Manufacturing with electric discharge machining of two-level hierarchical surfaces - experimental and computational study of the effect of roughness on the hydrophobicity of aluminum alloy surfaces

Σκόνδρας Γιούσιος, Δημήτριος; Skondras Giousios, Dimitrios

dc.contributor.author	Σκόνδρας Γιούσιος, Δημήτριος	el
dc.contributor.author	Skondras Giousios, Dimitrios	en
dc.date.accessioned	2024-08-27T09:24:52Z
dc.date.available	2024-08-27T09:24:52Z
dc.identifier.uri	https://dspace.lib.ntua.gr/xmlui/handle/123456789/60010
dc.identifier.uri	http://dx.doi.org/10.26240/heal.ntua.27706
dc.rights	Αναφορά Δημιουργού-Μη Εμπορική Χρήση-Όχι Παράγωγα Έργα 3.0 Ελλάδα	*
dc.rights.uri	http://creativecommons.org/licenses/by-nc-nd/3.0/gr/	*
dc.subject	Ηλεκροδιάβρωση	el
dc.subject	Επιφάνειες πολλαπλής κλίμακας	el
dc.subject	Διαβρεκτικότητα	el
dc.subject	Τραχύτητα	el
dc.subject	Μέθοδος πεδίου φάσεων	el
dc.subject	Electric Discharge Machining	en
dc.subject	Multiscale surfaces	el
dc.subject	Wettability	el
dc.subject	Roughness	el
dc.subject	Phase field method	el
dc.title	Manufacturing with electric discharge machining of two-level hierarchical surfaces - experimental and computational study of the effect of roughness on the hydrophobicity of aluminum alloy surfaces	en
dc.contributor.department	Εργαστήριο Τεχνολογίας των Κατεργασιών	el
heal.type	doctoralThesis
heal.secondaryTitle	Κατασκευή με ηλεκτροδιάβρωση ιεραρχικών επιφανειών δύο επιπέδων - πειραματική και υπολογιστική μελέτη της επίδρασης της τραχύτητας στην υδροφοβικότητα επιφανειών κραμάτων αλουμινίου	el
heal.classification	Κατεργασίες επιφανειών	el
heal.classification	Μέθοδος πεπερασμένων στοιχείων	el
heal.classification	Surface manufacturing	en
heal.classification	Finite element method	el
heal.language	en
heal.access	free
heal.recordProvider	ntua	el
heal.publicationDate	2024-01-12
heal.abstract	The wetting of solid surfaces is a scientific subject first developed in the early 17th century but yet not fully understood. Being inherently multiscale, wetting of surfaces is affected from the combination of each scale contribution, making it difficult to accurately predict. Surface energy that arises from the atomic scale increases the surface wettability with its increase. Roughness can affect wettability on multiple scales leading to different kinds of interfaces with various energy states, ranging from superhydrophilicity to superhydrophobicity. The study of remarkable attributes of self-cleaning, directional wetting or anti-fouling of natural surfaces such as lotus leaves, butterfly wings or water strider legs, unveiled the crucial role of hierarchical mechanisms that are able to manipulate wettability. Computational modeling approaches ranging from molecular dynamics to interphase tracking methods can offer insights on wettability aspects of surfaces at various scales. Hydrophobicity is a much-wanted attribute that can lead to functionalization of surfaces, offering added value in various applications. Inducing hydrophobicity to inherently hydrophilic materials, such as metals, poses a great manufacturing challenge related to accurate a customized surface texture realization. Design criteria including size, shape, aspect ratio, pitch, hierarchy and directionality have to be taken into account for fabricating features that can efficiently increase or stabilize the hydrophobicity of surfaces. Various manufacturing approaches have been employed for inducing hydrophobicity on metallic surfaces, ranging from etching and abrasive techniques to micromachining and laser processing of the surfaces. On many occasions, surface modification steps for inducing or enhancing hydrophobicity must be added during the fabrication procedure. Electric Discharge Machining (EDM) offers an efficient alternative that can be utilized for one-step fabrication of hydrophobic metallic surfaces. Wettability transition towards hydrophobicity and wettability decrease through hierarchical textures can be realized from the utilization of textured electrodes or by Wire Electric Discharge Machining. The effect of the complicated nature of the resulted surface and the wide range of possible higher order scale designs has been a topic of research during the past decade. Consequently, a wide, in-depth analysis on two-level surface structures of commercial materials produced by WEDM can provide a useful step towards the establishment of a facile, one-step manufacturing method for realizing hydrophobic and superhydrophobic metallic surfaces. Following that direction, the first step of this dissertation was the fabrication of planar surfaces on aluminum alloy 6082 surfaces by WEDM. Different combinations of the process parameters of peak current, pulse-on time and gap voltage were employed following an L-9 Taguchi experimental design, while the rest machining parameters were kept constant. Surface topography characterization was conducted by utilizing a profilometer and Scanning Electron Microscopy (SEM). Surface chemical composition was measured by Energy Dispersive X-ray Spectroscopy (EDS), while contact angle measurements were carried out with a goniometer apparatus. Wettability transition from initial hydrophilicity to hydrophobicity was observed in all machined samples, which exhibited hydrophobicity with contact angles reaching up to 132°. High carbon content was found on the machined surface, indicating a higher degree of hydrophobicity. Surface roughness parameters were affected more intensely by pulse-on time, while peak current was the most significant factor for altering the contact angle values. Moderate values of the peak current and pulse-on time parameters, between the ranges tested, were found to result in higher contact angles. Higher contact angles were observed for moderate values of the resulted surface parameters. Inspection of the SEM images revealed areas with micro/nano cavities, which can reportedly as air pockets, increasing the contact angle of the surfaces. Time-dependent 3D modeling was employed by coupling phase field and laminar flow on random rough surfaces having the same average mean surface value Sa with the measured values of the samples. Simulations suggested that an increase in surface roughness and contact area should increase the water contact angle. The analysis concluded that the multiple parameters that result in a multiscale surface complexity could utterly affect the wettability outcome of the surface. By applying the worst-case scenario regarding wettability at the smaller scale of roughness, two level roughness surfaces of various geometrical groove profiles were fabricated via the WEDM process. Roughness and contact angle measurements were repeated, for all the manufactured profiles. For selected cases, dynamic contact angle measurements and different droplet volumes and liquids were investigated. A further rise in contact angle was realized for the two-level structures, reaching up to superhydrophobic contact angles of 153°. Simple geometrical profiles of triangular, rectangular and circular shapes were found to be correlated with Ra, Rz and Rp values. Moderate values of those parameters within the ranges measured were found at the highest values of contact angles. Concerning the higher order roughness parameters of Rsk and Rku, near zero values of Rsk were found on the most hydrophobic samples, while negative Rsk or too high Rku values were found on the less hydrophobic samples. All samples showed directionality in wettability on different directions, induced by the grooved design. Air trapping inside the grooves was observed in some textures. Simulations for the assessment of the higher scale textures capability of retaining air inside the grooves showed that the dimensions of the grooves, and more critically their depth, are important for efficient air trapping. The re-entrant profiles were assessed as the most effective in inducing air trapping. Moreover, tilting tests on two and three directions on selected samples revealed strong adhesion of the surfaces with the droplets. Profiles with dual sized grooves were proved to better retain hydrophobicity in two front and side directions, while a step-like groove profile retained its hydrophobicity better at (+) and (-) side directions. In addition, experiments of static contact angle for different droplet volumes reveal that low volumes with dimensions in the vicinity of second scale or high volumes that pronounce gravitational effects have a negative impact on wettability. Samples tested with lower surface tension liquids proved to be superliquiphilic, which consists of a desired property for oil/water separation. Hydrophobicity was retained in selected samples that were tested after three months. Future directions are suggested for the investigation of the interplay between different hierarchical ratios of the two scales from an experimental and computational point of view.	el
heal.sponsor	«The implementation of the doctoral thesis was co-financed by Greece and the European Union (European Social Fund-ESF) through the Operational Programme «Human Resources Development, Education and Lifelong Learning» in the context of the Act “Enhancing Human Resources Research Potential by undertaking a Doctoral Research” Sub-action 2: IKY Scholarship Programme for PhD candidates in the Greek Universities».	el
heal.advisorName	Μαρκόπουλος, Άγγελος	el
heal.committeeMemberName	Μανωλάκος, Δημήτριος	el
heal.committeeMemberName	Παπαευθυμίου, Σπυρίδων	el
heal.committeeMemberName	Αριστομένης, Αντωνιάδης	el
heal.committeeMemberName	Βοσνιάκος, Γεώργιος Χριστόφορος	el
heal.committeeMemberName	Σπιτάς, Βασίλειος	el
heal.committeeMemberName	Μπενάρδος, Πανώριος	el
heal.academicPublisher	Εθνικό Μετσόβιο Πολυτεχνείο. Σχολή Μηχανολόγων Μηχανικών	el
heal.academicPublisherID	ntua
heal.numberOfPages	169 σ.	el
heal.fullTextAvailability	false