Environmental Engineering Reference
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pressure-sensitive neuromasts in the lateral line allow these creatures to measure
flow and changes in motion (Coombs 2001 ). Hairs on the skin or on the surface of
insects (like crickets) also enable biological systems to monitor changes in fluid
flow (Pfatteicher and Tongue 2002 ) or acoustic signals (Wiegerink et al. 2007 ).
Hairs or whiskers on mice are also used for object identification and avoidance.
Research by Dijkstra et al. ( 2005 ) and Wiegerink et al. ( 2007 ) have shown that
biomimetic filiform hairs could be used for sensing flow. Viscous flow would tilt
these micro-machined hairs and induce measureable capacitance changes. These
artificial hairs have also been organized into a large array for enhanced sensitivity
and performance. An array is also useful for enhancing sensing resolution, as well
as differentiating flow direction. Besides measuring capacitance changes, others
have used the measurement of changes in electrical current (Sarles et al. 2011 ),
field-effect response (Kim et al. 2009 ), piezoelectricity (Yu et al. 2010 ), or mag-
netostriction (McGary et al. 2006 ). These bio-inspired flow sensors have been
proposed for various applications (Pinto et al. 2011 ; Eberhardt et al. 2011 ; Tao
et al. 2011 ).
A specific application in which bio-inspired flow sensors have been used for
SHM is the case of bridge scour monitoring. Bridge scour is the disruption of
marine structure's foundations (e.g., overwater bridge piers and abutments) due to
rapid water flow, flooding, or severe weather events, among others (Whitehouse
1998 ). For instance, (Swartz et al. 2014 ) proposed a wireless smart scour sensing
post that consisted of bio-inspired magnetostrictive flow sensors attached to the
post surface (Fig. 11.7 ). When scour advances and exposes these Galfenol whis-
kers at different buried depths, the sensors would deflect due to fluid flow and drag.
A giant magnetostrictive sensor mounted at the base of the whisker would detect
such a change, and the response would be transmitted wirelessly to a base station.
A similar concept was also proposed by Wang et al. ( 2012 ), Loh et al. ( 2014 ), and
Azhari et al. ( 2014 ) but with piezoelectric sensors.
While hair-like structures that protrude from the surface of the skin could be
used for sensing flow, it is well known that the skin itself is a highly effective
distributed sensor. In fact, the skin has inspired the development of thin films and
coatings that are sensitive to different damage features. One particular technique
for creating artificial skin sensors is to incorporate randomly distributed carbon
nanotubes (CNT) in a thin, flexible, polymer matrix. Numerous techniques have
been developed, and they include examples such as evaporation (Dinh-Trong et al.
2009 ), spin coating (Yim et al. 2008 ), layer-by-layer (Loh et al. 2007 ), and
spraying (Kang et al. 2006 ), just to name a few.
In particular, Loh et al. ( 2007 , 2005 ) demonstrated strain sensing using layer-
by-layer thin films assembled with single-walled carbon nanotubes (SWNT) and
various polyelectrolyte species. It was observed that the film's electrical properties
(e.g., resistivity) varied linearly with applied strains up to at least 1 % strains, and
strain sensitivity could be controlled by modifying the concentration of the CNT
solution used during layer-by-layer fabrication (Loh et al. 2008 ). Like skin, these
materials could be applied onto the surfaces of structures for SHM. However, for
detecting damage within structural materials, these SWNT-based films were also
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