Biomedical Engineering Reference
In-Depth Information
a master for preparing a positive replica of the lotus leaf, which has nearly the
same surface morphology as the natural lotus leaf on both the micro- and nanoscale.
Small papillae hills with an average distance of 6
m and even the intricate nano
textures between the hills and in the valleys were successfully replicated in the
positive replica as shown in the SEM images (Fig.
9.4
, SEM images). The positive
replica also exhibits the same superhydrophobic property as the natural counterpart
with a contact angle of 160
ı
while the negative replica of the lotus leaf displays
hydrophobic properties with a contact angle of 110
ı
.
Master prepared by macromachining can also be used as template to fabricate
superhydrophobic substrates. In He et al.'s work [
39
], substrates contain squared
posts of 25
mwide,30
m deep, 8-80
m between the posts. A water drop can
both suspend on top of the posts and wet the posts determined by how the water
drop is attached. Gentle deposition causes a suspension of the water drop, with the
measured contact angle following the Cassie's theory prediction. While dropping
the water from some height induces a wetting of the gaps between the posts, with
the contact angle satisfying the Wenzel's theory.
Nanoimprint lithography is another kind of replication process with the pattern
replication achieved by heat- and pressure-driven process, and a hard master
pushed firmly onto a thermoplastic polymer layer whose temperature is above its
glass transition temperature [
101
]. And then the master is cooled and removed to
acquire a negative replica of the template. Impressively, nanoimprint lithography
is able to make very small features even down to a few nanometers based on
the master design. Take the work of Lee et al. [
102
], for example, nanoimprint
lithography is harnessed for the preparation of polystyrene substrates with disparate
nanostructures. In the nanoimprinting process, textured aluminum sheets and anodic
aluminum oxide (AAO) membranes were used as replication templates and the
master were pressed onto polystyrene (PS) polymer substrates with the temperature
raised simultaneously. After a temperature cooling down and pressure release
process, the replication template was eliminated from the polymer substrate by
dissolving with saturated HgCl
2
solution, achieving the successful replication
outcome, the large area, nanostructured PS surface. The diameters of PS nanofibers
were controlled by changing the pore diameter of the AAO replication templates,
which was adjusted as desired through wet-chemical etching. On the other hand, the
length of PS nanofibers could be tuned by appropriately varying the thickness of the
AAO template. Unlike imprint nanolithography using a hard master, capillary force
lithography utilizes a patterned elastomeric mold, which is directly laid onto a spin-
coated polymer film (Fig.
9.5
). In order to obtain the negative replica of the mold, the
temperature is raised above the polymer's glass transition temperature after solvent
evaporation, known as temperature-induced capillarity; or by direct molding prior
to solvent evaporation, known as solvent induced capillarity.
Similar to this, alumina membrane was exploited as the mold to facilitate aligned
PS nanotubes, as demonstrated by Jin et al. [
94
]. The membrane was brought into
contact with the PS solution which was previously cast on glass. After capillary
molding, the membrane was dissolved in NaOH to achieve aligned PS nanotubes.
The surface is superhydrophobic but very sticky, with the water droplet on it not
