Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session F15: Microscale Flows: Mixing and DynamicsMicro

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Chair: David Saintillan, University of California, San Diego Room: 601 
Monday, November 20, 2017 8:00AM  8:13AM 
F15.00001: Inertial effects in microfluidic flow focusing and mixing Ashwin Ramachandran, Wei Liao, Daniel P DePonte, Juan G Santiago A wide range of microfluidic devices focus a center stream between two symmetric sheath streams on either side. These devices are used to achieve a thin laminar stream with dimensions much smaller than the channel dimension, and to achieve rapid mixing to initiate reactions. We are using flow modeling and experiments to study the effects of flow inertia on such flow focusing. At low Reynolds numbers, these flows have been well studied and produce relatively simple flows with the center (sample) stream focused along the centerline of the output channel. At Reynolds (and Dean) numbers of order 10, we observe complex streamlines in these flows including three dimensional (3D) secondary flows such as 3D vortical structures. In some conditions, the center stream bifurcates into two or four streams which then flow near opposite walls of the outlet channel. We are also studying the effects of unwanted asymmetric imperfections in the inlet flows of these devices. We find inertial effects can determine which side of the outlet channel the output sample stream will flow. These studies have implications in microfluidic system design and estimates of mixing time scales for such flows. [Preview Abstract] 

F15.00002: ABSTRACT WITHDRAWN 
Monday, November 20, 2017 8:26AM  8:39AM 
F15.00003: Dynamical phase separation using a microfluidic device: experiments and modeling. Benjamin Aymard, Urbain Vaes, Anand Radhakrishnan, Marc Pradas, Asterios Gavriilidis, Serafim Kalliadasis We study the dynamical phase separation of a binary fluid by a microfluidic device both from the experimental and from the modeling points of view. The experimental device consists of a main channel (600$\mu$m wide) leading into an array of 276 trapezoidal capillaries of 5$\mu$m width arranged on both sides and separating the lateral channels from the main channel. Due to geometrical effects as well as wetting properties of the substrate, and under well chosen pressure boundary conditions, a multiphase flow introduced into the main channel gets separated at the capillaries. Understanding this dynamics via modeling and numerical simulation is a crucial step in designing future efficient microseparators. We propose a diffuseinterface model, based on the classical CahnHilliardNavierStokes system, with a new nonlinear mobility and new wetting boundary conditions. We also propose a novel numerical method using a finiteelement approach, together with an adaptive mesh refinement strategy. The complex geometry is captured using the same computeraided design files as the ones adopted in the fabrication of the actual device. Numerical simulations reveal a very good qualitative agreement between model and experiments, demonstrating also a clear separation of phases. [Preview Abstract] 
Monday, November 20, 2017 8:39AM  8:52AM 
F15.00004: Fluid rheological effects on particle migration in rectangular microchannels. Di Li, Xiangchun Xuan There has been an increasing interest in the use of viscoelastic solutions for particle focusing and separation in microfluidic devices. These passive manipulations arise from the flow induced elastic lift force that interacts with the inertial lift force for an enhanced control of particle motions. The rheological properties of the suspending fluid are supposed to have a significant impact on particle migration in microchannels. We present in this work an experimental investigation of the elastic and/or inertial focusing of polystyrene particles suspended in the flow of four types of fluids with varying rheological properties through a straight rectangular microchannel. Such a fundamental study is expected to provide useful data for fluid rheological effects on particle migration, which may be used to validate theoretical models. [Preview Abstract] 
Monday, November 20, 2017 8:52AM  9:05AM 
F15.00005: The fluid transport in inkjetprinted liquid rivulets Timothy Singler, Liang Liu, Xiaoze Sun, Yunheng Pei 
Monday, November 20, 2017 9:05AM  9:18AM 
F15.00006: Abstract Withdrawn The stability and dynamics of thin liquid films(\textless 100 nm) are of immense importance in industrial settings like coating and printing processes and extraction of oil from porous rocks. In this study a hydrodynamic model is introduced to capture the long term evolution of a Newtonian liquid film containing insoluble surfaceactive particles.We consider here the possibility of four distinct interaction regimes based on the surface rheological effects of the particles, such that either, both or neither of Marangoni and surface viscosity effects would be present at the leading order in the governing equations. The liquid film is bounded by a rigid impermeable solid below and covered by passive air phase above.A standard linear stability analysis and nonlinear simulations are performed on the set of highly coupled partial differential evolution equations. Linear stability analysis gives insights on whether a particular imposed perturbationwavenumber will grow or decay in time and also evaluating the fastest growing wavenumber. Parametric studies for all four regimes provides a strong confirmation that surface viscosity and Marangoni effects are indeed rupture delaying effects. 
Monday, November 20, 2017 9:18AM  9:31AM 
F15.00007: Taylor dispersion in peristaltic pumping David Saintillan, Brato Chakrabarti The diffusivity of a Brownian tracer in unidirectional flow is generally enhanced due to shear by the classic phenomenon of Taylor dispersion. At long times, the average concentration of the tracer follows a simplified advectiondiffusion equation with an effective sheardependent dispersivity. In this work, we make use of Brenner's generalized Taylor theory for periodic domains to study dispersion in peristaltic pumping. In channels with small aspect ratios, asymptotic expansions are employed to obtain analytical expressions for the dispersivity at both small and high Peclet numbers. Channels of arbitrary aspect ratios are also considered using a boundary integral formulation for the flow coupled to a hyperbolic conservation equation for the effective dispersivity, which is solved by the finitevolume method. Our numerical results show good agreement with theoretical predictions and provide a basis for understanding passive scalar transport in peristaltic flow, for instance in the ureter or in microfluidic peristaltic pumps. [Preview Abstract] 
Monday, November 20, 2017 9:31AM  9:44AM 
F15.00008: Dynamics of an elastic sphere containing a thin creeping region and immersed in an acoustic region for similar viscouselastic and acoustic time and lengthscales Amir Gat, Yonathan Friedman The characteristic time of lowReynolds number fluidstructure interaction scales linearly with the ratio of fluid viscosity to solid Young's modulus. For sufficiently large values of Young's modulus, both time and lengthscales of the viscouselastic dynamics may be similar to acoustic time and lengthscales. However, the requirement of dominant viscous effects limits the validity of such regimes to microconfigurations. We here study the dynamics of an acoustic plane wave impinging on the surface of a layered sphere, immersed within an inviscid fluid, and composed of an inner elastic sphere, a creeping fluid layer and an external elastic shell. We focus on configurations with similar viscouselastic and acoustic time and lengthscales, where the viscouselastic speed of interaction between the creeping layer and the elastic regions is similar to the speed of sound. By expanding the linearized spherical Reynolds equation into the relevant spectral series solution for the hyperbolic elastic regions, a global stiffness matrix of the layered elastic sphere was obtained. This work relates viscouselastic dynamics to acoustic scattering and may pave the way to the design of novel metamaterials with unique acoustic properties. [Preview Abstract] 
Monday, November 20, 2017 9:44AM  9:57AM 
F15.00009: Dynamics of anchored oscillating nanomenisci Thierry Ondar\c{c}uhu, Caroline Mortagne, Kevin Lippera, Philippe Tordjeman, Michael Benzaquen The study of liquid dynamics in the close vicinity of the contact line is fundamental to understand the physics of wetting. In this context, we present a selfcontained study of the dynamics of oscillating nanomenisci anchored on topographical defects around a cylindrical nanofiber (radius below 100 nm). Using frequencymodulation atomic force microscopy (FMAFM) with dedicated tips, we show that the friction coefficient surges as the contact angle is decreased. We propose a theoretical model within the lubrication approximation that reproduces the experimental data and provides a comprehensive description of the dynamics of the nanomeniscus. The dissipation pattern in the vicinity of the contact line and the anchoring properties of the defects are discussed as a function of liquid and surface properties in addition to the solicitation conditions and defects size. [Preview Abstract] 
Monday, November 20, 2017 9:57AM  10:10AM 
F15.00010: Generalized Knudsen Number for Oscillatory Flows Generated by MEMS and NEMS Resonators Kamil Ekinci, Vural Kara, Victor Yakhot We have explored the scaling behavior of oscillatory flows that are generated by the oscillations of MEMS and NEMS resonators in a gas. If the gas is gradually rarefied, the NavierStokes equations begin to fail and a kinetic description of the flow becomes more appropriate. The failure of the NavierStokes equations can be thought to take place via two different physical mechanisms: either the continuum hypothesis breaks down as a result of a finite size effect; or local equilibrium is violated due to the high rate of strain. By independently tuning the relevant linear dimensions and the frequencies of the MEMS and NEMS resonators, we experimentally observe these two different physical mechanisms. All the experimental data, however, can be collapsed using a single dimensionless scaling parameter that combines the linear dimension and the frequency of each resonator. This proposed Knudsen number for oscillatory flows is rooted in a fundamental symmetry principle, namely Galilean invariance. [Preview Abstract] 
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