Smalltalks "Controlling fluid flow direction in microfluidic systems through photothermal effects"

Abstract: 

In microfluidic environments, the transport of particles is typically governed by slow diffusion near interfaces. However, the introduction of localized fluid flow allows for active transport of suspended nano-objects within confined spaces. To achieve precise and dynamic control over fluid flow at the microscale, a promising approach is to harness photothermal effects by illuminating metallic or all-dielectric nanostructures. Previous research has demonstrated the induction of strong flow transients, with flow speeds reaching millimeters per second, when microscopic vapor bubbles nucleate on spatially isolated laser-heated plasmonic nanoantennas supported on a substrate. Nevertheless, in such structures, the flow pattern is cylindrically symmetric and always directed toward the nanoantenna at the substrate plane, limiting its applicability in, for example, particle manipulation schemes. In this context, we investigate the potential to manipulate the flow direction by adjusting the absorption characteristics of these metallic or dielectric nanostructures. Our research has shown 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 offer a means for rapid and precise mass transport near surfaces, with applications in microfluidics, bionanotechnology, and particle sorting.