Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session G8: Microscale Flows: Oscillatory Fluid Dynamics |
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Chair: Mark Paul, Virginia Tech Room: 108 |
Monday, November 23, 2015 8:00AM - 8:13AM |
G8.00001: The Fluid Coupled Dynamics of Small Oscillating Elastic Objects Mark Paul There is broad interest in the correlated motion of small elastic objects in a viscous fluid. In many cases, the coupled motion of an array of objects provides more information, and improved resolution, when compared with what is learned from the motion of a single object alone. The physical implications and subtleties of the fluid dynamics caused by oscillating objects for a range of applications will be discussed. The theoretical and numerical ideas needed for the quantitative description of these systems will be presented for external harmonic excitation and for dynamics driven by Brownian motion. An analytical approach for the correlated motion of two elastic objects in fluid will be presented. Each oscillating object is replaced with a two-dimensional cylinder of finite mass that is attached to a spring. The fluid motion is governed by the unsteady Stokes equations. Using linear response theory and the fluctuation-dissipation theorem, analytical expressions will be developed for the motion of the two fluid-coupled cylinders. A comparison of the analytical results with full finite element numerical simulations yields excellent agreement. These ideas can be extended to include different geometries, large arrays, and objects that are tethered together. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G8.00002: Effects of Length Scale and Frequency in Oscillatory Flows Induced by Micro- and Nano-mechanical Resonators Kamil Ekinci, Vural Kara, Victor Yakhot It is challenging to formulate a theory of flows induced by oscillating micro- and nano-mechanical resonators even in simple fluids. The characteristic length and time scales of these devices may lead to surprising deviations from classical fluid dynamics. Here, we study this problem in a near-ideal gas. By changing the gas pressure, we control the mean free path and the relaxation time of the gas. We measure, as a function of pressure, the fluidic dissipation of micro- and nano-mechanical resonators with length scales (sizes) and frequencies that span many orders of magnitude. We show conclusively how a subtle interplay between its length scale (size) and frequency determines the nature of flow around a mechanical resonator, resulting in a low-frequency regime dominated by length scale and a high-frequency regime dominated by frequency. We propose an analytical formula that incorporates both the dimensionless size and frequency and show excellent agreement over the entire parameter space between theory and experiment. Our results are significant for understanding high-frequency and nanoscale fluid-structure interactions as well as for designing improved devices. [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G8.00003: Measuring the Size and Slip Lengths of Individual Nanoparticles using Suspended Microchannel Resonators Jesse Collis, John Sader, Selim Olcum, Scott Manalis Characterizing nanometer-scale particles immersed in liquids using cantilever-based sensing methods can be challenging due to large hydrodynamic damping forces. Suspended Microchannel Resonators (SMRs) differ to conventional cantilever sensors by embedding a microfluidic channel within a vacuum-encased cantilever. These devices can be used as sensitive mass balances for individual nanoparticles flowing through the microfluidic channel; resolution at the attogram scale has been demonstrated recently. We explore a new modality for these devices, where the particle size and surface properties can be characterized. The theoretical framework for this modality is developed using both asymptotic and numerical methods, for which excellent agreement is observed. Comparison of experimental data with Monte-Carlo simulations shows we are able to accurately quantify the slip lengths of these particles. [Preview Abstract] |
Monday, November 23, 2015 8:39AM - 8:52AM |
G8.00004: Dynamic Wetting at MHz Vibration: Simple and Complex Liquid Films on an Ultrasonic Actuator Ofer Manor, Gennady Althshuler, Sameer Mhatre, Ludmila Abezgauz We excite simple and complex liquid films on ultrasonic actuators that produce a MHz substrate vibration in the form of a surface acoustic wave (SAW). Transfer of momentum from the MHz vibration in the solid substrate to the neighboring liquid translates to convective stresses within the liquid and on the film free boundary. These stresses further invoke various flow mechanisms, also known as acoustic streaming and may support dynamic wetting or dewetting of liquid films. In particular, we use theory and experiment to study the interplay between viscous, capillary, and the vibrational dynamics of liquid films and their internal structure. We employ MHz ultrasonic actuators to study the dynamic wetting and dewetting of free and confined films of oil and water/surfactant solutions on flat surfaces and within microfluidic channels. We further excite films of evaporating solutions and suspensions in order to study the active influence of the solid vibration on the geometry of the molecule and particle patterns that are deposited in this process. We show the physics underlying these different systems may be explained using the convective dynamics the MHz substrate vibration excites in liquid films. [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G8.00005: Resonance and streaming of armored microbubbles Tamsin Spelman, Nicolas Bertin, Olivier Stephen, Philippe Marmottant, Eric Lauga A new experimental technique involves building a hollow capsule which partially encompasses a microbubble, creating an "armored microbubble" with long lifespan. Under acoustic actuation, such bubble produces net streaming flows. In order to theoretically model the induced flow, we first extend classical models of free bubbles to describe the streaming flow around a spherical body for any known axisymmetric shape oscillation. A potential flow model is then employed to determine the resonance modes of the armored microbubble. We finally use a more detailed viscous model to calculate the surface shape oscillations at the experimental driving frequency, and from this we predict the generated streaming flows. [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G8.00006: Very long range vortices around microfluidic bubbles under ultrasound Philippe Marmottant, Flore Mekki-Berrada, Thomas Combriat, Pierre Thibault The acoustic vibration of a single bubble results in streaming flows, a non-linear effect that is localized near the bubble, and that is useful to mix and shear fluids in the vicinity only. Here we show that this streaming is much more extended around pair of bubbles because they interact. We perform experiments with flattened microfluidic bubbles, undergoing a volumic vibration mode in response to ultrasound. We observe very long-range recirculating flow around pairs of bubbles. Using a large lattice of these microbubbles, we obtain a unique acoustic bubble “pinball” driving fluid and particles in complex paths, following elaborate microstreaming vortices. We predict the streamlines to be the consequence of volumic and translational vibration of the bubbles. The translational vibration is the sign of the interaction between bubbles, here mediated by Rayleigh waves on the elastic channel walls. This work, part of the project Bubbleboost, gives a new insight into bubbles efficiency to trigger mixing in laminar flows. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G8.00007: Oscillatory Motion of a Bi-Phasic Slug in a Teflon Reactor Milad Abolhasani, Klavs Jensen Bi-phasic physical/chemical processes require transfer of solute/reagent molecules across the interface. Continuous multi-phase flow approaches (using gas as the continuous phase), usually fail in providing sufficient interfacial area for transfer of molecules between the aqueous and organic phases. In continuous segmented flow platforms (with a fluorinated polymer-based reactor), the higher surface tension of the aqueous phase compared to the organic phase of a bi-phasic slug, in combination with the low surface energy of the reactor wall result in a more facile motion of the aqueous phase. Thus, upon applying a pressure gradient across the bi-phasic slug, the aqueous phase of the slug moves through the organic phase and leads the bi-phasic slug, thereby limiting the available interfacial area for the bi-phasic mass transfer only to the semi-spherical interface between the two phases. Disrupting the quasi-equilibrium state of the bi-phasic slug through reversing the pressure gradient across the bi-phasic slug causes the aqueous phase to move back through the organic phase. In this work, we experimentally investigate the dynamics of periodic alteration of the pressure gradient across a bi-phasic slug, and characterize the resulting enhanced interfacial area on the bi-phasic mass transfer rate. We demonstrate the enhanced mass transfer rate of the oscillatory flow strategy compared to the continuous multi-phase approach using bi-phasic Pd catalyzed carbon-carbon and carbon-nitrogen cross coupling reactions. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G8.00008: Fluttering instabilities of cylinder in a Hele Shaw cell Harold Auradou, Jean-Pierre Hulin, Benoît Semin, Mario Cachile, Maria Veronica D'Angelo We found that a cylinder confined between two parallel plates displays a fluttering instabilities. The cylinder oscillates with respect to the horizontal. The characteristics of the instability (frequency, amplitude...) are found to be function of the Froude number. Compared to previous studies, this instability is triggered by the confinement and not by inertial effects. [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G8.00009: Formation of inverse Chladni patterns at microscale by acoustic streaming on a silicon membrane immersed in a liquid Cedric Poulain, Gael Vuillermet, Fabrice Casset High frequency acoustics (in the $MHz$ range) is known to be very efficient to handle micro particles or living cells in microfluidics by taking advantage of the acoustic radiation force. Here, we will show that low frequency ($\sim 50kHz$) together with use ultra thin silicon plate can give rise to a micro streaming that enables to move particles at will. Indeed, by means of silicon membranes excited in the low ultrasound range, we show that it is possible to form inverse two-dimensional Chladni patterns of micro-beads in liquid. Unlike the well-known effect in a gaseous environment at macroscale, where gravity effects are generally dominant, leading particles towards the nodal regions of displacement, we will show that the micro scale streaming in the vicinity of the plate tends to gather particles in antinodal regions. Moreover, a symmetry breaking effect together with the streaming can trigger a whole rotation of the beads in the fluidic cavity. We demonstrate that it is possible to make the patterns rotate at a well defined angular velocity where beads actually jump from one acoustic trap to another. [Preview Abstract] |
Monday, November 23, 2015 9:57AM - 10:10AM |
G8.00010: The acoustic radiation force on a small thermoviscous or thermoelastic particle suspended in a viscous and heat-conducting fluid Jonas Karlsen, Henrik Bruus We present a theoretical analysis (arxiv.org/abs/1507.01043) of the acoustic radiation force on a single small particle, either a thermoviscous fluid droplet or a thermoelastic solid particle, suspended in a viscous and heat-conducting fluid. Our analysis places no restrictions on the viscous and thermal boundary layer thicknesses relative to the particle radius, but it assumes the particle to be small in comparison to the acoustic wavelength. This is the limit relevant to scattering of ultrasound waves from sub-micrometer particles. For particle sizes smaller than the boundary layer widths, our theory leads to profound consequences for the acoustic radiation force. For example, for liquid droplets and solid particles suspended in gasses we predict forces orders of magnitude larger than expected from ideal-fluid theory. Moreover, for certain relevant choices of materials, we find a sign change in the acoustic radiation force on different-sized but otherwise identical particles. These findings lead to the concept of a particle-size-dependent acoustophoretic contrast factor, highly relevant to applications in acoustic levitation or separation of micro-particles in gases, as well as to handling of $\mu$m- and nm-sized particles such as bacteria and vira in lab-on-a-chip systems. [Preview Abstract] |
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