am5b10721_si_001.pdf (1.3 MB)
Time-Dependent Wetting Behavior of PDMS Surfaces with Bioinspired, Hierarchical Structures
journal contribution
posted on 2015-12-28, 00:00 authored by Himanshu Mishra, Alex M. Schrader, Dong Woog Lee, Adair Gallo, Szu-Ying Chen, Yair Kaufman, Saurabh Das, Jacob N. IsraelachviliWetting
of rough surfaces involves time-dependent effects, such as surface
deformations, nonuniform 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 1
× 10–7 to 1 × 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.