Generation and application of sub-kilohertz oscillatory flows in microchannels
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RESEARCH PAPER
Generation and application of sub‑kilohertz oscillatory flows in microchannels Giridar Vishwanathan1 · Gabriel Juarez1 Received: 6 November 2019 / Accepted: 22 July 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract We present an accessible and versatile experimental technique that generates sub-kilohertz sinusoidal oscillatory flows within microchannels. This method involves the direct interfacing of microfluidic tubing with a loudspeaker diaphragm, which allows independent control of oscillation frequency and amplitude. Oscillatory flows were generated with frequencies ranging from 10 to 1000 Hz and amplitudes ranging from 10 to 600 μ m. Fourier spectral analysis of particle trajectories, obtained by particle tracking velocimetry, was used to evaluate the oscillatory displacement in microchannels and shown to accurately represent simple harmonic motion specified by the loudspeaker. Oscillatory flow profiles in microchannels of square and rectangular cross-sections were characterized as a function of oscillation frequency, or Womersley number, and compared to theoretical benchmarks, such as Stokes flow and Stokes’ second problem. To highlight the versatility and effectiveness of the experimental method, prototypical applications were demonstrated utilizing pulsatile flow in microfluidic devices, such as inertial focusing and enhanced mixing at low Reynolds numbers.
1 Introduction The addition of oscillatory flow to steady unidirectional flow in microfluidic devices have been shown to be useful in a range of applications such as mixing at low Reynolds numbers (Phelan et al. 2008; Ahmed et al. 2009; Frommelt et al. 2008), particle sorting and focusing (Thameem et al. 2016; Schmid et al. 2014; Marmottant and Hilgenfeldt 2003; Mutlu et al. 2018), enhancement of heat transfer (Qu et al. 2017), flow control (Leslie et al. 2009; Phillips et al. 2016), microrheology (Vishwanathan and Juarez 2019a, b), and chemical extraction (Lestari et al. 2016; Xie et al. 2015). Nevertherless, the widespread use and study of the oscillatory flow component in microchannels remains uncommon due to challenges of implementation. At low frequencies ( 0.1 ≤ f ≤ 10 Hz), oscillatory flows are usually achieved with a programmed syringe pump, electromechanical relay valves (Abolhasani and Jensen 2016) or a pneumatic pressure controller (Zhou and Schroeder 2016). The fidelity of the desired waveform is limited by inertia * Gabriel Juarez [email protected] 1
Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
of the oscillatory driver. For low frequencies, the response time of syringe pumps and actuators in electromechanical valves and pnuematic pressure controllers is on the order of O(10 ms) , therefore preventing the realization of sinusoidal oscillations at higher frequencies. Some purely fluidic oscillators using a constant pressure head have also been developed in this range of frequencies (Dang and Kim 2017). At high frequenci
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