Oscillating light engine realized by photo-thermal solvent evaporation

The study addresses the challenge of directly converting solar energy into continuous mechanical work, avoiding losses inherent in multi-step energy conversion and storage. While actuators can produce motion under external stimuli, achieving self-sustained oscillation under diffuse or divergent light (like sunlight) is difficult because prior approaches rely on tightly focused beams and precise angles. The authors hypothesize that a symmetric, porous polymer film that undergoes rapid, reversible volume change via photothermal solvent evaporation and replenishment can sustain oscillatory bending under divergent light. They aim to design and characterize a light-driven oscillating actuator that functions as a solar engine to produce continuous mechanical work.

The paper situates the work within actuator technologies including dielectric elastomers, conducting polymers, shape memory materials, twisted fiber muscles, ionic composites, pneumatic actuators, and inorganic materials. Prior light-driven self-oscillators generally depend on localized, focused beams (e.g., lasers) where bending removes illumination until unbending re-exposes the area, requiring strict beam width and fixed incident angles—conditions incompatible with diffuse sunlight. Solvent evaporation from polymer films is known to induce volume shrinkage and bending (especially with porous films that enhance mass transport), but oscillating actuators leveraging solvent evaporation had not been reported. The authors outline requirements for oscillation under divergent light: symmetric shrinkage on both sides under illumination, rapid volume recovery during dark phases, and optimized mechanics, optical absorption, thermal rates, and actuation stress.

Materials: Commercial porous polypropylene/carbon black (PP/CB) films (Nitto SCF series) with acrylic resin binder, thicknesses 60, 100, 150 µm; composition ~99.7% polymer and 0.3% additives (Ca, Ti). A porous PDMS film was also fabricated by templating with NaCl and adding carbon black for photothermal absorption. Film structure and optics: The 100 µm PP/CB film exhibits hierarchical porosity: microscale pores averaging ~40 µm (mercury intrusion) and nanoscale pores ~91.83 nm (N2 adsorption/desorption). CB imparts broadband absorption (200–2600 nm). SEM characterized top and side morphologies. Wetting and swelling: The PP/CB surface is hydrophilic due to polyacrylic adhesive; ethanol drops are rapidly absorbed (contact angle ~10° at 1 s). Ethanol fully infiltrates pores (optical/fluorescence microscopy with rhodamine/ethanol). Swelling upon ethanol infiltration is anisotropic: length +18.21%, width +19.69%, thickness +48.50% (3×1 cm, 100 µm film), volume +110.10% to saturation within ~3–5 s; mechanical properties are largely retained post-infiltration. Thermal response and evaporation: Infrared (IR) illumination heats films rapidly via photothermal conversion: dry film reaches ~116 °C in ~10 s at 800 mW cm⁻², cooling to ~35 °C in ~5 s after light off. Ethanol content and evaporation kinetics were tracked via FTIR (O–H 3328 cm⁻¹, C–H 2972 cm⁻¹, C–O 1046 cm⁻¹) and gravimetry. Thinner films and higher IR intensities accelerate evaporation; for 100 µm films, average evaporation rate increased from ~0.53 to ~0.89% s⁻¹ as IR intensity rose 400→800 mW cm⁻². Wicking and solvent supply: Continuous solvent supply was provided by vertically positioning the film with a lower edge contacting ethanol (reservoir), enabling capillary rise. Wicking height and rate under varying IR intensities (400–800 mW cm⁻²) were measured; higher intensities decreased steady wicking height (13.55→10.19 mm) and showed a nonmonotonic wicking rate peaking at ~2.45 mm s⁻¹ at 650 mW cm⁻². Temperature profiles along the wetted film length were captured via IR imaging, revealing gradients. Oscillation setups: Films (typical 9 mm × 3 mm × 100 µm) were mounted with one end fixed and the other free. Experiments examined: (i) horizontal vs vertical mounting; (ii) IR irradiation at incident angles 0°, 45°, 90° relative to film surface; (iii) light intensity variation (400–800 mW cm⁻²); (iv) environmental temperature effects; (v) oscillation under sunlight (through window, direct) and simulated sunlight (Xe lamp, 130–320 mW cm⁻²). Displacement, curvature, frequency, and temperature were recorded via high-speed imaging and IR thermography. Stability tests covered ≥1000 cycles under continuous ethanol supply. Mechanical output: Load-lifting tests attached small masses (foam pieces or films) to the free end or via a thread to model a solar engine converting oscillation to work. Specific work and power were computed: W_act = m·g·h/m0 and P_act = m·g·h/(m0·t), where m is load mass, m0 actuator mass, h lift height, t lifting time. Weight-lift ratios were varied (0.5–6). Thickness (60, 100, 150 µm) effects on actuation stress, amplitude, frequency were measured. Solvent generality: Oscillation was evaluated with several solvents (ethanol, methanol, THF, acetone, ethyl acetate, dichloromethane); high-boiling polar and nonpolar solvents were also tested for comparison. A porous PDMS film was tested with ethanol and ethyl acetate. Characterization tools: SEM (MERLIN Compact), contact angle goniometer (Zhongchen JC2000D1), gravimetry for evaporation, IR thermography for temperature, optical and fluorescence microscopy for infiltration, and custom image analysis for displacement/curvature/frequency.

• A photothermal solvent-evaporation mechanism in a symmetric porous PP/CB film enables self-sustained oscillatory bending under divergent light (IR, sunlight, simulated sunlight) with continuous solvent supply.
• Thermal response: Dry films heat to ~116 °C in ~10 s under 800 mW cm⁻² IR and cool to ~35 °C in ~5 s after light off. Ethanol-infiltrated films show reduced temperatures due to evaporative cooling.
• Solvent handling: Ethanol swelling yields +110% volume increase; evaporation rate for 100 µm films increases from ~0.53 to ~0.89% s⁻¹ as IR intensity rises 400→800 mW cm⁻². Wicking height decreases (13.55→10.19 mm) and wicking rate peaks at ~2.45 mm s⁻¹ at 650 mW cm⁻² as IR intensity increases 400→800 mW cm⁻².
• Oscillation performance (typical 9×3×0.1 mm film, vertical mounting): At 800 mW cm⁻² IR, maximum displacement 15.7 mm, maximum curvature 7.3 cm⁻¹, period ~0.3 s (≈3.3 Hz), and light-to-work energy conversion efficiency ~0.9%. Minimum temperature during a cycle (~42.7 °C) occurs near maximum curvature; maximum (~48.5 °C) near the vertical position.
• Stability: Stable oscillation for ≥1000 cycles with continuous ethanol supply; solvent is recoverable in a sealed vessel.
• Intensity dependence: At 400 mW cm⁻², displacement ~1.1 mm, smaller temperature swing (25.9–30.3 °C), and oscillation to one side only. Increasing to 650 mW cm⁻² raises frequency to a maximum ~6 Hz and increases temperature swing (~9.4 °C). At 750–800 mW cm⁻², oscillation occurs across both sides with large displacement (~15 mm) and reduced temperature swing.
• Geometry/angle robustness: Oscillation persists for various film tilt angles and IR incidence (0°, 45°, 90°), with measurable displacements and curvatures across configurations.
• Thickness effects: For 60/100/150 µm films, actuation stress ~26.83/26.72/26.67 kPa, amplitudes ~9.98/15.73/14.15 mm, frequency ~3.3 Hz; 100 µm offers best overall performance in this set.
• Sunlight operation: Under ~50 mW cm⁻² sunlight (through window), displacement ~3.0 mm with irregular oscillation; under direct ~100 mW cm⁻² sunlight and ~36 °C ambient, displacement ~9.6 mm with no decay over 20 days. Under simulated sunlight (Xe lamp), periodic oscillation with displacements 15.1 and 16.2 mm at 250 and 320 mW cm⁻², periods ~2.13 and 2.06 s.
• Load handling and work output: The actuator lifts loads with weight-lift ratios up to 6. As ratio increases 0.5→4, displacement modestly decreases (17.9→13.3 mm), period 0.56→0.95 s; at 6, motion to one side only (2.5 mm). Specific work peaks at ~12.0×10⁻⁵ J g⁻¹ and specific power at ~2.0×10⁻⁴ W g⁻¹ at weight-lift ratio 4. In a thread-coupled engine configuration, maximum specific work 30.9×10⁻⁵ J g⁻¹ and maximum specific power 15.4×10⁻⁵ W g⁻¹ were achieved with a 1.9 mg load (displacement 12.4 mm, period 1.28 s).
• Comparative performance: Displacement, curvature, speed, and thickness-normalized metrics are superior or comparable to many reported light-driven oscillators; energy conversion efficiency (~0.9%) exceeds some CNT-based photoactuators and is within the range of advanced hybrid yarn and graphdiyne actuators.

The work demonstrates direct conversion of light (including diffuse sunlight) to continuous mechanical work by harnessing asymmetric, rapidly reversible volume changes via photothermal solvent evaporation in a symmetric porous film. Continuous solvent supply ensures sustained oscillation without external on/off control of light, overcoming limitations of prior systems that required localized beams and precise angles. Fast heating and solvent evaporation produce bending; evaporative cooling and rapid solvent diffusion enable quick recovery, supporting high frequencies (up to ~6 Hz) and large amplitudes. The actuator functions across various orientations and light incidence angles and can lift loads, allowing conversion of oscillatory motion into useful work (e.g., through thread coupling or eccentric shafts to rotary motion). Although actuation stress is modest compared to dense bimorphs, the large stroke, robustness, and ability to operate under real sunlight position this design as a viable solar engine concept. Environmental factors (temperature, light intensity) modulate performance, consistent with the evaporation-driven mechanism. The approach suggests applications in autonomous devices, solar-powered soft robotics, valves, sensors (light intensity/wavelength, solvent leakage), and signal modulation.

The authors present a self-oscillating, light-driven actuator based on a porous PP/CB film that, with continuous solvent supply, converts divergent light into continuous mechanical work. It achieves large displacement (up to 15.7 mm), high bending amplitude (7.3 cm⁻¹, ~224°), fast operation (up to ~6 Hz), stable long-term cycling, and light-to-work efficiency around 0.9%. The system operates under IR, simulated sunlight, and actual sunlight, and can lift loads to produce measurable specific work and power. Key enablers are rapid, reversible solvent-driven volume change, high photothermal absorption, appropriate film mechanics, and efficient solvent wicking. Future work should optimize pore architecture and film mechanics to enhance frequency and stroke, improve energy efficiency, integrate solvent-recirculation in sealed systems for continuous operation without loss, tailor light absorption across wavelengths, and design mechanisms to translate oscillation into rotary motion for practical engines and devices.

• Requires continuous supply of a volatile solvent; without replenishment oscillation diminishes as solvent depletes. A sealed, recirculating system is proposed but not yet implemented.
• Actuation stress (~26.7 kPa) is lower than many dense bimorph actuators (MPa range), potentially limiting force output despite large stroke.
• Performance depends on environmental conditions (ambient temperature, light intensity, and angle), leading to irregular oscillation under weak or variable sunlight.
• Solvent choice is constrained: high-boiling or nonpolar solvents show poor actuation due to slow evaporation or poor wetting/wicking; safety and handling of organic solvents are considerations.
• Optimization of pore size/type and comprehensive long-term durability under varied environments are deferred to future work.