Exploiting ferrofluidic wetting for miniature soft machines

Amoeba-like soft machines capable of dramatic shape change, splitting, and coalescing are promising for targeted drug delivery, minimally invasive surgery, cell transplantation, and catheter-based interventions. While magnetic actuation offers safe and dexterous control at small scales, most existing miniature soft robots rely on solid magnetic composites (e.g., PDMS, Ecoflex, hydrogels with hard magnetic particles), which limits deformability and hinders navigation through congested narrow spaces. Liquid magnetic materials such as ferrofluids provide extreme deformability and reconfigurability, potentially enabling passage through openings smaller than their nominal size and construction of multifunctional machines. Prior ferrofluid droplet robots mainly operated under low wettability conditions (high contact angle) and magnetic field gradients, yielding a limited motion set (e.g., stretching, rolling). The wetting dynamics across different substrates (low, high, and total wetting) and the torque-driven individual dynamics of millimeter-scale ferrofluid droplets have been underexplored. This work investigates how controlling ferrofluid wetting with interfaces enables multimodal motion, controllable fission–fusion, and the reconfiguration of droplets into diverse miniature machines.

Research on miniature soft machines spans optical, chemical, and magnetic actuation, with magnetic strategies enabling rapid and precise control. Solid magnetic elastomer-based robots have demonstrated reconfigurable morphologies and locomotion but face challenges in highly confined environments due to limited deformability. Ferrofluids are colloidal magnetic liquids of nanoscale particles stabilized by surfactants that exhibit tunable shapes under magnetic fields. Previous studies showed gradient-force-driven ferrofluid droplets can deliver cargo and actuate fluidic operations in lab-on-a-chip systems, but largely considered non-wetting or low-wettability regimes (90° ≤ θ < 180°), restricting functionality and motion modes. Wetting dynamics of ferrofluids on substrates with different hydrophilic/hydrophobic properties have been studied, as have field-induced deformation and breakup, but their application to constructing multifunctional soft robots exploiting high and total wetting has remained limited. This work addresses these gaps by integrating wetting-regime control with torque-driven dynamics to broaden droplet locomotion, splitting/merging, and functional reconfiguration.

Materials: An oil-based ferrofluid (dynamic viscosity 50 cP, saturation magnetization 43 mT, density 1.29 g/ml) with Fe3O4 nanoparticles dispersed in fluorocarbon oil was used. Biological tissues (porcine bladder, gastrointestinal and biliary systems) were sourced fresh; human placentas were obtained under ethical approval and prepared by draining blood and filling with PBS. Wetting characterization: Contact angles of 2 µL ferrofluid droplets on hydrogel, glass, resin, PMMA, copper (metal), and silicone were measured over 0–20 min without magnetic field; 3D optical microscopy captured morphology changes on silicone. Pull-off strength was quantified by the magnetic field required to detach droplets. Magnetic actuation systems: (1) Three-axis Helmholtz coil system produced uniform 1D/2D/3D fields, driven by ESCON 70/10 motor drivers and PC control; integrated with ultrasound imaging (Terason t3200) for real-time guidance. (2) A 6-DOF robotic arm carrying three coils was integrated with X-ray fluoroscopy (SIEMENS Artis zeego) to provide magnetic torque and gradient forces under imaging constraints. Locomotion and reconfiguration protocols: Low-wetting (hydrogel) substrates enabled torque-driven motions. 1D horizontal oscillating field Bs(t) = Bm sin(2πft) ey produced stretching; 1D vertical oscillating field led to stretching at f < 50 Hz and jumping at f > 50 Hz. 2D rotating fields generated rotating and tumbling; 3D conical fields produced kayaking and wobbling. Controlled fission/fusion employed: (i) a 2D oscillating field Bosc(t) comprising an orthogonal constant component and sinusoidal component, and (ii) a 3D wavy field Bwav(t) combining 1D oscillating and 2D rotating components with time-varying precession angle. Terrain construction: Complex hydrogel terrains (hurdles, stairs, wall holes, curved channels, sharp turns, gaps, comb-like channels, maze) were fabricated via 3D-printed molds and hydrogel casting. Bile duct phantoms were made from soft gel; ex vivo porcine organs were mounted to recreate uneven, folded, and narrow biological environments. Liquid capsule construction: Two strategies were used: (1) active adhesion of ferrofluid droplet to a passive pill to endow mobility and on-demand release by splitting, and (2) syringe injection of water-based liquid or solid payload (e.g., edible dyes, solids in saturated NaCl) into oil-based ferrofluid droplets, exploiting immiscibility to encapsulate. Typical 5 µL droplets were loaded with ~0.5 µL cargo to avoid instability; a vertical oscillating field increased internal pressure to eject cargo on demand. Liquid cilia fabrication and pumping: Resin substrates with semicircular pits (2 mm diameter, 1 mm depth, 4 mm pitch) were submerged in water and filled with 3 µL ferrofluid to form anchored droplets. A segmented magnetic program Bcilia(t) induced elongation–rotation–contraction cycles; arrays (1×7, 3×3, 9×9) produced synchronous waves for pumping. Liquid skin assembly: Oil-based ferrofluid wet, absorbed into, and penetrated silicone elastomer sheets to immobilize a surface layer of iron oxide nanoparticles, imparting magnetism. A high-frequency field split a large droplet into sub-droplets for uniform coating. The coated sheets were reconfigured into robots (spider, caterpillar, fish) and actuated by programmed fields. Modeling: A dilute emulsion model coupled magnetostatic Maxwell equations with incompressible Navier–Stokes and capillary forces to describe droplet deformation and splitting. The ferrohydrodynamic Bernoulli equation and boundary conditions quantified droplet elongation under uniform fields. COMSOL Multiphysics simulated flow fields for cilia arrays and vortex generation for the fish robot.

Wetting dynamics and adhesion: Without magnetic fields, ferrofluid droplets showed substrate-dependent contact angles: hydrogel ~147° (low wettability, non-adherent), glass ~80°, resin ~75°, PMMA ~65°, silicone ~30° initially decreasing to ~16° after 20 min due to solvent absorption and nanoparticle film formation. Silicone surfaces required ~300 mT to detach due to the rigid nanoparticle layer. Under fields up to 9 mT, droplet morphology varied across substrates; on silicone, lower fields did not change the nanoparticle film. Multimodal motion (low wettability): A 2 mm droplet under horizontal 1D oscillation stretched to 7.5 mm length with aspect ratio up to 9 at 9 mT; adjusting pitch angle induced net translation. Under vertical 1D oscillation, stretching persisted for f < 50 Hz (max aspect ratio ~5.5), while f > 50 Hz induced jumping with maximum height ~4 mm at 100 Hz, 9 mT, and controllable direction via pitch. 2D rotating fields produced rotating and tumbling; 3D conical fields led to kayaking and wobbling. Controllable fission–fusion: Under a 2D oscillating field Bosc(t) (f = 25 Hz, Ao = Co = 9 mT), a 2 mm droplet split along a line into 8 sub-droplets (average ~1 mm diameter) and re-fused upon lowering frequency. A 3D wavy field Bwav(t) split the same droplet into 17 sub-droplets (~400 µm average diameter) spread over a plane, followed by re-fusion with reduced frequency. Splitting alignment was controlled by field direction. Environmental adaptability: The droplet hurdled successive fences of heights 2.2, 3.6, and 4.5 mm via jumping; ascended three 2 mm steps; and jumped through an 8 mm wall hole. It deformed to traverse an annular channel (2 mm ID), a sharp 90° turn (1.8 mm ID) using stretching, and spanned a 3 mm gap via kayaking. For comb-like channels (1.5 mm ID), splitting into three sub-droplets (2D oscillation, f = 20 Hz, Ao·Co = 9 mT) enabled entry and subsequent fusion and locomotion (1D oscillation, f = 10 Hz, Bm = 9 mT). In ex vivo tissues, droplets tumbled along textured porcine bladder surfaces, overcame 10 mm gastric folds by splitting and re-fusion with pitch-increased tumbling, and navigated human placental vessels: a 1 mm droplet in a 1.5 mm main lumen split into three ~0.6 mm sub-droplets to enter ~0.8 mm branches and return. Liquid capsules (low wettability): Ferrofluid droplets adhered to pills to create mobile capsules enabling targeted, on-demand release by splitting inside an ex vivo stomach. Syringe-injected capsules (typical 0.5 µL payload in 5 µL droplet) maintained integrity during stretching and tumbling and ejected cargo via vertical oscillation. Solid dye payloads were released and mixed by droplet rotation, accelerating dissolution. Two capsules could coordinate delivery and fuse for retrieval. Capsules shuttled through narrow passages for 40 min without leakage before triggered release. In bile duct phantoms (3 mm ID, turns ~2 mm ID), capsules stretched to navigate and released cargo on demand. Real-time imaging (X-ray fluoroscopy with a robotic coil arm; ultrasound Doppler) enabled guidance; ultrasound tracked the capsule’s acoustic signature in natural bile ducts. Liquid cilia (high wettability): Anchored droplets in 2 mm pits elongated into cilia and executed nonreciprocal elongation–rotation–contraction cycles, with controllable swiping area via field amplitude and rotation angle. A 1×7 array generated directional pumping with measured dye flow speed ~3.9 mm/s; simulations confirmed flow during 135°→45° oscillation. Cilia length extended up to 5× the initial 1 mm at 18 mT. Cut-off (step-out) frequency increased from ~1.1 Hz at 9 mT to ~4.2 Hz at 18 mT. A 9×9 omnidirectional matrix steered pumping directions and transported solids; ultrasound Doppler visualized pumping of fresh porcine whole blood in enclosed channels. Liquid skin (total wetting): Ferrofluid wet, absorbed into, and penetrated silicone sheets, immobilizing iron oxide nanoparticles to create magnetic skins. The process, assisted by splitting into sub-droplets for uniform coverage, yielded: (i) a walking spider robot moving along a square trajectory (f = 4 Hz, Bm = 9 mT), (ii) a crawling caterpillar robot (two ferrofluid-coated ends under oscillation), and (iii) a swimming fish robot actuated by head oscillation inducing tail vortex shedding and forward thrust. A cilia-based robotic platform moved at ~0.3 mm/s under segmented fields (f = 2 Hz, Bm = 9 mT, θ = 30°). The liquid skin traversed a maze by jumping and stretching, then coated a target silicone sheet to enable underwater-to-surface transition and controlled locomotion.

By integrating wetting control with torque-driven ferrofluid droplet dynamics, the study demonstrates a unified platform for constructing multifunctional miniature soft machines. In low-wetting regimes, droplets exhibit rich, reprogrammable locomotion modes and reversible fission–fusion, enabling traversal of obstacles, confined channels, and complex biological environments. High-wetting anchoring with structured substrates yields liquid cilia that perform nonreciprocal strokes for directional pumping, with tunable length and beat characteristics to control flow rate and direction, including the handling of biofluids like whole blood. Under total wetting with silicone, droplets act as mobile skins that transform passive elastomeric components into magnetically actuated robots capable of walking, crawling, swimming, and complex environment transitions. These findings address limitations of solid magneto-elastomer robots by leveraging the extreme deformability, reconfigurability, and selective adhesion of ferrofluids, while expanding function from cargo delivery and lab-on-a-chip pumping to in situ robotic assembly and actuation. The quantitative characterization of wetting, contact angles, and adhesion, alongside field-programmable behaviors, provides design rules for selecting substrates and magnetic programs to achieve desired functions.

Ferrofluid droplets, when actuated by programmed alternating magnetic fields and coupled with controlled wetting at interfaces, can be reconfigured into versatile miniature soft machines. The platform supports multimodal motion, precise scale reconfiguration via fission–fusion, and robust adaptability in complex artificial and biological environments. At low wettability, droplets become liquid capsules for on-demand cargo delivery through tortuous ducts; at high wettability, they form liquid cilia arrays for wireless pumping and mixing; at total wetting, they serve as active liquid skins to convert passive elastomers into walking, crawling, and swimming robots. Compared to gradient-force-driven droplet systems and solid magneto-elastomers, torque-driven ferrofluid machines offer broader motion repertoires and dynamic reconfiguration. Future work could exploit stronger magnetic sources (e.g., Halbach arrays) to increase pumping speeds and downscale actuation, integrate other functional liquids (e.g., liquid metals), and program substrate wettability for reversible attachment/desorption to expand biomedical and microfluidic applications.

Experiments were primarily conducted in vitro or ex vivo phantoms and animal tissues; in vivo validation, long-term biocompatibility, and immune responses remain to be assessed. The silicone-wetting process forms a rigid nanoparticle layer requiring high fields (~300 mT) for detachment; removal involves IPA immersion and magnetic agitation. The syringe-based payload injection requires complete wetting between the needle and ferrofluid to prevent instability, limiting typical loads to ~0.5 µL in a 5 µL droplet. Magnetic actuation under X-ray fluoroscopy required a robotic arm with local coils due to Helmholtz system constraints on imaging volume. Liquid cilia performance (pumping speed, step-out frequency) depends on available field strengths; achieving higher throughputs and micron-scale cilia may require stronger magnets. Control of splitting number and uniformity depends on precise field programming and fluid properties, and environmental flows or substrate heterogeneities may affect repeatability.