Physics

Harnessing magnetic relaxation: the “Pacman effect” allows precise organization of superparamagnetic beads

The orbital representation of a magnetic colloid dipole (left) resembles a Pac-Man shape (right). Credit: Biswal lab/Rice University

Particles larger than ordinary molecules or atoms but invisible to the naked eye can form a variety of useful structures, including miniature propellers for microrobots, cell probes, and steerable microwheels designed for targeted drug delivery. Masu.

When Lisa Biswal and her team of chemical engineers at Rice University expose certain types of particles – micron-sized beads with special magnetic susceptibilities – to rapidly alternating rotating magnetic fields, the particles form orientation-dependent structures. discovered that it is organized into Or anisotropic. This discovery is important because the anisotropy can be tuned to develop new customizable material structures and properties.

“Our key discovery is that by changing the direction of rotation of the magnetic field with each rotation, we can create an anisotropic interaction potential between particles that has not been fully realized before.” said Aldo Spatafora Salazar of Chemistry and Biomolecules. He is an engineering research scientist at the Biswal Institute and one of the lead authors of a study published in the Proceedings of the National Academy of Sciences.

Dana Lobmeyer, the study’s other lead author, said the particles investigated in this study are collectively known as superparamagnetic colloids, and their responsiveness to magnetic fields allows them to function in a targeted manner. It has become a popular component of high-performance materials.

“This discovery is important for bottom-up advanced materials design, especially because it focuses on an aspect of colloid-magnetic field interaction that is usually overlooked: magnetic relaxation time,” Biswal said. said Lobmeyer, a Rice graduate who received the award. .

Relaxation time refers to the delay in the bead’s magnetic response to a change in the direction of the magnetic field. The researchers hypothesized that this delay and the effects of the alternating magnetic field influence the bead interactions, causing the beads to be arranged in a crystal lattice in two dimensions and to form elongated, aligned clusters in three dimensions. I erected it.

Harnessing magnetic relaxation: the “Pacman effect” allows precise organization of superparamagnetic beads

Microscopic image of cluster formation (left) and graphical representation of cluster arrangement (right). Credit: Biswal lab/Rice University

“The delay in the magnetic response of superparamagnetic beads, or magnetic relaxation time, was previously thought to be negligible, but by taking it into account and combining it with the effect of an alternating magnetic field, we can precisely control the magnetic response. “We found it to be a powerful way to analyze particles,” said the study’s corresponding author, William M. McCardell, Rice professor of chemical engineering, professor of materials science and nanoengineering, and senior associate in faculty development. Director Biswal said.

The study included a combination of experiments, simulations, and theoretical predictions. Experimentally, the research team examined both concentrated and dilute bead suspensions in combination with alternating magnetic fields of different strengths and frequencies.

“The concentrated beads formed elongated, aligned clusters, and we analyzed how different parameters affected their shape,” said Spatafora-Salazar. “The diluted suspension simplified the system and allowed us to study interactions between two beads, a version of the system known as a dimer.”

Experimental insights from dimers helped explain alignment and elongation in larger clusters. However, the experimental data agreed with the simulations only when magnetic relaxation time measurements (which will form the subject of another future study) are taken into account.

One interesting twist to the data was the Pac-Man shape described by the distribution of magnetization of the beads. In the magnetized state, each bead acquires a dipole, a pair of negative and positive charges, like a north-south axis.

In response to a rotating magnetic field, the dipole moves like a compass needle, aligning all the beads in the same direction. However, due to magnetic relaxation, the needle does not rotate a full 360 degrees, leaving behind a portion that appears as Pac-Man’s mouth when the data is mapped.

“The interaction is weakest along the mouth, but strongest along the head, causing alignment of dimers and clusters,” Lobmeyer said. “We would not have been able to understand this phenomenon without departing from the traditional assumptions used to study these beads.”

Further information: Aldo Spatafora-Salazar et al, Aligned colloid clusters in alternating current rotating magnetic fields revealed by magnetic relaxation, Proceedings of the National Academy of Sciences of the United States (2024). DOI: 10.1073/pnas.2404145121

Provided by Rice University

Citation: Harnessing magnetic relaxation: ‘Pacman effect’ enables precise organization of superparamagnetic beads (October 4, 2024) https://phys.org/news/2024-10-harnessing- Retrieved October 5, 2024 from magnetic-pac-effect-enables.html

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