Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session A03: Dynamic Surface Interactions I |
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Chair: Hernan Barrio Zhang, University of Edinburgh Room: Ballroom C |
Sunday, November 19, 2023 8:00AM - 8:13AM |
A03.00001: Two-phase flows with moving contact lines and surfactant above the critical micelle concentration Jalel Chergui, Debashis Panda, Lyes Kahouadji, Damir Juric, Seungwon Shin, Joao T Cabral, Omar K Matar In this work, we present three-dimensional direct numerical simulations of a two-phase moving contact line problem in the presence of surfactants. The model underlying the simulations accounts for surfactant solubility effects and is valid above the critical micelle concentration. The moving contact line model is incorporated using a Generalised Navier Boundary Condition where the viscous stresses along with the surfactant-dependent Young stresses are included. A parametric study is presented for the case of flow past a soil adhering to a substrate in a channel in connection with detergency-type applications. We assess the effects of Marangoni stresses, substrate kinetics near the contact line, and mass-action kinetics between the interface and the substrate that are relevant to cleaning and decontamination. |
Sunday, November 19, 2023 8:13AM - 8:26AM |
A03.00002: Contact-line dynamics on a liquid surface Rodrigo Ledesma-Aguilar, Sergi Granados Leyva, Ignacio Pagonabarraga, Aurora Hernández-Machado, Halim Kusumaatmaja, Abhnav Naga We study the dynamics of a three-fluid contact line on an underlying solid surface, similarly to the motion of a fluid-fluid front on Slippery Liquid-Infused Porous Surfaces (SLIPS). We present full hydrodynamics simulations based on the coupled Navier-Stokes and generalised Cahn-Hilliard equations. We focus on the dependence of the apparent contact angle of the front on the relevant capillary number, and show how, remarkably, the simulation data can be reproduced by the classical result of the Cox-Voinov theory of contact-line dynamics on a solid surface. By analysing the simulation data and deriving a simplified analytical model, we identify and rationalise the relevant length-scale governing the motion of the contact line, thus establishing a clear analogy between the classical problem of contact-line dynamics on a solid, and the emerging topic of dynamic wetting of a liquid surface. |
Sunday, November 19, 2023 8:26AM - 8:39AM |
A03.00003: Contact-Angle Hysteresis and Contact-Line Friction on Slippery Liquid-like Surfaces Hernan Barrio Zhang, Elfego Ruiz-Gutierrez, Steven Armstrong, glen McHale, Gary G Wells, Rodrigo Ledesma-Aguilar Contact-line pinning (static) and dynamic friction forces arise from the fundamental interaction between droplets and solid surfaces. In this work, we explore a unique low-friction surface called Slippery Omniphobic Covalently Attached Liquid (SOCAL). SOCAL surfaces exhibit a remarkable combination of contact-angle hysteresis and contact-line friction properties, where they present low pinning but a high dynamic friction against the motion of a droplet. Volume variation experiments show that the droplets contact angle with the surface relaxes back to equilibrium values after inflow/outflow, which can be used to measure contact-angle hysteresis with significant improvement in accuracy. Additionally, we observe that the relaxation dynamics of the contact line are uniquely slow in this low pinning surface, a concept that defies the intuition of low friction interactions. Hence, we compare predictions made by the Cox-Voinov model and the contact-line model based on Molecular Kinetic theory to understand the dynamics at play in the motion of a water droplet on SOCAL surfaces. Our results feature a special relation between the low static friction and the unexpectedly high dynamic friction observed this type of surface, which is driven by the kinetics of the contact line. This work highlights the unique wettability of SOCAL surfaces, provides us with insight on the relation of static and dynamic friction, and calls attention to potential application for low-pinning, slow droplet shedding surfaces. |
Sunday, November 19, 2023 8:39AM - 8:52AM |
A03.00004: Emergent collective motion of self-propelled condensate droplets Marcus Lin, Philseok Kim, Solomon Adera, Joanna Aizenberg, Xi Yao, Dan Daniel Recently, there has been much interest in droplet condensation on soft or liquid/liquid-like substrates. The ability of droplets to deform soft and liquid interfaces result in a wealth of phenomena not observed on hard, solid surfaces (e.g., increased nucleation, inter-droplet attraction). Here, we describe a unique complex collective motion of condensate water droplets that emerges spontaneously when a structured solid substrate is covered with a thin lubricant-oil film. Droplets move first in a serpentine, self-avoiding fashion before transitioning to circular motions. Through comprehensive experimental and theoretical study, we show that this self-propulsion (with speeds in the mm s-1 range) is fuelled by the interfacial energy release upon merging with newly condensed (but much smaller) droplets. The resultant emergent collective motion spans multiple length scales from sub-millimetre to several centimetres, with potentially important heat-transfer and water-harvesting applications. |
Sunday, November 19, 2023 8:52AM - 9:05AM |
A03.00005: Control of Droplet Evaporation Contact Line Dynamics by Silicone Oil Grafting Astrid Raynard, Anam Abbas, Steven Armstrong, Gary G Wells, glen McHale, Khellil Sefiane, Daniel Orejon Understanding the contact line dynamics during droplet evaporation is of importance to many everyday and industrial applications. To date, most of the research on hydrophobic coatings focuses on the development of low surface energy promoters leading to high contact angles and low contact angle hysteresis (CAH). While less effort has been placed on understanding the optimisation of fast and easy fabrication parameters for the control and tuning of such droplet-surface interactions. In this work, we address how different silicone oil grafting parameters such as viscosity of the oil (5 cSt, 20 cSt, and 100 cSt), deposition method (pipette or dip coating), number of layers (1, 2 3, 5), etc. can enable the control of the contact line dynamics during droplet evaporation. Contact angles as high as 108° with contact angle hysteresis ranging from 1° to 20° are found after grafting one single layer of oil on a smooth silicon substrate. Moreover, depending on the fabrication procedure, the duration of different evaporation modes, i.e., constant contact angle (CCA), constant contact radius (CCR), and mixed modes, can be controlled. In addition, stick-slip behaviour is also noticed during evaporation, where the magnitude of the jumps and changes in contact angle can also be tuned by the silicone oil grafting procedure adopted. The findings reported here provide the relevant guidelines for the control of the droplet-surface interactions by simple and scalable functionalisation of a smooth intrinsically hydrophilic substrate with potential applications in ink-jet printing, crop dusting, heat exchangers, and biomedical applications, amongst others. |
Sunday, November 19, 2023 9:05AM - 9:18AM |
A03.00006: Oscillation frequency of sessile droplets exposed to shearing gas flows Amir Abdollahpour, Sung Yong Jung, Mehdi Mortazavi Sessile drops exposed to shearing gas flows exhibit intriguing oscillatory motion before being removed from the surface. Our analysis has revealed that this oscillation plays a crucial role in the depinning of droplet contact lines, contributing up to around half of the force required to overcome the adhesion force. This behavior is speculated to be a result of the interplay between surface tension effects and the drag force from the shearing gas flow. The former aims to pin the droplet on the substrate, while the latter seeks to deform the droplet beyond its stable limits, defined by the advancing and receding contact angles. In this study, we evaluated the oscillating frequency of sessile drops under these conditions. Notably, we observed that the oscillating frequency of the droplets increases with the superficial gas velocity of the shearing gas flow. Across the range of flow conditions tested in our experiments, an oscillating frequency of approximately 235 Hz was observed, with 10.76 m/s being one of the tested values for the superficial gas velocity. |
Sunday, November 19, 2023 9:18AM - 9:31AM |
A03.00007: Solutal instabilities for patterning during evaporation Samantha A McBride, Severine Atis, Amir Pahlavan, Kripa K Varanasi A number of interfacial instabilities arise during evaporation of drops or thin films on hydrophilic materials due to the coexistence of solutal and thermal gradients. Here, we show that crystallization during evaporation can leave a record of multiple fluid instabilities to create a range of ordered patterns. For example, Marangoni instabilities are capable of generating ordered, hexagonal arrays of droplets from a moving fluid front. In our system, crystal clusters precipitate at the locations of the droplets organized by Marangoni forces to generate a crystalline pattern. We also find a novel instability in which a kink-like wave propagates along the fluid front and delineates regions of uniform but disparate film height. This unusual fluid behavior leads to generation of sawtooth structures composed of crystals of disparate heights with a large and distinct step change between the regions. The patterning regime that emerges can be controlled by alteration of the evaporation rate and surface forces between the drop and underlying substrate, and we present a phase diagram for controlling regimes. By harnessing instability for controlled patterning, we also demonstrate that these water-soluble crystalline patterns can be used as masks for sustainable micro-fabrication. |
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