Influence of nanofiber coating thickness and drop volume on spreading, imbibition, and evaporation

It is known that heat transfer resulting from the drop impact onto a hot substrate can be enhanced with the use of nanostructured coatings, such as nanofiber mats, on the substrate surface. One heat transfer enhancement mechanism is related to liquid imbibition into the porous structure and subsequent evaporation. The detailed mechanisms of liquid spreading, imbibition into the porous structure, and evaporation are still not understood. In this work, the influence of nanofiber coating thickness and drop volume on the kinetics of ethanol drop spreading, liquid imbibition and evaporation on a substrate without additional heating are studied. The initial phase of drop spreading as well as the overall processes are studied simultaneously which allows a direct comparison of influencing factors on the different wetting stages or subprocesses. Four different mat thicknesses in the range of 4-42 mu m were investigated, and the results for the drop spreading and evaporation were compared with the bare silicon surface. The results show that, for a mat thickness of 14 mu m and higher, the maximal imbibed area as well as the drying time depend only on the drop volume and are nearly independent of mat thickness. However, for the thinnest mat tested, the imbibed area was significantly smaller and the drying time was longer than for thicker coatings.