A Simple Way To Achieve Pattern-Dependent Tunable Adhesion in
Superhydrophobic Surfaces by a Femtosecond Laser
Dongshi Zhang, Feng Chen,* Qing Yang, Jiale Yong, Hao Bian, Yan Ou, Jinhai Si, Xiangwei Meng,
and Xun Hou
State Key Laboratory for Manufacturing System Engineering and Key Laboratory of Photonics Technology for Information of
Shaanxi Province, School of Electronics and Information Engineering, Xi’an Jiaotong University, Xi’an, 710049, People’s Republic of
China
*
S
Supporting Information
ABSTRACT: In this paper, we present a new approach to the
tunable adhesive superhydrophobic surfaces consisting of
periodic hydrophobic patterns and superhydrophobic struc-
tures by femtosecond (fs) laser irradiation on silicon. The
surfaces are composed of periodic hy drophobic patterns
(triangle, circle, and rhombus) and superhydrophobic
structures (dual-scale spikes induced by a fs laser). Our results
reveal that the adhesive forces of as-prepared surfaces can be
tuned by varying the area ratio (AR
s‑h
) of superhydrophobic
domain to hydrophobic domain, thus resulting in tunable static
and dynamic wettabilities. By increasing AR
s‑h
, (i) the static wetting property, which is characterized by the minimum water
droplet volume that enables a droplet to land on the surface, can be tailored from 1 μLto9μL; (ii) the sliding angle can be
flexibly adjusted, ranging from >90° (a droplet cannot slide off when the sample is positioned upside down) to 5°; and (iii) the
droplet rebound behaviors can be modulated from partial rebound to triple rebound. In addition, the Cassie−Baxter model and
the sliding angle model are used to speculate the contact angles and sliding angles to provide potentially theoretical models to
design slippery-to-sticky superhydrophobic surfaces. The tunable adhesive superhydrophobic surfaces achieved by fs laser
microfabrication may be potentially used in microfluidic systems to modulate the mobility of liquid droplets.
KEYWORDS: femtosecond laser, superhydrophobic surface, tunable wettability, silicon spikes, superhydrophobic−hydrophobic pattern,
water drop rebound
1. INTRODUCTION
Nature has illustrated a wide variety of superhydrophobic
surfaces with low or high adhesive forces. Low-adhes ive
superhydrophobic surfaces are characterized by water contact
angle (CA) larger than 150° and sliding angle (SA) less than
10°.
1
Water droplets on such surfaces can easily roll off to
remove loosely adherent dirt particles and debris from the
surfaces if the substrate is slightly tilted. This phenomenon is
well-known as the self-cleaning effect or lotus effect for its
original discovery on natural lotus leaf.
2
Other than low-
adhesive superhydrophobic surfaces, sticky or high-adhesive
superhydrophobic surfaces characterized by both large SA and
CA, represent another appealing branch of study.
3
A typical
example of sticky natural species is rose petal,
4
which exhibits
remarkable adhesive force with water droplet sticking to the
surface even when it is positioned upside down.
Recently, intense interest has been focused on the smart
superhydrophobic surfaces exhibiting tunable adhesive forces
and SAs.
5
Zhu et al. realized slippery-to-sticky superhydro-
phobic surfaces by tuning the topographies of polyimide (PI)
nanotube arrays using the porous alumina membrane molding
method.
6
Li et al. have prepared CuO superhydrophobic
surfaces ranging from extremely low to very high tunable
adhesive force by a combination of solution-immersion process
and fluoroalkylsilane coating.
7
Liu et al. reported the photo-
regulated stick−slip switch of water droplet mobility by using a
photoresponsive coating on anodized alumina blank substrate.
8
Su et al. reported a facile route to reversibly tune surface
wettability of In
x
Ga
1− x
N (InGaN) nanotip arrays by
octylphosphonic acid (OPA) modification and ultraviolet−
visible (UV−vis) light illuminations.
9
Nanoscale water
interaction with tungsten nanorods generated under various
Ar pressures and substrate tilting angles by using a glancing-
angle deposition technique have also shown tunable hydro-
phobic properties after their surfaces were modified with a thin
layer of Teflon.
10
By adjusting the deposition time, dissolution
time, concentration ratio, and solution concentration, Li et al.
also prepared a series of copper nanowire arrays with tunable
wetting behaviors via redox reaction through infiltrating
aqueous solutions of metal chloride salts into native porous
anodic aluminum oxide template.
11
Teisala found that variation
in the hierarchical structure of TiO
2
nanoparticle surfaces
Received: July 5, 2012
Accepted: August 21, 2012
Published: August 21, 2012
Research Article
www.acsami.org
© 2012 American Chemical Society 4905 dx.doi.org/10.1021/am3012388 | ACS Appl. Mater. Interfaces 2012, 4, 4905−4912