Stretchable, Fluorine‑Free Superhydrophobic Silicone Surfaces Fabricated by Femtosecond Laser Texturing

Abstract

Highly stretchable and durable superhydrophobic surfaces are of great interest for a range of applications. In this study we fabricated such surfaces on Ecoflex silicone elastomer using a femtosecond laser texturing approach. The resulting materials can withstand strains up to 400 % without loss of water‑repellency; on the contrary, stretching enhances their hydrophobicity. We also demonstrate a strain‑induced transition from a “petal” to a “lotus” wetting state, enabling reversible droplet capture and release. These laser‑ablated surfaces are fluorine‑free, mechanically robust, and amenable to scalable production, making them attractive for microfluidics, biomedical devices, and smart textiles.

Background

Artificial superhydrophobic surfaces play a pivotal role in applications such as drag reduction[1], anti‑biofouling[2], microfluidic manipulation[3], anti‑icing[4,5,6], water collection[7], and wearable electronics[8]. For use in artificial skin and wearable devices, the ideal surface must combine high stretchability, durability, biocompatibility, and straightforward fabrication. Traditionally, achieving stretchability required patterning rigid substrates or depositing micro/nanoparticles onto pre‑stretched elastomers[10,11], approaches that are complex and often rely on volatile organic solvents, conflicting with green manufacturing principles.

Femtosecond laser processing offers a clean, rapid route to create hierarchical micro/nanostructures on both rigid and flexible substrates[12–18]. Prior work has focused on PTFE and PDMS[20,21]; however, PTFE’s irreversible deformation[22] and PDMS’s limited modulus restrict superhydrophobic performance to strains below 100%[21]. Ecoflex, an ultra‑soft silicone elastomer capable of 500% stretch and biocompatible skin‑like compliance[23,24], presents an attractive substrate for laser texturing. Here we report, for the first time, the fabrication of highly stretchable, durable, and fluorine‑free superhydrophobic surfaces on Ecoflex via femtosecond laser ablation, with tunable adhesion and strain‑enhanced water repellency.

Methods and Experiment

Materials

The flexible rubber (Ecoflex 00‑20) was purchased from Smooth‑On, Inc., USA.

Preparation of Silicone Elastomers

As illustrated in Fig. 1a, a 2 mm thick Ecoflex sheet was produced by mixing liquid parts A and B in a 1:1 volume ratio and curing in a mold for 12 h at room temperature[23].

a Fabrication process of solid Ecoflex rubber. b Schematic device configuration and fabrication process. c Effect of laser processing parameters on CAs and SAs

Fabrication of Elastomeric Superhydrophobic Surfaces

Micro/nanostructures were produced by line‑by‑line femtosecond laser ablation in air (Fig. 1b). The elastomer was mounted on a nanotechnology stage (XY‑Tripod‑Theta 6‑Axis System, Alio Industries) and irradiated by a Ti:sapphire femtosecond laser (LIBRA, Conherent Inc., CA, USA) with a 100 fs pulse width, 1 kHz repetition rate, and 800 nm wavelength. A ×10 objective (NA 0.24) focused the beam; the scan speed was fixed at 2 mm s⁻¹. By varying the scanning spacing and laser fluence, we optimized surface morphology for superhydrophobic performance.

Characterization

Surface morphology was examined with a scanning electron microscope (SEM, JEOL JSM‑7001F) and a laser scanning confocal microscope (OLYMPUS, OSL4100). Energy‑dispersive X‑ray spectroscopy (EDS) assessed chemical changes. Contact angles (CAs) and sliding angles (SAs) were measured with a contact angle meter (SEO PHOENIX).

Results and Discussion

Structure and Superhydrophobic Properties

Superhydrophobicity arises from hierarchical surface topography inspired by natural surfaces[26]. Low‑adhesion (LA) surfaces, resembling lotus leaves, exhibit sliding angles below 10°[27], while high‑adhesion (HA) surfaces, akin to rose petals, retain droplets even when inverted[28]. By adjusting laser parameters, we fabricated both morphologies on Ecoflex.

Figures 1c and 2a–c display the wetting behavior and surface morphologies of the laser‑textured silicone elastomers. The HA surface (SA = 180°) shows a contact angle (CA) of 153.1° at a laser fluence of 45.4 J cm⁻² and a 10 µm spacing. Increasing the spacing reduces CA (to 128° at 80 µm) and transforms the surface from LA to HA. EDS spectra (Fig. 2d, e) reveal negligible chemical change, aside from a slight rise in oxygen content.

SEM images of the femtosecond laser‑induced rough microstructures with different laser fluences and scanning spacings. a 45.4 J cm⁻², 10 µm. b 45.4 J cm⁻², 50 µm. c 136.2 J cm⁻², 50 µm. EDS spectrum record for original sample (d) and laser‑ablated sample (e)

SEM images reveal a micro‑ridge network covered by 100–200 nm nanoparticles, formed by rapid cooling of the localized melt[30]. The ridge edges host rich nanoparticles, creating a Cassie‑Baxter wetting state and low adhesion; the ridge centers, lacking nanostructures, support a Wenzel state and high adhesion. By selecting a laser fluence of 136.2 J cm⁻² and spacings of 30 µm (LA) and 50 µm (HA), we achieved the desired wetting regimes.

Strain‑Modulated Structures and Wettability

We investigated CA and SA as functions of strain applied perpendicular (⊥) and parallel (∥) to the scanning direction. The strain ε = (L – L₀)/L₀. Figure 3a,b illustrate how the ridge period and groove width evolve under strain. Parallel stretching compresses the grating, shortening the period and narrowing grooves; the ridge centers fold and become enveloped by surrounding micro/nanostructures (Fig. 3e). At 400% strain, the period reduces to 20–30 µm (Fig. 3d), enriching surface topography. Perpendicular stretching elongates the period and groove width while preserving ridge geometry (Fig. 3f–h), creating a secondary 10 µm ridge array at the groove bottom.

Structural parameters of the HA superhydrophobic elastomer stretched at 0–400% strain in the parallel direction (a) and perpendicular direction (b). Surface morphologies of the HA superhydrophobic elastomer stretched at the strain of 400% in the parallel (c–e) and perpendicular (f–h) directions

Figure 4 shows that both LA and HA surfaces exhibit enhanced superhydrophobicity with increasing strain, contrary to prior reports[21,32]. For the HA surface, CA rises from 144.4° at 100% strain to 153.6° at 400% strain, while the SA drops to 12°. The transition from a “pinning” state (droplet stuck even upside‑down) to a “rolling” state (droplet slides at 43° tilt) occurs around 200% strain. Parallel and perpendicular stretching yield similar trends, with the HA surface achieving a maximum CA of 156.6° and a minimum SA of 9° at 400% strain.

CAs (a) and SAs (b) of the superhydrophobic elastomers at different parallel strain values. CAs (c) and SAs (d) of the surface at different perpendicular strain values

Mechanism of Stretching‑Enhanced Water Repellence

The enhancement originates from morphology changes rather than chemical transformations, as confirmed by EDS and the unchanged CA of pristine Ecoflex after 400% strain (Fig. 5). We model the wetting state as a combination of Cassie‑Baxter (air pockets at ridge edges) and Wenzel (full contact at ridge centers). Equation (3) shows that increasing the roughness factor r (due to newly formed micro‑ridge folding) or decreasing the solid fraction f_S (due to wider grooves) raises the apparent contact angle. Perpendicular stretching reduces f_S by increasing groove width, while parallel stretching increases r by adding hierarchical features, both contributing to higher CA and lower adhesion.

a CAs of the original elastomer at different strain values, and microscope images of the original elastomer with the strain of (b) 0 and (c) 400%

Durability

Durability was assessed by repeated kneading and torsion cycles. The LA surface maintained its superhydrophobicity after 50 cycles (Fig. 7a). For the HA surface, we performed 50 stretching‑relaxation cycles at 300% strain in both directions; the CA and SA remained stable across all cycles (Fig. 7b,c), confirming high reversibility and mechanical robustness.

a Processes of kneading and torsion and cyclic tests of stretching‑relaxing conducted in the (b) parallel and (c) perpendicular directions for the HA superhydrophobic elastomer

Droplet Transportation

The reversible pinning‑to‑rolling transition enables lossless droplet transfer. A 5 µL water droplet placed on an LA surface is captured by an approaching HA surface due to high adhesion; subsequent stretching reduces adhesion, allowing the droplet to release and fall under gravity. A supplementary video demonstrates the full cycle (Additional file 1: Video S1). The simplicity of this mechanism makes it suitable for automated microfluidic handling, and the scalability of femtosecond laser processing with high‑power (>100 W) lasers and high‑speed galvanometers[36,37] suggests industrial feasibility.

Demonstration of the lossless droplet transfer using the stretchable HA superhydrophobic elastomer

Conclusions

We have demonstrated, for the first time, fluorine‑free superhydrophobic surfaces on Ecoflex silicone elastomer that sustain strains up to 400 % while maintaining—and even enhancing—water repellency. By tuning laser fluence and spacing, we achieved both LA and HA morphologies, and strain‑induced transitions from pinning to rolling states enable reversible droplet capture and release. The surfaces exhibit excellent durability under repeated mechanical cycling and are compatible with scalable femtosecond laser fabrication. These highly stretchable, controllable, and robust surfaces hold great promise for biomedical devices, microfluidic systems, and smart wearable technologies.

Availability of Data and Materials

The datasets generated and/or analyzed during this study are available from the corresponding author on request.

Abbreviations

CA:

Contact angle

HA:

High adhesion

LA:

Low adhesion

PDMS:

Polydimethylsiloxane

PTFE:

Polytetrafluoroethylene

SA:

Sliding angle