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

Area of Science:

• Materials Science
• Microfluidics
• Surface Engineering

Background:

• Microfluidic systems require precise control over fluid behavior, heavily influenced by surface wettability.
• Traditional methods for modifying surface wettability often involve chemical coatings, which can be unstable or incompatible with certain applications.
• Topographic surface modifications offer a promising alternative for tuning wettability without chemicals.

Purpose of the Study:

• To develop and optimize a chemical-free surface engineering strategy for microfluidic devices using patterned nanoscale wrinkles.
• To investigate the impact of these engineered wrinkles on surface hydrophobicity and droplet generation efficiency.
• To evaluate the stability and compatibility of the wrinkled surfaces in microfluidic applications.

Main Methods:

• Fabrication of nanoscale wrinkles on polydimethylsiloxane (PDMS) surfaces through controlled oxygen-plasma treatment and mechanical prestrain.
• Characterization of surface topography using 3D laser profilometry and atomic force microscopy.
• Quantification of surface wettability via contact-angle measurements.
• Assessment of droplet generation performance and flow characteristics in microfluidic channels.
• Finite-element simulations to model fluid behavior at wrinkled surfaces.

Main Results:

• Optimized wrinkling parameters (40% prestrain, 100W plasma power, 12 min exposure) yielded stable hydrophobic PDMS surfaces.
• Wrinkled PDMS surfaces enabled reliable water-in-oil droplet generation without any additional coatings.
• Simulations revealed enhanced interfacial slip at wrinkled walls compared to flat surfaces.
• The engineered surfaces demonstrated orientation-dependent hydrophobicity and droplet generation behavior.
• The approach avoided common issues associated with silane-treated PDMS, such as hydrophobic recovery and coating incompatibility.

Conclusions:

• Mechanically engineered surface wrinkles provide an effective, chemical-free method for tuning hydrophobicity in microfluidic devices.
• This technique offers a robust solution for stable droplet generation and precise flow control.
• The findings have direct implications for advancing droplet-based assays and lab-on-a-chip technologies.