Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session U08: Microscale Flows: Interfaces and Wetting (8:45am - 9:30am CST)Interactive On Demand
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U08.00001: Phase separation of a confined ionic-liquid -- water mixture in a temperature gradient Marc Pascual, Alexandre Vilquin, Arthur Poquet, Marie-Caroline Jullien Ionic liquids have remarkable properties and are commonly harnessed for green chemistry, lubrication and energy applications. In this paper, we study a thermoresponsive Water - Ionic Liquid (IL) binary mixture which has the property of phase separating above a critical temperature (LCST system). For this purpose, we generate a temperature gradient in a microfluidic cavity where the confinement strengthens wetting effects and enhances the demixing. We show that the phase separation is performed by the joint effects of sedimentation and thermocapillary actuation giving rise to a 3D flow structure, which is quantitatively captured by our model. Thermocapillary forces impose the droplets direction of migration, while interactions with the walls play also a crucial role: a micrometer-thick wetting layer which undergoes thermal interfacial transport gives rise to shear flows oriented toward the warmer side. Altogether those mechanisms lead to the accumulation of the wetting phase near the heating source. The universality of the phenomenon is demonstrated with a Water-Lutidine mixture. We believe this work will find applications in the recycling of ionic liquids. [Preview Abstract] |
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U08.00002: Capillary-Flow Dynamics in Open Rectangular Microchannels Panayiotis Kolliopoulos, Krystopher Jochem, Daniel Johnson, Wieslaw Suszynski, Lorraine Francis, Satish Kumar Capillary flow of liquids plays a key role in many applications including lab-on-a-chip devices, heat pipes, and printed electronics manufacturing. Motivated by these applications, we combine theory and experiment to examine capillary-flow dynamics in open rectangular microchannels. SEM and profilometry are used to highlight the complex free-surface morphology. We develop a self-similar lubrication-theory-based (LTB) model accounting for this complexity and compare its predictions to those from the modified Lucas-Washburn (MLW) model, as well as experimental observations over a range of channel aspect ratios $\lambda$ and equilibrium contact angles $\theta$. For large $\lambda$ the two models are indistinguishable, whereas for smaller $\lambda$ the LTB model agrees better with experiments. The LTB model is in better agreement with experiments at smaller $\theta$, although as $\theta\rightarrow\pi/4$ it fails to account for important axial curvature contributions to the free surface and the agreement worsens. Finally, the LTB model also predicts the dynamics of fingers which extend ahead of the meniscus. These findings elucidate the MLW model limitations and demonstrate the importance of accounting for the complex free-surface morphology in open microchannel capillary flows. [Preview Abstract] |
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U08.00003: Design, fabrication, and analysis of a capillary diode for water-oil separation Dhiraj Nandyala, Zhen Wang, David Hwang, Thomas Cubaud, Carlos Colosqui A capillary slit is designed and fabricated with a micro/nanopatterned section enhancing surface wettability in order to work as a fluidic diode with vanishingly small hydrodynamic conductance for vertical imbibition when immersed in liquid above a critical depth. A large surface energy barrier at the junction between the patterned and non-patterned surface sections of the slit produces a finite immersion depth for liquid imbibition (i.e., ``forward'' capillary conduction). The finite critical immersion depth for forward conduction is controlled by the geometric design of the slit channel and micro/nanopattern, and the surface energy of the wetting fluids. The studied capillary device can work as a fluidic diode for water and behaves as a conventional capillary slit for the imbibition of oils. A prototype device with simple geometric design is demonstrated for the selective adsorption and separation of water and oil in vertical imbibition experiments at controlled immersion depths. Theoretical and experimental results indicate that the studied fundamental mechanism for selective liquid imbibition has potential application in simple devices for passive water-oil separation. [Preview Abstract] |
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U08.00004: Droplet-Coupled Wicking in Nanochannels with Micropores Sajag Poudel, An Zou, Shalabh Maroo Comprehending the dynamics of liquid spreading and subsequent evaporation on a nanostructured surface is key towards designing high heat flux dissipation devices. We performed experimental and numerical investigation of droplet-coupled wicking on a fabricated sample of cross-connected nanochannels (height \textasciitilde 728 nm) having micropores (radius \textasciitilde 1 \textmu m) at each intersection of channels. The overall phenomenon of droplet spreading is experimentally identified to have two distinct phases: (i) wicking dominant phase during which the liquid spread is governed by capillary pressure and viscous resistance and, (ii) evaporation dominant phase governed by thin-film evaporation from the menisci (\textit{The J. of Phy. Chem. C},~\textit{123}(38), 23529-23534). Computational fluid dynamics simulation of droplet wicking and evaporation is also performed on the nanochannels geometry (Langmuir, 36(27), 7801-7807) which helped estimate the spatial and temporal evaporation rate flux occurring at the menisci, as well as the wicking flux velocity and pressure gradient inside the nanochannels, thus quantifying key parameters of the evaporation dominant phase. This computational and experimental work provides a comprehensive understanding of droplet wicking mechanics and evaporation dynamics in such nanochannels. [Preview Abstract] |
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U08.00005: A Numerical Study of Pore Scale Partially Wet Interfacial Displacements Soheil Esmaeilzadeh, Zhipeng Qin, Amir Riaz, Hamdi A. Tchelepi Here, we study the pore-scale interfacial dynamics of immiscible two-phase flow in a partially wet regime using direct numerical simulation. We consider the displacement of a wetting fluid by a nonwetting one in a drainage scenario and a nonwetting fluid by a wetting one in an imbibition scenario. In a capillary tube we validate the previous experimental observations of pinch-off dynamics for a drainage scenario given a more viscous fluid being displaced by a less viscous one and extend such observations through a systematic study to an imbibition regime. We find that beyond a wettability dependent critical capillary number, a stable displacement by a less viscous fluid can transition into a growing finger that eventually leads to a series of pinch-off events in both wetting and non-wetting contact angles. For the phase separation in the form of pinch-off for drainage and imbibition, this work provides a quantitative study of the emerging length and time scales and their dependence on the wettability conditions, capillary effects, and viscous forces. [Preview Abstract] |
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U08.00006: Inertial effects on the flow near a moving contact line Akhil Varma, Anubhab Roy, Baburaj Puthenveettil Wetting phenomenon involves the motion of a liquid interface over a solid surface, characterized by a moving contact line; sliding drops and dip coating processes are some classic examples. While the traditional approach to model the flow near a contact line is to neglect inertia and assume Stokes flow, we show that this is inadequate to accurately describe the flow near fast-moving contact lines, where the flow lies in the visco-inertial regime. To this end, we implement the classical regular perturbation approach to determine the higher-order inertial corrections to the Stokes flow near a moving, straight contact line. We then pay particular attention to the influence of inertia on the flow velocity at the liquid interface. Furthermore, we formally show that at small length scales where the contact line physics become important, the inertial effects are negligible, and hence unlikely to affect the dynamic contact angle models. The analysis presents additional theoretical challenges in determining the solution at a few critical contact angles where modified, non-singular expressions are warranted, which we determine from first principles. [Preview Abstract] |
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U08.00007: Modeling capillary-assisted thin film evaporation on hierarchical surfaces. Arif Rokoni, Tejaswi Soori, Lige Zhang, Dong-Ook Kim, Ying Sun Capillary-assisted liquid delivery significantly enhances evaporation rate during thin film evaporation on structured surfaces. At equilibrium, evaporation rate is balanced by liquid delivery, whereas contact line starts to recede when evaporation rate becomes dominant. Hierarchical surfaces may not always improve evaporation rate over microstructures. In the present study, a mathematical model is developed to predict evaporation rate on hierarchical surfaces by considering evaporation rate, liquid delivery, and micropillar level meniscus shape. The model shows improvement in evaporation rate on sparsely spaced micropillars decorated with nanorods compared to bare micropillars. The instantaneous shape of the receding meniscus was captured using laser interferometry and then used as an input in the model. Experiments show that the bulk receding front follows a two-stage motion, slower around the micropillars but faster in-between pillars and the evaporation rates measured using thermocouples agree well with model predictions. The model sheds light on more effective designs of hierarchical surfaces for enhanced thin film evaporation. [Preview Abstract] |
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U08.00008: Intermediate states of wetting on hierarchical superhydrophobic surfaces Sebastian Dehe, Baruch Rofman, Valeri Frumkin, Moran Bercovici, Steffen Hardt We study the wetting transition on hierarchical superhydrophobic surfaces subjected to external forcing, and show that, in contrast to the common description of an abrupt jump between stable states, such surfaces experience a continuous set of intermediate states. We use two-tier hierarchical surfaces created by the random deposition of nanoparticles over a baseline microstructure, subject them to continuous electrical forcing, and image the light reflections from the gas-liquid interface to visualize the transitions. Using this method, we show that the transition is partially reversible and is limited only by localized Cassie to Wenzel transitions at nano defects in the structure. In addition, we show that even a surface containing many localized wetted regions can still exhibit extremely low contact angle hysteresis, thus remaining useful for many applications. Expanding the classical definition of the Cassie state in the context of hierarchical surfaces, from a single state to a continuum of meta-stable states ranging from the centimeter to the nanometer scale, is important for a better description of the slip properties of superhydrophobic surfaces, and provides new considerations for their design. [Preview Abstract] |
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U08.00009: Motion of contact line and reconfiguration of two-phase interface: realistic molecular dynamics simulations and Cahn-Hilliard phase field simulations. Ugis Lacis, Michele Pellegrino, Berk Hess, Gustav Amberg, Stephane Zaleski, Shervin Bagheri Two-phase flow is ubiquitous in both nature and engineering. One example is imbibition of a liquid in a porous material, involving many moving contact lines and reconfiguring two-phase interfaces. These processes are inherently molecular and it is still unclear how to quantitatively model these effects in continuum flow models. To a large extent, this ambiguousness stems from lack of reference data with sufficient accuracy for a wide range of parameters on a simple canonical problem. We choose a two-dimensional Couette problem of droplet between two moving plates and use atomistic molecular dynamics (MD) simulations to create reference data for different plate velocities. We capture steady moving contact line, as well as reconfiguration (splitting) of the two-phase interface, when plate velocity is larger than a critical value. We benchmark the Cahn-Hilliard phase field model against the MD results and provide the parameters for the best representation of the MD reality. Furthermore, we provide insights into key mechanisms responsible for the motion of the contact line as well as for splitting of the two-phase interface. These results will serve as a stepping stone towards developing accurate continuum modelling of many practical problems, such as imbibition in porous media. [Preview Abstract] |
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