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 G07: Microchannel Flows |
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Chair: Xiangchun Xuan, Clemson University Room: Georgia World Congress Center B212 |
Monday, November 19, 2018 10:35AM - 10:48AM |
G07.00001: Flow recirculation in microfluidic T-junctions and bends San To Chan, Simon J Haward, Amy Q Shen Flow recirculation occurs in the outlets of a dividing T-junction of square cross-section when the inlet Reynolds number Re exceeds a critical value Re_{c} ≈ 350 [1]. The recirculation zones can trap particles, which has important implications to the physics and engineering of inertial microfluidics. We investigate such flow phenomenon using novel glass microfluidic T-junction devices [2]. First, by micro-particle image velocimetry, we show the formation of two counter-rotating Dean vortices in the channel cross-section. Then, by a method that enables high-contrast imaging of recirculating streamlines, we visualize the complete structure of the recirculation zone for Re > Re_{c}. By varying the ratio of flowrate in the two outlets, we effectively decouple the swirl from the rate of vorticity decay. We demonstrate that even slight outflow imbalances can significantly alter both Re_{c} and the structure of the recirculation zones. Finally, we show that similar flow recirculation also occurs in sharp microfluidic bends. 1. Ault J T et al. (2016) Phys Rev Lett 117: 084501 2. Chan S T et al. (2018) Phys Rev Fluids 3: 072201(R) |
Monday, November 19, 2018 10:48AM - 11:01AM |
G07.00002: Experiment and depth-averaged analysis of water flow in contraction-expansion microchannels Xiangchun Xuan, Di Li The entry flow through a sudden contraction is a classical multidimensional problem that has been studied for decades. It becomes even more complicated in planar geometries that are typical in the microfluidics community because of the standard soft lithography technique. A three-dimensional simulation of such a flow, though proving accurate, is inevitably time-consuming and computationally expensive. Considering the fact that the planar microfluidic channels often have a small width/depth aspect ratio, we present in this work a generalized depth-averaged model suitable for studying the flow behavior in microfluidic devices with shallow-channel geometries. We demonstrate this idea by comparing the prediction of a depth-averaged analysis with both the three-dimensional simulation and the experimental observation for water flow through planar contraction-expansion microchannels of varying depths under varying flow rates. |
Monday, November 19, 2018 11:01AM - 11:14AM |
G07.00003: Viscous flow between soft planar surfaces Magnus Paludan, Keunhwan Park, Kaare Hartvig Jensen Various applications, from drug delivery to bioreactor homeostasis, require precise control of liquid flow rates. Fluid-structure interactions in miniaturized fluidic systems were recently proposed as a novel tool to achieve this goal (Duprat and Stone, Royal Society of Chemistry, 2015). We report a combined experimental, theoretical, and numerical study of pressure-driven radial flow in the narrow gap between a solid wall and a soft polymer membrane. Our experiments indicate that when the applied pressure is relatively small, the flow velocity (and hence flow rate) in the gap scales linearly with pressure. However, above a certain threshold value, the flow rate decreases with increasing applied pressure. This reversal of flow characteristics is due to elastic deformations of the soft membrane. A theory based on low-Reynolds-number lubrication theory and linear elasticity is developed which capture the main physical effects. The theoretical predictions agree qualitatively with the experimental results and quantitatively in certain configurations. |
Monday, November 19, 2018 11:14AM - 11:27AM |
G07.00004: Flow and Slippage of a Soft-Material by Controlled Surface Roughness Shape Francesca Pelusi, Mauro Sbragaglia, Andrea Scagliarini, Matteo Lulli, Massimo Bernaschi, Sauro Succi An assembly of soft jammed particles (“soft-material”) usually exhibits a complex rheology with a yield stress, i.e. a threshold value below which the material responds elastically to external perturbations. When flowing in a confined microfluidic channel, the material exhibits an effective slippage at the walls, which becomes the dominant contribution to the flow below or close to yield. Wall roughness is commonly accepted to decrease wall slippage, however quantitative assessments of the impact of surface roughness shape on wall slippage are scarce in literature. With the help of numerical simulations, in this work we systematically assess the impact of surface roughness shape on the scaling-laws that relate the wall slippage to wall stress. Both linear and quadratic scaling laws are observed for small and large values of wall stress respectively, provided that surfaces are flat/weakly rough. At moderate/large roughness, the linear scaling is suppressed, while the quadratic one persists. In all cases, the scaling observations are accompanied by a systematic analysis of the micro-mechanics in the rough boundary layer. |
Monday, November 19, 2018 11:27AM - 11:40AM |
G07.00005: Boundary Effects in Microchannel Flows Arezoo Hajesfandiari, Haoyu Zhang, James Chen Navier-Stokes (NS) theory becomes invalid as the near-wall rotation dominates. Recent experimental data suggest that NS fails to predict fundamental flow metrics, e.g. volume flow rate, in fluidics system at the micrometer range. Researchers have been resorting to first-principles based simulations; however such methods tend to be computationally expensive. This study introduces an additional term to the classical NS equations through the couple stress theory (CST) which is related to the volumetric collisions between the fluid elements. CST involves a new length scale parameter which leads to a new energy dissipation mechanism. This new mechanism is used to explain why NS is invalid for microfluidics system while demonstrating a theoretically rigorous correlation between CST and NS theory. These equations combined with appropriate boundary conditions are solved numerically to study the fully developed behavior of the flow in a rectangular channel. The detailed comparisons between the numerical results and experimental measurements present a clear picture for the complicated flow physics when the boundary effects dominate. |
Monday, November 19, 2018 11:40AM - 11:53AM |
G07.00006: Axial flow in a two-dimensional microchannel induced by a travelling temperature wave imposed at the bottom wall Chenguang Zhang, Harris H Wong, Krishnaswamy Nandakumar To explore driving mechanisms in microchannels, we studied the transport of fluid in a two-dimensional channel induced by a traveling temperature wave applied at the bottom wall. The Boussinesq approximation is used for the buoyancy effect. The system of equations is transformed to the coordinate moving with the temperature wave so time dependence is removed. Four dimensionless numbers emerge from the governing equations and boundary conditions: the Reynolds number Re, a Reynolds number Rc based on the wave speed, the Prandtl number Pr, and the dimensionless wavenumber K. The system of equations is solved by a finite-volume method and by a perturbation method in the limit Re→0. Surprisingly, the leading and first-order perturbation solutions agree well with the computed axial flow for Re<=1000. Thus, the analytic perturbation solutions are used to study systematically the effects of Re, Rc, Pr, and K on the axial flow Q. We find that Q varies linearly with Re, and Q/Re versus any of the three remaining dimensionless numbers always exhibits a maximum. The global maximum of Q/Re in the parameter space is determined for the first time. This axial flow exerts no net stress on channel walls and is driven solely by the Reynolds stress. |
Monday, November 19, 2018 11:53AM - 12:06PM |
G07.00007: 3D fluid flow manipulation on a microscopic scale Jeremias Gonzalez, David Quint, Ajay Gopinathan, Bin Liu Real-time control in a 3D flow chamber is essential for probing the force-free dynamics of swimming microorganisms. We present a novel device called a ``Stokes wind tunnel'' that permits direct control over a 3D fluidic chamber using independent pressure pumps that connect to a series of channels to a central chamber. Leveraging these basic components, we can create 3 orthogonal flow modes that act as a ``wind tunnel'' in which microorganisms become trapped in the center of the chamber under uniform Stokes flow due to a low Reynolds number condition. Our device also has the capacity to superimpose flow modes in order to form more complex flow patterns, such as shearing flow and extensional flow. These higher order flow patterns can be used to perturb microorganism dynamics and behavior in real-time through precise flow control. We additionally show the ability to perform high-throughput reconstruction of particle trajectory in 3D, which allows analysis of objects in the generated flow patterns. Our device represents the next generation in microfluidic control for probing fundamental microbial biomechanical systems. |
Monday, November 19, 2018 12:06PM - 12:19PM |
G07.00008: Flow analysis inside multihelical microchannel Pravat Rajbanshi, Animangsu Ghatak Flows through helical channels are important for host of scientific and engineering applications. The curvature and torsion of the helices have been revealed to stimulate secondary flow in addition to the primary axial flow, which enhances passive in-plane mixing between fluid streams. Most of these studies involve a single spiral with circular cross-section, which in essence is symmetric. The coupled effect of asymmetry of cross-section, curvature and torsion of channel would affect the flow profile inside such tubes which are yet to be explored and understood. In this perspective, here we have presented the analysis of fluid flow at low Reynolds number inside a novel triple helical channel which consists of three helical flow paths conjoined along their contour length leading to single multihelical flow geometry. We have performed both micro particle image velocimetry (µ- PIV) and 3D simulation in FLUENT of flow of a Newtonian fluid through such flow system, the results from simulation corroborates reasonably well with experimentally determined flow profiles. Our analysis also shows that in case of triple helical channels, number of vortices increases with the helix angle. |
Monday, November 19, 2018 12:19PM - 12:32PM |
G07.00009: Reducing the hydrodynamic resistance of viscous flows through needles Vishnu Jayaprakash, Caroline Taylor McCue, Maxime Costalonga, Kripa Varanasi Reducing the hydrodynamic resistance of viscous flows through confined geometries is of practical importance for a variety of applications such as drug delivery, additive manufacturing, and food processing. In the context of drug delivery, high concentration protein-based drugs are desirable due to their ability to be delivered subcutaneously, eliminating the need for the intravenous injections. However, high viscosity prevents manual injectability through standard medical needles; hence restricting the practical use of many such biologics. In this work, we present approaches to enhance the manual injectability of drug formulations through the use of multi-phase flows. Core annular flows and other lubrication techniques are used to achieve large reductions in hydrodynamic resistance. In addition, the dependence of resistance on (i) flow regimes, (ii) fluid properties such as viscosity and density and, (iii) interfacial parameters such as wettability and surface tension are characterized. Finally, we establish a regime map to minimize the hydrodynamic resistance of viscous flows through needles. |
Monday, November 19, 2018 12:32PM - 12:45PM |
G07.00010: Capillary Flow in V-Shaped Grooves: From Flatland to Curved 3D Trajectories Nicholas White, Sandra Troian Capillary flow in rectilinear V-shaped grooves inscribed on planar domains is finding widespread use in applications such as point-of-care biomedical devices, heat pipes for cooling microelectronics and spacecraft propellant management. Advances in 3D printing and microfabrication can now be used to extend simple rectilinear trajectories in 2D to arbitrarily curved compact trajectories in 3D. This introduces the potential for multi-layer and multi-functional operation of many types of microfluidic and optofluidic chips. Romero and Yost (1996) and Weislogel (1996) elucidated how the streamwise gradient in capillary pressure due to the change in radius of curvature of the circular fluid interface caused by differences in local film thickness leads to rapid wicking of Newtonian films in slender rectilinear V-grooves. We present an analytic model which extends that original work to arbitrarily curved V-grooves in 3D. Despite the complex flow trajectories allowed, a first order perturbation analysis yields a compact equation for the moving interface. This advance should be of use to the design and implementation of next generation 3D fluidic devices. |
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