Abstract:
This thesis focuses on the use of a design of experiment approach to examine
the significance of process factors and interactions on the fabrication of micro-
textured surfaces. The micro-textured surfaces examined contain pillar and hole
features ranging from 80 – 2 micrometers in diameter. The processes examined are the
deep reactive ion etching of silicon wafers for the production of silicon mould
inserts and the micro-injection moulding of polypropylene, high density
polyethylene and 316LS stainless steel replicate samples of the silicon mould
insert.
During the deep reactive ion etching of the silicon wafers the design of
experiment approach was used to determine the significant of platen power,
C4F8 gas flow and switching times to the presence of pillar undercut of 10 x 10,
5 x 5 and 2 x 2 micrometer
pillars. Undercuts occur when the pillar base has a smaller
cross-section than the apex of the pillar. Switching times was found to be the
only statistically significant parameter for both 10 x 10 and 5 x 5 micrometer
pillars.
The design of experiment approach is used in the micro-injection moulding of
polypropylene, high density polyethylene and 316LS stainless steel replicates to
examine the significance of mould temperature, cooling time, holding pressure
and injection speed on the part and buffer mass of the produce samples, the
height and width of pillar on the replicate surfaces and the variation of the
replicated pillars height and width from the original silicon mould insert.
Examination of the high density polyethylene replicates found that mould
temperature was the most significant factor regarding pillar dimensions (and
variation from the silicon mould insert) across the range of pillar sizes. Upon
examination of the polypropylene replicates it was found that the factor of most
significance on pillar dimensions varied across the different pillar sizes. Holding
pressure was identified as the most significant factor with regards to the 53 x 29
and 19 x 80 micrometer
pillars. Injection speed was found to be most significant for the
25 x 25 and 19 x 29 micrometer
pillars. Cooling time was found to be most significant
with regards to the 30 x 10, 25 x 10, 20 x 10 and 15 x 10 micrometer
pillars. While
ii
mould temperature was found to be most significant for the 20 x 20, 15 x 15 and
10 x 30 micrometer
pillars. The interaction between mould temperature and injection
speed was also found to be the most significant factor with regards to the 43 x
29 and 25 x 30 micrometer
pillars. Examination of the 316LS replicates found that
mould temperature was the most significant factor regarding pillar dimensions
for 80 x 80 and 19 x 80 micrometer
pillars. While holding pressure was found to be most
significant to the 29 x 29 micrometer
pillars and injection speed was identified as most
significant to the 53 x 80 micrometer
pillars.
The samples produced during the design of experiment investigations were
then used to examine the effect of surface texturing on droplet behaviour.
Droplet contact angles were examined on polypropylene, high density
polyethylene and silicon samples structured with 10 – 2 micrometer
pillar. Initial droplet
contact angles were found to be higher on the polypropylene samples than the
high density polyethylene or silicon samples. With the lowest initial contact
angles being found for the silicon inserts. Droplet ‘channelling’ and evaporation
were examined on silicon, polypropylene, high density polyethylene and 316LS
samples structured with micro-channel surface pillars and holes ranging from 80
– 2 micrometer
in diameter. Contact pinning of the droplet to the surface via the three-
phase contact-line was noted during observations of droplet ‘channelling’. This
pinning effect was observed at all sample tilt angles (30 - 90
o
). With regards to
droplet evaporation, the droplets were noted to evaporate evenly (with no or
limited contact pinning) on all unstructured surfaces and the surfaces structured
with hole features. On the surfaces structured with pillar features, the droplets
appeared too evaporated along the surface gradient from the smallest pillars to
the largest.