Directional Control of Transient Flows Generated by Thermoplasmonic Bubble Nucleation

It has previously been shown that strong flow transients, reaching mm/s flow speeds, are induced when microscopic vapor bubbles nucleate on spatially isolated laser-heated plasmonic nanoantennas supported on a substrate. However, the flow pattern is cylindrically symmetric and always directed toward the nanoantenna at the substrate plane. This limits its applicability in, for example, particle manipulation schemes. Here, we show that the flow direction can be locally reversed by breaking the photothermal symmetry using two nearby nanoantennas that differ either in size or polarization response. The in-plane flow transient is strong enough to push microparticles tens of microns across a surface. Directional flow control may provide the means for rapid and precise mass transport near surfaces for applications in microfluidics, bionanotechnology, and particle sorting.