Bubble Dynamics and Directional Marangoni Flow Induced by Laser Heating of Silicon Nanodisk Arrays

Gold nanostructures have been extensively used as photothermal heat sources in a variety of studies due to their chemical inertness, biocompatibility, and advantageous thermoplasmonic properties. However, gold nanostructures are prone to surface melting and thermal deformation, which, in some cases, limit their applicability. In this study, we investigate micrometer-sized amorphous silicon nanodisk arrays as a stable and biocompatible alternative for the particular application of photothermally induced microbubble formation and generation of strong directional Marangoni flows in water. By using time-modulated continuous-wave laser heating, we show that the induced flows can move microparticles tens of micrometers across a substrate surface. The direction of particle movement can be preselected by utilizing asymmetric pairs of nanodisk arrays as heat sources or dynamically controlled by altering the laser spot position relative to a symmetric pair of arrays. We also demonstrate that average bubble size and particle displacement positively correlate and crucially depend on the laser modulation frequency. These results are discussed in terms of the temporal dynamics of bubble growth following nucleation. Our findings highlight the potential of using silicon nanostructures as substrates for generating strong thermocapillary flows on the micrometer scale, with potential applications in chemical mixing, pumping, particle sorting, and mass transport.