Lithographically fabricated SU8 composite structures for wettability control

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