In the fabrication of chemical sensors via inkjet printing, the dynamic behavior of droplets spans the entire lifecycle of “nozzle ejection – in-flight transport – substrate impact – spreading and film formation.” The stability and controllability at each stage directly determine the uniformity, density, and interfacial quality of the sensitive layer, thereby influencing sensor response sensitivity, detection limits, and long-term reliability. From the precise shaping of picoliter-scale droplets to the orderly deposition of nanoscale sensing materials, droplet dynamics serve as the critical bridge connecting ink formulation, equipment parameters, and device performance.
In recent years, with breakthroughs in high-speed imaging and interface engineering, research on droplet dynamics has progressed from qualitative observation to quantitative regulation, laying a solid foundation for the controllable fabrication of high-performance chemical sensors.
I. Full Lifecycle Evolution Mechanism of Droplet Dynamics
Droplet dynamic behavior results from the synergistic interaction among ink physical properties, device driving parameters, and substrate interface characteristics. The evolution process can be divided into four key stages, each governed by distinct mechanical and physicochemical principles.
(1) Ejection Formation Stage: Precise Shaping from Ink Chamber to Droplet
Droplet ejection marks the starting point of dynamic behavior. The core objective is to use driving energy to overcome surface tension and generate stable droplets, while suppressing satellite droplets and filament breakage.
In piezoelectric inkjet systems (currently the mainstream technology for chemical sensor fabrication), the piezoelectric element undergoes mechanical deformation under pulsed voltage, generating pressure waves inside the ink chamber. When the pressure exceeds surface tension, ink is expelled from the nozzle, forming a liquid column.
The stability of this liquid column directly determines droplet quality. Its breakup position and morphology are regulated by both ink viscoelasticity and pulse waveform design:
- Low-viscosity inks (< 5 mPa·s) tend to experience excessive elongation, leading to Plateau–Rayleigh instability and the formation of a main droplet accompanied by multiple satellite droplets.
- High-viscosity inks (> 25 mPa·s) are prone to incomplete column breakage, resulting in “tailing” phenomena.
Pulse waveform optimization is a key control strategy. Early monopolar trapezoidal waves often generated residual oscillations that produced satellite droplets. In contrast, bipolar pulse waveforms suppress residual vibration through reverse voltage, improving satellite suppression rates by over 90%. Epson’s PrecisionCore printing technology applies this principle to achieve stable ejection of ultra-small 3.2 pL droplets.
(2) In-Flight Stage: Trajectory Stability and Shape Preservation
During flight, the primary objective is to ensure that droplets reach the designated deposition site along a predefined trajectory while maintaining structural integrity.
For nanoparticle-based inks commonly used in chemical sensors (such as MXene or molecularly imprinted polymer inks), morphological distortion may occur due to particle distribution non-uniformity or solvent evaporation. Key influencing factors include:
- Surface tension and density of the ink: Surface tension below 25 mN/m can cause droplet deformation in flight, while large density differences may induce particle sedimentation. Researchers at Xihua University regulated MXene ink surface tension to 32 mN/m using sodium alginate modification, ensuring spherical droplet morphology during flight.
- Flight distance and ambient humidity: Short flight distances (< 5 mm) minimize solvent evaporation and airflow disturbance, making them ideal for micro-sensor array fabrication. When environmental humidity falls below 40%, rapid solvent evaporation causes droplet shrinkage; therefore, humidity control between 50–60% is recommended.
(3) Substrate Impact Stage: Interface Regulation from Collision to Spreading
The impact stage governs the initial deposition morphology of the sensitive material. Droplet behavior can be categorized into spreading, retraction, and rebound modes.
The key regulatory factor is the matching between substrate wettability and droplet kinetic energy:
- On hydrophilic substrates (contact angle < 30°), droplets spread rapidly upon impact. Spreading diameter is inversely proportional to viscosity. For example, polyaniline ink printed on hydrophilic aluminum foil spreads up to five times the nozzle diameter.
- On hydrophobic substrates (contact angle > 90°), droplets tend to retract after spreading. The research group led by Prof. Song Yanlin at the Institute of Chemistry, Chinese Academy of Sciences, demonstrated that asymmetric adhesion on patterned wettability substrates can induce angular momentum during retraction, causing droplets to rotate at speeds up to 7300 rpm. This rotational behavior promotes uniform nanoparticle distribution and enhances conductive properties of the sensing layer.
(4) Film Formation Stage: Structural Solidification from Liquid to Solid
Film formation is the final stage, where solvent evaporation and post-treatment processes convert droplets into functional layers. This stage directly determines the sensing performance of chemical sensors.
Two key challenges must be addressed:
- Prevention of particle aggregation: Rapid solvent evaporation may create a surface “skin,” trapping internal solvent and forming bubbles, resulting in porous and non-uniform films. Gradient drying (60°C pre-drying followed by 120°C curing) allows controlled solvent evaporation. For MXene inks, this approach reduced porosity fluctuation from ±15% to ±3%.
- Enhancement of interfacial bonding: Flexible substrates (e.g., PET, PI) exhibit weak interfacial adhesion. By modifying substrates with chitosan, hydrogen bonding between amino groups and MXene hydroxyl groups increased film adhesion strength by 2.5 times, maintaining integrity after 1000 bending cycles.
II. Core Control Dimensions and Key Technologies for Droplet Dynamics
Effective regulation requires coordinated optimization across three dimensions: ink formulation, equipment parameters, and substrate interface engineering.
(1) Ink Formulation: Precise Matching of Physical Properties
Optimal droplet behavior requires balancing printability and sensing functionality. Key parameters include viscosity, surface tension, and dispersion stability:
- Viscosity window: 5–20 mPa·s ensures stable jetting and flight integrity. Adding 5% PEG to MXene ink increased viscosity from 3 to 12 mPa·s, improving satellite suppression from 65% to 98%.
- Surface tension: Maintained between 25–40 mN/m. Using formic acid as a dispersant, Donghua University researchers achieved 31 mN/m surface tension and 50 μm resolution patterning.
- Dispersion stability: Polyelectrolyte modification (e.g., sodium alginate for MXene) builds electrostatic repulsion layers, preventing sedimentation for 15 days and improving particle uniformity by 40%.
(2) Equipment Parameters: Precision in Driving and Motion Control
External control mechanisms ensure droplet size consistency and accurate trajectory:
- Piezo drive optimization: Pulse amplitude linearly determines droplet volume. Epson S3200 printheads allow continuous adjustment from 3.2 pL to 12 pL. Jetting frequency can reach 50,000 Hz, enabling rapid sensor array fabrication.
- Motion precision: Micrometer-level positioning accuracy is required. Closed-loop systems reduce deposition deviation from ±2 μm to ±0.5 μm, essential for arrays with spacing below 10 μm.
(3) Substrate Interface: Patterned Wettability Engineering
Substrate wettability dictates spreading and film morphology:
- Uniform wettability: Hydrophilic chitosan substrates (contact angle 25°) enhance spreading for high-sensitivity humidity sensors; hydrophobic PI substrates (contact angle 105°) enable 10 μm microelectrode patterning.
- Patterned wettability: Alternating hydrophilic–hydrophobic regions created via lithography or nanoimprinting regulate droplet retraction. Research by the Chinese Academy of Sciences achieved 95% satellite suppression and 98% material utilization efficiency using this approach.
III. Impact of Droplet Dynamics on Chemical Sensor Performance
Stability in droplet dynamics directly correlates with sensor performance. Defects in any stage may degrade functionality or cause device failure.
(1) Satellite Droplets and Tailing: Reduced Uniformity
Satellite droplets create surface protrusions, leading to ±15% porosity variation and over 20% response deviation in MXene humidity sensors. Tailing blurs electrode edges, causing spacing deviations >5 μm and 30% signal-to-noise reduction. Bipolar waveform optimization and viscosity adjustment to 15 mPa·s reduce defect rates from 40% to below 5%.
(2) Flight Deviation and Shape Distortion: Reduced Accuracy
Deposition deviation >2 μm leads to cross-talk in sensor arrays. In molecularly imprinted arrays, misalignment reduces specificity by 40%. Droplet sphericity reduction from 95% to 80% decreases the temperature coefficient of resistance (TCR) by 33%. Airflow control and substrate flatness (<10 nm roughness) reduce deviation below 0.5 μm.
(3) Uneven Spreading and Interface Delamination: Reduced Stability
Uneven thickness causes stress concentration and cracking in flexible devices after 100 bending cycles. Increased contact resistance by 10 Ω reduces signal efficiency by 15%. Chitosan modification and gradient drying improve spreading uniformity to 92%, enhance adhesion 2.5-fold, and maintain performance degradation below 10% after 1000 bending cycles.
In summary, precise regulation of droplet dynamic behavior is fundamental to achieving high-performance inkjet-printed chemical sensors. It is not merely a fluid mechanics challenge, but an integrated materials–process–device engineering strategy that defines the ultimate sensing performance and long-term reliability.