soft Matter. 2014 Apr 7;10(13):2102-9. doi: 10.1039/c3sm52326h.
Droplets impacting on a superheated surface can either exhibit a contact boiling regime, in which they make direct contact with the surface and boil violently, or a film boiling regime, in which they remain separated from the surface by their own vapor. The transition from the contact to the film boiling regime depends not only on the temperature of the surface and the kinetic energy of the droplet, but also on the size of the structures fabricated on the surface. Here we experimentally show that surfaces covered with carbon-nanofibers delay the transition to film boiling to much higher temperatures compared to smooth surfaces. We present physical arguments showing that, because of the small scale of the carbon fibers, they are cooled by the vapor flow just before the liquid impact, thus permitting contact boiling up to much higher temperatures than on smooth surfaces. We also show that as long as the impact is in the film boiling regime, the spreading factor of impacting droplets is consistent with the We(3/10) scaling (with We being the Weber number) as predicted for large We by a scaling analysis.
撞击过热表面的液滴可能呈现接触沸腾状态，即液滴与表面直接接触并剧烈沸腾；也可能呈现膜态沸腾状态，即液滴被自身蒸汽与表面隔开。从接触沸腾到膜态沸腾的转变不仅取决于表面温度和液滴的动能，还取决于表面上所制备结构的尺寸。在此，我们通过实验表明，与光滑表面相比，覆盖有碳纳米纤维的表面会将向膜态沸腾的转变延迟到高得多的温度。我们给出了物理论据，表明由于碳纤维的尺度小，在液体撞击前它们会被蒸汽流冷却，从而使得接触沸腾能够在比光滑表面高得多的温度下发生。我们还表明，只要撞击处于膜态沸腾状态，撞击液滴的铺展因子就与标度分析针对大韦伯数（We）所预测的We(3/10)标度一致。
