Observation of the dynamics of magnetic nanoparticles induced by a focused laser beam by using dark-field microscopy

• The dynamics of magnetic nanoparticles induced by a focused laser beam was investigated by using dark-field microscopy.
• The experimental results revealed that the dynamics of magnetic nanoparticles was dominated by different mechanisms.
• A convenient technique to measure the Soret coefficient of nanoparticles was provided.

The dynamics of Fe3O4 magnetic nanoparticles under the irradiation of a tightly focused laser beam was investigated by using a high-intensity dark-field microscopy. A depletion region of magnetic nanoparticles was found at the center of the laser beam where the dissipative force (absorption and scattering forces) dominated the dynamics of the magnetic nanoparticles. In contrast, the dynamics of magnetic nanoparticles was dominated by thermal and mass diffusions at the edge of the laser beam where the dissipative force was negligible. In addition, the transient variation in the concentration of magnetic nanoparticles was characterized by recording the transient scattering light intensity. The coefficients of thermal diffusion, mass diffusion and the Soret effect for this kind of magnetic nanoparticles were successfully extracted by using this technique.