Continuous 3D printing from one single droplet

The paper addresses the inefficiencies and precision limitations of photocuring-based 3D printing (e.g., DLP/SLA and volumetric methods), which typically require large resin vats, suffer from low wet and net material utilization, residual resin on printed parts, heat accumulation at high UV intensities, and extra curing that degrades resolution. The central hypothesis is that introducing a receding three-phase contact line (TCL) at a suitably engineered curing interface enables continuous conversion of a single resin droplet into a 3D structure with minimal residue, improved utilization efficiency, and enhanced printing stability and precision. The study explores interfacial designs that reduce adhesion between liquid/cured resin and the curing interface, using the droplet’s free contact surfaces and inner circulation to sustain continuous printing while suppressing excessive curing.

Prior work in additive manufacturing highlights photocuring approaches (DLP, SLA, volumetric AM) for high-resolution fabrication across biomimetics, microfluidics, sensors, and shape-morphing systems. However, these methods have low material utilization due to vat-based processes, resin waste, and exothermic heat management issues at high speeds. Interfacial science shows that surface chemistry and roughness modulate TCL dynamics; bioinspired superamphiphobic and slippery surfaces reduce adhesion via trapped air or lubricants, enabling droplet mobility. Building on these insights, the paper applies interfacial manipulation (fluorinated flat, superamphiphobic, and lubricant-infused slippery surfaces) to control TCL receding during UV curing for one-droplet printing.

• Printing concept and configuration: A single resin droplet is placed on a UV-transparent curing interface at the bottom of a custom vat. A supporting plate contacts and penetrates the droplet from above. UV patterns are projected from below while the plate elevates at constant speed, curing resin at the interface and forming a continuous 3D structure as the droplet TCL recedes until the entire droplet converts to solid.
• Hardware: Custom setup with LED UV projector (PRO4500; projection area 32.2 mm × 51.6 mm; 912 × 1140 px; 0–65 mW/cm²), a UV-transparent curing window, and an aluminum supporting plate on a programmable stage (velocity 1.5–100 mm/min; resolution 10 µm). Continuous elevation speeds up to 200 µm/s were used. A load cell (Mark-10 M5-05 on ESM303 stage; 0.5 mN force resolution; 20 µm displacement resolution) measured adhesion/peel forces during separation.
• Curing interfaces tested: (i) fluorinated quartz (F-quartz) by grafting fluorinated molecules; (ii) candle-soot-derived superamphiphobic coating (hierarchical roughness + low surface energy); (iii) lubricant-infused slippery PDMS (S-PDMS) forming a liquid lubricant layer over PDMS micro-roughness. All have high UV transparency.
• Printing patterns: Circular UV patterns with diameters 0.5–3.0 mm; also V-grooved patterns with varying intersection angle α (0–180°) and specific length-to-width ratios (L/W = 4:1, 17:1, 30:1). Initial droplet masses varied (~24–151 mg) to study utilization.
• Measurements and characterization: Contact angles (OCA20) with 3.0 µL droplets; SEM (JEOL JSM-7500F) for surface and sidewall morphology; micro-CT (Skyscan 1272) for internal structures; real-time optical monitoring (Nikon D750) of curing; interfacial adhesion energies via OWRK method for liquid/solid pairs and via peel-off/load measurements for cured resin/cured interface.
• Comparative study: One-droplet printing versus traditional vat polymerization on S-PDMS interfaces, tracking width evolution and D/D_design ratio over time, as well as material utilization (wet: printed wet mass/initial resin; net: dry mass/initial resin).
• Analytical framework: Defined three interfaces with adhesion energies γ1 (liquid resin/cured resin), γ2 (cured resin/curing interface), γ3 (liquid resin/curing interface). Derived criteria for successful one-droplet printing: γ1 > γ3 and γ3 > γ2. Applied capillarity theory (Bretherton scaling and capillary rise in corners) to rationalize residue scaling and utilization trends with droplet size, pattern radius, and V-groove angle.

• Demonstration of one-droplet printing: A 24-mm-long cylindrical grid structure was fabricated from a single droplet with wet material utilization of 99.6%; only 0.4% resin remained on the supporting plate.
• Stable, precise continuous printing: On S-PDMS, one-droplet printing maintained uniform widths (left edge 492.9–507.1 µm; right edge 492.9–500.0 µm) and stable D/D_design = 0.986–1.007 over time. In contrast, vat polymerization widths became highly nonuniform (left 259.1–663.4 µm; right 445.4–704.5 µm) with D/D_design increasing up to 1.336 over 500 s, showing protrusions/steps/bubbles due to excess curing.
• Role of curing interface: • Quartz/F-quartz: TCL pinning, large solid–solid adhesion on curing, breakage; one-droplet printing fails. • Superamphiphobic: TCL recedes; columnar structures form but sidewalls exhibit vertical stripes (frozen TCL distortion) due to solid–air–solid composite interface. • S-PDMS: TCL recedes; smooth sidewalls; stable continuous printing due to solid–liquid–solid composite interface with lubricant shielding and low adhesion.
• Interfacial adhesion metrics (mean ± s.d.): • Contact angle (°): Quartz 34.0 ± 1.3; F-quartz 84.8 ± 1.9; S-PDMS 42.4 ± 2.1; Superamphiphobic 151.7 ± 2.5. • γ2 (cured resin/curing interface, mJ/m²): Quartz 125.5 ± 3.4; F-quartz 67.3 ± 2.1; S-PDMS 1.8 ± 0.3; Superamphiphobic 1.5 ± 0.1. • γ3 (liquid resin/curing interface, mJ/m²): Quartz 69.4 ± 1.4; F-quartz 48.8 ± 3.6; S-PDMS 56.9 ± 4.3; Superamphiphobic 3.7 ± 0.6. • γ1 (liquid resin/cured resin, mJ/m²): 59.4 ± 2.9 (resin-specific constant). Satisfaction of criteria γ1 > γ3 and γ3 > γ2 explains success on S-PDMS and superamphiphobic interfaces, failure on quartz, and partial success on F-quartz.
• Material utilization efficiencies: • One-droplet (S-PDMS): Wet efficiencies generally >96% across tested structures. Net efficiencies up to ~92.5% (e.g., V-groove L/W 17:1, 30°). Net efficiency increases when initial droplet mass decreases (smaller r) and when UV pattern radius increases, due to curvature-driven reduction of adhered layer thickness scaling from R to r (e ~ r·Ca^(2/3)). • Traditional vat printing: Wet ~87.5%, net ~66.7% under comparable tests. • V-grooved patterns: In one-droplet printing, smaller intersection angle α increases capillary rise h(x) and capillary dragging force, reducing residue and increasing net efficiency; opposite trend in vat printing.
• Process advantages: Receding TCL yields free liquid/air and liquid/interface boundaries, enhancing inner droplet circulation and suppressing x–y overcuring and afterglow-induced extra curing at high speeds.
• Application: Successfully printed dental crown structures (molar, incisor, canine) from individual droplets with smooth sidewalls and good fit, demonstrating controllability and potential for on-demand, material-efficient fabrication.

The findings confirm that controlling interfacial adhesion to enforce a receding TCL is key to converting an entire droplet into a prescribed 3D structure with minimal residue. Meeting γ1 > γ3 ensures the liquid remains preferentially attached to the growing solid rather than the substrate, driving continuous TCL retraction; γ3 > γ2 ensures that newly cured layers detach cleanly from the interface to allow uninterrupted growth. S-PDMS provides the optimal composite interface (solid–lubricant–solid) that minimizes cured-part adhesion while maintaining sufficient liquid adhesion to sustain TCL motion, yielding smooth sidewalls and stable dimensions. Compared to vat polymerization—where constrained contact surfaces and residual resin lead to overcuring, bubbles, and dimensional drift—the one-droplet approach leverages free surfaces and vigorous internal circulation to maintain high fidelity (stable D/D_design) and dramatically improves material utilization. The capillarity-based analysis rationalizes how droplet size, UV pattern radius, and V-groove geometry tune residue and net efficiency via curvature-dependent film thickness and corner rise, providing design rules for optimizing efficiency and precision across geometries.

This work introduces a one-droplet 3D printing strategy that, by engineering the curing interface to support a receding TCL, continuously transforms a single resin droplet into complex 3D structures with high material utilization and precision. Using an S-PDMS slippery interface satisfies the necessary adhesion criteria (γ1 > γ3 and γ3 > γ2), enabling smooth, stable printing and suppressing extra curing typical in vat processes. The approach achieves wet efficiencies ≥96% and net efficiencies up to ~92.5%, maintains dimensional stability (D/D_design ≈ 1), and extends to practical shapes such as dental crowns. Theoretical analysis links utilization to droplet curvature, UV pattern size, and capillary rise in V-grooves, offering guidelines for process design. This interfacial strategy can reduce resin waste, save valuable inks, and inform on-demand, precise photocuring-based AM. Future work could expand material compatibility, explore dynamic control of interface properties and lubricant layers, and scale to larger or more complex multi-material structures.

• Dependence on curing interface: Quartz and fluorinated quartz surfaces fail to support continuous one-droplet printing due to high cured-part adhesion (γ2) relative to liquid adhesion (γ3); superamphiphobic surfaces cause striped sidewalls due to TCL distortion. Effective operation relies on UV-transparent, low-adhesion, lubricant-infused (S-PDMS) interfaces.
• Process window: Demonstrated continuous elevation speeds up to 200 µm/s; higher speeds or UV intensities were not characterized here.
• Residuals and handling: A small fraction of resin (~0.4% in demonstration) can remain on the supporting plate due to plate–liquid adhesion.
• Resin specificity: Adhesion values and criteria validation were measured for specific resin systems; generalization to broader chemistries requires further testing.