Tunable Wetting and Chemical Coating-Free Slip on Wrinkled PDMS for Droplet Microfluidics

Microfluidic systems offer significant advantages in handling small sample volumes while enabling high-throughput experimentation and precise flow control. Surface wettability critically governs droplet-generation efficiency and flow characteristics in microfluidic channels. Topographic modulation of polymer surfaces provides a simple, chemical-free route to tune wettability. We present a surface-engineering strategy that patterns nanoscale wrinkles on polydimethylsiloxane (PDMS), optimized at 40 ϵ prestrain, 100 W oxygen-plasma power, and 12 min exposure. The resulting wrinkled PDMS exhibited stable hydrophobicity sufficient for reliable water-in-oil droplet generation without any coating. Wettability was quantified by contact-angle measurements, and wrinkle morphology was verified by 3D laser profilometry and atomic force microscopy. Orientation-dependent hydrophobicity and droplet-generation behavior were also assessed. Finite-element simulations showed enhanced interfacial slip at wrinkled walls relative to flat walls under matched driving and captured orientation effects on near-wall kinematics. The approach circumvents hydrophobic-recovery and coating-compatibility issues commonly encountered with silane-treated PDMS. Therefore, the results establish mechanically engineered surface wrinkles as an effective, chemical-free means to tune hydrophobicity in microfluidic devices, with direct relevance to droplet-based assays and lab-on-a-chip platforms.