I. Abstract of the Paper’s Core Research

This work uses carboxyl‑functionalized single‑walled carbon nanotube aqueous ink as the feedstock and employs aerosol jet printing on a polyimide flexible substrate. By leveraging capillary flow induced by droplet evaporation, it achieves spontaneous alignment of the carbon nanotubes. The study systematically investigates the effects of nozzle size, sheath‑gas and atomizing‑gas flow rates, printing speed, and substrate temperature on line morphology, nanotube orientation, and electrical conductivity, thereby fully elucidating the self‑assembly formation mechanism. The key findings are as follows:

Aerosol-Driven Coffee-Ring Self-Assembly–Directed Mechanism for Forming Double CNT Lines

After aerosol droplets deposit, the contact line becomes pinned; a high‑speed capillary flow at the solvent edge drives carbon nanotubes to aggregate along both sides of the droplet, forming oriented CNT double‑lines parallel to the line edges. SEM images confirm that the carbon nanotubes are preferentially aligned along the edge lines, with an average orientation half‑width at half‑maximum of only 55°, indicating excellent order.

Multi-parameter quantitative control of the forming window

Nozzle, temperature, and printing speed are simultaneously optimized to achieve a total line width of 30–80 μm, with individual conductive double lines (coffee-ring effect) measuring 600–1500 nm in width. The optimal gas‑flow configuration—atomization gas at 30–35 sccm and low sheath gas below 50 sccm—enables the formation of a fully continuous double‑line structure as narrow as 16 μm.

Carbon nanotube alignment is strongly correlated with electrical properties.

At low ambient gas pressure and low printing speeds, carbon nanotubes exhibit higher structural order, shorter electron transport pathways, and a resistivity that can drop to several tens of kΩ/cm per unit length. Multi-layer stacking during printing significantly increases the density of the conductive network, leading to an exponential reduction in resistance.

Complete modeling of the forming mechanism

A model incorporating droplet volume conservation and capillary‑convection coupling is established to elucidate the competitive interplay between evaporation rate and carbon‑nanotube diffusion: at faster drying rates, the carbon nanotubes do not have sufficient time to align, yielding narrow lines with reduced structural order; conversely, when the deposition amount is adequate, highly oriented conductive double‑line patterns can be obtained.

Advantages of Template-Free Room-Temperature Forming

The entire process is carried out at room temperature and ambient pressure, eliminating the need for orientation‑inducing agents, high‑temperature annealing, or lithographic templates. It causes no damage to thermally sensitive flexible substrates such as PI and exhibits exceptionally strong process compatibility.

II. Complete Preparation Procedure for Directed Carbon Nanotube Aerosols

Ink Pre-treatment: Carboxyl-functionalized single-walled carbon nanotube aqueous ink, featuring low-defect, long carbon nanotubes (average length 1300 nm), with a defoaming agent added during storage to ensure stable atomization.

Base treatment: The PI film is cleaned and dried by blowing, with a hydrophilic surface that ensures the contact line of droplets remains pinned, thereby providing a foundation for coffee-ring formation.

Aerosol Equipment Commissioning: Ultrasonic atomization at 0.6 A, paired with multi‑size nozzles of 150/200/300 μm; atomizing gas flow rate 25–40 sccm, sheath gas flow rate 10–150 sccm, printing speed 1–5 mm/s, and substrate temperature ramped from 25 to 80°C in gradient experiments.

Single-layer / Multi-layer comparison printing: Single‑track and five‑track repeated lines were used to fabricate gradient‑oriented CNT double‑wire arrays.

Dries naturally at room temperature; no high-temperature sintering is required.

Complete characterization: Laser confocal profilometry, FESEM for observing carbon nanotube alignment, ImageJ-based orientation factor analysis, and four-probe measurements of resistivity per unit length.

III. Five Core Process Advantages of Aerosol Jet–Based Directed CNT Fabrication

Far surpassing the micro‑graphic precision of inkjet printing, with a minimum line width of 16 μm.

For self-assembled CNTs of the same type, the inkjet line width is as low as 130 μm, and aerosol‑flow focusing achieves a tenfold improvement in precision, making it suitable for MEMS microtransistors and microelectrodes.

Spontaneous orientation without an additional template / electric field

By leveraging solvent-driven capillary flow–induced self-assembly, this approach eliminates the need for oriented lithography and external-field‑assisted alignment steps, significantly reducing both R&D and mass-production costs.

Fully flexible and compatible with low temperatures, without damaging the polymer substrate.

The entire process involves no high-temperature steps, enabling stable processing of various heat-sensitive substrates, including PET, PI, and tattoo paper.

Process parameters are adjustable, and orientation and resistivity can be precisely controlled.

Nozzle design, gas flow rate, and printing speed are dynamically optimized to fabricate two types of circuits—high‑conductivity and high‑resolution—on demand.

Single-device compatibility: disordered thin films and oriented double lines

By simply adjusting the sheath gas flow rate, you can seamlessly switch between two structural configurations: a uniform carbon film and a directional double‑line design, enabling a single system to support the development of multiple device types.

IV. Five Core Technological Innovations of This Study

Innovation 1: A novel process for the directional self-assembly of CNT double wires via aerosol‑mediated evaporation

The aerosol direct-writing system leverages the coffee-ring effect to achieve preferential alignment of carbon nanotubes, thereby overcoming the limitations of inkjet‑printing resolution.

Innovation 2: A Quantitative Model for Multi-Fluid Coupled Forming

By integrating droplet volume conservation and the competitive mechanism of capillary convection, we can elucidate the equilibrium relationship between evaporation and carbon‑nanotube diffusion, thereby enabling predictions of line dimensions and structural order.

Innovative Three-Channel System – A Quantitative Correspondence Framework for Orientation Degree

By calibrating the effects of sheath gas and nebulization gas on carbon nanotube alignment using the Herman orientation factor, a high‑orientation, stable process window is established.

Innovative Multi-Layer Stacked Regulation Scheme for Enhancing Electrical Conductivity

Multi-pass printing widens the conductive pathways of coffee-ring patterns, significantly reducing line resistance without compromising precision.

Innovative 5: A Template-Free, Ambient-Temperature Paradigm for Oriented Carbon Nanotube Fabrication

By abandoning high-temperature CVD, alignment inks, and photolithography‑induced patterning, a new pathway has been opened for low‑cost flexible carbon‑based electronics.

V. Current Industry Challenges and Corresponding Solutions

Challenge 1: In conventional printing, carbon nanotubes tend to stack randomly, resulting in low charge-carrier mobility.

✅ Aerosol capillary flow drives the oriented alignment of carbon nanotubes along the edge, shortening the electron transport path and enhancing device switching performance.

Challenge 2: Inkjet‑based self‑assembly produces CNT lines that are too thick to enable chip miniaturization.

✅ The airflow focuses and compresses the mist stream, with a minimum droplet size of just 16 μm, making it ideal for micron‑scale transistor interconnects.

Challenge 3: CVD‑grown aligned carbon nanotubes require high‑temperature, rigid silicon substrates, which preclude flexibility.

✅ Ambient‑temperature, ambient‑pressure aerosol process; direct formation of PI flexible films; compatible with wearable devices.

Challenge 4: Carbon nanotube alignment relies on complex templates and electric fields, resulting in a cumbersome and costly fabrication process.

✅ Pure solvent evaporation–driven self-assembly, with no additional induction steps, streamlines the production line.

Challenge 5: The electrical conductivity is fixed and non-tunable, making it difficult to meet the diverse requirements of sensing and transistor applications.

✅ Adjust print speed and layer overlap to freely control the circuit resistance range, ensuring compatibility with a wide variety of components.

VI. Future Diversified Industrial Application Scenarios

1 Flexible Carbon Nanotube Thin-Film Transistor

Highly oriented CNT microchannels enhance current‑carrying mobility, enabling flexible display driver chips.

2 Ultra-High-Sensitivity Micro-Strain Sensor

A directional carbon‑nanotube double‑wire conductive network delivers more stable strain‑signal response, enabling wearable physiological patches.

3 Microfluidic Chip-Based Electrodes for Trace-Level Detection

16 μm high-precision conductive microcircuits, single-cell and biochemical micro-sensing arrays.

4 Flexible Transparent Conductive Micro-Interconnects

The dual‑wire structure offers high transmittance and enables wiring for lightweight, flexible optoelectronic devices.

5 High-Throughput R&D at the Carbon-Based Electronics Laboratory

Rapidly switch between ordered and disordered carbon film structures, enabling high-throughput screening of carbon nanotube device formulations.

Image and Text Guide

• (a)   AJP full process: SWNT ink is passed through Ultrasonic nebulizer Atomization into 1–5 μm microdroplets → transport by carrier gas (N₂) to the deposition head → ejected through the nozzle and impinging on a flexible substrate.
• (b)   Magnification at the nozzle: annular Sheath flow Encapsulate and focus aerosol bundles containing SWNTs; droplets coalesce on the substrate to form a continuous thin line (ink pool cross-sectional area A, volumetric flow rate Ṽ_ink, line width w).
• (c,d)   Coffee-ring effect (side view/top view + time series): Contact-line pinning and faster edge evaporation → capillary/peripheral flow drives SWNTs toward the edge → formation upon complete solvent evaporation. Conductive dual-wire structure (Evaporation-Driven Self-Assembly, EDSA).

• (a) Stability line — good continuous twin-line : The edges have sufficient SWNTs to form a continuous conductive pathway.
• (b) Sparse discontinuous twin-line : SWNT deficiency results in discontinuities at the edges.
• (c) Wire breakage : Contact-line pinning (due to surface inhomogeneity or surface-energy differences) leads to trajectory breakage.
• (d) Distension : Excessive ink flow can lead to localized buildup and bulging; this can be prevented by cleaning the substrate and optimizing printing parameters.

• (a)   Schematic of measurements: line width versus coffee-ring width; the trends in coffee-ring width and line width are highly consistent (n ≥ 10, error bars represent standard errors).
• (b) Print speed : 1→5 mm/s, both the line width and the coffee-ring width decrease—resulting in a reduction of SWNTs received per unit area.
• (c) Nozzle size : 150→300 μm, line width increases—larger nozzles produce greater ink flow; a 150 μm nozzle can achieve a line width of approximately 40 μm.
• (d) Substrate temperature : 25→80 °C, both the line width and the coffee-ring width decrease—higher temperatures accelerate evaporation and reduce spreading (the anomaly at 40 °C may be due to printing fluctuations).
• Summary : Line width 38.5–80 μm and coffee-ring width 500–1500 nm are tunable; the volume-conservation model w ∝ √(Ṽ_ink/v) can account for the influence of each parameter.

• Five types of forms : Straight CNT twin-line, bulging twin-line, straight homogeneous line, bulging homogeneous line, blurred line.
• Optimal window : Medium atomization flow (AF 30–35 sccm) + low sheath flow (SF < 50 sccm) →   Straight double track 。
• Mechanism At a high sheath flow rate (≈150 sccm), the deposition line is narrower and solvent evaporation is faster, preventing SWNTs from being carried to the contact line by convection; consequently, the double‑line pattern transitions into a uniform line. At a low atomization flow rate (AF 25 sccm), blurring or overspray may occur—though the overspray is extremely slight and does not affect conductivity.

• (a–d)   The edges of the printed line can be divided into an unprinted region, a coffee-ring region (a densely packed network of SWNTs), and a sparsely populated region within the line; SWNTs at the edge… Select the optimal configuration parallel to the contact line/coffee-ring periphery. Arrangement.
• (e) Five-stage mechanism : ① Aerosol particles undergo random deposition, coexisting with axial flow (driven by Laplace pressure differences) and peripheral flow; ② SWNTs align along the overall flow direction in shear flow and migrate toward the edge; ③ One end of an SWNT reaches and becomes pinned at the contact line, while surface tension pulls it toward the boundary to minimize surface energy; ④ The contact line becomes fully filled with SWNTs; ⑤ Van der Waals forces arising from the accumulated SWNTs generate a torque, causing subsequent SWNTs to rotate and align along the boundary.

• (a)   Visual orientation map (right-hand color map): Most of the colors fall within the ±45° range, indicating that the CNTs are aligned along the direction around the droplet perimeter. Preferential orientation (Parallel to the contact line.)
• (b)   Orientation distribution curve: A distinct peak is observed at 0° (parallel to the contact line direction). Orientation degree is quantified by the full width at half maximum (FWHM); a smaller FWHM indicates higher orientation.
• Quantitative results : Orientation distribution FWHM, average approximately   55°±3.59° , indicating that CNTs exhibit preferential orientation along the twin line but are not perfectly aligned; misorientation arises from airflow disturbances, non-uniform adsorption, and uneven evaporation, among other factors.

• Herman orientation factor f : f = 1 corresponds to perfect parallelism, f = 0 to randomness, and f = −0.5 to perpendicularity; the principal axis is defined as being parallel to the line of contact (θ = 0°).
• (a) Atomization flow : AF decreased from 40 to 25 sccm, and the orientation factor declined (ANOVA p = 0.00275, n ≥ 10; the difference was statistically significant).
• (b) Sheath current : SF increased from 10 to 100 sccm, and the orientation factor decreased (p = 0.00734, n ≥ 13).
• Pattern High atomization flow + low sheath flow → wider nanotube bundles and longer solvent evaporation time → CNTs have more time to align → improved alignment; there is a trade-off between bundle width and alignment degree.

• (a,b)   Optical images and 3D reconstruction profiles: After multiple printings, both the line width and the coffee-ring width increase (with the contact line progressively shifting outward and CNTs accumulating at different locations).
• (c)   The wire thickness remains nearly constant: 1 pass, 3.49 ± 0.20 μm; 5 passes, 3.45 ± 0.50 μm—whereas the number of passes primarily affects the width rather than the thickness.
• (d)   Specific resistance per unit length: for a single pass, approximately 600 Ω/cm to 11 MΩ/cm (at printing speeds of 1–5 mm/s); with five passes, it decreases to 41–370 kΩ/cm—multi‑layer printing deposits more CNTs, significantly enhancing conductivity; the resistance increases with printing speed in an approximately exponential (logarithmic‑linear) manner.