results demonstrate that this limitation can be circumvented by operating below the length scale
determined by the electron mean free path.
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We thank E. Demler, A. Bleszynski Jayich, B. Myers, A. Yacoby,
M. Vavilov, R. Joynt, A. Poudel, and L. Langsjoen for helpful
discussions and insightful comments. Financial support was
provided by the Center for Ultracold Atoms, the National Science
Foundation (NSF), the Defense Advanced Research Projects Agency
Quantum-Assisted Sensing and Readout program, the Air Force
Office of Scientific Research Multidisciplinary University Research
Initiative, and the Gordon and Betty Moore Foundation. S. K. and A. S.
acknowledge financial support from the National Defense Science
and Engineering Graduate fellowship, V.E.M. from the Society
of Fellows of Harvard University, and S.K. from the NSF Graduate
Research Fellowship. All fabrication and metrology were performed
at the Center for Nanoscale Systems (CNS), a member of the
National Nanotechnology Infrastructure Network, which is supported
by the NSF under award no. ECS-0335765. The CNS is part of
Materials and Methods
Figs. S1 to S7
Tables S1 to S3
4 December 2014; accepted 16 January 2015
Published online 29 January 2015;
Robust self-cleaning surfaces that
function when exposed to either
air or oil
Yao Lu,1 Sanjayan Sathasivam,1 Jinlong Song,2 Colin R. Crick,3
Claire J. Carmalt,1 Ivan P. Parkin1*
Superhydrophobic self-cleaning surfaces are based on the surface micro/nanomorphologies;
however, such surfaces are mechanically weak and stop functioning when exposed to oil. We
have created an ethanolic suspension of perfluorosilane-coated titanium dioxide nanoparticles
that forms a paint that can be sprayed, dipped, or extruded onto both hard and soft materials to
create a self-cleaning surface that functions even upon emersion in oil. Commercial adhesives
were used to bond the paint to various substrates and promote robustness. These surfaces
maintained their water repellency after finger-wipe, knife-scratch, and even 40 abrasion cycles
with sandpaper. The formulations developed can be used on clothes, paper, glass, and steel for a
myriad of self-cleaning applications.
Artificial self-cleaning surfaces work through extreme water repellence (superhydropho- bicity) so that water forms near spherical shapes that roll on the surface; the rolling motion picks up and removes dirt, viruses,
and bacteria (1–3). To achieve near spherical water droplets, the surfaces must be highly textured
(rough) combined with extremely low water affinity (waxy) (4, 5). The big drawback of these
artificial surfaces is that they are readily abraded
(6–8), sometimes with little more than brushing
with a tissue, and readily contaminated by oil
(9–11). We report here a facile method for making superhydrophobic surfaces from both soft
(cotton or paper) and hard (metal or glass) materials. The process uses dual-scale nanoparticles
of titanium dioxide (TiO2) that are coated with
perfluorooctyltriethoxysilane. We created an ethanol-based suspension that can be sprayed, dipped, or
painted onto surfaces to create a resilient water-repellent surface. By combining the paint and
adhesives, we created a superhydrophobic surface that showed resilience and maintained its
performance after various types of damage, including finger-wipe, knife-scratch, and multiple
abrasion cycles with sandpaper. This method can
also be used for components that require self-cleaning and lubricating such as bearings and
gears, to which superamphiphobic (repels oil and
water) surfaces (9–11) are not applicable.
A paint was created by mixing two different
size ranges of TiO2 nanoparticles (~60 to 200 nm
and ~21 nm) in an ethanol solution containing
perfluorooctyltriethoxysilane (12). Scanning elec-
tron microscopy (SEM) and transmission electron
microscopy (TEM) of the constituent particles
of the paint (Fig. 1A) show the dual-scale na-
ture of the TiO2 nanoparticles. X-ray photo-
electron spectroscopy (XPS) (Fig. 1B) showed
that the titanium dioxide particles were coated
We used many different coating methods to
create the water-repellent surfaces, including an
artist’s spray-gun to coat hard substrates such
as glass and steel, dip-coating for cotton wool,
and a syringe (movie S1) to extrude the paint
onto filter paper. After allowing the ethanol to
evaporate for ~180 s at room temperature, the
treated areas of the substrates supported water
as near spherical droplets, whereas the untreated
parts were readily wetted (it required ~30 min for
the ethanol to fully evaporate from cotton wool
and filter paper at room temperature) (fig. S1). We
used x-ray diffraction (XRD) (Fig. 1C) to analyze
the coatings on hard and soft substrates. The
diffraction peaks show the expected patterns for
On a surface that shows water repellence,
water droplets tend to bounce instead of wetting
the surface (13, 14). However, for soft substrates,
extreme superhydrophobicity is required to achieve
the bouncing phenomenon because the water
droplets tend to be trapped onto the threads of
the substrates (cotton wool) (15). Shown in fig. S2
are the water dropping tests on untreated glass,
steel, cotton wool, and filter paper, which were
readily wetted (the contact moment of the water
droplets and the solid surfaces is defined as 0).
Shown in Fig. 2 is the water bouncing process
on dip-coated glass, steel, cotton wool, and filter
paper surfaces. Water droplets completely leave
the surface without wetting or even contaminating the surfaces (the water was dyed blue to
aid visualization), indicating that the surfaces
were superhydrophobic. In movie S2, we compare the water-affecting behavior between untreated and treated glass, steel, cotton wool, and
filter paper, respectively. The effect of artificial
rain on the treated surfaces is shown in movie
S3; the drop sizes varied with random impact
1Department of Chemistry, University College London, 20
Gordon Street, London, WC1H 0AJ, UK. 2Key Laboratory for
Precision and Non-traditional Machining Technology of
Ministry of Education, Dalian University of Technology,
Dalian, 116024, People’s Republic of China. 3Department of
Chemistry, Imperial College London, South Kensington
Campus, London, SW7 2AZ, UK.
*Corresponding author. E-mail: firstname.lastname@example.org