Introduction
Controlled surface topographies on soft polymer substrates are of great interest for applications in microfluidics, optics, sensors, and flexible electronics. Wrinkling via plasma oxidation offers a lithography-free route to generate such patterns. This work systematically investigates the formation and tunability of plasma-induced wrinkles on uncross-linked polydimethylsiloxane (PDMS) thin films.

Methods
Uncured PDMS films were exposed to oxygen plasma to create a stiff silica-like (SiOx) surface layer atop a compliant elastomer substrate. This bilayer system, upon cooling, developed compressive stresses that triggered buckling instability. Wrinkle evolution was monitored using optical microscopy, and topographical features were quantified using atomic force microscopy (AFM). Plasma exposure duration and power were varied to study their influence on wrinkle morphology. Additional post-treatment thermal annealing and sequential short-duration plasma exposures were performed to explore secondary tuning strategies.

Results
Wrinkle wavelength and amplitude increased with prolonged plasma treatment, correlating with greater oxidized layer thickness and mechanical contrast. Sequential short plasma exposures and thermal annealing further enhanced compressive strain, leading to reduced wrinkle wavelengths and improved pattern uniformity. The process demonstrated high reproducibility and tunability, with feature sizes controlled by simple adjustments of plasma parameters.

Conclusions
Oxygen plasma treatment provides a scalable, metal-free method to engineer wrinkle morphologies on PDMS. By controlling plasma parameters and applying secondary treatments, nanoscale topographies with tailored periodicity can be achieved without complex lithography. This approach offers a reliable framework for designing functional surfaces with tunable wettability, adhesion, and optical properties, advancing the development of soft material interfaces for emerging technologies.