Water Wicking and Droplet Spreading on Randomly Structured Thin Nanoporous Layers

Abstract

Growing thin, nanostructured layers on metallic surfaces is an attractive, new approach to create superhydrophilic coatings on heat exchangers that enhance spray cooling heat transfer. This paper presents results of an experimental study of enhanced droplet spreading on zinc oxide, nanostructured surfaces of this type that were thermally grown on copper substrates. The spreading rate data obtained from experimental high speed videos was used to develop a model specifically for this type of ultrathin, nanoporous layer. This investigation differs from previous related studies of droplet spreading on porous surfaces, which have generally considered either ordered, thin, moderately permeable layers, or thicker, microporous layers. Our layers are both very thin and have nanoscale porosity, making them low-permeability layers that exhibit strong wicking. An added benefit is that the thermally grown, stochastic nature of our surfaces make manufacturing easily scalable and particularly attractive for spray-cooled heat exchanger applications. The model presented here can predict the spreading rate for the wetted footprint of a deposited water droplet over two spreading stages: an early synchronous spreading stage, followed by hemispreading. The comparison of experimental data and model predictions confirms the presence of these two specific spreading stages. The model defines the transition conditions between synchronous and hemispreading regimes based on the change in spreading mechanisms, and we demonstrate that the model predictions of spreading rate are in good agreement with the experimental determinations of droplet footprint variation with time. The results indicate that the early synchronous spreading regime is characterized by flow in the porous layer that is primarily localized near the upper droplet contact line. The potential use of these experimental findings and model for optimizing superhydrophilic, nanostructured surface coatings is also discussed, as it pertains to the surface's ability to enhance water vaporization processes.

The Design of Hydrophilic Nanochannel‐Macrostripe Fog Collector: Enabling Wicking‐Assisted Vertical Liquid Delivery for the Enhancement in Fog Collection Efficiency

“…This droplet spreading can be classified into two mechanisms: synchronous‐spreading and hemi‐spreading. [ 45 ] In the early synchronous‐spreading regime, the droplet front propagates spontaneously with the wicking front; when the droplet reaches a certain radius that minimizes the free energy of the system, the droplet starts wicking through the surface structures beyond the droplet front, which is called the hemi‐spreading. The synchronous‐spreading (<67 ms) and hemi‐spreading (>67 ms) of the nanochannels after the plasma treatment are described in Figure 4d.…”

Capillary rise and evaporation of a liquid in a corner between a plane and a cylinder: A model of imbibition into a nanofiber mat coating

“…The velocity field in a meniscus cross-section has been determined by the solution of equation (8). The geometry of the computational domain is fully determined by the contact angles θ 1 and θ 2 and by the dimensionless wetted substrate length H. The latter varies between zero and H m .…”

“…Depending on those properties, different behavior types can been observed, including perfect wetting, partial wetting, non wetting, and liquid imbibition into the layer, both in the direction parallel and normal to the substrate. The phenomenon of imbibition is present in many different types of structures and geometries that are able to cause a capillary action between liquid and solid media [4][5][6][7][8][9]. The topography of the substrate, the geometry and orientation of the pores and geometrical features such as pillars can determine preferential liquid spreading directions and wetting patterns [10].…”

“…The capillary rise in porous medium or in a porous layer connected to infinite reservoir is also determined by the balance between the viscous and capillary forces, apart from the early stage of the process. Many theoretical models of imbibition are based on the assumption of Darcy flow within a porous layer or along the wall topography [6,8,20,21]. If evaporation is not significant, the resulting imbibition length still follows the t 1/2 -dependence, In addition, the imbibition length is proportional to the quadratic root of the permeability of the porous medium and the capillary pressure.…”

Mapping the transition to superwetting state for nanotextured surfaces templated from block-copolymer self-assembly

“…This implies that superwetting of nanotextured surfaces is closely related to their ability to support spontaneous hemiwicking; i.e. flow in-between the open surface textures driven by uncompensated Laplace pressure for the advancing liquid menisci; [30][31][32] contrary, surfaces not supporting hemiwicking will tend to pin droplets. The Laplace pressure, which may either act as a driving force or a barrier for the flow, is of order / , where is the surface tension of the liquid, and is the characteristic pore size.…”