velocities, and the droplets could not wet the
The paint had good self-cleaning properties
when applied on various substrates, especially
for soft porous materials, such as those used in
making clothes and paper. The coated surfaces
show water-proofing properties from the water-
bouncing and artificial rain tests. Further tests
on cotton wool and filter paper are shown in figs.
S3 (the experimental scheme) and S4 (the ex-
perimental results). As shown in fig. S4, A and
B, the dip-coated cotton wool inserted into the
methylene blue–dyed water formed a negative
meniscus on the solid-liquid-vapor interfaces
because of hydrophobicity (16). The cotton wool
was removed from the water and remained fully
white with no trace of contamination by the dyed
water (fig. S3). A dirt removal test when an ar-
tificial dust (MnO powder) was put on the spray-
coated filter paper, which was then cleaned by
pouring water, is shown in fig. S4, C and D. The
untreated piece of filter paper (placed below) was
wet and polluted by the dirt, whereas the treated
piece stayed dry and clean (fig. S3). The self-
cleaning tests on the dip-coated cotton wool and
spray-coated filter paper are shown in movie S4; a
time-lapsed video clip of water droplets (dyed blue)
staying on the dip-coated cotton wool and syringe-
coated filter paper for 10 min is shown in movie
S5, and neither the cotton wool nor the filter paper
had blue left after the droplets were removed.
These tests indicate that the soft substrates (cotton
and paper) gained the nonwetting and self-
cleaning properties after treating with the paint.
Dirt removal tests were also carried out on dip-
coated glass and steel surfaces; as shown in fig. S4,
E and F, the droplet took the dirt (MnO powder)
away, and the surfaces were cleaned along the path
of the water droplet movement. The self-cleaning
property of dip-coated glass and steel surfaces is
shown in movie S6 in a high-speed motion capture.
Very few reports have shown any self-cleaning
tests in oil because superhydrophobic surfaces
normally lose their water repellency when even
partially contaminated by oil. This is because the
surface tension of the oil is lower than that of
water, resulting in the oil penetrating through
the surfaces. Making superamphiphobic surfaces
(that repel both water and oils) is an effective
way to solve this problem (9, 10, 17). However, there
are many instances that require both self-cleaning
from water repellency and a smooth coating of oil,
such as lubricating bearings and gears; under these
conditions, superamphiphobic surfaces cannot be
used because they will also repel lubricating oils.
SCIENCE sciencemag.org 6 MARCH 2015 • VOL 347 ISSUE 6226 1133
Fig. 1. Paint characterizations. (A) SEM (top) and TEM (bottom) of the constituent nanoparticles in the paint. Sizes varied from ~60 to 200 nm for the TiO2
nanoparticles (Aldrich), whereas ~21 nm in size refers to P25. (B) XPS of the paint, where “F” refers to perfluorooctyltriethoxysilane and “Ti” refers to TiO2.
(C) XRD patterns of treated and untreated substrates compared with the respective standard patterns for TiO2 anatase (the P25 particles had a small rutile
component, as expected).
Fig. 2. Time-lapse
photographs of water
droplets bouncing on the
treated glass, steel, cotton
wool, and filter paper
surfaces. Droplet sizes,
~6.3 T 0.2 mL.