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

The heat transfer rate during spray cooling and drop impact can be significantly enhanced by applying nano- and micro-structured layers on substrate surfaces. These layers affect the substrate-liquid hydrodynamics interactions and the associated heat and mass transport mechanisms. Specifically, nanofiber mats have been identified as suitable nanostructured coatings for enhancing heat transfer in practical applications because they are easily applicable to many base substrates. Additionally, it is possible to vary their geometric parameters and chemical composition over a wide range.

The hydrodynamics of the liquid-surface interaction is the main factor affecting drop impact cooling efficiency. On smooth surfaces, the drop impact outcomes like splash and deposition also depend on temperature, velocity, drop diameter and surface wettability. Splashing and bouncing limit heat transportation from the substrate to the fluid. Interestingly, applying nanofiber coatings on substrates could suppress these phenomena limiting heat transport. Moreover, drop spreading and imbibition have been shown in membranes, porous layers and solid substrates covered with porous coatings. Therefore, another important heat transfer enhancement mechanism is related to liquid spreading/ imbibition into the porous structure followed by evaporation.

Nevertheless, this area remains largely unexplored despite a significant amount of research devoted to studying heat transfer enhancement mechanisms and related effects of nanofiber coatings. In particular, there is still a lack of detailed understanding of the effects of flow mechanisms like spreading, imbibition and evaporation of liquids on substrates covered with nanofiber coatings. This can be attributed to numerous factors, including the highly chaotic geometric structure of the nanofiber mats, which hinders accurate estimation of the capillary pressure and the permeability of the porous layer. Therefore, developing effective models for studying heat enhancement mechanisms in liquids on substates covered with nanofiber coatings requires a thorough understanding of the influence of nanofiber coatings and fluid properties o liquid wetting and transport processes.

Herein, Michael Heinz, Professor Peter Stephan and Professor Tatiana Gambaryan-Roisman from the Technical University of Darmstadt studied the influence of polyacrylonitrile nanofiber coating thickness and drop volume on the kinetic of pure ethanol drop spreading, imbibition and evaporation on unheated substrates. Four different nanofiber mat thicknesses ranging from 4 – 42 µm were investigated. The initial drop spreading together with the overall processes were simultaneously studied to allow direct comparison of the various influencing factors on various wetting stages. Finally, the results of evaporation and drop spreading were compared to that of bare silicon surfaces. Their research work is currently published in the journal, Colloids and Surfaces A: Physicochemical and Engineering Aspects.

The authors showed that for a mat thickness of 14 µm and above, the drying time and maximal imbibed area were mainly affected only by the drop volume and not the mat thickness. Compared with the thicker mat coatings, thinner coatings exhibited significantly smaller imbibed areas and longer drying time. Thus, the smallest coating thickness exhibited the worst performance among the tested thickness samples. After the drop impact, the drop radius grew to each maximum value before eventually shrinking due to the simultaneous imbibition and evaporation effects. Compared with uncoated substrates surfaces, a 60% reduction in drying time was reported for coating thickness from 14 to 41.5 µm.

In summary, a carefully designed experimental study of the drop impact, imbibition and evaporation of pure ethanol on polyacrylonitrile nanofiber coatings fabricated via electrospinning was conducted by Michael Heinz and colleagues. While the overall time scale of the evaporation- and imbibition-dominated stages were strongly influenced by the coating thickness, the dimensionless drop base radius exhibited universal behaviors and was not affected by changes in drop size or coating thickness. In a statement to Advances in Engineering, the authors said their findings provided detailed insights into the wetting of polyacrylonitrile nanofiber coatings and how it is influenced by different factors, contributing to future studies o heat transfer enhancement mechanisms.