I. Abstract of the Paper’s Core Research

This study prepared a hydrophobic-specific aerosol ink based on nano‑silica and developed a pneumatic aerosol‑printing system. The system was used to systematically investigate how printing speed, substrate temperature, and sheath‑gas flow rate influence the width of hydrophobic lines, edge roughness (LER), and film thickness, enabling the selective fabrication of superhydrophobic patterns on hydrophilic substrates. Furthermore, two key functional validations were conducted—microfluidic manipulation and localized waterproofing on printed circuit boards. The core findings are as follows:

Low-viscosity hydrophobic aerosol ink system

Nano‑sized gas‑phase silica, combined with an epoxy curing system, exhibits a viscosity of only 11 cP, making it compatible with pneumatic ultrasonic atomization. It ensures no nozzle clogging during long‑term printing, can be formulated at room temperature, and forms a stable, hydrophobic film upon low‑temperature curing at 170°C.

Quantitative Control Model for the Three Major Process Intervals

Three forming regimes—overflow, standard, and turbulent—are defined for three key parameters: printing speed, substrate temperature, and sheath‑gas flow rate. The standard operating window is as follows: printing speed 20–50 mm/s, substrate temperature 40–70°C, and focus ratio FR = 1.2–3.2. Under these conditions, the edge roughness of the printed lines remains stable at less than 10 μm, with an average line width of 60 μm and a thickness of 1.32 μm.

Controllable Overlap Enables All-Superhydrophobic Surfaces

When the line overlap ratio is 60%, the thin film develops a nano‑textured Cassie structure, achieving a water contact angle of 154° and exhibiting stable superhydrophobic properties; by tuning the overlap ratio, isotropic or anisotropic wetting can be switched.

Excellent durable hydrophobic coating

After 80 minutes of hydrochloric acid corrosion, 100 tape‑peel tests, and 30 sandpaper abrasions, the surface still retains its superhydrophobic properties, with the nanostructure remaining robust and resistant to detachment, making it well suited for demanding service environments.

Validation of the implementation of two major scenario-based features

① Microfluidic grid: Hydrophobic barriers enclose hydrophilic micro‑pools, precisely confining 4 μL droplets to enable array‑based, compartmentalized culture of biological cells; ② Electronically localized waterproofing: A hydrophobic protective layer is printed only on the LED conductive traces; after simulated seawater immersion for 30 minutes, the circuit remains fully functional, whereas the uncoated control sample shorts within one minute.

II. Complete Preparation Procedure for Superhydrophobic Micro-Patterned Aerosols

Ink Formulation: Nano-silica, epoxy resin, a curing agent, and 2-butoxyethanol solvent are mixed and stirred, then subjected to ultrasonic dispersion to obtain a stable hydrophobic ink.

Ultrasonic cleaning of hydrophilic glass substrates with isopropyl alcohol;

Aerosol Equipment Commissioning: 200 μm nozzle, carrier gas at 450 sccm, exhaust gas at 425 sccm, sheath gas at 30–80 sccm, and the substrate maintained at a constant temperature of 50°C;

Print in split mode: Single-channel hydrophobic boundary / Multi-layered overlapping superhydrophobic regions, with customizable square and triangular micro‑grids;

Cure in a 170°C oven for 30 min, then allow to stand for 24 h to stabilize the surface wetting structure.

Characterization: Contact angle measurement, SEM microstructural analysis, 3D profilometry for film thickness measurement, durability testing for resistance to friction, corrosion, and delamination, and verification of waterproofing functionality in microfluidic devices and electronic circuits.

III. Five Core Advantages of Aerosol-Based Wetting Pattern Formation (Comparison with Lithography, Plasma Processing, and All‑Area Spraying)

Maskless digital定点 deposition, no custom templates required.

CAD‑based free‑form design of hydrophobic patterns enables on‑demand printing only in the target regions, eliminating the need for photolithography and mask fabrication, and supporting rapid iteration in small batches.

Without damaging the substrate’s intrinsic structure.

No acid–alkali etching or plasma bombardment is employed, ensuring that the intrinsic properties of glass, flexible PET, and PCB substrates remain intact.

Material utilization has improved significantly.

Hydrophobic coatings are applied only to functional areas, as opposed to full‑surface spraying, which wastes large quantities of material and reduces the cost of hydrophobic silica consumables.

Micron-level precision with controllable boundaries, enabling microfluidic patterning.

Minimum stable line width: 60 μm; edge roughness < 10 μm; perfectly compatible with microfluidic droplet‑encapsulation arrays.

Low-temperature, gentle forming; compatible with heat-sensitive flexible substrates.

Curing occurs at only 170°C, enabling direct processing of high-temperature‑unstable substrates such as PI and flexible circuit boards.

IV. Five Core Technological Innovations of This Study

Innovation 1: Complete formulation for hydrophobic silica aerosol ink

An epoxy‑based composite nano‑silica system, with low viscosity for pneumatic atomization, forms a wear‑resistant, nano‑rough, hydrophobic surface upon curing, balancing processability and durability.

Innovation 2: A Three-Parameter Interval-Based Standardization and Regulation System

Define velocity, temperature, and sheath‑gas overflow ranges—standard and turbulent‑flow shaping windows—and establish a universal process‑parameter library for mass production to lower the debugging threshold.

Innovative 3D printing strategy for controllable overlap ratio and superhydrophobicity regulation

No ink modification is required; by simply adjusting the path overlap, seamless switching between conventional hydrophobicity and 154° superhydrophobicity is achieved, with anisotropic wetting customizable to suit specific needs.

Innovation 4: A novel, selectively targeted waterproofing solution with cutting-edge electronic protection.

Breaking away from the conventional one-piece PCB lamination approach, this solution protects only the conductive traces while leaving device thermal management unaffected, delivering lightweight, moisture‑resistant protection.

Innovative 5‑step additive manufacturing route for damage‑free microfluidic chips

A hydrophobic enclosure is engineered to form a hydrophilic micro‑pool, enabling droplet partitioning and cell arraying, thereby replacing complex photolithographic microfluidic fabrication processes.

V. Current Industry Challenges and Corresponding Solutions

Challenge 1: Conventional mask‑based spray coating and photolithography processes require custom templates, making design modifications and retooling costly.

✅ Aerosol-based digital direct writing enables one-click updates to design files, eliminating the need for any masks and significantly shortening the R&D cycle.

Challenge 2: Etching and plasma modification can corrode the substrate, degrading its optical and electrical properties.

✅ Pure physical droplet deposition, with no chemical corrosion and high-energy surface treatment, leaving the substrate’s original properties fully intact.

Challenge 3: Whole‑area hydrophobic coating cannot provide localized protection, leading to impaired device heat dissipation and wasted consumables.

✅ Targeted selective printing: only the circuitry and micro‑pool areas are laminated, balancing waterproofing with heat dissipation while reducing material consumption.

Challenge 4: Conventional hydrophobic coatings exhibit poor wear and corrosion resistance, leading to a decline in their hydrophobic performance over long-term use.

✅ A silica nanoparticle–microstructure composite epoxy framework that maintains a high contact angle even after acid etching, friction, and tape‑peel testing.

Challenge 5: Conventional fabrication methods struggle to produce micron-scale, precision liquid‑control microgrids, and microfluidic processing is costly.

✅60 μm high-precision hydrophobic boundaries enable the batch fabrication of microdroplet‑based isolation arrays, offering a low‑cost solution for biochip prototyping.

VI. Future Diversified Industrial Application Scenarios

1 Microfluidic Bioassay Chip

Hydrophobic confinement of hydrophilic micro‑arrays enables partitioned dispensing of trace reagents and single‑cell isolation for culture, thereby preventing droplet cross‑contamination.

2 Vehicle-mounted / Marine circuit board spot‑on waterproof protection

Selective superhydrophobic coatings for sensor and LED circuits enable long-term stable operation in seawater and high-humidity environments.

3 Biomimetic Fog Collection Device

Patterned hydrophobic–hydrophilic composite surface, efficiently capturing atmospheric water vapor, serves as the core structure of outdoor water‑saving devices.

4 Moisture-Resistant Flexible Wearable Sensing Electrodes

Localized hydrophobic protection for conductive circuits in human sweat environments, extending the service life of wearable devices.

5 High-Throughput Wetting Materials R&D Platform for Laboratories

Rapidly print hydrophobic samples with varying degrees of overlap, enabling high-throughput testing of contact angles and durability performance, and accelerating formulation screening.

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• At speeds below 20 mm/s, the solvent spreads excessively (bleeding); at speeds above 50 mm/s, the lines become too thin (thickness <1 μm), resulting in insufficient reliability.
• In the conventional regime, the linewidth and thickness both decrease uniformly, with the thickness exceeding 1 μm—comparable to the micro‑roughness of the cuticular surface on naturally hydrophobic plants (approximately 1–5 μm).
• LER (3σ) is essentially independent of speed, at approximately 6 μm; acceptable edge quality can be achieved at speeds of 20 mm/s and above.

• Sheath gas is used to compress and collimate the droplets to control the line width; the focusing ratio FR, defined as the sheath‑gas flow rate divided by the atomizing‑gas flow rate, has an optimal range of 1–10 for conductive silver inks.
• At 30–80 sccm (FR ≈ 1.2–3.2), the region is in focus, and the line width decreases linearly with flow rate; at 90 sccm (FR = 3.6), turbulence develops inside the nozzle, causing the line width to increase.
• For a superhydrophobic line of approximately 60 μm, an LER below 10 μm is recommended (corresponding to about a 16% variation); under turbulent flow conditions, the LER can increase to nearly 30 μm.

• A 2-mm square hydrophilic spot can accommodate a water droplet of approximately 4 μL, while the hydrophobic boundary forms a “wall” that confines the droplet within the hydrophilic region until it fully evaporates.
• The grid size can be reduced to several hundred micrometers, making it suitable for biomedical droplet‑positioning applications such as cell culture.
• The triangular mesh demonstrates precise droplet positioning, with no cross-contamination between droplets of different colors.

• Overlapping lines give rise to micro‑ and nano‑scale structures arranged periodically along the length, exhibiting a wavy morphology, with silica nanoparticles uniformly dispersed.
• The FIB cross-section shows that the average groove thickness increases from 0.80 μm at 0% overlap to 2.22 μm at 75% overlap.
• The overlap ratio is adjusted via a line pitch of 60–15 μm, enabling full or partial coverage of the hydrophobic strip.

• When unoverlapped, the WCA in the parallel and perpendicular directions is 122° and 126°, respectively (anisotropic hydrophobicity); at 60% overlap, the difference diminishes to nearly zero, reaching 154° (superhydrophobicity, isotropic).
• The bare glass WCA is 32°; Ra/Rq increases from 74.82/91.58 nm in the non-overlapping case to 95.74/120.25 nm at 75% overlap.
• The wavy morphology traps air, giving rise to the Cassie–Baxter state, and the micro‑nano roughness is key to achieving superhydrophobicity.

• After 80 minutes of chemical corrosion, the nanostructure remains virtually undamaged, with only a slight increase in surface energy; short-term immersion does not result in significant degradation.
• Mechanical wear: After 30 cycles, the contact angle decreases to 130° (transitioning from superhydrophobic to hydrophobic), attributable to the loss of nano‑scale roughness.
• After 100 tape‑peel cycles, the contact angle remains at 148°, demonstrating excellent peel resistance of the nanolayer and meeting the reliability requirements of practical applications.

• Directional waterproofing is achieved by AJP printing a hydrophobic gas-phase silica sol along the conductive path (60% overlap, coating width 1 mm).
• Simulated seawater conductivity: 53.2 mS/cm; the uncoated circuit lost connectivity within 1 minute, whereas the coated circuit maintained connection for over 30 minutes.
• It demonstrates that AJP can provide directional waterproofing for sensitive circuit segments without requiring full‑board encapsulation, making it suitable for maritime electronic systems.

• The carrier gas, at 450 sccm, is passed through an atomizer to generate polydisperse particles; the exhaust gas, at 425 sccm, removes excess carrier gas and small particles; and the sheath gas, at 50 sccm, compresses and focuses the aerosol stream.
• Nozzle–substrate spacing: 1–5 mm; capable of printing on non‑planar substrates and compatible with inks spanning a wide viscosity range of 1–1000 cP (this ink: 11 cP, surface tension: 23.17 mN/m).
• The ink was formulated from an average 16 nm fumed silica, epoxy resin, a curing agent, and 2-butoxyethanol; the pattern was cured at 170 °C for 30 minutes.