Acoustofluidics exfoliation mechanism of metal-organic framework nanosheets

Liquid-phase exfoliation using surface acoustic waves (SAWs) has been applied to two-dimensional transition metal materials and layered metal-organic frameworks (MOFs). Nevertheless, the underlying mechanisms are not fully understood. According to the proposal, the acoustic stripping mechanism is mainly caused by the effect of fluid shear force on bulk layered materials induced by acoustic streaming. Due to the lack of actual real-time in situ experimental evidence, most of the research described to date has been inconclusive. This paper reports that the motion of MOF particles is driven by acoustic streaming generated by standing SAWs in a de-ionized water droplet. The distributions of acoustic pressure and acoustic streaming inside a droplet, and the trajectories of polystyrene beads of different diameters under the influence of acoustic radiation force and drag force induced by acoustic streaming were calculated by numerical simulation to predict the motion of MOF particles. The whole process of MOF exfoliation is observed using a high speed camera. The dynamic sequence of exfoliation events was determined using ultrafast optical imaging, which has not been reported before. The exfoliation of MOFs is not only caused by acoustic streaming but also by particle-particle collisions, as shown by the in situ observation.