Controlled splitting and focusing of a stream of nanoparticles in a converging-diverging microchannel.

We demonstrate the potential of a converging-diverging microchannel to split a stream of nanoparticles towards the interfacial region of the dispersed and the carrier phases, introduced through the middle inlet and through the remaining two inlets respectively, while maintaining a low Reynolds number limit (<10) for the flow of both phases. In addition to the splitting of passive tracer particles, such as polystyrene beads as used herein, the present setup has the potential to be utilized for a controlled reaction and thereby the separation of products towards an intended location, as observed from the experimentation with silver-nanoparticles and hydrogen-peroxide solution. Moreover, the microscale dimension of the channel allows controlled deposition of the reaction product over the bottom surface of the channel, allowing the possibility of bottom-up fabrication of microscale features. We unveil the underlying hydrodynamics that lead to such behaviours through numerical simulations, which are consistent with the experimental observations. The phenomenological features are found to be guided by the splitting of the intrinsic streamlines conforming to the flow geometry under consideration.