Scientists demonstrate precise control over artificial microswimmers using electric fields

In a new study in Physical Review Letters, scientists have demonstrated a method to control artificial microswimmers using electric fields and fluid flow. These microscopic droplets could pave the way for targeted drug delivery and microrobotics.

In the natural world, biological swimmers, like algae and bacteria, can change their direction of movement (or swimming) in response to an external stimulus, like light or electricity. The ability of biological swimmers to change directions in response to electrical fields is known as electrotaxis.

Artificial swimmers that can respond to external stimuli can be extremely helpful for targeted drug delivery applications. In this study, researchers chose to model artificial swimmers that respond to electric fields.

Phys.org spoke to the co-authors of the paper: Ranabir Dey, an Assistant Professor at the Indian Institute of Technology Hyderabad; and Corinna Maaß, an Associate Professor at the University of Twente. Both were formerly at the Max Planck Institute for Dynamics and Self-Organization Göttingen, where the study germinated.

Speaking of their motivation behind the study, Prof. Dey said, “The physics driving active, intrinsic motion is fascinatingly rich and different from the one governing passive, externally driven matter, and we find many complex, even counterintuitive phenomena.”

Prof. Maaß added, “Discovering the working principle behind such effects in a simple model system can help us understand and control far more complicated, even biological systems.”

Artificial swimmers

Artificial swimmers mainly belong to two categories, active colloids (also known as Janus particles) and active droplets. They are called “active” because they move in response to a stimulus.

Janus particles, named after the two-faced Roman god Janus, have two distinct surfaces with different chemical or physical properties. The design allows these surfaces to have an asymmetry for self-propulsion. For example, one side might attract water while the other repels it.

However, Janus particles require specialized materials, external stimuli to move, and asymmetry complications. They can be challenging to study and work with.

Active droplets, on the other hand, are much simpler in structure. They are oil-based droplets suspended in an aqueous solution. They do not require external stimuli to self-propel, instead relying on internal reactions.

External stimuli like electric fields can be used to change their motion, making them very useful in confined environments like microchannels, which are narrow channels often used in lab-on-a-chip devices and microfluidic systems.

Electrotaxis in artificial swimmers is understudied, especially in confined spaces involving flowing fluids (like microchannels). Electrotaxis offers advantages over other taxis, such as the ability to be instantly turned on and off, adjusting the swimmers’ motion for direction and speed, and it can also be scaled to operate over short and long distances.

Biological swimmers respond naturally to electric fields generated by potential differences across cellular boundaries or tissue structure. However, artificial swimmers don’t, and must be engineered to do so.

Active droplets in microchannels

The researchers aimed to study how active droplets respond to external electric fields in confined microchannels.

“Swimmers have to communicate with the world outside their local environment via interactions with the system boundaries. Imagine guiding a swimmer along a channel—one might want to avoid the swimmer crashing into or adhering to the walls, reorienting it in a specific direction, or staying in a specific area,” explained Prof. Maaß.

Prof. Dey added, “This can be engineered for a wide range of swimmers by choosing appropriate values for an externally applied flow and electric field in the channel.”

The researchers used oil droplets containing a compound called CB15 (commonly used for active droplet studies) mixed in with a surfactant. These droplets were placed in microchannels, with electrodes placed at the ends to apply electric fields. The radius of these droplets was roughly 21 micrometers.

Along with the electric field, the researchers could also control the fluid flow, i.e., the pressure for more comprehensive control. The voltage varied up to 30 volts.

To analyze the trajectories of the active droplets, the researchers used video tracking and particle image velocimetry, which can measure the velocities in fluid flows.

Additionally, they developed a hydrodynamic model incorporating the droplet’s surface charge, movement direction, flow interactions, and electric field orientation to predict electrotactic dynamics.

Controlling flow and electric fields

The experiment found that the droplets showed a range of responses to the varying electric field. The researchers observed that the active droplets perform U-turns when the electric field opposes their motion. They also noted that the velocity of the droplets increases with the strength of the electric field.

By controlling the electric field in conjunction with the flow, the researchers could direct the precise motion of the droplets. This is known as electrorheotaxis.

When the electric field opposed the flow of the droplets, their oscillatory motion was reduced, and the researchers were able to achieve stable centerline swimming.

When the electric field aligned with the flow of the droplets, the researchers were able to maintain upstream swimming with modified oscillations. At high voltages, this switched to downstream swimming, following the wall of the microchannel.

The hydrodynamic modeling revealed the reason behind the motion of the droplets in the electric field. They found that these droplets carry an inherent electric charge, which affects their movement when exposed to an electric field.

They further found that the channel walls also played a role in affecting the droplets’ movement, due to their interactions with the surrounding fluid dynamics. The observed data aligned well with the predictions made by the researchers’ hydrodynamic model.

“We demonstrated that tuning two parameters (flow and electric field) gives access to a distinct number of motility states, encompassing upstream oscillation, wall and centerline motion, and motion reversal (U-turns),” said Prof. Dey.

Potential for more

The study demonstrates that simple droplets can mimic complex biological behaviors, making it a very promising avenue for biomedical applications.

Electric field and pressure-driven flow are readily available methods, which makes this application extremely appealing.

Discussing potential applications, Prof. Maaß said, “Since these guidance principles apply to any swimmer with surface charges in a narrow environment, they could be used to guide motile cells in medical applications, lab-on-a-chip or bioreactor scenarios, and in the design of motile carriers, such as microreactors or intelligent sensors.”

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