PFAS-Free Biphilic Surface Design for Fog and Dew Atmospheric Water Harvesting

Original Authors: Konstantinos Taliantzis and Kosmas Ellinas

Original paper is accessible at: https://doi.org/10.1039/d5nr05114b

Why surface patterning matters in atmospheric water collection

Atmospheric water harvesting is not limited to sorbents such as MOFs, hydrogels, or hygroscopic salts. Another important pathway is passive atmospheric water collection, where water is collected directly from fog droplets or from dew formation on cooled surfaces.

In these systems, the surface itself becomes the key technology. Its wettability, microstructure, and ability to move droplets determine whether captured water can be efficiently collected or becomes trapped, pinned, or re-evaporated.

This study focuses on one of the most important questions in surface-based AWH:

How should hydrophilic and hydrophobic regions be patterned to maximize fog and dew collection?

The authors systematically investigated PFAS-free biphilic surfaces, meaning surfaces that combine superhydrophobic regions with superhydrophilic patterns, without relying on fluorinated PFAS-based chemistries. This is important because many high-performance water-repellent coatings still depend on PFAS compounds, which raise environmental and regulatory concerns.

Fog and dew require different surface strategies

A major contribution of the paper is its clear distinction between fog collection and dew collection.

Fog consists of liquid droplets already suspended in air. Therefore, fog harvesting is mostly governed by:

• droplet impact

• capture

• coalescence

• mobility

• shedding

In contrast, dew forms when water vapor condenses on a cooled surface. Dew collection depends on:

• nucleation site density

• latent heat dissipation

• droplet growth

• coalescence

• drainage

This distinction is essential because a surface design that improves fog collection does not necessarily improve dew collection. In fog harvesting, too much hydrophilic area can slow droplet removal. In dew harvesting, however, hydrophilic areas can promote nucleation and guide drainage.

PFAS-free biphilic surfaces: design and fabrication

The authors fabricated a library of biphilic PDMS surfaces using soft lithography and patterned hydrophilic domains. The approach was designed to be both PFAS-free and scalable, offering a more environmentally compatible pathway for water-collection interfaces.

Three deterministic microtopographies were tested:

• Pillars

• Trapezoids

• Honeycombs

These microstructures were designed to provide a superhydrophobic background. The authors then added superhydrophilic patterns in two main forms:

1. Circular spots

   • 500 µm diameter

   • 1000 µm diameter

2. Parallel stripes

   • 1000 µm width / 1000 µm spacing

   • 1000 µm width / 2000 µm spacing

This produced 12 biphilic surface combinations, allowing a systematic comparison of pattern type, feature size, and wettability ratio across both fog and dew harvesting.

Wettability contrast and microstructure

The surfaces showed strong wetting contrast between hydrophobic and hydrophilic domains.

For the superhydrophobic regions:

• Pillars: WCA 162°, CAH 9°

• Trapezoids: WCA 145°, CAH >45°

• Honeycombs: WCA 140°, CAH 40°

For the hydrophilic/superhydrophilic regions:

• Pillars: WCA 20°

• Trapezoids: WCA 17°

• Honeycombs: WCA 16°

This strong wettability contrast allowed the authors to test how water behaves when it encounters highly mobile hydrophobic regions next to water-attracting hydrophilic domains.

The pillar surface showed the best droplet mobility because of its high contact angle and low hysteresis. In contrast, honeycomb surfaces suffered from droplet pinning inside cavities, which limited collection performance.

How biphilic stripes work

The paper provides a useful mechanistic explanation for stripe-patterned surfaces.

On stripe patterns, droplets first nucleate and grow on the superhydrophobic “runway.” Once they reach a critical size, they contact the neighboring superhydrophilic stripe. Capillary forces then pull the droplet into the hydrophilic channel, and gravity helps drain it away.

This creates a repeated five-stage cycle:

1. droplet nucleation and growth

2. boundary interception

3. capillary migration

4. surface clearing

5. cycle renewal

The key advantage of stripe patterns is that they act as water guides, converting scattered droplets into directional flow pathways.

How biphilic spots work

Spot patterns behave differently. Instead of guiding water continuously, they create localized capture points.

Droplets accumulate on the hydrophilic spots, grow into larger formations, and remain confined by the surrounding superhydrophobic background. This confinement helps prevent the formation of continuous water films.

In the study, these discrete liquid formations reached an average size of approximately 3 mm before gravity-driven removal. The complete clearing cycle occurred at an average frequency of about 160 seconds, or 2 min 40 s.

This mechanism is especially useful when the goal is to increase nucleation without sacrificing overall droplet mobility.

Fog collection: mobility dominates

For fog harvesting, the main finding is that droplet mobility is more important than adding hydrophilic capture regions.

The flat PDMS reference collected:

• 4.01 ± 0.76 g cm⁻² h⁻¹

Among single-wettability microstructured surfaces, the pillar surface performed best:

• 4.39 g cm⁻² h⁻¹

This was attributed to its low adhesion and fast droplet shedding.

However, after biphilic patterning, fog performance generally did not improve. In many cases, adding hydrophilic regions reduced WCR by approximately 11–30%, because the hydrophilic domains delayed shedding and increased wetted surface area.

For pillar substrates, the biphilic patterns gave relatively similar fog yields:

• 500 µm spots: 3.86 ± 0.32 g cm⁻² h⁻¹

• 1000 µm spots: 3.72 ± 0.69 g cm⁻² h⁻¹

• 1000:1000 µm stripes: 3.66 ± 0.55 g cm⁻² h⁻¹

• 1000:2000 µm stripes: 3.76 ± 0.48 g cm⁻² h⁻¹

The best biphilic pillar design remained slightly below flat PDMS and below the unpatterned superhydrophobic pillar surface.

The conclusion for fog collection is clear:

If a surface already has excellent droplet mobility, adding too much hydrophilic patterning can hurt rather than help.

Honeycomb surfaces: biphilicity helps, but cannot fully overcome pinning

Honeycomb surfaces behaved differently because they had low intrinsic droplet mobility and strong retention inside cavities.

The unpatterned honeycomb surface collected only:

• 2.01 ± 0.90 g cm⁻² h⁻¹

Adding 500 µm hydrophilic spots increased this to:

• 3.67 ± 1.08 g cm⁻² h⁻¹

This represents an 83% improvement compared to the unpatterned honeycomb surface.

However, even with this improvement, honeycomb-based designs still remained below the flat PDMS reference. This shows that biphilicity can improve poor surface geometries, but it cannot fully compensate for a topography that strongly pins droplets.

Dew collection: pattern design becomes more important

Dew harvesting showed a more complex and more pattern-sensitive behavior than fog harvesting.

Under intermediate humidity conditions:

• 70% RH

• ΔT = 15°C

the best-performing surface was:

• pillar microstructure

• parallel stripes

• 1000 µm width / 2000 µm spacing

• 35% hydrophilic coverage

This surface achieved a WCR of:

• 0.0417 ± 0.0050 g cm⁻² h⁻¹

corresponding to a:

• 156% improvement over flat PDMS 

This performance was attributed to the balance between two functions:

• hydrophilic stripes promoted nucleation and drainage

• the remaining 65% superhydrophobic area preserved droplet mobility

This is one of the most important design lessons of the paper: in dew harvesting, the best surface is not fully hydrophilic or fully hydrophobic, but carefully balanced.

High-humidity dew collection

At higher humidity and lower cooling strength:

• 90% RH

• ΔT = 5°C

the best-performing surfaces included:

• pillars with 1000:1000 stripes: 0.039 g cm⁻² h⁻¹

• trapezoids with 1000 µm spots: 0.0408 g cm⁻² h⁻¹

• trapezoids with 1000:1000 stripes: 0.0363 g cm⁻² h⁻¹

These represented improvements of over 25% compared to PDMS.

At high humidity and stronger cooling:

• 90% RH

• ΔT = 15°C

the best performance came from 1000 µm hydrophilic spots:

• 0.0717 g cm⁻² h⁻¹ on trapezoids

• 0.065 g cm⁻² h⁻¹ on pillars

These corresponded to gains of:

• 53.5% for trapezoids

• 39% for pillars

Under these conditions, larger hydrophilic spots appear beneficial because they provide more nucleation sites while maintaining relatively low hydrophilic coverage.

Design rules emerging from the study

The most valuable contribution of this paper is not just reporting the best-performing surface, but extracting design rules for PFAS-free atmospheric water collection interfaces.

For fog collection:

• droplet mobility dominates

• superhydrophobic pillars perform well

• excessive hydrophilic patterning reduces shedding

• low hydrophilic coverage is preferable

• small hydrophilic spots can help poor geometries but may not improve already mobile surfaces

For dew collection:

• both nucleation and drainage matter

• stripes are effective under intermediate RH because they guide drainage

• 35% hydrophilic coverage provides a strong balance between nucleation and mobility

• under high RH, 1000 µm spots can enhance nucleation without causing early film formation

• pattern type, size, and wettability ratio must be matched to environmental conditions

Why PFAS-free matters

Many high-performance superhydrophobic and biphilic surfaces rely on fluorinated chemistries. While effective, PFAS-based approaches raise concerns about persistence, environmental accumulation, and long-term safety.

This work is important because it demonstrates that structured, high-contrast biphilic surfaces for AWH can be fabricated through a PFAS-free and scalable route. That makes the study relevant not only for water collection performance, but also for environmental sustainability and future deployment.

In other words, the paper is not simply asking:

Which surface collects the most water?

It is also asking:

Can we design water-harvesting surfaces that are efficient, scalable, and safer for the environment?

Key insights

1. PFAS-free biphilic surfaces can be used for both fog and dew harvesting.

2. Fog collection is mainly controlled by droplet mobility.

3. Dew collection is controlled by a balance of nucleation, coalescence, heat transfer, and drainage.

4. Pillar microstructures offer high droplet mobility and strong fog performance.

5. Honeycomb structures suffer from pinning and flooding.

6. For dew collection at 70% RH and ΔT = 15°C, 1000:2000 µm stripes on pillars improved WCR by 156%.

7. At 90% RH and ΔT = 15°C, 1000 µm spots achieved the highest WCR: 0.0717 g cm⁻² h⁻¹ on trapezoids.

8. The best pattern depends strongly on the environmental regime.

9. Hydrophilic coverage must be optimized; more hydrophilic area is not always better.

10. The work provides practical design guidelines for scalable, PFAS-free AWH surfaces.

Takeaway

This study shows that atmospheric water harvesting surfaces cannot be designed with a single universal rule.

For fog, the priority is fast droplet shedding.For dew, the priority is balancing nucleation with drainage.

By systematically comparing biphilic pattern type, size, wettability ratio, and microtopography, the authors demonstrate that PFAS-free surface engineering can provide practical design rules for next-generation fog and dew water harvesting systems.