Circadian humidity fluctuation induced capillary flow for sustainable mobile energy

The study investigates whether natural daily fluctuations in ambient humidity can be harnessed as a ubiquitous, sustainable energy source. The authors hypothesize that humidity-driven absorption/desorption of water by a static ionic liquid (IL) droplet pinned on a poly(dimethylsiloxane) (PDMS) nanowire array will induce directional capillary flows along the solid/liquid interface. These flows, in turn, re-distribute ions within the IL and across the interfacial electrical double layer, generating a measurable potential difference and current. This mechanism is distinct from prior drop-motion-based generators (spreading, impacting, bouncing, dragging) that harvest transient energy from dynamic droplets. By leveraging circadian humidity cycles, the goal is continuous, low-cost power generation available anytime and anywhere.

Prior works converted droplet motion (e.g., spreading, falling/impinging on PTFE, bouncing on lubricated surfaces, liquid–liquid triboelectric interactions) into electricity, typically producing transient outputs dependent on drop dynamics and velocity. Humidity-related devices previously yielded brief signals limited by rapid saturation. The present work diverges by employing static IL droplets and exploiting routine environmental humidity fluctuations to maintain sustained directional interfacial flows and continuous power output. References cited include droplet–membrane interactions, tunable bouncing, vapor-mediated droplet behaviors, and humidity-induced actuation, positioning the current approach as a continuous, humidity-fluctuation-driven energy harvester rather than an impact-based transient generator.

• Device concept and materials: A spherical-cap IL droplet (1-octyl-3-methylimidazolium chloride, OmimCl) is placed on a PDMS nanowire array (diameter ~90 nm, length ~2 μm) whose nanostructure pins the three-phase contact line. PDMS surface was chemically modified. The IL is selected for negligible evaporation and strong hygroscopicity.
• Outdoor tests: Open-circuit voltage (Voc) measured by digital multimeter while the device was exposed to outdoor air (no direct sunlight), capturing correlation between relative humidity (RH) fluctuations and voltage polarity/magnitude over multiple days.
• Controlled indoor experiments: Windless, constant temperature (25 °C) and RH (~40%). Three initial water content (WC) states in IL were tested: WC < WCs (undersaturated), WC > WCs (oversaturated), and WC = WCs (equilibrium). Streaming potentials/currents and I–V characteristics were recorded over time to link absorption/desorption to voltage polarity and magnitude.
• In-situ flow visualization: Polystyrene microspheres (~18 μm diameter) were dispersed in IL droplets to track flow under constant RH (~40%). Optical microscopy captured inward (edge-to-center) and outward (center-to-edge) flows depending on absorption vs desorption. Flow velocities near the edge were statistically quantified across RH 5–90%.
• Humidity dependence: Depth-averaged microsphere velocities and initial Voc were measured as functions of RH to establish moisture-dependence. IL water uptake/desorption rates were quantified gravimetrically versus time.
• Wind effect: Streaming potentials and absorption/desorption rates were measured with and without a gentle wind (~2.5 km/h) to assess convective enhancement.
• Energy storage demonstrations: Single droplet charging of commercial capacitors (1–100 μF) within 1 min; arrayed capacitors (16 × 220 μF in series) charged to power a red LED (~1.8 V) and an LCD screen (1.5 V).
• Molecular dynamics (MD) simulations: LAMMPS used with coarse-grained IL (Omim+ and Cl−) and all-atom/force-field models (OPLS-AA/SPC/UFF) for PDMS and water. Periodic boundary in x–y, open in z, 300 K thermostat. Radial distribution functions (RDFs) evaluated for PDMS–ion/water and ion–ion/water pairs at varying water contents x(H2O)=0,4,13 (mol ratio). Non-equilibrium simulations under pressure gradients probed near-wall velocity profiles of ions and water and charge stratification within PDMS nanowire confinement.
• Fabrication: PDMS prepolymer (Sylgard 184, 10:1) mixed, degassed, cast using AAO templates (pore ~60 nm, depth 2 mm), Ag/AgCl electrodes integrated, AAO removed by 1 M NaOH at 60 °C to yield PDMS nanowire arrays.
• Characterization and measurements: SEM, EDS, XPS, optical microscopy of microspheres; electrical outputs (Voc, short-circuit current) recorded with digital multimeter; environmental RH/temperature monitored; humidity adjusted with salts (e.g., CaCl2).

• Outdoor correlation with RH fluctuation: As RH increased from 28.7% to 39.1%, peak Voc reached ~110 mV; during RH decrease, polarity reversed with peak ~−128 mV. Over 3 days with RH 81–83%, continuous voltage output of ~0.14–0.15 V was recorded.
• Controlled RH experiments: When IL WC < saturation (WCs), absorption drove inward edge-to-center flow and a positive streaming potential reaching ~129 mV after 600 s. When WC > WCs, desorption drove outward flow and inverted voltage/current. At WC ≈ WCs, both streaming potential and current approached 0, consistent with I–V curves. Short-circuit currents of ~2 μA (absorption) and −1.4 μA (desorption) were observed, higher than many prior humidity generators.
• Direct flow visualization: Microscopy with microspheres confirmed humidity-driven directional capillary flows. At RH ~40%, both inward (absorption) and outward (desorption) flows along the bottom/top regions of the droplet were observed, consistent with contact line pinning maintaining a spherical-cap profile.
• Humidity dependence of flow: Near-edge bead velocities scaled strongly with RH: ~0.2–0.8 μm/s at RH 5% versus ~65–100 μm/s at RH ~90%, aligning with diffusion-driven flux and interfacial flow expectations.
• Wind enhancement: With ~2.5 km/h wind, streaming potentials increased to ~232 mV (absorption) and ~168 mV (desorption), attributed to enhanced mass transfer; water absorption/desorption rates increased compared to still air.
• Energy storage demonstrations: A single droplet charged 1–100 μF capacitors within 1 min. A bank of 16 capacitors (220 μF each, in series) reached ~2.18 V, lighting a red LED (~1.8 V). The device also powered a 1.5 V LCD screen.
• MD insights into mechanism: Increased water content strengthened PDMS–cation (Omim+) affinity and weakened PDMS–anion (Cl−) affinity; water also modified cation–anion and ion–water RDFs, enhancing Cl− association with water and facilitating ion separation under flow. Under applied pressure gradients, near-wall Cl− moved faster than Omim+ within ~1 nm of PDMS, leading to charge separation and interfacial potential differences. Confined PDMS nanowire gaps promoted Omim+ clustering, further slowing cation motion and enabling charge stratification. Overall, absorbed water critically mediates interfacial interactions and ion transport asymmetry to produce streaming potentials.

Findings support a two-step mechanism whereby ambient humidity fluctuations drive IL water absorption/desorption at the droplet interface, generating directional capillary flow constrained by contact line pinning on PDMS nanowires. This flow redistributes ions along the electrical double layer, creating a streaming potential whose polarity follows the absorption (inward) versus desorption (outward) flow direction. The voltage magnitude scales with humidity (and is enhanced by mild airflow), enabling continuous power output synchronized to circadian RH cycles. MD simulations corroborate the critical role of water in modulating PDMS–ion and ion–ion/water interactions and establishing asymmetric near-wall ion velocities and charge separation during flow. Compared with drop-impact or transient humidity devices, this approach provides sustained, passive energy harvesting from small, ubiquitous RH fluctuations, broadening the landscape of ambient energy sources beyond solar and wind.

The work demonstrates a humidity-fluctuation-driven energy harvester based on static ionic liquid droplets pinned on PDMS nanowire arrays. Circadian RH variations induce moisture absorption/desorption, leading to directional capillary flows, interfacial ion redistribution, and continuous electrical output. Experiments (outdoor/indoor), flow visualization, and MD simulations collectively validate the mechanism. Practicality is shown by charging capacitors rapidly, lighting an LED, and powering an LCD screen, with performance enhanced by mild airflow. This establishes ambient humidity fluctuation as a widely accessible, low-cost green energy resource. Potential future work could optimize nanowire geometry and surface chemistry, scale up device arrays, integrate with energy storage and power management, and investigate long-term stability and diverse IL chemistries across climates.