Studies on Improving Quality Factor of 2D GMR Gratings and on Nonwetting Properties of Plasma-treated Polymer Surfaces Towards Liquid Metal Microfluidics

Polydimethylsiloxane (PDMS) is a biocompatible elastomer that is used widely in microfabrication, and this work presents three studies that utilize this elastomer in the areas of strain sensors, thin film coatings, and liquid metal microfluidics. The first study is on improving the quality factor of a 2D guided-mode resonance (GMR) strain sensor, which is a binary grating made of PDMS and titanium dioxide. To improve the quality factor, a slotting design rule is developed that can be applied to any grating design. To study the effect of the slotting design rule, finite element analysis simulations were performed, and the results indicate that the design rule helps produce resonances with at least a 6-fold increase in quality factor over the original design as well as more axially-symmetric sensitivities. The second study concerns the CF4/O2 plasma-treatment of polymers (PDMS being one of several studied) which creates a nonwetting surface toward gallium-based liquid metals. Gallium-based liquid metals tend to wet a variety of materials, and a method that allows conversion of a previously wetting polymer surface to a nonwetting one can help open new areas of research for liquid metal applications. The study conducts a variety of surface-level analyses – contact angle goniometry, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and apparent surface free energy analysis – to show that the cause of the nonwetting property is primarily due to surface roughness. The third study is on the feasibility of CF4/O2 plasma-treated PDMS channels to allow actuation and generation of surface-oxidized gallium based liquid metal (oxLM) droplets. The results of the study indicate that actuation and generation of oxLM droplets is not feasible due to the surface oxide of the liquid metal.