Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session J30: Micro/Nano scale Flows: Interfaces, Particles, and Channels II 
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Chair: Jesse Collis, University of Melbourne Room: 154AB 
Sunday, November 19, 2023 4:35PM  4:48PM 
J30.00001: Study on the evolution and stability of gas–liquid interfaces based on composite structures on the sidewall surface of a microchannel Zhaohui Yao The gas–liquid interface plays a crucial role in reducing the flow resistance of superhydrophobic surfaces. However, this interface is highly unstable and prone to collapse under flow shear, environmental pressure fluctuations, phase transitions, and diffusion between dissolved gases and free gases. Once the gas–liquid interface collapses, the flow resistance increases rapidly. Therefore, it is necessary to study the stability of the gas–liquid interface. This paper considers a three dimensional (3D)printed composite structure combining transverse posts and reentrant structures in a microchannel. This structure effectively improves the stability of the gas–liquid interface, allowing it to maintain stability even on surfaces made of hydrophilic materials. Under the effect of the transverse posts, the length of the gas–liquid interface above the groove increases from micrometers to millimeters. The lattice Boltzmann method is applied to analyze how the composite structure effectively improves the stability of the gas–liquid interface. Through analysis of the interface collapse process, the factors affecting the stability of the gas–liquid interface in this structure are explored, providing a theoretical foundation for structural optimization. 
Sunday, November 19, 2023 4:48PM  5:01PM 
J30.00002: Nearly spherical particles vibrating in viscous fluids: A second order asymptotic theory Jesse F Collis, Alex R Nunn, John E Sader

Sunday, November 19, 2023 5:01PM  5:14PM 
J30.00003: Steady Streaming and Pumping Driven by Two Frequency Oscillations Hyun Lee, Robert Guy, William D Ristenpart Recent experiments showed net transport of an object sliding on a surface undergoing two mode vibrations for certain frequency pairs. Inspired by these experiments, we revisit the classical problem of steady streaming in fluids (i.e. nonzero mean flow produced from periodic forcing) driven by multifrequency oscillations. Using numerical simulations, we examine the flow generated by twofrequency oscillations of a rigid object immersed in fluid. Our results show that net pumping occurs when these twofrequency pairs produce a nonantiperiodic driving force. Furthermore, we use small amplitude analysis and extend the past results of steady streaming to the double frequency case. While steady streaming occurs at secondorder in amplitude, pumping is a thirdorder effect which is analytically challenging to compute. Therefore, we use a hybrid numerical and analytical method to explain the mechanism behind pumping of twofrequency oscillations. 
Sunday, November 19, 2023 5:14PM  5:27PM 
J30.00004: Mechanisms driving the acoustic propulsion of nanoparticles Peijing Li, Jesse F Collis, Douglas R Brumley, John E Sader A particle possessing shape or density asymmetries trapped in an acoustic standing wave can undergo propulsion due to a steady streaming flow generated by the particle. Remarkably, a reversal occurs in the propulsion direction at a critical value of dimensionless frequency (scaled by the viscous diffusion time), which varies depending on the shape and/or density asymmetries of the particle. We investigate the origin of this reversal in propulsion by studying a canonical problem of the streaming flow around a sphere that undergoes oscillatory rotations, which is suspended in an unbounded fluid exhibiting rectilinear oscillations (which may be phaseshifted relative to the sphere rotations). An analytical expression of the flow field is derived in the limit of the infinitesimal amplitude of both the applied oscillatory flow and the particle's rotational oscillations. We find that two distinct bifurcations occur as a function of frequency. At the first bifurcation, a stagnation point forms in the interior of the flow which then splits into a saddle node and a vortex centre as the dimensionless frequency increases. A reversal happens at the second bifurcation when the stagnation point approaches infinity and then flips from the equator to the poles. Furthermore, we show that the streaming flow in the far field coincides with that of a Stokeslet whose magnitude is the net force exerted on the particle. 
Sunday, November 19, 2023 5:27PM  5:40PM 
J30.00005: Molecular Dynamics Study on the Effects of InPlane Pore Oscillations on Flow through Nanoporous 2D Membranes Juan Pablo Martinez Cordeiro, Narayana R Aluru Although extensive studies have been performed into the use of nanoporous 2D membranes for filtration, most have focused on static and quasiequilibrium behavior. Many natural and practical systems, however, involve dynamic coupling between the fluid and the material through which the fluid flows. This requires understanding the physical mechanisms that arise in nonequilibrium mechanonanofluidic systems. In this work, we first develop the necessary theoretical foundation to perform fundamental studies of nanofluidic dynamic behavior that seclude the effects of adding arbitrary heat into the system. Then we use Molecular Dynamics (MD) to analyze the effects of inplane harmonic pore oscillations on water transport through nanoporous and subnanoporous graphene. We report an increase in average axial velocity and a decrease in average water density inside the pore. The relative competition between these effects determines the degree to which flux through the membrane is enhanced, and flux enhancement occurs for all the considered oscillatory schemes in comparison to the static case. We use hydrodynamic models and analysis of nanoscopic behavior to explain the observed phenomena as a result of dynamic coupling between the membrane and the fluid. 
Sunday, November 19, 2023 5:40PM  5:53PM 
J30.00006: Secondary flows and forces around a sphere in a nonuniform oscillatory flow Jake Minten, Xiaokang Zhang, Bhargav Rallabandi Oscillatory flows provide a powerful means of harnessing inertial effects to direct the motion of suspended particles. This motion has varyingly been described using secondary radiation forces and streaming theory, with contrary results. In this work, we model the timeaveraged secondary flow around a sphere suspended in a known, spatially varying, oscillatory flow. We first decompose the oscillatory flow into translating, dilating, and straining components and obtain the corresponding particle disturbance flow analytically. We then show that the timeaveraged secondary flow is driven by a combination of an effective body force due to the inertia of the primary flow, and an effective slip condition arising from the Stokes drift of the primary flow. Assuming small oscillation amplitudes and axial symmetry, we solve for the secondary flow using a vorticitystream function formulation over a wide range of oscillatory Stokes layer thicknesses. We then use the secondary flow to compute the secondary timeaveraged hydrodynamic force felt by the sphere, and find it to be in excellent agreement with an analytical theory based on the Lorentz reciprocal theorem. In particular, the force changes sign when the Stokes layer thickness is comparable to the particle radius, and corresponds to a reversal in the secondary flow around the particle. We then show how the framework presented here smoothly connects the streamingdominated and radiationforcedominated pictures of timeaveraged particle dynamics. 
Sunday, November 19, 2023 5:53PM  6:06PM 
J30.00007: The Lorentz reciprocal theorem applied to oscillatory flows in rigid and deformable channels Shrihari D Pande, Evgeniy Boyko, Ivan C Christov We demonstrate the use of the Lorentz reciprocal theorem in obtaining corrections to the flow ratepressure drop relation for oscillatory flow in channels. We start from the unsteady Stokes equations and derive the suitable reciprocity relations for both rigid and deformable channels, assuming all quantities can be expressed as timeharmonic phasors. For the case of a rigid channel, the auxiliary problem is a Poiseuille flow, and the reciprocal theorem allows us to calculate the firstorder correction to the pressure drop in the Womersley number. For the case of a deformable channel, we account for the fluidstructure interaction using a domain perturbation approach under the assumption of a small compliance number. Using the Womersley flow solution in a rigid channel as an auxiliary problem, we obtain the first correction to the flow ratepressure drop relations due to compliance of the channel. 
Sunday, November 19, 2023 6:06PM  6:19PM 
J30.00008: Effect of geometric design on the motion of microrobots due to acoustic streaming flows Ritu R Raj, Jin G Lee, Ankur Gupta, Wyatt Shields Recent work has shown that the geometric design of microrobots powered by acoustic streaming has a significant impact on their motion. To date, most studies focus on a single acoustically responsive structure, either a bubble or thin structure, to power microrobot locomotion. By actuating such microrobots containing multiple acoustically responsive structures with discrete, single frequency acoustic fields, multiple patterns of streaming flows can be generated, leading to frequencydependent motion with distinct trajectories. 
Sunday, November 19, 2023 6:19PM  6:32PM 
J30.00009: OJAWS (Oscillating Jet Assisted Wet Spinning): A structured microfiber production methodology Barath Venkateswaran, Zehao Pan, Janine K. Nunes, PierreThomas Brun, Howard A Stone Jet Assisted Wet Spinning (JAWS) is an emerging method that makes sub100 micron fibers without the confinement and potential clogging issues of microfluidic channels. JAWS uses a fast water jet to stretch an adjacent jet of a monomer solution while both jets are submerged in a water bath. The monomer jet is bent, entrained, and thinned through the influence of the adjacent and significantly faster water jet. The ensuing dynamics allow for the production of fibers that are much thinner than the jet nozzles used. Oscillation of the fluid jets allows for making structured fibers as the monomer jet traverses across the velocity gradients of the entraining flow. We observe in our experiments that the oscillations of the twojet setup transverse to the flow direction allow us to make structured fibers, including looped, crimped, and interweaving shapes. The morphologies of the obtained microfibers will be discussed. 
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