dc.contributor.advisor |
Willmott, G |
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dc.contributor.author |
Broom, Matheu |
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dc.date.accessioned |
2020-02-25T00:48:10Z |
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dc.date.issued |
2020 |
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dc.identifier.uri |
http://hdl.handle.net/2292/50108 |
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dc.description.abstract |
This Thesis describes the study of water drop impacts on microstructured surfaces. Previous studies of liquid droplets impacting on solid surfaces have mostly been concerned with smooth flat surfaces. The introduction of an array of micrometre-scale pillars (microstructure) adds new research avenues. This work systematically studies the effect of the microstructure design on droplet impact phenomena. The experiments in this study include over 800 high speed videos of water drop impacts on 23 different surface designs. Droplets with an average diameter D0 = 2.51 ± 0.04 mm are produced with 11 discrete impact velocities in the range of 1.5 - 2.5 m/s producing a range of impact Weber numbers (We) from 50 to 250. Soft-lithography methods are used for the production of polydimethylsiloxane (PDMS) surfaces. The microstructures consist of 20 µm width pillars of both circular and square cross sectional shape arranged in square arrays. A selection of pillar spacings (40 µm, 60 µm and 80 µm) and pillar heights (15 µm, 22 µm and 30 µm) is investigated. The creation of a dual view time-synchronized high-speed imaging system is demonstrated. Using this system, drop impact events can be recorded at frame rates between 10,000 and 42,000 frames per second (fps), with an exposure time between 10 µs and 2 µs. High magnification optics produce a spatial resolution range of ± 20 µm to ± 4 µm. A Matlab image processing script is created for the automatic extraction of data during real time (25 fps) playback. These methods are first employed to study phenomena that are observed promptly after the impact event (< 100 µs). Four distinct zones of formation of microbubbles on the same size scale as individual micropillars are shown. Bubbles formed directly under the droplet are found to be independent of both the surface and the We of impact, and all other regions increase in diameter with We. Jets (small streams of ejected liquid) in the rim of the droplet are mostly observed on surfaces with large pitch (80 µm). Increasing the height of the pillars is found to control the angular divergence of the jets. The liquid-air interface between individual pillars is recorded and movement through the microstructure is found to be induced by a zipping mechanism. The size and shape of the entire impact region (wetted region under the drop) is then studied. It is found that surfaces with 40 µm and 60 µm pitch can support a semi-wetting state, whereas all surfaces with 80 µm pitch completely wet for the range of We studied. Directional dependent wetting of the microstructure is observed on surfaces with 60 µm pitch with the formation of protrusions at 45° to the microstructure array. When studying the size and shape of the whole droplet (outer region), it is found that all of the microstructure patterns studied reduce the relative spread of droplets when compared to at PDMS. Spreading on surfaces that are semi-wetting is shown to be mostly independent of the pillar height, whereas completely wetting surfaces spread less on taller pillars. Doublet and triplet fingers are identified and it is shown that by increasing the pillar height the angle of separation reduces. Finally, similar methods are used to study droplet impact near a single micrometre-scale ridge. This investigation is motivated by the wide range of ridged structures in nature. Three outcomes (pinning, wetting and splashing) of the spreading droplet and the ridge are identified. It is found that an energy threshold can be used to describe the transition between the different outcomes. |
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dc.publisher |
ResearchSpace@Auckland |
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dc.relation.ispartof |
PhD Thesis - University of Auckland |
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dc.relation.isreferencedby |
UoA99265291413902091 |
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dc.rights |
Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated. Previously published items are made available in accordance with the copyright policy of the publisher. |
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dc.rights.uri |
https://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm |
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dc.rights.uri |
http://creativecommons.org/licenses/by-nc-sa/3.0/nz/ |
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dc.title |
Imaging and Analysis of Water Drop Impacts on Microstructured Surfaces |
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dc.type |
Thesis |
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thesis.degree.discipline |
Physics |
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thesis.degree.grantor |
The University of Auckland |
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thesis.degree.level |
Doctoral |
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thesis.degree.name |
PhD |
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dc.rights.holder |
Copyright: The author |
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dc.rights.accessrights |
http://purl.org/eprint/accessRights/OpenAccess |
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pubs.elements-id |
795292 |
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pubs.record-created-at-source-date |
2020-02-25 |
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dc.identifier.wikidata |
Q112951480 |
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