the surface texture, such as functionalized
ceramics or metal oxides, liquid repellency can
be maintained for up to an order of magnitude
larger abrasion severity ( 3 , 126 , 132 ) (Fig. 6G).
Finally, thermosetting or cross-linked poly-
mers such as different polyurethanes, epoxies,
and phenolics are widely used as coatings in
many different industries (aerospace, automo-
tive, and so forth). By incorporating various
fillers within these matrices, different durable
hydrophobic and superhydrophobic coatings
have been developed ( 123 , 133 ). To aid in the
selection of adequately miscible fillers and
binders, the use of Hansen solubility parame-
ters can be particularly useful ( 123 ). The use of
polymers as a coating treatment on metallic
texture, or for infusion within a metal matrix,
has also yielded extremely abrasion-resistant,
water-repellent materials ( 134 , 135 ).
Scale-up and commercialization
Apart from performance, the commercial fea-
sibility of a solid- or liquid-repellent coating is
largely determined by the materials used and
the fabrication methodology. Lithographically
fabricated surfaces can be difficult to inex-
pensively manufacture on a large scale, which
limits their practical utilization. Current large-
scale coating processes, such as spray coating
( 123 ), electrochemical machining ( 136 ), chem-
ical etching, and physical and chemical vapor
deposition ( 125 ), can allow for easier scale-up
and commercial transition of new coatings.
The chemistry of the materials used for fab-
ricating and processing the coating is also
critical. Use of high quantities of corrosive,
fluorinated, or heavy metalÐbased chemicals
and volatile organic compounds (VOCs) during
manufacturing can prevent commercial transi-
tion because of environmental and regulatory
concerns ( 137 ). The current move away from
bio-accumulating fluorinated chemicals is par-
ticularly worth mentioning in this regard.
High-quality, waterproof outer wear has uti-
lized long-chain fluorinated chemicals for
decades. However, recent understanding re-
garding the toxic by-products and environ-
mental accumulation of these chemicals has
left companies around the world searching
for fluorine-free alternatives that can provide
water repellency and stain resistance similar
to those of long-chain fluorocarbons. Moving
forward, collaborative efforts among scientists
and engineers across different disciplines will
be needed to develop the next generation of
surface coatings that can prevent the accretion
of a broad spectrum of foulants while also
satisfying industrial standards of coating scal-
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Fig. 6. Comparing durability of different
liquid and solid repellant surfaces.
(A) Taber Rotary abrader on a superhydro-
phobic surface. Reprinted with permission
from ( 123 ), copyright the American Chemical
Society (2017). (BtoF) SEM micrographs
of commonly used abraders used to test
abrasion. (G) Abrasion severity of sandpaper
and Taber abrasion experiments conducted on
different hydrophobic and superhydrophobic
materials, plotted against changes in water
contact angle. Degree of deviation from
0% is proportional to loss of surface
performance. All data start at the origin.
Data are compiled from previous work
( 123 , 124 , 126 , 131 – 133 , 135 , 185 , 186 ).
RESEARCH | REVIEW