Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session Q01: Focus Session: Acoustofluidics II |
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Chair: Ofer Manor, Technion Room: Sagamore 123 |
Monday, November 21, 2022 1:25PM - 1:38PM |
Q01.00001: Amplifying secondary Bjerknes forces for precise acoustofluidic actuation Athanasios G Athanassiadis, Rahul Goyal, Zhichao Ma, Peer Fischer Acoustofluidic forces, such as streaming and radiation forces, are often enhanced by resonant bubbles built into devices. However, since bubbles are deeply-subwavelength resonators, there is limited ability to use these effects for precise manipulations. While the secondary radiation forces between bubbles could lead to more precise actuation, such forces are extremely weak – typically nanonewtons – and thus have been largely ignored. Here, we demonstrate that patterning microbubbles into arrays can geometrically amplify the secondary Bjerknes forces, producing surprisingly high forces that overcome the effects of streaming and primary radiation forces. We experimentally demonstrate the ability of a Bjerknes actuator to precisely position a 1 cm object with 15 μm accuracy, using sound with a 50 cm wavelength. By modifying the bubble patterns, the Bjerknes actuator can also act as a rotational motor, driving unidirectional motion of a freely-floating structure. We describe the amplification and manipulation behavior using a theoretical model, and show that the amplified Bjerknes forces can outperform comparable magnetic actuators. By embedding arrays of resonant bubbles, new capabilities can be integrated into acoustofluidic devices, providing precise, switchable actuation. |
Monday, November 21, 2022 1:38PM - 1:51PM |
Q01.00002: Single beam selective acoustical tweezers: from acoustical vortices to focused beams. Invited Speaker: Michael Baudoin Single beam acoustical tweezers open some tremendous perspectives for the precise selective manipulation, assembly and mechanical properties testing of microscopic objects, cells and microorganisms. In this presentation we will first review the 2D and 3D trapping capabilities of acoustical vortices (some helical wave spinning around a phase singularity) and focused beam. Then we will discuss our latest experimental developments on ultra-high frequency microparticles manipulation with nanoNewton forces and 3D manipulation of particles with frequency induced axial displacement of the trapped object. Finally, we will introduce a new mixed finite element/angular spectrum code, which enables to compute precisely the field generated by active holographic transducers. |
Monday, November 21, 2022 1:51PM - 2:04PM |
Q01.00003: Experimental and numerical investigation of microscale acoustic streaming for mixing fluids Beomseok Cha, Jinsoo Park In microfluidics, homogeneous flow mixing is essential in broad applications including biochemical assays and chemical synthesis. However, low Reynolds number flows render flow mixing challenging in the microchannel. Recently, acoustofluidic mixing based on acoustic streaming flow (ASF) has been proposed to address the limitations of the previous mixing methods. However, in-depth investigation of the ASF is still in demand to characterize the flow mixing phenomenon. Here, we experimentally and numerically visualize the microscale ASF under varying conditions of total flow rate, acoustic wave amplitude, and fluid viscosity using particle image velocimetry and computational fluid dynamics, respectively. We utilized a prism-embedded vertical-type acoustofluidic device to experimentally visualize the streamwise-spanwise plane of the ASF for the first time. For the numerical investigation, we adopted wave attenuation model and compared the results with those obtained from the experiments. Based on the findings, we applied the ASF-based mixing to rapid, continuous, automated chemical synthesis. |
Monday, November 21, 2022 2:04PM - 2:17PM |
Q01.00004: Surface acoustic waves for particle fractionation – physical mechanisms relevant for device design Christian Cierpka, Sebastian Sachs, Jörg König By integrating surface acoustic waves (SAW) into microfluidic devices, microparticle systems can be separated precisely. The driving mechanism exploited here is the acoustic radiation force, which depends on the size and acoustic properties of the suspended particles. Thus a multidimensional particle separation is possible. However, fluid motion caused by the acoustic streaming effect is superimposed and can further manipulate particle trajectories. This might have a negative influence on the separation result. A characterization of the crucial parameters that affect the pattern and scaling of the acoustically induced flow is thus essential for the design of acoustofluidic separation systems. In the presentation, the fluid flow induced by pseudo-standing acoustic wave fields with a wavelength much smaller than the width of the confined microchannel is revealed in detail, using quantitative three-dimensional measurements of all three velocity components (3D3C) accompanied by numerical simulation. |
Monday, November 21, 2022 2:17PM - 2:30PM |
Q01.00005: Data driven investigation of turbulent capillary waves William Connacher, Jeremy Orosco, James Friend We study a millimeter scale liquid meniscus subjected to ~7 MHz ultrasound. It is well understood that similar systems at lower frequencies generate droplets via a Faraday wave mechanism, but there is considerable debate about the atomization mechanisms at higher frequencies. Progress has been limited by the extremely small, and multi-scale, space and time scales involved relative to available experimental and numerical techniques. We present data captured at 10 us and 10 nm resolution using high speed digital holographic microscopy (DHM) and subject that data to principle component analysis (PCA) and Koopman based methods. More typical time series frequency analysis (e.g. FFT) shows that our system exhibits wave turbulence, which is well known to exchange energy between scales. We are also able to show, using spatial data of the entire surface (not single point), that energy is transfering towards smaller wavelengths as the input power to the system increases. Furthermore we identify multiple power thresholds at which turbulent wave behavior suddenly change, which are difficult to identify using frequency analysis. |
Monday, November 21, 2022 2:30PM - 2:43PM |
Q01.00006: Analysis of Transient Acoustic Streaming Invited Speaker: James Friend Acoustic streaming has traditionally been assumed to be a steady-state, relatively slow fluid response to passing acoustic waves. This slow streaming assumption was first made by Lord Rayleigh a century and a half ago. From intractable nonlinear governing equations, the method produces useful solutions by first solving the acoustic field independently of the streaming it generates and other hydrodynamic phenomena. The resolved acoustic field is then used to determine the streaming through a time average of the nonlinear phenomenon. However, in modern use, acoustic streaming exhibits large velocities comparable to the acoustic field, rendering the traditional approach flawed if not outright invalid. However, the method remains widely used today, as there is no suitable alternative. We provide a novel approach to supplant this method, seeking to properly treat the spatiotemporal scale disparities present between the acoustics and remaining fluid dynamics. The separation of the governing equations between the fast (acoustic) and slow (hydrodynamic) spatiotemporal scales are shown to naturally arise from the intrinsic properties of the fluid under forcing, not by arbitrary assumption beforehand. Solution of the unsteady streaming field equations provides physical insight into observed temporal evolution of bulk streaming flows that, to date, have not been modeled. A Burgers equation is derived from the new method to represent unsteady flow. By then assuming steady flow, a Riccati equation is found to represent it. Solving these equations produces direct, concise insight into the nonlinearity of the acoustic streaming phenomenon. |
Monday, November 21, 2022 2:43PM - 2:56PM |
Q01.00007: Inertial Forces on Microparticles in Oscillatory Flows Invited Speaker: Sascha Hilgenfeldt Particle manipulation is a core task of microfluidic devices. Understanding in detail how and when a particle’s trajectory deviates from streamlines of the flow informs the control necessary for concentration, separation, and sorting of microscale objects in applications. Recently, it has been recognized that inertial forces can be a powerful tool for accomplishing such tasks, even in low-Reynolds-number microfluidics. Our work focuses on oscillatory flows, where inertia can be brought to bear on smaller length and time scales than in steady-flow scenarios. Revisiting the classical theory of particle trajectories by Maxey and Riley, we derive a rigorous generalization of their equation of motion, which, for neutrally buoyant particles, gives rise to a universal inertial force expression based on gradients and curvature of the background flow field [1]. A similar analysis for particles with density contrast [2] reveals additional force terms and incorporates earlier theoretical descriptions such as Auton’s correction and the secondary radiation force of acoustofluidics as limiting cases. This body of work shows that time-averaged inertial forces in microscale oscillatory flows are strong compared to other effects and that their sign and spatial dependence can be altered by controlling the flow field. The theory spans the entire range of Stokes numbers, dovetails with acoustofluidics in the inviscid limit, and has been rigorously checked against direct numerical simulations. For many experiments such as bubble microfluidics relying on oscillatory flow this formalism provides a quantitative guide for device design. |
Monday, November 21, 2022 2:56PM - 3:09PM |
Q01.00008: Spreading of a thin film exposed to a surface acoustic wave Yifan Li, Joseph D'Addesa, Javier A Diez, Ofer Manor, Linda J Cummings, Lou Kondic We report experimental, theretical, and computational study of the dynamics of a thin silicon oil film spreading due to the forcing resulting from applied suface acoustic wave (SAW). In experiments, we consider a drop spreading either on a flat substrate, or on a substrate with superimposed topology. Starting from the first principles, our model includes the forcing due to SAW into the standard long wave formulation. Finally our simulations allow for discussion of the influence of SAW on the time-dependent film evolution. Combining experimental and theoretical/computational efforts allows us to formulate a realistic model accounting accurately for the coupling between acoustic and fluids aspects of the considered problem. |
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