Lithographically fabricated SU8 composite structures for wettability control

SU8 is a negative resist which is widely used for the fabrication of micron scale lateral 13 features over a wide range of heights using photolithographic methods. This has been extensively 14 used as a method to produce surface structure to which hydrophobicity can be added and a model 15 super-water repellent surface of achieved. However, such an approach requires at least two-steps 16 and does not embed the desired properties as part of the structure itself or create multiple levels 17 of topographical structure. In other applications, a variety of inclusions have previously been 18 studied to tailor the properties of SU8. In this work we report an approach to, and results from, 19 incorporating inclusions, in our case glass beads, of different wettabilities into the SU8 structures 20 produced by photolithography. In particular, we focus on ridge structures expected to be of use 21 in flow systems as drag reducing surfaces. We used scanning electron microscopy and 22 profilometry to investigate how the inclusion of either hydrophobic or hydrophilic glass beads 23 (sieve size of 20-30 μm) affects the definition of the structures formed and contact angle 24 goniometry to define what effect such inclusions has on the wettability of the SU8 composite 25 structure. It was found that the inclusion of hydrophobic glass beads in the SU8 resist resulted in 26 poorly defined structures compared to both SU8 on its own and SU8 to which hydrophobic glass 27 beads were added. In contrast, inclusion of hydrophilic glass beads resulted in a well-defined 28 ridge structure with the majority of the beads located in the ‘valleys’. Both EDX analysis and 29 contact angle data indicated that the surface chemistry of the beads themselves (both the 30 hydrophobic and hydrophilic beads) were masked by the SU8. Counter-intuitively, the inclusion 31 of hydrophobic beads in the SU8 composite resin resulted in ridges with increased wettability, 32 compared to SU8 ridges, as opposed to the inclusions of hydrophilic beads that resulted in 33 surfaces with increased effective hydrophobicity