Optical tweezers-based velocimetry: a method to measure microscale unsteady flows
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RESEARCH ARTICLE
Optical tweezers‑based velocimetry: a method to measure microscale unsteady flows P. Ghoddoosi Dehnavi1 · D. Wei2 · M.‑E. Aubin‑Tam2 · D. S. W. Tam1 Received: 10 June 2020 / Revised: 24 July 2020 / Accepted: 3 August 2020 © The Author(s) 2020
Abstract In the study of micro-scale biological flows, velocimetry methods based on passive tracers, such as micro-PIV and microPTV, 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 μm 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 μm 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
* D. S. W. Tam [email protected] Extended author information available on the last page of the article
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1 Introduction 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). Determining the appropriate velocimetry method strongly depends on the flow: the characteristic length scale L, the velocity magnitude U, and the timescale of the velocity variations 𝜏 . On the micron scale, flow velocity measurements are widely preformed using methods based on passive tracer particles such as micro-PIV (Akbaridoust et al. 2018; Adhikari et al. 2016; Kowalczyk et al. 2007; Kondratieva et al. 2008; Yan et al. 2007; Guasto et al. 2010; Drescher et al. 2010; Williams and Wereley 2009), micro-PTV (Chen et al. 2014; Park and Kihm 2006), and micro-holographic-PTV (Lee et al. 2016; Kim a
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