Researchers have discovered that when a specific type of particles—tiny beads with unique magnetic properties—are exposed to a quickly changing, rotating magnetic field, they tend to form structures that depend on direction, known as anisotropic structures. This discovery is important because anisotropy can be adjusted to create new material structures with varying properties.
These particles are larger than regular molecules or atoms but too small to be seen without a microscope, allowing them to form various useful structures like tiny propellers for microrobots, probes for cells, and controllable microwheels for targeted drug delivery.
A team of chemical engineers from Rice University, led by Lisa Biswal, has identified that exposing these specially magnetized micron-sized beads to a rapidly changing rotating magnetic field prompts them to self-assemble into anisotropic arrangements. This is an important breakthrough since it allows for the manipulation of anisotropy to engineer new material properties and structures.
“Our main finding is that by changing the rotational direction of the magnetic field after each turn, we can create an anisotropic interaction potential between these particles, which has not been fully explored until now,” explained Aldo Spatafora-Salazar, a research scientist in Biswal’s lab and a lead author of the study published in the Proceedings of the National Academy of Sciences.
The particles examined are known as superparamagnetic colloids, which respond strongly to magnetic fields and are valuable for developing advanced materials with specific functionalities, according to Dana Lobmeyer, the other first author of the research.
“This discovery is crucial for the design of advanced materials at a granular level, especially since we focused on a commonly overlooked aspect of colloids’ interaction with magnetic fields—magnetic relaxation time,” she noted, having earned her doctorate at Rice under Biswal’s guidance.
Magnetic relaxation time refers to the lag in the beads’ response to shifts in the magnetic field direction. The researchers proposed that this delay, combined with the rotating magnetic field, plays a significant role in how the beads interact, leading to a two-dimensional crystal lattice formation and the creation of aligned, elongated clusters in three dimensions.
“The delayed response of superparamagnetic beads was previously seen as unimportant, but our findings show that considering it alongside the alternating magnetic field’s effects allows for greater control over the particle arrangements,” stated Biswal, the study’s lead author and a prominent professor at Rice University.
The study incorporated a mix of practical experiments, simulations, and theoretical analyses. In their experiments, the team examined both dense and dilute suspensions of beads combined with magnetic fields of varying strengths and frequencies.
“In concentrated settings, the beads formed elongated, aligned clusters, and we investigated how different factors influenced their shape,” Spatafora-Salazar said. “Dilute suspensions provided a clearer view, allowing us to focus on the interactions between two beads, referred to as a dimer.”
Insights from the dimer experiments helped clarify how larger clusters align and elongate. However, the match between experimental data and simulations was accurate only after accounting for the measured magnetic relaxation time, which will be detailed in an upcoming study.
Interestingly, the data revealed a Pac-Man shape in the magnetization distribution of the beads: when magnetized, each bead becomes a dipole, having a positive and negative side similar to a north-south orientation. In the presence of a rotating magnetic field, these dipoles act like compass needles, aligning together. However, due to magnetic relaxation, these needles do not rotate a complete 360 degrees, which appears as a “mouth” when visually interpreted.
“The interactions are weakest along the mouth but strongest at the head, which leads to the alignment of dimers and clusters,” said Lobmeyer. “Understanding this behavior required us to move away from traditional assumptions used to study these beads.”
Additional authors of the study include Rice alumni Lucas H.P. Cunha ’23 and former postdoctoral fellow Kedar Joshi. This research was funded by the National Science Foundation and the ACS Petroleum Research Fund.
