Light-driven dandelion-inspired microfliers

The study addresses the challenge of achieving energetically efficient, untethered heavier-than-air flight at insect scale. Conventional micro aerial vehicles (MAVs) rely on complex airframes, motors, batteries, and electronics that become inefficient when scaled down due to reduced power density, transmission losses, and increased viscous effects. Prior attempts using flapping-wing MAVs, piezoelectric or dielectric actuators, and wireless power have limitations including added weight, disturbed balance, and short untethered flight durations. Nature offers efficient wind-dispersal strategies, particularly dandelion seeds whose porous pappus generates a separated vortex ring and enhanced drag for long-range dispersal. This work proposes a dandelion-inspired, light-responsive microflier that can modulate its pappus opening under light to control falling speed and achieve mid-air flight via light-induced updrafts, aiming to provide a simple, lightweight, and controllable platform for energy-efficient flight without onboard power.

• Flapping-wing insect-scale MAVs have demonstrated tethered and limited untethered flight using piezoelectric and dielectric elastomer actuators, but are constrained by battery mass and integration complexity.
• Wireless power approaches (RF, light with photovoltaics) enable untethered operation but still add complexity and mass that disturb flight balance.
• Wind-dispersed seeds (e.g., dandelion) achieve efficient passive flight via pappus-induced drag and a separated vortex ring, enabling long-distance dispersal. Prior dandelion-inspired devices enabled battery-free sensing and backscatter communication with wide-area dispersal but suffered from high randomness and limited controllability.
• A recent dandelion-inspired disperser using light-responsive LCE films required combined wind and light, had 2D asymmetry, and achieved <25 mm flight height and <1 s flight time at high incident power density, highlighting the need for 3D symmetry and better controllability. These works motivate a 3D symmetric, light-programmable device that passively leverages aerodynamics for controllable, efficient flight.

Actuator design and fabrication: An ultralight bimorph soft actuator was fabricated from polyimide (PI; CTE ~−20 K⁻¹) and low-density polyethylene (LDPE; CTE ~−280 K⁻¹) to exploit large thermal expansion mismatch for bending/twisting. PI film surfaces were rendered positively charged via layer-by-layer deposition of PAH/PSS polyelectrolytes, finishing with PAH. LDPE films were plasma-treated to be negatively charged and spin-coated with inks of surface-modified ultrasmall Au nanorods (AuNRs) for strong NIR photothermal conversion. PI and LDPE layers were electrostatically laminated to ensure robust interfacial bonding. COMSOL simulations evaluated interfacial stresses and deformation under ΔT. Shape programmability: Leveraging LDPE anisotropy, cutting the laminate at different angles (α = 45°, 90°, 135°) produced distinct modes (cylindrical bending and left/right twist) upon heating, enabling programmable strip twisting. Microflier construction: Rectangular bimorphs (α = 90°, 14 mm × 12 mm) were scissor-cut on one end into uniform thin strips (~5 mm long, ~0.7 mm wide). The laminate was rolled into a tube (LDPE inner, PI outer) forming an "achene". Approximately 40 fiberglass strands (length 10–20 mm, diameter ~25 μm, density 2.4 g cm⁻³) were attached to the strip ends as the pappus. Total microflier mass was adjusted to ~4 mg. Photoactuation characterization: Under 808 nm NIR irradiation (10–150 mW cm⁻²), temperature rises and bending angles were measured: temperature increased from ~18 to 54.2 °C in 8 s at 150 mW cm⁻²; bending reached 180° in 0.33 s and recovered in ~1.62 s after light off. Actuation stress was quantified up to ~0.91 MPa. Cyclic durability exceeded 1000 cycles without fatigue at various intensities/frequencies. Areal mass ~2 mg cm⁻². Light-controlled falling tests: Conducted in a confined space to avoid air disturbances. A Philips infrared lamp (0–350 W, 420–1000 nm) above the test area controlled pappus opening. High-speed imaging (Canon EOS 6D Mark II) captured descent from set heights; velocities were extracted using video analysis. Light-powered mid-air flight setup: An airflow tunnel (length 40 cm, diameter 10 cm) was placed above a light source (Philips IR lamp 0–350 W or simulated solar lamp 0–300 W, 350–1000 nm) to create a vertical updraft and a spatially symmetric light field at the outlet. Microfliers were released from specified heights above the outlet with tweezers; trajectories, flight time, and height were recorded. Flow visualization: A fog machine seeded the flow; a 532 nm Nd:YLF laser sheet illuminated smoke for TR-PIV measurements (field 135 mm × 70 mm, 400 Hz, 4000 snapshots). Data were processed with Dantec Dynamic Studio to visualize updrafts and vortex rings and quantify updraft velocities (0–1.25 m s⁻¹ depending on lamp power).

• Ultralight, sensitive actuator: NIR light (808 nm, 150 mW cm⁻²) increased actuator temperature from ~18 to 54.2 °C in 8 s; bending angle reached 180° in 0.33 s and recovered in ~1.62 s after light off. Maximum actuation stress ~0.91 MPa; stable over >1000 cycles. Areal mass ~2 mg cm⁻².
• Programmable deformation: Cutting angle α enabled cylindrical and handed twisting modes due to LDPE anisotropy; hand temperature could trigger deformation.
• Light-controlled falling: Without light (pappus closed), dropping 20→3.4 cm took ~0.17 s with terminal velocity ~0.98 m s⁻¹. With light (50 mW cm⁻²) opening the pappus, falling time increased to 0.43 s and terminal velocity decreased to ~−0.41 m s⁻¹. Without pappus, falling time ~0.30 s and velocity ~−0.57 m s⁻¹ under similar light. Pappus opening angle increased from ~4° to ~92° as light intensity rose 0→60 mW cm⁻², enabling continuous tuning of terminal velocity. Sunlight (~80 mW cm⁻²) opened the pappus in ~2.5 s. More fiberglass strands further reduced terminal velocity.
• Payload effects: Adding 0–24 mg payload increased terminal velocities for both closed and open states; with light, the open state remained slower. At high payload (>25 mg), open/closed velocities converged.
• Mid-air flight: In a symmetric tunnel-updraft and light field, microfliers released near the outlet achieved average upward velocity ~72 mm s⁻¹, sustained flight time ~8.9 s, and maximum height ~350 mm (at release height near 0 mm, lamp 300 W). Flight success depended on release height; at 250 mm release height, flight failed. Flight height increased with lamp power (150–350 W) and with longer/more numerous pappus fibers; ~40 strands was a practical optimum.
• Solar-like illumination: With a simulated solar lamp (300 W), sustained flight time ~2.4 s and height ~160 mm were achieved.
• Updraft and vortex ring: Light-induced heating generated updrafts tunable from 0 to ~1.25 m s⁻¹ (by lamp power). Flow visualization revealed a separated vortex ring above the microflier; vortex size increased with pappus opening (0°, 60°, 90°, 120°), providing drag enhancement analogous to natural dandelion seeds.
• Programmable autorotation: Microfliers exhibited spontaneous clockwise or counterclockwise rotation during ascent due to slight handed twists of the strips from cutting. Deliberately fabricated right-handed strips produced clockwise rotation; left-handed strips produced counterclockwise rotation, enabling control over rotation mode.

The work demonstrates that a dandelion-inspired, light-responsive microflier can leverage a symmetric pappus morphing mechanism to modulate aerodynamic drag and interact effectively with a light-induced vertical updraft. Opening the pappus under illumination increases projected area and fosters the formation of a separated vortex ring above the flier, reducing pressure and enhancing drag to balance weight, thus enabling prolonged, controllable flight. The axisymmetric actuator geometry and colocation of light and updraft (from the same source) improve stability and flight efficiency compared with prior 2D asymmetric designs requiring wind plus light. Additionally, actuator shape programmability provides a simple means to control autorotation direction, offering a pathway toward controlled flight behaviors. These findings directly address the goal of energetically efficient, untethered heavier-than-air flight at small scales by minimizing onboard mass and exploiting ambient light as both energy source and control signal. The relevance spans environmental monitoring and wireless communication platforms, and suggests principles useful for solar-sail-like propulsion and small spacecraft attitude control, where lightweight morphing structures and radiative heating can be harnessed.

This study introduces an ultralight, super-sensitive PI/LDPE bimorph actuator with AuNR-enabled photothermal response and uses it to build a 3D dandelion-inspired microflier with an actuated pappus. Key contributions include: (1) a robust, scissor-cuttable, reversibly photoactuated bimorph with large stress and high sensitivity; (2) light-controlled modulation of falling velocity and demonstration of light-fueled mid-air flight with ~8.9 s duration and ~350 mm height; (3) direct flow visualization of a separated vortex ring underlying the lift/drag enhancement; and (4) programmable clockwise/counterclockwise autorotation via actuator geometry. Future work should improve photoactuation efficiency, enhance lift, and integrate miniaturized sensors and control electronics for in-flight state control and feedback. Advances toward sunlight-level (<100 mW cm⁻²) operation and adaptive morphology may broaden practical applications in environmental sensing, communication, solar sails, and robotic spacecraft.

• The difference in terminal velocity between open and closed pappus states diminishes with added payload and becomes negligible beyond ~25 mg.
• Limited in-flight control: the current design lacks onboard mechanisms to change state during flight, restricting control authority.
• Absence of integrated systems: no embedded sensors or microcontrollers (e.g., pressure, temperature, humidity, attitude) are included, preventing closed-loop flight attitude control and environmental monitoring. Additional challenges include aligning symmetric light and wind fields; simultaneous wind tunnel and light use increased failures and suppressed autorotation.