Study of mechanical rectification effects in colloidal dispersions in a non-inertial frame

2024 Fiscal Year Final Research Report

• Project/Area Number: 22K14593
• Research Category: Grant-in-Aid for Early-Career Scientists
• Allocation Type: Multi-year Fund
• Review Section: Basic Section 29010:Applied physical properties-related
• Research Institution: Japan Atomic Energy Agency
• Principal Investigator: Umeda Maki 国立研究開発法人日本原子力研究開発機構, 原子力科学研究所 先端基礎研究センター, 研究職 (90914060)
• Project Period (FY): 2022-04-01 – 2025-03-31
• Keywords: 磁性 / 磁気回転効果 / 磁性流体

Outline of Final Research Achievements

The Barnett effect is a phenomenon in which mechanical rotation of a magnetic material causes its spin angular momentum to align along the rotation axis via spin-rotation coupling, resulting in magnetization. For materials with a known gyromagnetic ratio, the rotation speed of the magnet can be estimated from the strength of the Barnett field, an inertial field acting on the material. In this study, we developed a measurement system to detect the Barnett field acting on colloidal magnetic particles in a ferrofluid and measured their rotational velocity in a rotating frame. As a result, we found that the rotational speed of nanoparticles in the fluid exceeds that of the external rotation. Furthermore, we achieved control over the rotational speed depending on the solvent. These findings indicate that the particles in the rotating frame are driven by inertial and viscous hydrodynamic forces, demonstrating mechanical rectification of rotational motion in a fluid.

Free Research Field

スピントロニクス

Academic Significance and Societal Importance of the Research Achievements

磁気回転効果は物質中の角運動量と物体の運動を結びつける本質的な現象だが、従来は固体が中心だった。本研究は、磁性ナノ粒子を含む機能性流体に測定対象を拡張することで、慣性磁場を高感度に測定し、回転系における揺らぎ駆動の自発回転を初めて実証した。回転座標系下で熱揺動が整流されることでナノ粒子が角運動量を獲得する新しい力学機構を明らかにし、スピンと軌道の角運動量が流体中で結合する様子を観測した。本成果は、スピン流の非平衡ダイナミクスや非慣性系の流体力学の理解を深める基盤となる。社会的には、ナノスケール回転センサや流体駆動素子、対称性を利用した整流技術の創出に貢献する可能性を有する。