Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session G25: Microscale Flows: Drops and Bubbles-I |
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Chair: Thomas Cubaud, Stony Brook University Room: E145 |
Monday, November 21, 2016 8:00AM - 8:13AM |
G25.00001: Microfluidic destabilization of viscous stratifications: Interfacial waves and droplets Xiaoyi Hu, Thomas Cubaud Microfluidic two-fluid flows with large differences in viscosity are experimentally investigated to examine the role of fluid properties on hydrodynamic destabilization processes at the small scale. Two- and three-layer flow configurations are systematically studied in straight square microchannels using miscible and immiscible fluid pairs. We focus our attention on symmetric three-layer stratifications with a fast central stream made of low-viscosity fluid and a slow sheath flow composed of high-viscosity fluid. We quantify the influence of the capillary and the Reynolds numbers on the formation and evolution of droplets and wavy stratifications. Several functional relationships are developed for the morphology and dynamics of droplets and interfacial waves including size, celerity and frequency. In the wavy stratification regime, the formation and entrainment of thin viscous ligaments from wave crests display a rich variety of dynamics either in the presence or in the absence of interfacial tension between liquids. [Preview Abstract] |
Monday, November 21, 2016 8:13AM - 8:26AM |
G25.00002: ABSTRACT WITHDRAWN |
Monday, November 21, 2016 8:26AM - 8:39AM |
G25.00003: Polymer Droplet Dynamic Wetting Measurement at the Nanometer Scale on Smooth Surfaces Using Atomic Force Microscopy Mohammadreza Soleymaniha, Jonathan Robert Felts Fluid spreading is a complex phenomenon driven strongly by intermolecular forces that requires nanometer scale microscopy to observe and understand. We present a technique for measuring molten polymer spreading dynamics with nanometer scale spatial resolution at elevated temperatures on sapphire, silicon oxide and mica using tapping-mode atomic force microscopy (AFM). The experimental setup is used to measure the spreading dynamics of polystyrene droplets with $2\mu$m diameters at 115-175 C. Custom image processing algorithms realize the droplet height, radius, volume and contact angle of the droplet over time. The contact angle evolution followed a power law with time with experimental exponent values of -0.26, -0.08, and -0.2 for sapphire, silicon oxide, and mica, respectively at 115 C. ~The non-zero steady state contact angles result in a slower evolution of contact angle with time compared to Tanner's Law, as expected. We observe local crystallinity on the molten droplet surface, where crystalline structures appear to nucleate at the contact line and migrate toward the top of the droplet. Increasing the temperature from 115 C to 175 C reduced surface crystallinity from 35{\%} to 12{\%}, consistent with increasingly energetically favorable amorphous phase as the temperature approaches the melting temperature. This platform provides a way to measure spreading dynamics of extremely small volumes of heterogeneously complex fluids not possible through other means. [Preview Abstract] |
Monday, November 21, 2016 8:39AM - 8:52AM |
G25.00004: Bubble deformations in corrugated microchannels at large capillary numbers Thomas Cubaud, Martin Sauzade Multiphase flows in confined microgeometries display a variety of intriguing dynamics. Here, we experimentally examine trains of monodisperse gas bubbles of different sizes and concentrations passing through a series of extensions and constrictions from low to large capillary numbers. Using highly viscous carrier fluids, we show in particular that bubbles strongly deform in velocity fields set with the channel geometry. We measure the instantaneous front and rear velocities of periodically distorted capillary surfaces and develop functional relationships for predicting the morphology of multiphase flow patterns at the pore scale. [Preview Abstract] |
Monday, November 21, 2016 8:52AM - 9:05AM |
G25.00005: A Numerical Analysis of Droplet Breakup in Asymmetric T-Junctions with Different Outlet Pressure Gradients. Way Lee Cheng, Arum Han, Reza Sadr Droplet splitting is the breakup of a parent droplet into two or more daughter droplets of desired sizes. It is done to improve production efficiency and investigational capacity in microfluidic devices. Passive splitting is the breakup of droplets into precise volume ratios at predetermined locations without external power sources. In this study, a 3-D simulation was conducted using the Volume-of-Fluid method to analysis the breakup process of a droplet in asymmetric T-junctions with different outlet arm lengths. The arrangement allows a droplet to be split into two smaller droplets of different sizes, where the volumetric ratio of the daughter droplets depends on the length ratios of the outlet arms. The study identified different breakup regimes such as primary, transition, bubble and non-breakup under different flow conditions and channel configurations. Furthermore, a close analysis to the primary breakup regimes were done to determine the breakup mechanisms at various flow conditions. The analysis show that the breakup mechanisms in asymmetric T-junctions is different than a regular split. A pseudo-phenomenological model for the breakup criteria was presented at the end. The model was an expanded version to a theoretically derived model for the symmetric droplet breakup. [Preview Abstract] |
Monday, November 21, 2016 9:05AM - 9:18AM |
G25.00006: Using Microfluidics for Droplet and Particle Characterization of Environmental Fluids Andrew Metcalf, Chris Hogan, Cari Dutcher Two-phase flows in microfluidic platforms enable high-throughput experiments for measurement of rheological, thermodynamic, and kinetic interfacial properties. In this talk, I will highlight biphasic microfluidic studies with environmental applications, including atmospheric aerosol properties and water contamination. For example, the fate of atmospheric aerosol particles can be profoundly affected by the presence of surface-active species within the aerosol liquid. In this work, the presence of these species is detected with microfluidic interfacial tensiometry, in which the behavior of the droplet interface under extensional shear is measured. Both secondary organic aerosol chemical mimics and aerosol filter extracts are used in droplet generation. In addition, preliminary work for use of the platform for water treatment applications will be highlighted. Particulate contamination in water can be detected by freezing contaminated droplets at different temperatures to study the perturbed thermodynamic state. [Preview Abstract] |
Monday, November 21, 2016 9:18AM - 9:31AM |
G25.00007: Movement of liquid droplets containing polymers on substrate Guohui Hu, Heng Wang It is of both fundamental and practical interests to study the flow physics in the manipulation of droplets. As a microreactor, the macromolecules or particles inside the droplets might have significant influences on their movement. In the present study, the many-body dissipative particle dynamics (MDPD) is utilized to investigate the translocation of droplets containing polymer on a substrate driven by the wettability gradient, where the polymer is modelled as worm-like chain (WLC). The internal flows of the droplets are analyzed, as well as the comparison to the polymer-free moving droplets. The effects of physical parameters, such as the interaction potential between liquid particle and polymer beads, the mass of the beads, on the translocation speed are also addressed in the present study. These results might be helpful to the optimization in design of the microfluidic systems. [Preview Abstract] |
Monday, November 21, 2016 9:31AM - 9:44AM |
G25.00008: Simulation of the self-assembly of colloidal droplets in a micro-channel Zhouyang Ge, Luca Brandt In colloidal sciences, much progress has been made on the synthesis of complex building blocks mimicking molecular structures to elaborate innovative materials. The basic elements of such colloidal molecules are particles or droplets less than one millimeter in size. Their self-assembly relies on either lengthy brownian motion or careful microfludic designs, on top of typical colloidal interactions, e.g. depletion attraction. Regardless of the approach, however, questions remain why the colloids undergo certain path to organize themselves and how such process can be optimized. Here, we perform direct numerical simulations using a Navier-Stokes solver at low Reynolds number, combined with either the immersed boundary method (IBM) or a newly-proposed level set (LS) method for interface description. In the IBM simulations, the colloids are treated as rigid, spherical particles under a Lennard-Jones-like potential, reproducing attractive depletion force. Results show that, for four particles, a planar diamond is formed under a weak potential while a 3D tetrahedron is formed under a strong potential, which agree qualitatively with experiments. In the next step, LS simulation of colloidal droplets will be performed to investigate the roles of surface tension in the self-assembly. [Preview Abstract] |
Monday, November 21, 2016 9:44AM - 9:57AM |
G25.00009: Inertia effects on bubble generation in thin T-junction microchannel Kazuyasu Sugiyama, Hidehiko Okubo, Seigo Nabeshima, Tomoaki Watamura A numerical study on gas-liquid interface dynamics of bubble generation in a thin microchannel with a squeezed T-junction is performed. In consideration of liquid inertia, the basic equations consist of the Laplace law and the two-dimensional Euler-Darcy equation under the assumption of Hele-Shaw's flow owing to a large width-to-thickness aspect ratio of the channel cross-section. The velocity potential and the interface motion are numerically predicted by means of a boundary element method. The simulated results reasonably capture the experimentally observed behaviors that the interface pinches off at the channel junction and then a bubble forms. For a fixed liquid velocity, the generated bubble is likely to be smaller with decreasing the gas pressure, but the bubble is no longer generated at the gas pressure below a threshold. The bubble size minimized at the generation limit is arranged using the capillary, Reynolds and Weber numbers, and the results imply the significance of the liquid inertia in the bubble generation process in spite of the micrometer-scale phenomena. [Preview Abstract] |
Monday, November 21, 2016 9:57AM - 10:10AM |
G25.00010: A close-up view of a pancake droplet in the microfluidic chips Lailai Zhu, Francois Gallaire We develop a boundary integral method to study the droplet dynamics in confined geometries in the low-capillary-number regime, where the the lubrication film between the droplet and solid boundaries becomes important. We investigate a translating droplet tightly squeezed in a Hele-Shaw cell. The cell gap width is around $0.5 \sim 0.85$ the radius of a relaxed droplet and the capillary number is in the range $\left[0.007,0.16\right]$. We highlight the three-dimensional feature of the droplet interface and flow filed. The interface develops an arc-shaped ridge near the rear-half rim with a protrusion in the rear and a laterally symmetric pair of higher peaks; this pair of protrusions has been identified by recent experiments~\footnote{Huerre et al., Phys. Rev. Lett., vol. 115 (6), 2015, 064501} and predicted asymptotically~\footnote{Burgess & Foster, Phys. Fluids A, vol. 2 (7), 1990, pp. 1105-1117}. The mean film thickness is well predicted by the extended Bretherton model with fitting parameters. Flow fields with recirculation patterns are presented. On the horizontal plane, a dipolar disturbance flow field is identified and its $1/r^2$ spatial decay is confirmed numerically. [Preview Abstract] |
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