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    Time-Dependent Wetting Behavior of PDMS Surfaces with Bio-Inspired, Hierarchical Structures

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    Type
    Article
    Authors
    Mishra, Himanshu cc
    Schrader, Alex M.
    Lee, Dong Woog
    Gallo, Adair
    Chen, Szu-Ying cc
    Kaufman, Yair cc
    Das, Saurabh
    Israelachvili, Jacob N.
    KAUST Department
    Water Desalination and Reuse Research Center (WDRC)
    Biological and Environmental Sciences and Engineering (BESE) Division
    Date
    2015-12-28
    Permanent link to this record
    http://hdl.handle.net/10754/592756
    
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    Abstract
    Wetting of rough surfaces involves time-dependent effects, such as surface deformations, non-uniform filling of surface pores within or outside the contact area, and surface chemistries, but the detailed impact of these phenomena on wetting is not entirely clear. Understanding these effects is crucial for designing coatings for a wide range of applications, such as membrane-based oil-water separation and desalination, waterproof linings/windows for automobiles, aircrafts, and naval vessels, and antibiofouling. Herein, we report on time-dependent contact angles of water droplets on a rough polydimethylsiloxane (PDMS) surface that cannot be completely described by the conventional Cassie-Baxter or Wenzel models or the recently proposed Cassie-impregnated model. Shells of sand dollars (Dendraster excentricus) were used as lithography-free, robust templates to produce rough PDMS surfaces with hierarchical, periodic features ranging from 10-7-10-4 m. Under saturated vapor conditions, we found that in the short-term (<1 min), the contact angle of a sessile water droplet on the templated PDMS, θSDT = 140° ± 3°, was accurately described by the Cassie-Baxter model (predicted θSDT = 137°); however, after 90 min, θSDT fell to 110°. Fluorescent confocal microscopy confirmed that the initial reduction in θSDT to 110° (the Wenzel limit) was primarily a Cassie-Baxter to Wenzel transition during which pores within the contact area filled gradually, and more rapidly for ethanol-water mixtures. After 90 min, the contact line of the water droplet became pinned, perhaps caused by viscoelastic deformation of the PDMS around the contact line, and a significant volume of water began to flow from the droplet to pores outside the contact region, causing θSDT to decrease to 65° over 48 h on the rough surface. The system we present here to explore the concept of contact angle time dependence (dynamics) and modeling of natural surfaces provides insights into the design and development of long- and short-lived coatings.
    Citation
    Time-Dependent Wetting Behavior of PDMS Surfaces with Bio-Inspired, Hierarchical Structures 2015 ACS Applied Materials & Interfaces
    Publisher
    American Chemical Society (ACS)
    Journal
    ACS Applied Materials & Interfaces
    ISSN
    1944-8244
    1944-8252
    DOI
    10.1021/acsami.5b10721
    Additional Links
    http://pubs.acs.org/doi/10.1021/acsami.5b10721
    ae974a485f413a2113503eed53cd6c53
    10.1021/acsami.5b10721
    Scopus Count
    Collections
    Articles; Biological and Environmental Sciences and Engineering (BESE) Division; Water Desalination and Reuse Research Center (WDRC)

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