Abstract
In the study of micro-scale biological flows, velocimetry methods based on passive tracers, such as micro-PIV and micro-PTV, are well established to characterize steady flows. However, these methods become inappropriate for measuring unsteady flows of small amplitude, because, on these scales, the motion of passive tracers cannot be distinguished from Brownian motion. In this study, we use optical tweezers (OTs) in combination with Kalman filtering, to measure unsteady microscopic flows with high temporal accuracy. This method is referred to as optical tweezers-based velocimetry (OTV). The OTV method measures the nanometric displacements of a trapped bead, and predicts the instantaneous velocity of the flow by employing a Kalman filter. We discuss the accuracy of OTV in measuring unsteady flows with 1.5–70 upmum s^{-1} amplitudes and 10–90 Hz frequencies. We quantify how the bead size and the laser power affect the velocimetry accuracy, and specify the optimal choices for the bead size and laser power to measure different unsteady flows. OTV accurately measures unsteady flows with amplitudes as small as 3–6 upmum s^{-1}. We compare the accuracy of OTV and micro-PTV, and characterize the flow regime for which OTV outperforms micro-PTV. We also demonstrate the robustness of OTV by measuring the unsteady flow created by the cilia of green alga Chlamydomonas reinhardtii, and comparing with numerical predictions based on Stokes equations. An open-source implementation of the OTV software in Matlab is available through the 4TU.Centre for Research Data.Graphic abstract
Highlights
Accurate flow velocimetry measurements are crucial in a range of applications from engineering to physics and biology
Established velocimetry methods, developed to measure and visualize flows, include pitot tubes, hot-wire anemometry as well as optical methods relying on passive tracer particles, Laser Doppler Anemometry (LDA), Particle Tracking Velocimetry (PTV) and Particle Image Velocimetry (PIV) (Adrian et al 2011; Lindken et al 2009)
We characterized the accuracy of the optical tweezers-based velocimetry (OTV) measurements for different bead sizes and corner frequencies of the optical tweezers (OTs)
Summary
Accurate flow velocimetry measurements are crucial in a range of applications from engineering to physics and biology. Tracer-based methods cannot accurately measure unsteady flows with low Péclet numbers, because the particle displacements due to the flow become comparable to the displacements due to thermal diffusion. Previous studies have used OTs as a positioning tool to facilitate flow measurements by trapping a particle at a given location, before releasing it to use it as a passive tracer for micro-PTV (Di Leonardo et al 2006; Leach et al 2006; Knöner et al 2005; Nève et al 2008; Padgett and Di Leonardo 2011). Videos of the microscopy are synchronously recorded by an sCMOS camera (LaVision PCO.edge) at 100–1000 Hz
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