Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session U11: Microscale Flows: Oscillations (8:45am  9:30am CST)Interactive On Demand

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U11.00001: Elasticityinduced viscous streaming phenomenon Yashraj Bhosale, Tejaswin Parthasarathy, Mattia Gazzola Rectified flows associated with the oscillatory motion of immersed solid boundaries, known as viscous streaming, are an efficient way of manipulating and controlling fluids via inertial effects in microfluidic settings. Despite its potential, we know surprisingly little about viscous streaming beyond the classical cases of vibrating solid cylinders, plates, and spheres. Here we extend our understanding by numerically investigating the effects of solidboundary elasticity, via a recently developed framework based on remeshed vortex methods coupled to reference map techniques. Preliminary results indicate that a complex interplay between inertial, viscous, and elastic forces leads to rich behaviors. [Preview Abstract] 

U11.00002: Viscous streaming in 3D: Effects of geometry and topology Fan Kiat Chan, Yashraj Bhosale, Tejaswin Parthasarathy, Mattia Gazzola Recent studies on viscous streaming flows generated by oscillating bodies highlighted the importance of curvature effects in 2D geometries. Here we extend our understanding to 3D systems by investigating a series of shapes, starting from wellunderstood spheres and progressively breaking geometric and actuation symmetry. We leverage tools from direct numerical simulation and flow visualization to establish a sparse representation of often complex and dense 3D flows, and employ dynamical system theory to systematically analyze and characterize the underlying streaming dynamics. Capitalizing on the same protocol, we further explore streaming flows generated by topologically distinct bodies. [Preview Abstract] 

U11.00003: Transient microscale compressible flow in a viscoelastic tube Vishal Anand, Ivan Christov We analyze transient compressible flow at low Reynolds number conveyed in a compliant, linearly viscoelastic tube. A linear equation of state accounts for the compressibility of the fluid, whilst the slenderness of the tube allows the use of the lubrication approximation. The structural mechanics is governed by Donnell shell theory, augmented with KelvinVoigt viscoelasticity. The hydrodynamic pressure couples the fluid mechanics of the flow with the structural mechanics of the tube. Within this theoretical framework, we study the startup flow created by an oscillatory inlet pressure imposed on an initially static fluid and structure. We show that the frequency response of the deformed viscoelastic tube is akin to that of a bandpass filter; the deformation reaches a peak value at the resonant frequency determined by a balance of inertia and viscoelastic damping. We demonstrate that the interplay of compressibility and compliance leads to acoustic streaming: the hydrodynamic pressure and velocity fields have nonzero means when averaged over a period of the inlet pressure's oscillations. A frequencydependent flow rate enhancement is induced by streaming, exhibiting a lowpass response. [Preview Abstract] 

U11.00004: Robustness and Sensitivity of Streaming Flow Patterns Partha Kumar Das, Sascha Hilgenfeldt Steady streaming flows result from rectification of periodic flow induced by an oscillating interface, and have been used extensively in microfluidic device design. Even simple objects executing simple motion can give rise to complex streaming patterns that sensitively depend on parameters, such as the prototypical case of a cylinder oscillating translationally. We argue that this complexity and sensitivity is not typical for vigorous streaming flows encountered in microfluidic applications, chiefly relying on mixedmode oscillations of deformable objects with pinned contact lines, such as bubbles or droplets. Experiments varying the modality, channel geometry, and the dynamic boundary condition at the interface (nostress, tangential stress continuous, noslip) find an extremely robust vortexpair streaming pattern independent of frequency or viscosity contrast. Comparing and contrasting the theoretical modeling of these flows with that of classical singlemode streaming patterns, we identify the conditions under which robust or sensitive streaming is expected. These results allow for the design of microfluidic devices guided by physical principles and tailored to applications that either require unvarying, robust flows or easily tunable changes in the streaming. [Preview Abstract] 

U11.00005: Flow Curvature Induced Inertial Forces on Particles in Oscillatory Flows Siddhansh Agarwal, Fan Kiat Chan, Mattia Gazzola, Sascha Hilgenfeldt Oscillatory flow is a very effective way of exploiting inertia in a typical low \emph{Re} setting. Localized oscillating objects such as a volumetrically pulsating bubble have been used to efficiently induce migration of micronscale objects in microfluidic devices. In this work, we describe a new kind of attractive inertial force that is generically present in this situation and induced by the curvature tensor of the flow field, while not relying on contrasts of density or compressibility. In the spirit of Oseen's and Saffman's formalisms, we present a simple uniformly valid expression for the force that interpolates accurately between the two limits of purely viscous (inner) and inviscid (outer) solutions. The particle dynamics is ultimately reduced to an overdamped equation of motion that is simple to use and quantitatively accurate. Extensive direct numerical simulations of the NavierStokes equations show excellent quantitative agreement with theory throughout the entire parameter range. Comparison is also made with experiments showcasing the predictive power of our theory and its practical relevance for object manipulation in microfluidics. [Preview Abstract] 

U11.00006: Viscous flywheel sensing of~nanoparticles Georgios Katsikis, Jesse Collis, Scott M Knudsen, Vincent Agache, John Sader, Scott R Manalis From accelerometers and gyroscopes to microresonators, inertia is often sensed to control motion or measure the physical properties of an analyte. Here, we demonstrate inertial sensing of the physical properties of nanoparticles by local rotation of a microfluidic channel. Through experiments with fluid suspended nanoparticles in hollow microcantilevers and an analytical theory, we show that inertial sensing in our system can directly measure nanoparticle volume. Paradoxically, particle mass only emerges when viscous effects in the fluid become dominant over inertia. We explain this paradox via a viscositydriven hydrodynamic coupling between the particle and the microfluidic channel, that turns the former into a `viscous flywheel'. Our modality now enables the simultaneous measurement of nanoparticle volume and mass, using a single measurement. [Preview Abstract] 

U11.00007: Simulations, experiments and applications with streaming lattices Gabriel Juarez, Giridar Vishwanathan, Yashraj Bhosale, Tejaswin Parthasarathy, Mattia Gazzola Steady streaming refers to the rectified flow patterns produced when solid boundaries interact with high frequency oscillatory flows. This phenomenon is arguably the most efficient way to exploit interia at the microscale, with several practical applications from mixing to particle manipulation. Here, we present numerical predictions and experimental verifications of steady streaming realized in a periodic lattice of cylinders with alternating curvatures. The interplay between multiple curvatures and oscillation frequency leads to a rich variety of flow topologies, beyond classically understood ones. We leverage this setup for tunable noncontact filtration and enhanced particle trapping. [Preview Abstract] 

U11.00008: Enhanced diffusivity and skewness of a diffusing tracer in the presence of an oscillating wall Richard McLaughlin, Lingyun Ding, Robert Hunt, Hunter Woodie We examine a passive scalar diffusing in timevarying flows which are induced by a periodically oscillating wall in a Newtonian fluid between two infinite parallel plates. These shear flows yield the generalized Ferry waves which are exact solutions of the NavierStokes equations. First, we calculate the second Aris moment for all time, and its long time limiting effective diffusivity as a function of the geometrical parameters, frequency, viscosity, and diffusivity. We show that the viscous dominated limit results in a linear shear layer for which the effective diffusivity is bounded with upper bound $\kappa(1+A^2/(2L^2))$, where $\kappa$ is the tracer diffusivity, $A$ is the amplitude of oscillation, and $L$ is the gap thickness. Alternatively, we show that for finite viscosities the enhanced diffusion is unbounded, diverging in the high frequency limit. Physical arguments are given to explain these striking differences. Physical experiments are performed in water using Particle Tracking Velocimetry to quantitatively measure the fluid flow. We document that the theory is quantitatively accurate. Further, we show that the scalar skewness is zero for linear shear at all times, whereas for the nonlinear Ferry wave, using MonteCarlo simulations is nonzero. [Preview Abstract] 

U11.00009: An optimization technique for precise pulsatile flow generation in pressuredriven microfluidic devices Steffen Recktenwald, Thomas John, Christian Wagner Pulsatile flows are ubiquitous in nature and technology. In microfluidic devices, pulsatile or oscillatory driving of the flow enhances a broad variety of microscale operations and is also used for biomimicry in physiological studies. To study biological systems under physiologically relevant flow conditions, precise control of the timedependent flow field is paramount. However, generating a welldefined pulsatile flow with pneumatically operated pressure pumps remains challenging and significant deviations from the desired waveform can arise. In this study, we present a method to generate an optimized pulsatile flow in pressuredriven microfluidic systems using two commercially available pressure controllers. Therefore, we derive an adapted input signal based on the amplitude response of the pumps. This adapted input results in an optimized pressure output for various pulsatile waveforms, which significantly improves the timedependent flow of microparticles and red blood cells in microfluidic channels. Our technique does not require any hardware modifications of the commercial pumps and can be easily implemented in standard pressuredriven microfluidic setups. [Preview Abstract] 
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