Public defence in Engineering Physics, M.Sc. Xianhu Liu

Title of the thesis: Magnetic field-induced particle assembly and jamming

Doctoral student: Xianhu Liu
Opponent: Associate Professor Germán Salazar Alvarez, Uppsala University, Sweden
Custos: Professor Olli Ikkala, Aalto University School of Science, Department of Applied Physics

Ferromagnetic materials possess the remarkable ability to magnetize when exposed to an external magnetic field and exhibit swift responsiveness to magnetic stimuli. This makes them ideal carriers for responsive materials. Moreover, leveraging the assembly of ferromagnetic particles under magnetic influence enables the creation of diverse superstructures, catering to specific application needs.

In this thesis, ferromagnetic cobalt (Co) and nickel (Ni) particles with varying surface roughness were synthesized. These particles served as the foundation for magnetic field-induced assembly, resulting in the formation of unique superstructures. The distinctive traits of these superstructures were systematically characterized, and their potential applications were explored.

Initially, utilizing relatively smooth-surfaced ferromagnetic Co particles facilitated the assembly of weakly jammed superstructures under magnetic influence. The morphology of these superstructures could be adjusted by modifying the magnetic field strength, offering customizable sensitivity to pressure stimuli. To enhance jamming, ferromagnetic Ni particles with comparatively rougher surfaces were utilized. Increased interparticle friction lowered the critical packing density necessary for jamming. Consequently, tightly packed superstructures formed under the magnetic field. This robust jamming effect induced structural memory in the assembled superstructures, allowing for adjustable jamming through magnetic field kinetics and providing extensive sensitivity to stimuli. Lastly, the conversion of jamming-induced structural memory into electrical signals, further transformable into visible light signals, was demonstrated.

This assembled ferromagnetic particles system suggests an avenue for designing trainable and adaptable life-inspired materials, for example, for soft robotics and interactive autonomous devices.

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/ 

Doctoral theses at the School of Science: https://aaltodoc.aalto.fi/handle/123456789/52