Abstract
This thesis describes the study of metal topographic surface wettability gradients, from their manufacturing methods to the analysis of their possible applications for enhanced water management and ice removal, examined at the laboratory level. Previous studies of surface wettability gradients are typically limited to considering only one aspect of them (such as production, design or examination). Another limitation was the substrate used. In most cases, the wetting gradients were fabricated on a silicon substrate with or without coatings. The introduction of topographically modified aluminium structures raises several research possibilities, such as the investigation of gradient manufacturing, implementation and modification of the examination techniques and development of a theoretical description of water droplet behaviour.
This thesis aims to answer the following research question:
- how a surface should be modified;
- what theory could explain and predict wetting properties of such a surface;
- which design(s) will be more beneficial for a certain application.
To answer them, this work systematically studies the features of all-metal surface wettability gradients and their impact on water droplet phenomena: wetting, spontaneous motion, directional bouncing and considers ice adhesion to this kind of surface.
This thesis covers the theory of the droplet motion on a topographically modified surface and applies it to the case of coating-less topographically modified structures. The possible designs for various applications are also discussed. The experiments in this study include:
- various methods of topography modification, such as, micro-milling, laser-etching and ion-beam implantation;
- goniometric investigation of the wetting of microstructured aluminium as well as SEM and AFM imaging of the surface;
- imaging of spontaneous droplet motion on horizontally aligned and tilted surfaces;
- high-speed imaging of droplet impact on topographical wettability gradients with analysis of droplet outcomes.
This thesis clarifies the roles of line-edge roughness and microstructural geometry from each microfabrication technique, which manifests in technique-specific nano- to mid-micro-scale roughness, producing a hierarchical structure. For example, laser-etched surfaces exhibit line-edge roughness with a microstructure of 3−20 μm width and 5−10 μm height superimposed with evenly spread spikes of 50−250 nm. This results in a high contact angle (>150°) coupled with a low contact angle hysteresis (<15°), promoting superhydrophobicity on a coating-free aluminium surface. Ion-beam post-processing was used to create an additional nanoroughness on a microstructure and a controllable Gibbs surface free energy change of the substrate material.
This work presents the design, fabrication, and investigation of topographical surface wettability gradients on aluminium, which are aimed at spontaneous water droplet movement for improved surface water management. One surface aligned horizontally demonstrates a droplet travel distance of up to almost 2 mm for 4.5-11 µL droplets. A map of the theoretical ranges of the tested surface wettability gradients is also presented.
The proposed force balance model allows analysis of the impact of the topography on the forces acting on the droplet. The discrepancy between modelled and observed contact angles in most cases does not exceed 10%. The modelled droplet footprint fits the experimentally measured ones with an error of less than 10% for most cases. Though modelled motion distances were twice as large as experimentally observed ones, the comparison of the proposed model with the originally developed theoretical model showed that the difference in the net force was less than 5%. Both observed and modelled average velocities were within less than 30% difference. Like the traditional models, the new model overestimates droplet kinematics; however, it does not require knowledge a priori of all the contact angles across the gradient during droplet motion, relying only on the material’s surface wettability and the local surface area fraction. Therefore, the model presents a simplified and convenient means of designing a linear topographical gradient for spontaneous droplet motion.
The droplet impact on these surfaces involves the analysis of images obtained via high-speed capturing at 3000 frames per second. The analysis of the droplet behaviour demonstrates the ability of the surface wettability gradients to impose a vectorial direction to the droplet bouncing. Depending on the impact parameters and the gradient strength, a droplet can reach a height of almost 10 mm after bouncing off the horizontally aligned gradient and can land up to 10 mm away from the deposition point.
Finally, this study uses a shear stress test to evaluate the ice adhesion strength on raw, polished and micro/nanoengineered hierarchical superhydrophobic aluminium (produced via one-step laser etching) with fixed- and gradient-pitch structures. The influence of the ice-surface contact area and the mould shape and material on the ice adhesion results were observed for all samples. Moreover, the impact of the direction of the removal force was investigated on the wetting gradients. Key results are that topographically altered surfaces can be hydrophobic but not icephobic, whereas non-altered surfaces are hydrophilic but with much lower ice adhesion. Additionally, gradient surfaces were observed to provide hydrophobicity for most areas of the surface while allowing the removal of the ice column at lower forces from certain directions.