Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session B35: Microscale Flows: Oscillations 
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Chair: Henry Chu, Carnegie Mellon Room: 617 
Saturday, November 23, 2019 4:40PM  4:53PM 
B35.00001: The force on a slender particle under oscillatory translational motion in unsteady Stokes flow Jason Kabarowski, Aditya Khair Asymptotic approximations are derived for the hydrodynamic force on a rigid, axisymmetric particle executing longitudinal or transverse oscillation in unsteady Stokes flow. The slender particle has an aspect ratio $\epsilon = a/L << 1$, where $L$ is the halflength of the particle, and $a$ is its characteristic crosssectional width. The frequency of oscillation is parameterized by the complex quantity $\lambda^2 = i L^2 \omega / \nu$, where $\nu$ is kinematic viscosity, $\omega$ is particle oscillation frequency, and $i = \sqrt{1}$. Asymptotic approximations for the force are obtained in three frequency regimes: (i) low frequency, $\epsilon  \lambda  << 1$; (ii) moderate frequency, $\epsilon  \lambda  \sim O(1)$; and (iii) high frequency, $\epsilon \lambda>> 1$. Physical interpretations of the force in each regime are made and compared between the longitudinal and transverse oscillation cases. Our asymptotic predictions are in good agreement with the numerically computed frequencydependent force on a prolate spheroid ($\epsilon = 0.1$) for longitudinal and transverse oscillations by Lawrence and Weinbaum (J. Fluid Mech., vol. 189, 1988) and Pozrikidis (Phys. Fluids, vol. 1, 1989), respectively. [Preview Abstract] 
Saturday, November 23, 2019 4:53PM  5:06PM 
B35.00002: Rectified inertial forces in oscillatory flows: theory, simulation, experiment Siddhansh Agarwal, Fan Kiat Chan, Mattia Gazzola, Sascha Hilgenfeldt While the longtime effects of oscillatory interface motion on fluid elements are well understood (leading to rectified flow known as steady streaming), the rectification of forces on particles in such flows is much less understood, and of immediate practical relevance. In previous work, we analytically described physical effects leading to steady forces of particle attraction to or repulsion from the interface in a setting where streaming is negligible and for weak viscous effects. Here we present results from direct numerical simulations of the NavierStokes equations quantifying rectification forces on spherical particles in a wider range of situations. The outcomes suggest an improved version of the theory that more generally and systematically takes into account inertial forces on particles in oscillatory flows, revealing the dependence of these inertial forces on the strength of background flow gradients as well as on the particle size and density contrast. Comparison is also made with experiments using oscillating microbubbles. The improved fundamental understanding of attractive/repulsive forces manipulating microparticles benefits experimental design in many applications. [Preview Abstract] 
Saturday, November 23, 2019 5:06PM  5:19PM 
B35.00003: A simple and effect method for generating subkilohertz oscillatory flows in microchannels Gabriel Juarez, Giridar Vishwanathan Oscillatory flows present a wide range of exciting possibilities in microfluidics ranging from propulsion to trapping to mixing. Explorations involving oscillatory flows in microfluidics are often made challenging by the need for fabrication of complex microfluidic devices or miniaturized actuators.~We report the construction and evaluation of a detachable speakerbased apparatus capable of producing highly sinusoidal oscillatory flows in microchannels with frequencies ranging from 201000 Hz and maximum amplitudes at least as large as 100 microns. Useful applications using very simple microchannel geometries are demonstrated. [Preview Abstract] 
Saturday, November 23, 2019 5:19PM  5:32PM 
B35.00004: New insights into the migration patterns of a single capsule flowing in a pulsating microchannel. Ali Lafzi, Amir Hossein Raffiee, Sadegh Dabiri Investigating the migration patterns of deformable capsules in inertial microchannels has been of great interest among researchers duo to its numerous biological applications such as sorting, separation, and filteration of cells. A huge drawback in conventional microfluidics is the inability to focus extremely small particles in the order of nanometers due to the requirement of designing a practically impossible elongated microchannel. Exploiting an oscillatory flow is one solution to this issue where the length that the capsule needs to travel to focus is virtually extended beyond the physical length of the device. Here we present results of simulation of such oscillatory flow of capsules in microchannels. The aforementioned improvement has been observed for capsule deformabilities and mechanisms to induce the pulsation used in our study. However, there is a limit to the system throughput beyond which, there is no single focal point for the capsule. Another advantage of having a pulsating microchannel is the ability to control the capsule focal point by changing the oscillation frequency according to the cases presented in this study. The capsule focusing point also depends on its deformability, flow rate, and the form of the imposed periodic pressure gradient. [Preview Abstract] 
Saturday, November 23, 2019 5:32PM  5:45PM 
B35.00005: Continuum breakdown of the acoustic field generated by a pulsating microsphere Yaron BenAmi, Avshalom Manela The flow field of a pulsating sphere is a canonical problem in acoustics. We consider the counterpart problem at small length scales of the sphere, where its radius is comparable with the gas mean free path. Such small scales are often encountered in the process of timeresolved spectroscopy of microparticles. At these conditions, the gas rarefaction effects become important and the continuum description breaks down. The acoustic field is analyzed in the entire range of sphere lengthscales. Numerical calculations are carried out via the direct simulation Monte Carlo method; analytical predictions are obtained in the freemolecular and nearcontinuum regimes. In the latter, the regularized thirteen moments model is applied, to capture the system response at states where the NavierStokesFourier equations break down. The results quantitate the dampening effect of gas rarefaction on the acoustic field. At nearcontinuum conditions, the acoustic field is composed of exponentially decaying 'compression', 'thermal' and 'Knudsenlayer' modes, reflecting thermoviscous and higherorder rarefaction effects. Stronger attenuation is obtained in the freemolecular regime, where boundary sphericity results in a â€˜geometric reductionâ€™ of the molecular layer affected by the source. [Preview Abstract] 

B35.00006: ABSTRACT WITHDRAWN 

B35.00007: ABSTRACT WITHDRAWN 
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