Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session F24: Microscale Flows: Complex Fluids |
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Chair: Cheng Wang, Missouri University of Science and Technology Room: Georgia World Congress Center B312 |
Monday, November 19, 2018 8:00AM - 8:13AM |
F24.00001: Dynamics of paramagnetic and ferromagnetic prolate spheroids in simple shear flow and uniform magnetic field Christopher A Sobecki, Jie Zhang, Yanzhi Zhang, Cheng Wang We present a theoretical analysis of rotational dynamics for paramagnetic and ferromagnetic particles of prolate ellipsoidal shapes in an unbounded Newtonian shear flow at zero-Reynolds numbers and under a uniform magnetic field. In the absence of a magnetic field, both particles will perform periodic rotations known as Jeffery’s Orbit. The application of a magnetic field affects the rotational dynamics of paramagnetic and ferromagnetic particles differently due to their magnetic properties. To describe the relative strength between the magnetic and hydrodynamic torques, we introduce two dimensionless parameters and determine their critical values above which particle rotations are impeded. In a weak magnetic field, both particles perform periodic rotations but with different symmetry properties. We discuss the relationship between the symmetry of their rotation and the direction of the magnetic field. In a strong magnetic field, the paramagnetic and ferromagnetic particles are impeded at different steady angles and their stability is analyzed. The differences of particle dynamics result in different lateral migration behaviors in wall-bounded shear flows, which are demonstrated by numerical simulations. |
Monday, November 19, 2018 8:13AM - 8:26AM |
F24.00002: Study of deformation of a ferrofluid droplet in simple shear flows under a uniform magnetic field Cheng Wang, Md. Rifat Hassan When a droplet is subjected to simple shear flows, it is deformed by the flow field. Magnetic field provides an additional means of controlling the deformation of ferrofluid droplets. Here, we have performed a thorough numerical investigation on the deformation and orientation of a ferrofluid droplet in a simple shear flow under a uniform magnetic field. Based on the finite element method, our two-dimensional simulation couples the magnetic field with the flow field. A level set method is used to track the dynamic motion of the droplet interface. Focusing on low Reynolds number flows (Re $\le 0.02$), we found that at a small capillary number (Ca $\approx 0.02$), with increasing magnetic field strength, the magnetic field dominates over shear flow and determines the deformation of the droplet. The orientation of the droplet is also found to be aligned with the direction of the magnetic field more as the magnetic field strength is increased. For small capillary number cases, the droplet deformation is found to be maximum at a = 45° (direction of magnetic field relative to the flow) and minimum at a = 135°. Additionally, the flow field inside and outside of the droplet are different due to different magnetic field conditions. |
Monday, November 19, 2018 8:26AM - 8:39AM |
F24.00003: Lateral migration of ferrofluid droplets in a microchannel under uniform magnetic fields Cheng Wang, Jie Zhang Manipulation of microfluidic droplets is an important step for applications in chemical and biological assays. In this work, we experimentally studied the lateral migration of ferrofluid micro-droplets in a microchannel under uniform magnetic fields, where an aqueous ferrofluid works as the discrete phase and an oil works as the continuous phase. When the uniform magnetic field is applied, the droplet is deformed and migrates laterally. The effect of the direction and strength of magnetic field, interfacial tension on the lateral migration of magnetic droplet is investigated. The results show that the direction of droplet deformation and lateral migration depends on the direction of magnetic field. The degree of deformation and the net lateral migration depends on the strength of magnetic field. This method provides a simple and flexible means to manipulate and sort microfluidics droplets, as compared with other force-based manipulation techniques. |
Monday, November 19, 2018 8:39AM - 8:52AM |
F24.00004: Stabilization of the out-of-plane precession of magnetic nanorods in Magnetic Rotational Spectroscopy experiments Vaibhav Palkar, Pavel Aprelev, Olga Kuksenok, Konstantin Kornev Magnetic Rotational Spectroscopy (MRS) is a nanorheological technique offering analysis of the rheological response of a complex fluid on weak shear loads caused by rotating nanorods. MRS relies on imaging complete revolution of nanorods and thus offers high sensitivity. The original technique involves analysis of planar rotation of nanorods and determining a critical frequency of rotating magnetic field at which a synchronous rotation turns into an asynchronous rotation. Magnetic nanorods are subject to the out-of-plane perturbations of the ambient magnetic field and it becomes a real difficulty for an experimentalist to cancel the bias field: Earth’s field appears to be a significant obstacle for the MRS experiments. We theoretically predicted and experimentally validated that a bias field surprisingly acts favorably for MRS in viscous fluids as it stabilizes synchronous precession of the nanorod. The phase portrait of the associated dynamic system predicts unexpectedly complex dynamics of 3D rotation of nanorods. We report on the impact of these dynamics on studying viscosity of nanoliter droplets. |
Monday, November 19, 2018 8:52AM - 9:05AM |
F24.00005: Cell encapsulation in a flow focusing microchannel: Effects of viscoelasticity Mohammad Nooranidoost, Daulet Izbassarov, Ranganathan Kumar The effect of viscoelasticity of working fluids on cell encapsulation dynamics is investigated in an axisymmetric flow focusing configuration using a three-phase front-tracking method. A series of cells with predefined size and frequency are encapsulated by the disperse phase forming compound droplets suspended in an outer fluid. Viscoelasticity of the fluids is modeled using a model called FENE-CR. Following Nooranidoost et al [1,2], compound droplet formations are examined for viscoelastic parameters including Weissenberg number, polymeric viscosity ratio, and extensibility parameter. It is found that these parameters have a significant influence on droplet size, size distribution and frequency of droplet generation. Depending on the flow rate of the outer and inner fluids, viscoelasticity of the fluids may increase/decrease the droplet size and its distribution. It is also found that the viscoelasticity has a similar effect as decreasing flow rate ratio and acts to delay transition from squeezing to dripping regime. This study can be useful to improve single cell encapsulation. ^{1}M. Nooranidoost, D. Izbassarov and M. Muradoglu, Phys. Fluids 28, 123102 (2016). ^{2}M. Nooranidoost, M. Haghshenas, M. Muradoglu and R. Kumar, Bull. Am. Phys. Soc. Volume 62, Number 14, (2017). |
Monday, November 19, 2018 9:05AM - 9:18AM |
F24.00006: A numerical study on viscoelastic droplet migration on substrates with wetting gradients Fan Bai, Hongna Zhang, Sang Joo The study of spontaneous motions of a droplet migrating due to substrate wettability gradient is extended to viscoelastic fluids using numerical simulations based on the multiphase-field model of OpenFOAM with new boundary conditions, ensuring correct contact angles at multiphase junctions. The migration speed and the shape of the droplet, dependent on the Weissenberg number, are examined along with other parameters studied previously. For the parameter ranges studied it is found that the fluid elasticity affects the droplet motion indirectly more by changing the droplet footprint than other ways. |
Monday, November 19, 2018 9:18AM - 9:31AM |
F24.00007: Steady streaming rheology for non-Newtonian fluids Gabriel Juarez, Giridar Vishwanathan Microfluidic rheometry of complex fluids has received much attention in the last decade and methods are in place to measure shear viscosity, extensional viscosity, and more recently, relaxation time. In this work, we discuss an experimental study on the use of steady streaming for rheology of dilute viscoelastic liquids. An oscillatory flow field is setup in a microfluidic channel using an electroacoustic transducer over a range of frequencies (≤ 1 kHz). The resulting steady streaming flow around a cylindrical post is observed using stroboscopic and high-speed imaging of passive tracer particles on an inverted microscope. The velocity fields are acquired through particle tracking velocimetry methods and the effect of elasticity is quantified by a systematic comparison with a Newtonian liquid as a function of excitation frequency and non-dimensional oscillation amplitude. |
Monday, November 19, 2018 9:31AM - 9:44AM |
F24.00008: Non-Newtonian fluid flows in a contraction-expansion microchannel Purva Jagdale, Joshua Issacks, Jeb Gary, Di Li, Xiangchun Xuan Non-Newtonian fluids have been studied widely for various biological and industrial applications. Microfluidics provides a simple and efficient way to understand non-Newtonian fluid flow on small length scales for a wide range of Reynolds and/or Weissenberg number. In this study we show through systematic experiments the behavior of non-Newtonian fluid flow in a planar contraction- expansion microchannel. Five important types of fluids are tested in this geometry to investigate the sole and combined effects of fluid elasticity, shear thinning and inertia. Each characteristic behavior of the fluid plays a role in inducing lip or corner vortices, instability in the flow and bending of streamlines at the channel constriction. Such a fundamental study will possibly help in understanding the flow of non-Newtonian fluids through complex channel geometries of lab-on-a-chip devices. |
Monday, November 19, 2018 9:44AM - 9:57AM |
F24.00009: High-Throughput Microfluidic Creep Relaxation Experiments in an Extensional Flow Device Huda Irshad, Deqiang Xu, Joanna B Dahl Many microfluidic platforms that measure the mechanical properties of single cells rely on extensive approximations and empirical calibration due to complicated cell deformations and viscous stresses. We present a microfluidic system that closely matches the mechanical modeling field equations and boundary conditions so that true mechanical properties can be extracted from observed deformations. We previously demonstrated the feasibility of a microfluidic extensional flow device that stretches single cells to measure viscoelastic mechanical properties (stiffness and fluidity) of cells using a phenomenological mechanical modeling approach. Here we rigorously derive the mechanical equation for this microfluidic mechanical measurement system using the elastic-viscoelastic correspondence principle. The new equation is applied to a creep relaxation technique to measure the properties of alginate hydrogel microparticles. With this mechanically-consistent analytic formula, the measured mechanical properties are independent of the measurement platform. The properties can be used in numerical simulations that investigate physiologically relevant situations with confidence that the predicted mechanical responses are quantitatively accurate. |
Monday, November 19, 2018 9:57AM - 10:10AM |
F24.00010: Viscoelastic secondary flows in curved microchannels Lucie Ducloue, Laura Casanellas, Simon J Haward, Robert J. Poole, Manuel A. Alves, Sandra Lerouge, Amy Q Shen, Anke Lindner The flow of viscoelastic fluids is well-known to develop purely elastic instabilities in curved geometries in the absence of inertia. Below the critical shear rate at which the instability is triggered, a steady, secondary flow driven by the first normal stress difference and the curvature of the streamlines develops in the cross-section of the channel. For channels of constant curvature and square cross-section, numerical calculations have shown that this flow takes the shape of two counter-rotating vortices. We present the first experimental visualization evidence and characterization of this steady secondary flow. Using a dilute solution of polymer, we capture the nature of the flow by performing confocal imaging of the stream-dyed fluid in the channel cross-section. We show that the observed dye transport is in good qualitative agreement with the flow lines computed numerically. We then use micro-PIV techniques to measure the components of the flow velocity in the plane of the microchannel, half-way between the top and bottom walls. We show that the measured streamlines and the relative velocity magnitude of the secondary flow are in quantitative agreement with the numerical results. |
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