Fab-free Micropatterned Surfaces for Multiscale Droplet Generation and Its Applications

Abstract

The growing need for low-cost, customizable, and accessible microfabrication tools has catalysed the exploration of non-conventional platforms for biological analysis, electronics printing, and open-channel microfluidics. Traditional fabrication techniques such as photolithography, soft lithography, and screen-printing offer precision and scalability but remain tethered to cleanroom facilities, photomask fabrication, and multi-step processing. These limitations restrict faster development of prototype and deployment in decentralized or resource-limited settings. This thesis originated from the objective of developing a fab-free, reusable, and low-cost microarray platform. The initial approach leveraged superhydrophobic copper meshes, chemically etched to generate nanowires and then silanized or PTFE-coated to induce extreme water repellence. When these meshes were brought into contact with PDMS substrates, nanowires selectively transferred to the PDMS surface through the mesh pores, creating micropatterned hydrophilic islands on a superhydrophobic background. This maskless method produced droplet microarrays capable of trapping sub-microliter droplets in predefined locations. The simplicity and reusability of this stamp-like patterning method led to further exploration, which showed that the wettability contrast principle could be used in more ways than just droplet confinement. Controlling ink confinement on a patterned surface was a great way to print high-viscosity inks, nanoparticle suspensions, and biofluids. These are still hard to do with nozzle-based systems like inkjet or aerosol printing due to the problem of nozzle clogging. This led to the evolution of the project: instead of transferring nanowires alone, laser engraving was employed to directly modify the wettability of chemically treated copper surface. By selectively removing the hydrophobic coating, hydrophilic paths could be created on a superhydrophobic base, generating heterogeneous patterned surfaces with controllable wettability contrast. These patterned surfaces were then used as reusable stamps to print functional inks onto a variety of substrates like flexible polymers, paper, and cloth. The thesis provides a comprehensive analysis of heterogeneous surface engineering for multi-scale droplet management and printed device fabrication, advancing from droplet-trapping microarrays to complex ink printing techniques. This approach is novel as it provides a low-cost, cleanroom-free process. Its functionality is also extended to enablement of droplet confinement, bio-patterning, and conductive ink printing across curved, flexible, and bio-inert surfaces.