(425b) Wetting on Strain Induced Microstructured Surfaces

Wetting on Strain Induced Microstructured Surfaces

Vartika Parihar^a, Soumen Das^b and Sunando Dasgupta^a

^aDepartment of
Chemical Engineering, IIT Kharagpur 721302, India

^bSchool of Medical
Science and Technology, IIT Kharagpur 721302, India

E-mail: pari92vartika@gmail.com, sou@smst.iitkgp.ernet.in, sunando@che.iitkgp.ernet.in

Abstract

Wetting characteristics on hydrophobic
micro-structured surfaces have remained a subject of eminent significance due
to its characteristic properties and relevance in a plethora of industrial
applications and scientific studies, such as anti-biofouling, self-cleaning and
microfluidicsetc¹. Numerous processes for the
fabrication and characterization of such specialty surfaces have been proposed
and practiced². However, most of them have limitations, i.e. they
are inherently complex, intrinsically infrastructure dependent i.e. they
require elaborate micro-fabrication facilities, and Most importantly, it is
limited by the feature size of the available mask or the stamp³. Fabrication of microstructured
surfaces with the controlled tuning of feature size has rarely been reported⁴.  In this work, a new method for the fabrication
of controlled dual scale roughened surfaces is reported involving relatively
simpler processes compared to the existing processes⁵. The study enumerates a straightforward, masks-less process for the creation of microstructured
surfaces, comprising of longitudinal wrinkles with controlled feature sizes. The microstructures result in the
different wetting behavior of a sessile droplet with possible advantages in
various applications. These
wrinkles have been characterized by two morphological parameters, namely the amplitude
and wavelength and they are found to be strong functions of the applied
pre-strain. Wetting states over such wrinkled surfaces are observed to be
governed by the underlying pattern of wrinkles. Modulation of the pre-strain
leads to contrasting wetting regimes of Cassie-Baxter for high values of strain
and Wenzel state for lower values of strain. The resulting equilibrium contact angles have been measured as a function of the
buckle wavelength and amplitude. Thus, we have demonstrated a new and simple
design paradigm, consisting of parallel grooves of appropriate aspect ratio
with controlled anisotropic wetting.

The wrinkle process comprises stretching
of a soft Polydimethylsiloxane(PDMS) (10:1) substrate and subsequent physical
vapor deposition (sputtering)⁶, resulting in a hard metal (Nichrome) layer on the surface. Upon relaxation, sinusoidal
wrinkles with a uniform wrinkle wavelength (λ) and amplitude (A) are
formed due to the buckling instability (mismatch of strain in between the two
materials) that relieves stresses in the form of sinusoidal undulation. Subsequently, after preparation of the surfaces, the
topography is transferred on a PDMS (10:1) spin-coated, glass slide for
enhancing the hydrophobicity on surfaces with anisotropic wetting behavior. The final structure of the fabricated
micro wrinkles on a PDMS substrate is obtained as shown in the 3D Optical
Surface Profiler (3D-OSP) image (Figure 1).

The variations in the buckle
wavelength and amplitude are studied after complete relaxation of initial
applied pre-strains on different samples. Effect of
this varying topography on the wetting behavior is also studied by measuring
the contact angles of 2µl DI water droplet for different substrates by using a Rame-Hart Goniometer. The contact angles measured on
the substrate are shown along with the images of the sessile droplets in Figure
2. The images of the droplets are captured in two prime directions, i.e. along
the wrinkles and across the wrinkles, demonstrating the strong anisotropic
wetting behavior of the surfaces with nearly superhydrophobic
behavior in a direction perpendicular to the direction of wrinkles on surfaces
at high values of pre-strains.

Keywords: Microstructures, Superhydrophobic,
Microfluidics, Contact Angle, Anisotropic Wetting.

Figure 1. 3D optical surface profilometry
of the wrinkled PDMS surface for 50 % applied pre-strain; (a) 3D image, and

 (b) corresponding surface profile.

Figure
2. Contact angles subtended
by a DI water droplet of 2 µl volume on different surfaces. (a-c) shows
contact angles measured along the direction of wrinkles on surfaces with
pre-strains of 20%, 50%, and 80% respectively; whereas,

 (d-f) shows contact angles measured perpendicular to the direction
of wrinkles on surfaces with pre-strains of 20%, 50%, and 80% respectively.

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