Tilting and tumbling of Janus nanoparticles at sheared interfaces

We use molecular dynamics simulations to investigate the response of individual Janus nanoparticles adsorbed at liquid-fluid interfaces to imposed symmetric shear. Depending on particle shape, amphiphilicity, and the applied shear rate, two distinct rotational dynamics toward a steady particle orientation are observed: smooth tilting, and tumbling. Particles adopt a steady orientation when imposing shear-induced torque is balanced with the opposing capillary-induced one. The tumbling dynamics can be perhaps explained based on the free energy of different states relative to the thermal energy. We construct phase diagrams correlating particle dynamics to shear rate, hydrophobicity, and shape. Our results have direct implication on flow-induced alignment and assembly of Janus particles at fluid interfaces.