Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session D7: Microfluids: Oscillation |
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Chair: Sascha Hilgenfeldt, University of Illinois Room: 329 |
Sunday, November 24, 2013 2:15PM - 2:28PM |
D7.00001: Probing the rheology of viscous fluids using microcantilevers and the fluctuation-dissipation theorem Brian Robbins, Milad Radiom, John Walz, William Ducker, Mark Paul A microscopic understanding of the rheology of fluids at high frequencies remains an important and open challenge. Current microrheology approaches include the use of micron-scale beads held in optical traps as well as micron-scale cantilevers. Typically, these approaches have been limited in their range of accessible frequencies and dynamic viscosities. In this talk we are interested in the high-frequency regime for very viscous fluids where one must include inertial effects and the frequency dependence of the viscous damping. We present experimental results of the noise spectrum in displacement of the tip of a microcantilever for a variety of fluids that cover a range of viscosities. Using analytical predictions based upon the fluctuation-dissipation theorem, we present an approach to quantify the density and viscosity of the fluid from measurements of the noise spectrum. We are particularly interested in exploring fluids much more viscous than water. We use insights from this study to explore the dynamics of an oscillating elastic object in a power-law fluid to probe the rheology of a non-Newtonian fluid at high frequency. [Preview Abstract] |
Sunday, November 24, 2013 2:28PM - 2:41PM |
D7.00002: Controlled microparticle transport in arrays of oscillating probes Kwitae Chong, Jeff D. Eldredge A probe of circular cross section, undergoing rectilinear oscillation, creates large-scale steady circulatory cells by viscous streaming. In previous work, we have shown that inertial particles can be trapped inside these streaming cells, regardless of particle size and density and Reynolds number (Chong et al., Physics of Fluids, 2013). In the present work, we extend this study to various arrangements of oscillating probes. High fidelity computations are used to simulate the flow field, and a modified form of the Maxey-Riley equation is used to capture particle transport. It is shown that, by controlling the sequence of starting and stopping the oscillation of individual probes, inertial particles can be transported in a predictable manner between trapping points. In order to reduce the considerable expense of generating the flow field, we also explore the use of steady Stokes flow to serve as an approximate surrogate for the flow between probes. The boundary conditions for this flow are obtained by matching with the inner Stokes layer solution. [Preview Abstract] |
Sunday, November 24, 2013 2:41PM - 2:54PM |
D7.00003: Streaming driven by sessile microbubbles: Explaining flow patterns and frequency response Bhargav Rallabandi, Cheng Wang, Lin Guo, Sascha Hilgenfeldt Ultrasound excitation of bubbles drives powerful steady streaming flows which have found widespread applications in microfluidics, where bubbles are typically of semicircular cross section and attached to walls of the device (sessile). While bubble-driven streaming in bulk fluid is well understood, this practically relevant case presents additional complexity introduced by the wall and contact lines. We develop an asymptotic theory that takes into account the presence of the wall as well as the oscillation dynamics of the bubble, providing a complete description of the streaming flow as a function only of the driving frequency, the bubble size, and the physical properties of the fluid. We show that the coupling between different bubble oscillation modes sustains the experimentally observed streaming flow vortex pattern over a broad range of frequencies, greatly exceeding the widths of individual mode resonances. Above a threshold frequency, we predict, and observe in experiment, reversal of the flow direction. Our analytical theory can be used to guide the design of microfluidic devices, both in situations where robust flow patterns insensitive to parameter changes are desired (e.g. lab-on-a-chip sorters), and in cases where intentional modulation of the flow field appearance is key (e.g. efficient mixers). [Preview Abstract] |
Sunday, November 24, 2013 2:54PM - 3:07PM |
D7.00004: Calibration of the Modal Parameters of a Microcantilever from Gas Dissipation Charles Lissandrello, Kamil L. Ekinci We determine the modal mass and the spring constant of a microcantilever from fluidic dissipation measurements. In all experiments the device is held in a vacuum chamber, and its oscillations are monitored using a sensitive heterodyne interferometer. First, thermal fluctuations of the device are measured, and the modal parameters are established. Second, the microcantilever is driven, and its dissipation is measured as a function of the gas pressure in the chamber. These dissipation measurements, combined with a theory to describe gas damping in the kinetic regime, allow us to estimate the effective modal mass and the spring constant. The measurements are repeated for multiple mechanical modes of the same device and for multiple devices. All modal parameters from the dissipation measurements are compared to those obtained from the thermal noise measurements and are found to be in excellent agreement. [Preview Abstract] |
Sunday, November 24, 2013 3:07PM - 3:20PM |
D7.00005: Flow induced vibrations of high-frequency microcantilevers Taejoon Kouh, Seth Hodson, Victor Yakhot, Kamil Ekinci Here we present a parametric study of flow induced vibrations of high-frequency microcantilevers with resonance frequencies in the range 70 kHz to 400 kHz. In the experiments, the microcantilevers are placed in a microchannel; subsequently, a known air flow rate is established through this microchannel, while the pressure drop across is monitored. The resulting transverse vibrations of the microcantilevers are monitored optically with a displacement sensitivity at the level of thermal fluctuations as a function of the air flow rate. As the flow rate is increased, we detect a sudden increase in the vibration amplitudes of the microcantilevers. In addition, the resonance frequencies and the line-widths of the microcantilevers shift as a function of the imposed flow rate. We discuss possible trigger mechanisms for the observed vibrations, including vortex shedding and turbulent fluctuations, and obtain scaling relations in terms of the experimental parameters. [Preview Abstract] |
Sunday, November 24, 2013 3:20PM - 3:33PM |
D7.00006: Analysis of flow characteristics for viscosity sensing applications of suspended microchannel resonators Wook Lee, Jungchul Lee, Seongwon Kang In this study, we analyzed the flow characteristics and performance of a viscosity sensor based on a suspended microchannel resonator (SMR). First, we verified the assumptions of Sader et al. (2010) for their analytic solution using the approach of direction numerical simulation. Second, the relationship between monotonicity of the quality factor and the changes of integrated energy variables was investigated. It was found that the monotonicity to the Reynolds number is strongly dependent on a source term of the kinetic energy equation. Based on this, a change in the quality factor was related to specific patterns of the velocity and vorticity fields. Third, the effects of geometrical parameters of the SMR on performance as a viscosity sensor were investigated. The variations in the measurable viscosity range as well as the viscosity resolution were investigated in terms of the flow characteristics affected by the design parameters. It was found that the off-axis displacement shows a significant but consistent effect on performance of the SMR viscometer regardless of the flow condition. In contrast, the other geometric parameters show more complicated effects, as they are also related to the resonant frequency of the SMR and affected by the compressibility of a fluid. [Preview Abstract] |
Sunday, November 24, 2013 3:33PM - 3:46PM |
D7.00007: Mode Coupling of phonons in a Dense One-Dimensional Microfluidic Crystal Jean-Baptiste Fleury, Ulf D. Schiller, Shashi Thutupalli, Gerhard Gompper, Ralf Seemann Microfluidic crystals are highly ordered arrangements of water-in-oil droplets flowing in microchannels. Their collective dynamics can exhibit a rich behaviour due to the long-range hydrodynamic interactions mediated by the surrounding phase. In this work, we report the specific excitation of long-lived phonon modes in a dense microfluidic crystal. The excited vibrations show transverse modes that originate from the dipolar flow field around the droplet, whereas the longitudinal modes arise from a non-linear coupling due to the breaking of translational invariance under confinement. [Preview Abstract] |
Sunday, November 24, 2013 3:46PM - 3:59PM |
D7.00008: Viscous damping of a periodic perforated microstructure Dorel Homentcovschi, Bruce Murray, Ron Miles The study of a thin air layer squeezed between a moving plate and a rigid plate is important in many microelectromechanical systems such as microphones, microaccelerometers and resonators as some examples. The horizontal motion of the thin air gap in a planar microstructure yields squeeze-film damping that can adversely affect the dynamic response of the device. The backplate often contains a regular array of perforations in order to reduce the time required for fabrication. In order to analyze the viscous damping in some cases, it is possible to take advantage of the regular hole pattern by assuming periodicity. Here a method is developed to calculate the damping coefficient in microstructures with periodic perforations. An approximate analytic solution as well as numerical solutions to the incompressible Stokes equations are obtained. The results can be used to minimize squeeze film damping. In addition, since micromachined devices have finite dimensions, the periodic model for the perforated microstructure requires the calculation of some frame (edge) corrections. Analysis of the edge corrections has also been performed. Results from analytical formulas and numerical simulations match very well with published measured data. [Preview Abstract] |
Sunday, November 24, 2013 3:59PM - 4:12PM |
D7.00009: Three-dimensional tracking of acoustophoretic particle trajectories in a Poiseuille flow Rune Barnkob, Massimiliano Rossi, Alvaro G. Marin, Christian J. K\"{a}hler Acoustics in microfluidics has proved as an excellent technique for particle separation. The technique is often based on advecting the particles by a Poiseuille flow, while acoustic forces push the particles transversely across the flow according to particle size, density, and compressibility. In this work we study such particle trajectories in a microchannel containing a dilute particle suspension. The microchannel is excited in its transverse ultrasound half-wave resonance, while a Poiseuille flow is imposed along the channel. In addition to the viscous drag force from the imposed flow, the particles are subject to forces from acoustic radiation as well as viscous drag from acoustic streaming (Muller et al., PRE, in press, 2013). In the microchannel cross-section, the acoustic streaming is two-dimensional, while the acoustic radiation force is one-dimensional. However, the actual particle velocity induced by the acoustic radiation force has a two-dimensional character due to wall-enhancement of the viscous Stokes drag. In the experiments, we use a 3D astigmatic particle tracking technique (APTV, Cierpka et al., Meas Sci Technol 22, 2011) to determine the particle trajectories, which we compare to theoretical predictions for future optimization of acoustic separation systems. [Preview Abstract] |
Sunday, November 24, 2013 4:12PM - 4:25PM |
D7.00010: Shape Morphing of an Elastic Cylinder via Time-Varying Internal Viscous Flows Shai Elbaz, Amir Gat Viscous flows in contact with an elastic body apply both pressure and shear stress on the solid-liquid interface and thus create internal stress- and deformation-fields within the solid structure. We study the interaction between elastic slender axi-symmetric structures and internal time-varying viscous flows as a tool to create controlled shape-morphing of such elastic cylindrical structures. We neglect inertia in the liquid and solid and focus on two cases. Case 1 is viscous flow through a hollow elastic cylinder and case 2 is axial flow in the shallow gap created by two concentric cylinders, where the internal cylinder is rigid and the external elastic. For case 1, we obtain a linear diffusion equation and for case 2 we obtain a non-linear diffusion equation governing the deformation. Solutions for both cases allowing control of the time varying deformation field by way of controlling the liquid pressure at the inlet and outlet are presented. This research is of interest to applications such as micro-swimmers and soft-robotics. [Preview Abstract] |
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