Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session Z34: Micro/Nano Flows: Interfaces |
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Chair: Ikuya Kinefuchi, Univ of Tokyo; Ali Beskok, SMU Room: 242 |
Tuesday, November 22, 2022 12:50PM - 1:03PM Author not Attending |
Z34.00001: Surfactant based control of droplet size in step emulsifiers Punnag Padhy, Mohammad A Zaman, Lambertus Hesselink In the step emulsification method, the sharp change in capillary pressure due to sudden release of fluidic confinement in a microchannel trigger's droplet pinch-off. Unlike most other microfluidic methods, it does not rely on the shear force due to the flow of the continuous phase to break off the droplet. This makes it a robust approach to generate highly monodisperse droplets across a wide range of fluid flow conditions and fluctuations in the microfluidic device. However, the downside of this remarkable stability is the lack of handles to tune the droplet size once the device geometry is fixed during fabrication. We explore the choice of surfactants as a potential route to control droplet sizes in step emulsifiers without changing the device geometry. Studying water droplet generation in a silicon-on-glass microfluidic device, we see that different surfactants reduce the water-oil interfacial tension to different values to tune the droplet size. Furthermore, while multiple surfactants can yield the same interfacial tension, the wetting of channel walls varies vastly. This enables droplet formation in one case while preventing its break-up in the other. Our study shows that surfactants can tune interfacial tension and channel wetting to control droplet size in step emulsifiers. |
Tuesday, November 22, 2022 1:03PM - 1:16PM |
Z34.00002: Oil-water interactions under nonequilibrium conditions with nanoparticles and surfactants Thao X Nguyen, Sepideh Razavi, Dimitrios V Papavassiliou The combined effects of surfactants and nanoparticles at the oil-water interface under shear flow are investigated with dissipative particle dynamics computations. Anisotropic Janus particles (JPs)1,2 with amidine polystyrene latex and gold surfaces were simulated3 at a dodecane-water interface and with octaethylene glycol monododecyl ether (C12E8). The water flow was driven by a body force on water. It was found that when the driving force for the flow was higher than a critical value the droplets of oil could pinch off one of the channel walls and transition to a stratified flow, or they could extend along the channel wall in the form of slag. Compared to bare oil, nanoparticles provided a more stable interface; but pinch-off occurred at the same three-phase contact angle, regardless of particle numbers or properties. The presence of surfactants caused the oil to become unstable at lower pressure drops. Computational methodologies will be discussed and their validation, as well as the mechanisms of stabilizing the oil-water interface. |
Tuesday, November 22, 2022 1:16PM - 1:29PM |
Z34.00003: Delaying dynamic wetting failure using thermal Marangoni flow Ninad V Mhatre, Marcio S Carvalho, Satish Kumar Coating processes are limited by the onset of dynamic wetting failure beyond a critical substrate speed. In this work, we study the influence of thermal Marangoni flow on dynamic wetting with the objective of delaying wetting failure to higher substrate speeds. A two-dimensional hydrodynamic model is developed to examine steady-state dynamic wetting of a Newtonian liquid in a parallel-plate geometry where a temperature gradient between the plates generates thermal Marangoni flow. The dynamics of the air displaced by the liquid are accounted for, and the Galerkin finite element method is used to calculate steady-state solutions and find the critical substrate (bottom plate) speed at which wetting failure occurs. It is found that thermal Marangoni flow directed toward the dynamic contact line at the substrate delays wetting failure to a higher speed, whereas flow away from the contact line causes wetting failure at a lower speed. Flow toward the contact line reduces the bending of the air-liquid interface and increases the thickness of the air film near the contact line. This lowers the magnitude of pressure gradients in the air phase and facilitates removal of air from the contact line, delaying the entrainment of air and consequently wetting failure to higher speeds. In contrast, flow away from the contact line increases interface bending at a given speed, leading to thinning of the air film near the contact line and causing wetting failure at lower speeds. The findings presented in this study suggest a novel strategy for designing faster coating processes through the application of thermal Marangoni flow. |
Tuesday, November 22, 2022 1:29PM - 1:42PM |
Z34.00004: Morphological transition in the instability of surface-attached hydrogel films Caroline Kopecz-Muller, Clémence Gaunand, Marjan Abdorahim, Patrick J Tabeling, Yvette Tran, Thomas Salez, Finn Box, Elie Raphael, Joshua D McGraw Hydrogels may swell drastically when brought in contact with solvent, as driven by gradients of osmotic pressure, resulting in a change in volume that depends on the swelling ratio. For poly-N-isopropylacrylamide gels (PNIPAM), the ratio of wet-to-dry volume is usually a few hundred percent. However, grafting hydrogel films onto a rigid substrate geometrically constrains the swelling. The formation of a surface pattern can result of swelling-induced in-plane stresses. |
Tuesday, November 22, 2022 1:42PM - 1:55PM |
Z34.00005: Constructing a measurement system for the nonequilibrium velocity distribution of evaporating water molecules from a liquid-vapor interface Ikuya Kinefuchi, Atsushi Matsushima, Takehiro Shiraishi, Yuta Yoshimoto, Shu Takagi Multiphase fluid analyses involving phase change require an appropriate boundary condition at the liquid-vapor interface that gives evaporation or condensation mass flux, the interface temperature, etc. The Hertz-Knudsen-Schrage equation is widely adopted for describing the net mass flux. However, the validity of this equation is often questioned because its derivation assumes the Maxwell-Boltzmann velocity distribution for evaporating molecules, which seems inconsistent with the nonequilibrium nature of evaporation/condensation. Molecular dynamics simulations indicated that the velocity distribution of evaporating molecules deviates from the Maxwell-Boltzmann distribution under highly nonequilibrium conditions, whereas such nonequilibrium velocity distributions have not yet been verified by experiments. Here we show an experimental setup for measuring the velocity distribution of evaporating water molecules from a liquid-vapor interface, which is kept in a vacuum using a nanoporous membrane. We discuss the initial result of the nonequilibrium velocity distribution of evaporating water molecules obtained by the time-of-flight method. |
Tuesday, November 22, 2022 1:55PM - 2:08PM |
Z34.00006: Adsorption of Hydrophilic Silica Nanoparticles at Oil−Water Interfaces with Reversible Emulsion Stabilization by Ion Partitioning Robert K Keane, Wei Hong, Wei He, Robbie Bancroft, Sam Teale, Anthony Dinsmore Adsorption of particles at oil−water interfaces is the basis of Pickering emulsions, which are common in nature and industry. For hydrophilic anionic particles, electrostatic repulsion |
Tuesday, November 22, 2022 2:08PM - 2:21PM |
Z34.00007: Molecular Dynamics Study of Thin Film Evaporation in Nanochannels Ali Beskok, Mustafa Ozsipahi Evaporation studies focus on the identification and characterization of heat transfer and flow dynamics in the vicinity of the solid-liquid-vapor contact line. The meniscus is often characterized by the non-evaporating adsorbed layer, thin-film, and capillary regions. The adsorbed layer, which has a thickness on the order of a nanometer, is traditionally believed to be non-evaporating due to the strong intermolecular forces producing a strong disjoining pressure that suppresses evaporation. Despite this classical view, recent molecular dynamics (MD) simulations have shown that adsorbed layer plays a significant role during thin film evaporation. Utilizing a new energy-based interface detection method, we present nonequilibrium MD simulation results of thin film evaporation of liquid argon sandwiched between two parallel platinum plates. One end of the platinum channel is heated by energy addition, while the other end is cooled at the same rate to ensure constant energy of the simulation system. Liquid argon evaporates in the heater and travels to the condenser region. Unlike the transient simulations of evaporating droplets and interfaces prevalent in the literature, the presented simulation system exhibits statistically steady transport. In this talk, we present the shapes of the evaporating menisci for 4 different channel heights varying from 2 nm, 4 nm, 8 nm, and 16 nm, at three different wall-fluid interaction parameters and under several different heating/cooling rates. |
Tuesday, November 22, 2022 2:21PM - 2:34PM |
Z34.00008: Bi-stability of non-equilibrated aqueous two-phase flows in microchannels Niki Abbasi, Janine K Nunes, Zehao Pan, Tejas Dethe, Andrej Košmrlj, Howard A Stone In recent years, there has been great interest in controlling aqueous multi-phase systems in microfluidic systems. Besides their inherent biocompatibility, it is possible to utilize these systems away from their chemical equilibrium, where the transport of species and the formation of interfacial tension give rise to a dynamic process, enabling design of novel microstructures. In our work, we study the evolution of non-equilibrated two-phase flow within a flow-focusing microfluidic device. We find that depending on the flow history, which is controlled by the order in which the inner and outer streams are first flowed in the microchannel, there are two different flow configurations that can arise. One is the conventional two-phase parallel flow. The other is the formation of fronts that propagate transverse to the main flow direction on the top and bottom walls of the channel from the outer stream towards the inner stream, which can lead to the splitting of the inner stream into two discrete flows and can affect the kinetics of phase separation. We characterize these fronts at different positions within the length of the microchannel by tuning the composition of two liquid phases, varying the flow rates, and by changing the order in which the two fluids are introduced into the microchannel. We believe these different flow configurations have important implications for the design and utilization of systems involving non-equilibrated solutions within miniaturized devices, and the transport of species within such systems. |
Tuesday, November 22, 2022 2:34PM - 2:47PM |
Z34.00009: Flow resistance of grooved heat pipes with axially varying meniscus curvature Toby Kirk, Marc Hodes, Andrew Daetz We present a semi-analytical procedure to capture the effect of slowly-varying (streamwise) meniscus curvature on the flow resistance of an axial-groove heat pipe (AGHP). The relevant small parameter in the geometry of an AGHP is the ratio of the groove period to the length of the adiabatic section. Prescribed are the AGHP geometry, its orientation with respect to gravity and relevant thermophysical properties (and, by implication, the capillary pressure driving the flow in the grooves). We focus on meniscus variation in the adiabatic region due to pressure gradients using an asymptotic expansion, and invoke the standard assumptions in the evaporator and condenser sections, i.e., that the meniscus curvature is constant and equal to that of the cylinder. In the adiabatic section, the deviation of the meniscus from this curvature is captured using a boundary perturbation. A local analysis at the contact lines ensures the flow singularities are fully resolved. Our procedure enables more accurate prediction of the capillary limit in an AGHP, and the results are compared to those in the literature. |
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