Universal droplet propulsion by dynamic surface-charge wetting

The study addresses how to achieve universal, high-performance, and programmable droplet propulsion on solid surfaces without additives or complex electrode architectures. Droplet transport is essential in microfluidics, water harvesting, self-cleaning, condensation heat transfer, energy harvesting, and 3D printing. Motion on solids is hindered by contact angle hysteresis; effective propulsion requires overcoming lateral adhesion. Existing passive strategies (wettability gradients via chemistry or topography, and SLIPS) enable self-motion or reduced hysteresis but suffer from limited handling distance, flexibility, durability issues, and viscous losses. Active strategies using thermal, vapor, optical, acoustic, magnetic, and electric stimuli improve control; among them, electric methods are attractive due to material generality, additive-free operation, and real-time control, but conventional electrowetting/dielectrowetting require complex electrode patterns and control. Unipolar charged surfaces without electrodes can move droplets but typically only on superhydrophobic/slippery surfaces with limited driving force. The research question is whether strong, reconfigurable electric fields can be created by alternately depositing opposite charges to realize universal, high-force, contactless droplet propulsion on diverse, unmodified dielectric surfaces.

• Passive manipulation: Wettability gradients via chemical composition or topographic patterns can induce self-propulsion and enable dynamic control, but distance and flexibility are limited. SLIPS reduces contact angle hysteresis for gravity-assisted transport across a range of surface tensions; however, lubricant durability and viscous resistance hinder use in harsh environments such as condensation.
• Active manipulation: Thermal, vapor, and optical stimuli create Marangoni or asymmetric wetting but often yield low velocities and have strict material constraints. Acoustic, magnetic, and electric fields offer high controllability. Electrowetting/dielectrowetting produce strong forces but require patterned electrodes and complex control. Electrode-free unipolar charged surfaces enable long-distance handling with high precision yet generally require superhydrophobic or slippery substrates and provide relatively small driving forces. A knowledge gap remains on leveraging simultaneous opposite surface charges to boost forces while maintaining universality and simplicity.

• Principle and setup: Dynamic surface-charge wetting is achieved by sequentially depositing opposite charges onto a dielectric polymer film mounted on a conductive substrate (e.g., ITO-coated glass). Two steel needle electrodes generate corona discharge using battery-powered step-up transformers (low current ~4 µA, power ~0.1 W). The top (vertical) needle deposits positive charge; the side (horizontal) needle deposits negative charge.
• Geometry and voltages: Polymer thickness 0.05–2 mm; top needle-surface distance ~3 cm; side needle-surface distance ~1 cm; inter-needle horizontal distance 3–7 cm. Applied voltages: top needle 0–15 kV (typ. 11 kV for 10 s), side needle 0 to −9 kV (typ. −7 kV). An optimized working distance governs surface charge density and coverage.
• Operating sequence: (1) Positively charge the surface with the top needle for ~10 s. (2) Deposit negative charge from the side needle; the droplet is then transported along or against the resulting electric field lines. A single deposition step does not propel droplets.
• Materials and surfaces: Tested dielectric films include acrylic, PET, BOPP, Kapton, and PTFE. Conductive backings include ITO glass, graphite, copper foil, and tin foil to enable flat, curved, and flexible configurations. Liquids include water, ethanol, and nonconductive silicone oil. Everyday substrates (e.g., plastic gloves, glasses, wrapping paper, woodware) were also demonstrated.
• Measurements and characterization: Contact angles and contact angle hysteresis were measured; droplet velocities were recorded (side/top views). Surface potentials over a 45 × 55 mm area were mapped off-line with an electrostatic voltmeter after sequential charging. Surface charge density σ was obtained via σ = Uc with c = εε0/d (ε: permittivity, d: film thickness). Finite element method simulations yielded charge and field distributions on the surface and on/within the droplet. The micropipette-based method quantified resisting forces at barriers (edge/wettability gradients). The dependence of force distribution and maximum electrostatic force position on applied negative voltage was analyzed.
• Performance and programmability: Sequential charge spreading synchronized with droplet motion enabled merging, trapping, and tracking; bidirectional motion via alternating voltages on opposing side electrodes; spatially patterned electrodes (e.g., trident) guided multiple droplets to coalesce; multi-vertical needle arrays provided relayed, long-distance transport. Curved and vertical surface transport was demonstrated.

• Universal propulsion on diverse, unmodified dielectric films (acrylic, PET, BOPP, Kapton, PTFE) and conductive backings (ITO glass, graphite, copper, tin), and on everyday objects.
• Liquids: Conductive (water, ethanol) and dielectric (silicone oil) droplets are propelled.
• Performance metrics:
  • Speed: Up to ~130 mm/s (10 µL water on PET). Representative average velocities over ~2 cm: acrylic/water 18 mm/s; PET/water 130 mm/s; PTFE/water 57 mm/s; BOPP/water 43 mm/s; Kapton/water 26 mm/s; PTFE/ethanol 20 mm/s.
  • Volume range: 1 µL to 1 mL.
  • Contact angles: 40°–106° across surfaces; CAH up to 35°; smaller CAH correlates with higher velocity.
  • Residue: Minimal/lossless transport indicated by fluorescence imaging.
  • Geometry: Effective lateral transport distance per charging cycle ~2 cm; continuous motion on curved and vertical surfaces (against gravity).
• Mechanism and fields:
  • Dynamic surface-charge wetting creates persistent imbalance with front contact angle smaller than rear; left contact line remains initially pinned while right spreads; both decrease until ~72° (below receding angle) when the left line depins.
  • Anisotropic spreading: D_y/D_x ≈ 1.3; contact area increases; max diameter ~1.5× initial.
  • Surface potential mapping: After positive charging, uniform ~+1.1 kV; subsequent negative charging forms a propagating semi-elliptical negative region (min ~−0.9 kV) until saturation.
  • Charge density from FEM: Initial positive surface charge up to ~6.1 × 10^−4 C/m²; negative charge redeposits and neutralizes/reverses polarity, enabling reconfigurability.
  • Droplet charges: Transition from positive to negative as the negative pattern reaches it; charge concentrates at front edge and droplet bottom near the three-phase line, producing strong electrostatic force at the boundary of opposite polarity.
  • Force capability: Maximum electrostatic driving forces can exceed droplet weight; reported driving force up to ~6× gravitational force; droplets surmount edge and adverse wettability gradients; measured resisting force at a barrier ~3× droplet gravitational force.
  • Voltage control: Increasing negative side voltage enlarges negative potential area and shifts the opposite-charge boundary forward; the position of maximum electrostatic force aligns with this boundary rather than peak potential; higher voltage yields larger force over longer distance and higher average velocity.

The work demonstrates that alternately depositing opposite charges on a dielectric surface generates strong, reconfigurable electric field gradients that can overcome contact angle hysteresis and lateral adhesion without requiring patterned electrodes or lubricants. The mechanism—dynamic surface-charge wetting—creates a sustained imbalance with a smaller advancing (front) contact angle than the receding (rear) contact angle, enabling continuous propulsion. Charge neutralization and redeposition allow rapid reconfiguration of force landscapes on a single film. Concentration of charge at the droplet front edge yields large electrostatic forces at the boundary between opposite polarities, sufficient to overcome strong pinning, traverse edge barriers, and move against gravity on vertical surfaces. The approach achieves high speeds, broad volume compatibility (1 µL–1 mL), low residue, and applicability across a wide range of wettabilities and substrates. Programmable spatiotemporal charge control enables advanced operations such as bidirectional transport, merging, tracking, and relayed long-distance motion, supporting applications in microreactions, defogging, and photovoltaic cleaning.

The study introduces a universal, electrode-free droplet propulsion strategy based on dynamic surface-charge wetting achieved via sequential corona deposition of opposite charges. It delivers high driving forces (up to ~6× weight), high velocities (~130 mm/s), minimal residue, broad liquid and volume compatibility, and operation on hydrophilic to hydrophobic, flat to curved/vertical surfaces. Spatiotemporal charge programming affords versatile manipulation, including merging, trapping, tracking, and relayed transport, illustrating strong potential for microreactors, antifogging, and surface cleaning (e.g., photovoltaic panels). Future work could optimize electrode arrangements for longer single-pass distances, integrate closed-loop sensing for precise trajectory control, and explore scalability and robustness under environmental factors (e.g., humidity, contamination, condensation).

• Effective single-cycle transport distance is limited (effective lateral electrode-driven distance ~2 cm), and droplet velocity decreases to zero after several centimeters without recharging/relay.
• Performance depends on substrate wettability/contact angle hysteresis (smaller CAH yields higher velocity), though large CAH (up to 35°) can still be overcome with higher forces.
• Requires a dielectric film on a conductive backing and corona discharge hardware; working distances and voltages must be optimized for sufficient charge density and coverage.
• Publication text does not report long-term durability under repeated cycling or in harsh environments (e.g., high humidity, contaminants), nor quantitative residue over many cycles.