Travel time after photobleaching velocimetry

Abstract

In interfacial science and microfluidics, there is an increasing need for improving the ability to measure flow velocity profiles in the sub-micrometer range to better understand transport phenomena at interfaces, such as liquid–solid interfaces. Current standard methods of velocimetry typically use particles as tracers. However, seed particles can encounter issues at liquid and solid interfaces, where charge interactions between particles and surfaces can limit their ability to measure near-wall flows accurately. Furthermore, in many flows, seed particles have a different velocity from that of their surrounding fluid, which the particles are intended to represent. Several molecular tracer-based velocimeters have been developed which can bypass these issues. However, they either have limited resolution for measurement near solid surfaces, such as for slip flows, or require pre-calibration. Laser-induced fluorescence photobleaching anemometry (LIFPA) is one such technique that is noninvasive and has achieved unprecedented nanoscopic resolution for flow velocity profile measurement. However, it also requires pre-calibration, which is unavailable for unknown flows. Here, we present a novel, calibration-free technique called travel time after photobleaching (TTAP) velocimetry, which can measure flow velocity profiles and near-wall flow with high spatiotemporal resolution. Furthermore, TTAP velocimetry is compatible with LIFPA, and thus, the two systems can be coupled to satisfy LIFPA’s long-anticipated need for pre-calibration, enabling measurement of flow velocity profiles in unknown flows with salient resolution.