Brownian Dynamics Simulations of Carbon Nanotubes Breaking During Sonication

The extraordinary mechanical, thermal, and electrical properties of carbon nanotubes (CNTs) are well-known; however, the processing of CNTs into high-performance macroscopic materials is hampered by CNTs' poor solubility in virtually all common aqueous and organic solvents. CNTs naturally tend to form insoluble bundles and ropes due to van der Waals interactions. One of the most common approaches to liquid-phase dispersion is ultrasonication, which breaks the bundles apart. Stabilizing agents such as surfactants and polymers are then used to prevent the CNTs from reaggregating. However, a number of experiments indicate that the average length of CNTs in solution decreases with sonication time; this is due to CNT breaking due to the extreme flow field around cavitating bubbles.

We couple Brownian Dynamics simulations of CNTs with a model for the formation and collapse of bubbles during sonication. These simulations show that CNTs are prone to breakage when they become oriented in a direction tangent to the bubble wall during bubble growth; when the bubble collapses, such CNTs become compressed and can break. This contradicts previously-suggested mechanisms for CNT shortening. The simulations suggest phase diagram for nanotube breakage as a function of length and stiffness; we compare simulation results to experimental data for the scaling of average length as a function of sonication time.