Design of experiment studies for the fabrication processes involved in the micro-texturing of surfaces for fluid control

摘要

This thesis focuses on the use of a design of experiment approach to examinethe significance of process factors and interactions on the fabrication of micro-textured surfaces. The micro-textured surfaces examined contain pillar and holefeatures ranging from 80 – 2 micrometers in diameter. The processes examined are thedeep reactive ion etching of silicon wafers for the production of silicon mouldinserts and the micro-injection moulding of polypropylene, high densitypolyethylene and 316LS stainless steel replicate samples of the silicon mouldinsert.During the deep reactive ion etching of the silicon wafers the design ofexperiment 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 micrometerpillars. Undercuts occur when the pillar base has a smallercross-section than the apex of the pillar. Switching times was found to be theonly statistically significant parameter for both 10 x 10 and 5 x 5 micrometerpillars.The design of experiment approach is used in the micro-injection moulding ofpolypropylene, high density polyethylene and 316LS stainless steel replicates toexamine the significance of mould temperature, cooling time, holding pressureand injection speed on the part and buffer mass of the produce samples, theheight and width of pillar on the replicate surfaces and the variation of thereplicated pillars height and width from the original silicon mould insert.Examination of the high density polyethylene replicates found that mouldtemperature was the most significant factor regarding pillar dimensions (andvariation from the silicon mould insert) across the range of pillar sizes. Uponexamination of the polypropylene replicates it was found that the factor of mostsignificance on pillar dimensions varied across the different pillar sizes. Holdingpressure was identified as the most significant factor with regards to the 53 x 29and 19 x 80 micrometerpillars. Injection speed was found to be most significant for the25 x 25 and 19 x 29 micrometer pillars. Cooling time was found to be most significantwith regards to the 30 x 10, 25 x 10, 20 x 10 and 15 x 10 micrometer pillars. Whileiimould temperature was found to be most significant for the 20 x 20, 15 x 15 and10 x 30 micrometer pillars. The interaction between mould temperature and injectionspeed was also found to be the most significant factor with regards to the 43 x29 and 25 x 30 micrometerpillars. Examination of the 316LS replicates found thatmould temperature was the most significant factor regarding pillar dimensionsfor 80 x 80 and 19 x 80 micrometer pillars. While holding pressure was found to be mostsignificant to the 29 x 29 micrometer pillars and injection speed was identified as mostsignificant to the 53 x 80 micrometer pillars.The samples produced during the design of experiment investigations werethen used to examine the effect of surface texturing on droplet behaviour.Droplet contact angles were examined on polypropylene, high densitypolyethylene and silicon samples structured with 10 – 2 micrometer pillar. Initial dropletcontact angles were found to be higher on the polypropylene samples than thehigh density polyethylene or silicon samples. With the lowest initial contactangles being found for the silicon inserts. Droplet ‘channelling’ and evaporationwere examined on silicon, polypropylene, high density polyethylene and 316LSsamples 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’. Thispinning effect was observed at all sample tilt angles (30 - 90o). With regards todroplet evaporation, the droplets were noted to evaporate evenly (with no orlimited contact pinning) on all unstructured surfaces and the surfaces structuredwith hole features. On the surfaces structured with pillar features, the dropletsappeared too evaporated along the surface gradient from the smallest pillars tothe largest.