Magnetic oil droplets with switchable rolling and swimming modes enable adaptive navigation, particle transport, and control in chemically varied environments.
(Nanowerk Spotlight) Artificial microswimmers have emerged as powerful tools for studying dynamic transport, fluid-interface interactions, and autonomous sensing. One class of such systems—self-propelled oil droplets suspended in surfactant solutions—has been widely explored as a model for chemically driven motility. These droplets can mimic aspects of biological behavior, such as chemotaxis, by using internal and surface flows to move along concentration gradients. But despite this biomimicry, synthetic droplets remain highly constrained by their chemical environment.
In particular, their motion is tightly coupled to the availability of surfactants, limiting their utility in regions where chemical fuel is sparse or gradients are weak. They also lack the adaptive motion strategies that biological cells use to switch between different forms of locomotion depending on physical or chemical cues.
Attempts to overcome these constraints have typically focused on tuning the chemistry of the droplet–surfactant system. Adjusting the oil phase or modifying the surfactant can change propulsion speed and direction, but such approaches still tether motion to environmental chemistry. In parallel, research in microrobotics has shown that magnetic fields can be used to control the movement of microstructures such as solid microrollers.
These systems are mechanically robust and externally steerable, but they lack the soft, responsive interfaces of fluid droplets and cannot autonomously react to their surroundings. Bridging these two approaches—combining the adaptive interfacial dynamics of soft droplets with the programmability of magnetic actuation—has remained a major challenge.
In a study published in Advanced Science (“Surface Rolling Active Magnetic Emulsions”), researchers from the Max Planck Institute for Intelligent Systems and Koç University report a hybrid system that achieves precisely this integration. They engineered oil droplets containing freely rotating clusters of ferromagnetic nanoparticles. These droplets can autonomously swim via chemically induced interfacial flows or, when placed in a rotating magnetic field, roll along solid surfaces. This dual-mode propulsion allows droplets to move in both fuel-rich and fuel-deficient environments, and to switch between locomotion strategies depending on the spatial constraints and chemical landscape.
Switchable two locomotion modes of active magnetic droplets augment chemotactic navigation in confined spaces. a) Schematic depicting the motion of a self-propelling droplet along a chemical gradient and interaction with obstacles on its way. Visualization of chemical gradients using fluorescence microscopy in b) chemically isolating walls and c) diffusion of surfactants through pillars in chemically non-isolating pillars. d) Trajectory of two leading droplets that successfully navigate through a chemically isolating environment. e) Trajectory of four leading droplets navigating in a nonisolating environment, where all droplets get stuck at pillars and fail to navigate through the region. f) Trajectory of a droplet successfully navigating through the chemically non-isolating environments by switching between locomotion modes. g) Schematic depicting switchable locomotion enabling entry into narrow channels. h–i) Droplet accelerating towards the channel via surface rolling. Entry into the channel via surface rolling is prohibited by the flow field around the rolling droplet. j) Upon switching off the magnetic field, the droplet enters the channel due to propulsion driven by Marangoni stresses. Trajectories in (h-j) are color coded in instantaneous speed, with color representing speeds in μm s−1. Scale bar represents 1 mm in (d), 500 μm in (h), and 100 μm in (i,j). (Image: Reprinted from DOI:10.1002/advs.202501866, CC BY) (click on image to enlarge)
At the core of the system is a compact cluster of FePt nanoparticles encapsulated inside the droplet. These clusters can rotate freely within the oil phase. When an out-of-plane rotating magnetic field is applied, the cluster spins, generating rotational flow inside the droplet. This internal circulation couples to the nearby surface, breaking symmetry and causing the droplet to roll along the substrate. At surfactant concentrations below 1 wt%, where Marangoni propulsion is negligible, this rolling mechanism enables the droplets to translate effectively—even in the absence of chemical gradients.
Experiments showed that the rolling speed increases with magnetic field frequency up to a point. For 80-micron diameter droplets, speeds rise linearly with frequency up to around 30 Hz, after which they plateau. This saturation occurs because, at higher frequencies, the magnetic cluster begins to fragment into smaller rotating units, weakening the coherence of the induced flow. The team confirmed this behavior by observing the internal structure of the cluster at various frequencies and correlating it with translational speed.
They also showed that droplet viscosity plays a critical role: droplets with low-viscosity silicone oil (10 cP) could sustain rolling up to 140 Hz, while those with high-viscosity oil (100 cP) stopped responding above 30 Hz. The internal viscosity of the droplet imposes a limit on how fast the cluster can rotate before decoupling from the applied field.
Computational fluid dynamics simulations supported the experimental findings. By modeling the droplet as a viscous sphere with a rotating internal cluster, the researchers quantified how torque is transmitted to the droplet interface. The model incorporated parameters such as viscosity ratio, droplet and cluster radius, and lubrication distance between the droplet and substrate. Simulations showed that rotational flows were the dominant contributor to rolling motion, and that as the lubrication distance increased—due to higher lift forces at greater speeds—the effective rolling efficiency dropped. This matched the experimental observation that the slipping coefficient (a dimensionless measure of rolling efficiency) decreases with increasing magnetic field frequency.
At higher surfactant concentrations (20 wt%), the same droplets autonomously propel via Marangoni flows caused by uneven interfacial tension. In this mode, propulsion results from gradients in surfactant adsorption and dissolution of oil into the surrounding fluid. The droplets swim along these gradients without external input. However, when a rotating magnetic field is introduced, the rolling flow competes with and can eventually overpower the spontaneous propulsion. The researchers found that at 20 Hz, the droplet transitions rapidly to rolling, overriding the Marangoni effect. At lower frequencies, the two flows interfere, producing erratic or oscillatory motion.
This controllable switching between swimming and rolling was tested in various constrained geometries. In confined channels with patterned chemical gradients, self-propelling droplets could navigate toward higher surfactant concentrations. However, when multiple droplets were introduced, trailing droplets encountered repulsive gradients created by the trails of leading droplets, which caused them to become trapped.
The magnetic field enabled selective recovery: rolling allowed a trapped droplet to escape by moving against the chemical gradient, and then resume swimming once a clear path was found. This capability is particularly relevant for environments with non-uniform permeability, such as arrays of micropillars where local concentration traps form due to diffusion through the gaps.
The researchers also explored the interaction between the droplets and external micro-objects. Rolling droplets were able to collect and transport polystyrene particles (3 microns in diameter) along their path. Upon switching back to Marangoni propulsion, these cargo particles were released. This mechanism was used to create defined patterns of particles by controlling the direction of rolling with the magnetic field.
In a separate experiment, the team demonstrated adsorption and release of chemically modified silica particles on the droplet interface. As more particles accumulated on the surface, the speed of propulsion decreased, reflecting the added drag. Rolling was again used to release the particles efficiently.
Importantly, this hybrid propulsion is reversible and minimally disruptive. The shape and interfacial structure of the droplets remained stable across multiple transitions. There was no evidence of droplet shrinking or significant solubilization during short-term rolling, and the system maintained performance across different oil chemistries. These features suggest a high degree of robustness and versatility for further exploration.
The ability to integrate external magnetic control with internal autonomous behavior offers a flexible platform for navigating heterogeneous environments. Droplets can be guided through traps, manipulate micro-objects, and adapt to different spatial conditions by switching propulsion strategies. While the current implementation uses a global magnetic field—limiting control to one droplet at a time—future designs with localized or programmable fields could enable coordinated multi-droplet systems.
This work provides a clear example of how combining synthetic and naturalistic flow mechanisms can expand the functionality of soft microscale systems. Potential applications include programmable drug delivery, responsive materials, and in situ sensing. Further studies could explore how these droplets behave in more complex fluids, at higher flow rates, or under collective conditions. But even in their current form, the ability to dynamically switch between autonomous swimming and externally guided rolling makes these droplets a promising tool for navigating and interacting with the microscale world.
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