Particles larger than odd molecules or atoms yet still sufficiently small to be invisible to the naked eye may give rise to many alternative sorts of useful structures reminiscent of tiny propellers for microrobots, cellular probes and steerable microwheels for targeted drug delivery.
A team of Rice University chemical engineers led by Lisa Biswal has found that exposing a certain class of such particles — micron-sized beads endowed with a special magnetic sensitivity — to a rapidly alternating, rotating magnetic field causes them to prepare into structures which can be direction-dependent or anisotropic. The finding is critical because anisotropy may be manipulated to create latest, tunable material structures and properties.
“Our key finding is that by alternating the direction of the rotation of the magnetic field after each revolution, we will create an anisotropic interaction potential between particles, which has not been fully realized before,” said Aldo Spatafora-Salazar, a chemical and biomolecular engineering research scientist within the Biswal lab and certainly one of the lead authors on a study concerning the research published in Proceedings of the National Academy of Sciences.
Dana Lobmeyer, the opposite first creator on the study, explained that the particles under scrutiny within the study are collectively generally known as superparamagnetic colloids whose responsiveness to magnetic fields makes them a preferred constructing block for high-performance materials with tailored functionality.
“This discovery is critical for bottom-up advanced materials design, especially because we honed in on a side of the interaction between the colloids and magnetic fields that is often ignored — magnetic rest time,” said Lobmeyer, a Rice doctoral alumna advised by Biswal.
The comfort time refers back to the delay within the beads’ magnetic response to changes in field direction. The researchers hypothesized that this delay combined with the effect of the alternating magnetic field affects the beads’ interactions, causing them to rearrange right into a crystal lattice in two dimensions and to form elongated, aligned clusters in three dimensions.
“The delayed magnetic response, or magnetic rest time, of superparamagnetic beads was previously considered negligible, but what we found is that taking it under consideration and coupling it with the effect of the alternating magnetic field is a strong strategy to exercise precise control over the particles,” said Biswal, the corresponding creator on the study and Rice’s William M. McCardell Professor in Chemical Engineering, professor of materials science and nanoengineering and senior associate dean for faculty development.
The research involved a mixture of experiments, simulations and theoretical predictions. Experimentally, the team checked out each concentrated and dilute bead suspensions combined with alternating magnetic fields of various intensities and frequencies.
“Concentrated beads formed elongated, aligned clusters, and we analyzed how different parameters influenced their shape,” said Spatafora-Salazar. “Dilute suspensions simplified the system, allowing us to check interactions between two beads — a version of the system generally known as a dimer.”
Experimental insights from dimers helped explain the alignment and elongation in larger clusters. Nevertheless, experimental data only matched simulations once the magnetic rest time measurements (which form the topic of a separate forthcoming study) were considered.
One fun twist to the info was the Pac-Man shape described by the distribution of a bead’s magnetization: In a 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 of the beads along the identical orientation. Nevertheless, attributable to magnetic rest, the needle doesn’t turn a full 360 degrees, leaving what shows up as Pac-Man’s mouth when the info is mapped out.
“The interactions are weakest along the mouth but strongest along the pinnacle, causing the alignment of dimers and clusters,” Lobmeyer said. “We’d not have been capable of understand this phenomenon without deviating from the standard assumptions used to check these beads.”
Other authors include Rice alumni Lucas H.P. Cunha ’23 and former Rice postdoctoral fellow Kedar Joshi. The research was supported by the National Science Foundation (214112, 1828869) and the ACS Petroleum Research Fund (65274-ND9).
