Engineering Superomniphobic Surfaces on Flexible PDMS
Imagine a surface that remains completely dry even when submerged in oil, a protective coating that causes both water and corrosive acids to bead up and roll away, or a flexible electronic device that continues to function perfectly even when drenched in rain or spilled coffee. This is the promise of superomniphobic surfaces—materials engineered to repel virtually any liquid. The term "omniphobic" literally means "fears all," and these remarkable surfaces live up to their name by displaying extreme repellency to both high surface tension liquids like water and low surface tension liquids like oils, alcohols, and even concentrated acids 3 7 .
What makes this field particularly exciting today is the development of sticky superomniphobic surfaces—materials that repel liquids but can also controllably adhere to droplets. This seemingly paradoxical combination of properties opens up revolutionary applications in liquid manipulation, lab-on-a-chip devices, and targeted drug delivery.
Recent breakthroughs in creating these surfaces on flexible, transparent PDMS (polydimethylsiloxane) substrates represent a significant step toward practical real-world applications 1 . In this article, we'll explore the fascinating science behind these surfaces, examine a groundbreaking experiment in detail, and discover how they're poised to transform industries from healthcare to consumer electronics.
The angle formed where liquid meets solid surface. Higher angles (closer to 180°) indicate greater repellency.
Difference between advancing and receding contact angles as droplets move across a surface.
Minimum tilt angle required to make a droplet slide off the surface.
| Liquid Type | Example | Surface Tension (mN/m) | Repellency Difficulty |
|---|---|---|---|
| High Surface Tension | Water | 72.0 | Easier to repel |
| Medium Surface Tension | Ethanol | 22.1 | Harder to repel |
| Low Surface Tension | Hexadecane | 27.5 | Difficult to repel |
| Very Low Surface Tension | Silicone Oil | ~20.0 | Most difficult to repel |
The re-entrant texture—mushroom-shaped or overhanging geometries—creates capillary forces that effectively "pin" the liquid-air interface, preventing liquid from contacting the base of the structure 6 .
Flexible PDMS substrates offer a unique advantage in creating sticky superomniphobic surfaces. Their elastic nature allows for mechanical interlocking with droplets—the surface can slightly deform to "grip" a droplet without penetrating the liquid-solid-air composite interface that enables repellency 1 .
Liquid droplet makes contact with the flexible PDMS surface with micropillar array.
PDMS micropillars slightly deform, creating mechanical interlocking with the droplet.
Droplet adheres to surface while maintaining spherical shape and repellent properties.
Droplet released cleanly through specific stimuli (vibration, electrical fields, compression).
Silicon nanowires serve as master mold for creating micropillar array.
PDMS mixture (10:1 base-to-curing agent) poured onto silicon nanowire mold.
Cured at 70°C for 2 hours, then solidified PDMS peeled from mold.
Optional fluorinated compound treatment for enhanced repellency.
| Property | Performance | Test Method |
|---|---|---|
| Water Contact Angle | >150° | Static contact angle measurement |
| Hexadecane Contact Angle | >150° | Static contact angle measurement |
| Sliding Angle | <6° | Tilting stage measurement |
| Mechanical Resilience | Maintained super-repellency after mechanical damage | Abrasion testing |
| Chemical Stability | Withstood acid/base corrosion | Immersion in pH 3-11 solutions |
| Flexibility | Retained properties when bent to 2mm radius | Bending test |
| Transparency | >90% visible light transmission | Spectrophotometry |
Protecting sensitive components from moisture, sweat, and environmental liquids while maintaining flexibility and comfort 2 .
Liquid-resistant coatings that prevent biofouling and contamination in implantable and external medical equipment.
Precise droplet manipulation without absorption or cross-contamination in microfluidic devices and lab-on-a-chip applications 6 .
Surfaces that initially adhere to specific bodily locations then release therapeutic compounds in response to physiological cues .
| Material/Method | Function | Example Use Cases |
|---|---|---|
| PDMS (Polydimethylsiloxane) | Flexible, transparent substrate | Creating stretchable, bendable repellent surfaces 1 |
| Fluorinated Silica Particles | Provides re-entrant texture and low surface energy | Spray coating to create hierarchical structures 2 |
| Fluorodecyl POSS | Ultra-low surface energy coating | Achieving contact angles >150° with oils 3 |
| Electrospraying | Fabrication of 3D papillose micro-textures | Creating hierarchical structures with overhanging features 6 |
| Two-Photon Polymerization | High-resolution 3D printing of microstructures | Fabricating doubly reentrant pillars (DRPs) |
| Silicon Nanowire Molds | Template for micropillar arrays | Creating precise re-entrant geometries on PDMS 1 |
The engineering of sticky superomniphobic surfaces on flexible PDMS represents a remarkable convergence of materials science, surface chemistry, and mechanical engineering. By solving the apparent paradox of simultaneous repellency and adhesion, researchers have opened new possibilities for manipulating liquids without contamination—a capability with profound implications for fields ranging from medicine to consumer electronics.