Analysis of the Role and Impact of Inkjet Deposition Fluids
1. Core Function of Deposition Fluids
The core function of inkjet deposition fluids is to balance the inherent contradiction between functionality and jetting performance.
Functionality:
The fluid must carry active components (such as conductive silver paste or photosensitive resins) to ensure that the deposited material achieves its intended properties (e.g., electrical conductivity or curing strength).
Jetting Performance:
The fluid must maintain low viscosity (<20 mPa·s) and appropriate surface tension (20–70 mN/m) to form stable droplets, preventing satellite droplets or nozzle misfiring.
2. Trade-Off Between Functionality and Jetting Performance
Conflicting Relationship
Functional additives (such as nanoparticles or crosslinking agents) may increase viscosity or alter surface tension, leading to abnormal droplet breakup or nozzle clogging.
For example, high solid-content conductive inks require dispersants to maintain stability, but this may compromise jetting precision.
Solutions
Dispersant Optimization:
Water-based or solvent-based dispersants can improve particle suspension and reduce flocculation. For instance, TiO₂-based inks require anti-settling formulations.
Dynamic Viscosity Control:
Online monitoring and shear rate adjustment ensure that the fluid maintains Newtonian behavior during jetting.
3. Key Indicators of Jetting Performance
Droplet Size and Accuracy:
Piezoelectric inkjet technology uses the inverse piezoelectric effect to generate micron-scale droplets (10–50 μm diameter). Proper matching between fluid surface tension and nozzle geometry is essential.
Satellite Droplet Suppression:
Static mixers help reduce internal air bubbles in the fluid, preventing the formation of secondary droplets during ejection.
4. Development and Observation Technologies
Drop Watching Systems:
Real-time observation of droplet formation, including meniscus formation speed and flight trajectory, is used to validate jetting stability.
Filtration Testing:
GF/B filter membranes (1 μm) detect impurities, while SUS filter plates (15 μm) simulate internal printhead flow resistance to evaluate fluid smoothness and flow behavior.
5. Application Cases
High-Precision Printing:
UV-curable inks require low viscosity (<10 mPa·s) to ensure small droplet formation (<30 μm), while photoinitiators must be incorporated to guarantee rapid curing speed.
Industrial-Grade Deposition:
Ceramic slurries achieve high solid content (>50%) through multi-jet technology and rely on thixotropic control to prevent sedimentation.
The Complex Capillary Flow Characteristics of Ink
The wetting and flow behavior of ink within capillaries is a complex multiphysics coupling problem, involving fluid mechanics, surface chemistry, and materials science.
The following analysis is presented from three perspectives: mechanisms, influencing factors, and current research challenges.
I. Physical Mechanisms of Wetting and Flow
1. Capillary Action and Surface Tension
Ink flow within capillaries is governed by surface tension (γ) and contact angle (θ), following the Young–Laplace equation:
ΔP=2γcosθr\Delta P = \frac{2\gamma \cos \theta}{r}ΔP=r2γcosθ
where r is the capillary radius.
- When θ < 90°, the ink can spontaneously wet the capillary wall.
- When θ > 90°, external force is required to drive the flow.
2. Non-Newtonian Fluid Behavior
Most inks exhibit shear-thinning (pseudoplastic) behavior, meaning their apparent viscosity (η) changes with shear rate (γ̇):
η=K⋅γ˙ n−1\eta = K \cdot \dot{\gamma}^{\,n-1}η=K⋅γ˙n−1
High pigment-content inks (such as UV inks) often exhibit yield stress (τᵧ), which significantly hinders capillary flow.
II. Key Influencing Factors
| Factor Category | Specific Parameters | Influence Mechanism |
| Fluid Properties | Viscosity (1–1000 mPa·s) | High-viscosity inks require higher capillary pressure (ΔP) but are prone to flow hysteresis |
| Interfacial Properties | Contact angle (30°–120°) | Hydrophilic capillaries (θ < 90°) promote spontaneous penetration; hydrophobic surfaces require modification |
| Structural Parameters | Capillary diameter (10–500 μm) | Smaller diameters increase capillary force dominance, but flow resistance increases quadratically |
| Environmental Conditions | Temperature (20–60°C) | For every 10°C increase, viscosity decreases by ~15%, significantly altering flow rate |
III. Current Research Challenges
1. Dynamic Wetting Behavior
The contact angle of ink inside a capillary changes over time (dynamic contact angle hysteresis). Existing models struggle to accurately describe the migration of the three-phase contact line.
2. Multicomponent Interactions
Pigment particles (0.1–10 μm) may undergo phase separation from binders, altering local rheological properties and leading to non-uniform flow.
3. Experimental Observation Limitations
Flow inside micrometer-scale capillaries is difficult to monitor in real time. Current microscopic imaging techniques (e.g., confocal microscopy) face trade-offs between resolution and temporal response.
(3D X-ray imaging of printhead internal structure is often used to analyze such flow behavior.)
IV. Directions for Technological Breakthrough
1. Microfluidic Chip Simulation
PDMS microchannel arrays can simulate printing capillary networks, enabling quantitative evaluation of ink wetting dynamics.
2. Intelligent Material Applications
Photo-responsive binders (e.g., azobenzene-modified resins) can regulate interfacial energy under UV irradiation, enabling active control of flow behavior.
Current research still requires integration of molecular dynamics simulations with high-precision experimental validation to establish more universal predictive models for capillary flow behavior.