Key influence factors in magneto-controlled motion of micro-nano graphite flakes.

Magneto-controlling micro-nano materials' motion is a promising way that enable the noncontact, remote, and nondestructive controlling of their macrostructure as well as functionalities. Here, an optical microscope with an electromagnet was constructed to in-situ monitor the magneto-controlled motion process microscopically. Taking micro-nano graphite flake (MGF) as a model system, we experimentally demonstrate the key factors which influence the magneto-controlling of materials' motion. First, the product of intensity and gradient of the magnetic field (B▽B) has been confirmed as the dominant driving force and the flipping direction of the MGFs is accordingly determined by the vector direction of B×▽B. Second, quantitatively comparative experiments further revealed that the threshold driving force has an exponential relationship with the structural aspect ratio (b/a) of MGFs. Third, the critical magneto-driving force is found as proportional to the viscosity of the solvent. In addition, we also discovered the delay effect, fatigue effects, and the multiple cycle acceleration effect in magneto-controlled flakes flipping. Accordingly, a dynamic model is developed that describes the flip of the diamagnetic flake under external magnetic field excitation considering the shape factor. It is shown experimentally that the model accurately predicts the flip dynamics of the flake under different magnetic field conditions. These findings can be used to achieve magneto-controlling materials' macrostructure as well as their functionalities.