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    An Experimental and Theoretical Investigation of Pressure-Induced Wetting Transitions

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    MS-Thesis_Zain Ahmad_Spring 2020.pdf
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    Description:
    MS Thesis
    Embargo End Date:
    2021-05-07
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    Type
    Thesis
    Authors
    Ahmad, Zain cc
    Advisors
    Mishra, Himanshu cc
    Committee members
    Nunes, Suzana Pereira cc
    Farooq, Aamir cc
    Ghaffour, NorEddine cc
    Program
    Environmental Science and Engineering
    KAUST Department
    Biological and Environmental Sciences and Engineering (BESE) Division
    Date
    2020-05
    Embargo End Date
    2021-05-07
    Permanent link to this record
    http://hdl.handle.net/10754/662792
    
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    Access Restrictions
    At the time of archiving, the student author of this thesis opted to temporarily restrict access to it. The full text of this thesis will become available to the public after the expiration of the embargo on 2021-05-07.
    Abstract
    A number of industries suffer from inefficient use of energy resources due to frictional drag manifesting at solid-liquid interfaces. A simple method to reduce frictional drag under laminar flow conditions is to entrap air at the liquid-solid interface – in wetting state known as Cassie state. Over time, however, the entrapped air can be lost, and the Cassie state transitions to the fully-filled or the Wenzel state, thereby increasing the frictional drag dramatically. In particular, many practical applications expose surfaces to elevated pressures, and it is thus crucial to investigate pressure-induced Cassie-to-Wenzel transitions in gas-entrapping microtextured surfaces. However, there is a dearth of experimental techniques that can provide high-resolution optical images during wetting transitions at elevated pressures. In this thesis, we address this challenge designing and developing an inexpensive and robust pressure device that can act as an accessory for confocal laser scanning microscopy (CLSM). Equipped with this platform, we set out to visualize Cassie-to-Wenzel transitions in FDTS-coated circular doubly reentrant cavities (DRCs) and simple cavities. We demonstrate that on immersion in water, DRCs stabilize water-air interface, such that on the application of the external pressure as water penetrates into the DRCs, the liquid meniscus at the inlet remains pinned. In stark contrast, in SCs the water meniscus does not get pinned at the inlet, and it keeps on advancing with the increasing pressure along the cavity walls. Since localized laser heating in CLSM can influence wetting transitions, we utilized another custom-built pressure cell connected with upright optical microscopy as a complementary platform. We investigated the following wetting transitions: (i) breakthrough pressures (BtPs), defined as the pressure at which the liquid-vapor meniscus touches the cavity floor, by gradually ramping the external pressure, and (ii) wetting transitions at fixed pressures below the BtP. To understand the physical mechanisms underlying our experimental results, we utilized the Fick’s diffusion model and found that the consideration of air diffusion into water under elevated pressures is crucial. To conclude, we hope that the experimental and theoretical results presented here would advance the rational development of robust gas-entrapping microtextured surfaces for a myriad of applications
    Citation
    Ahmad, Z. (2020). An Experimental and Theoretical Investigation of Pressure-Induced Wetting Transitions. KAUST Research Repository. https://doi.org/10.25781/KAUST-NI3PU
    DOI
    10.25781/KAUST-NI3PU
    ae974a485f413a2113503eed53cd6c53
    10.25781/KAUST-NI3PU
    Scopus Count
    Collections
    Biological and Environmental Sciences and Engineering (BESE) Division; Environmental Science and Engineering Program; Theses

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