Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session T39: Interfacial Phenomena I 
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Chair: Fernando TempranoColeto, Princeton University Room: 204B 
Monday, November 20, 2023 4:25PM  4:38PM 
T39.00001: Synchronization and collective motion of interfacial capillary spinners JackWilliam Barotta, Giuseppe Pucci, Alireza Hooshanginejad, Eli Silver, Daniel M Harris When a millimetric body is deposited onto the interface of a vibrating liquid bath, the relative motion between the object and interface generates outward propagating waves which leads to steady propulsion. Prior work has shown that chiral objects or "spinners" in isolation can rotate at a steady rate, with both the direction of rotation and angular speed depending sensitively on the spinner geometry and driving parameters. Here, we consider the capillary wavemediated interactions of multiple spinners. We first characterize the twospinner problem, demonstrating that spinners are able to stably synchronize their rotation in certain parameter regimes via their mutual wavefield. The manybody problem is then considered where a rich variety of static and dynamic interaction modes are found. A model of the spinner interaction is proposed, allowing us to draw analogy to other systems exhibiting spontaneous synchronization. This highly tunable and accessible system represents a new platform for studying active and driven systems interacting through deformable substrates. 
Monday, November 20, 2023 4:38PM  4:51PM 
T39.00002: Spontaneous rotation by symmetry breaking of a capillary wave source Basile Dhote, JackWilliam Barotta, Eli Silver, Daniel M Harris When a millimetric object is deposited onto the interface of a vibrating liquid bath, the floating body generates an extended capillary wavefield with associated surface streaming flows. It has recently been shown that chiral objects placed at the interface are able to steadily rotate in a determined direction via an imbalance of wave stresses as a consequence of their imposed geometric asymmetry. Here, we consider symmetric (achiral) objects which spontaneously begin to rotate in either direction. This symmetry breaking phenomenon occurs above a critical driving acceleration, with this threshold depending on the driving frequency, fluid parameters, and wave source geometry. We characterize the dependence of the steady rotation speed on the experimental parameters, and rationalize our observations with a simple mathematical model drawing inspiration from other physical systems that exhibit spontaneous symmetry breaking. 
Monday, November 20, 2023 4:51PM  5:04PM 
T39.00003: Selfsimilar regimes in the viscous Marangoni spreading of surfactant on a deep subphase Fernando TempranoColeto, Howard A Stone The spreading of an insoluble surfactant on a fluid interface is a fundamental fluid mechanics problem that has been investigated extensively due to its implications for flows in biology, the environment, and in technological applications. While many different regimes have been studied theoretically, analytical progress has proven challenging in the limit of a deep subphase at low Reynolds and high Péclet numbers, due to the nonlocal nature of the coupling between the interfacial velocity field and the concentration of surfactant [Thess, Phys. Rev. Lett. (1995)]. Recently, the problem was shown to be equivalent to the complex Burgers equation [Crowdy, SIAM J. Appl. Math. (2021)], paving the way to exact solutions obtained through the method of characteristics [Bickel, Phys. Rev. E (2022)]. Here, we study the selfsimilarity of the problem, providing further insights into its structure. First, a phaseplane formalism is used to identify different selfsimilar regimes. In the case of a pulse of outwardspreading surfactant, we find a globally valid solution exhibiting selfsimilarity of the first kind. On the other hand, surfactantfree holes collapsing inward exhibit selfsimilarity of the second kind, which only holds locally. We find that the exponents in the powerlaw behavior of these hole solutions can be obtained exactly using stability arguments, and distinguish two different powerlaw exponents that hold depending on the nature of the initial condition. We also discuss the importance of higherorder effects like surface diffusion and endogenous surfactant on these idealized solutions. 
Monday, November 20, 2023 5:04PM  5:17PM 
T39.00004: Receding contact lines in waterice systems Rodolphe Grivet, Christophe Josserand, Thomas Seon, Axel Huerre, Laurent Duchemin This presentation will be about theoretical and experimental investigations on the wetting behavior of water on ice. Several previous experimental observations and theoretical considerations points towards the idea that the icewater system is not perfectly wetting. However, investigating that phenomenon systematically using classical techniques used to measure wettability is an experimental challenge, as ice and liquid water are usually not at equilibrium when coexisting. Our experiments force a wellcontrolled outofequilibrium situation, namely the receding motion of a contact line on a vertical plate of ice, and we compare the results with a model for the solidifying film. Our results show that solidification allows contact lines to stay stationary in regimes of the capillary number that are out of reach for inert systems. These results also point towards the idea that water does not perfectly wet ice. 
Monday, November 20, 2023 5:17PM  5:30PM 
T39.00005: Exogenousendogenous surfactant iteraction yields heterogenous spreading in complex branching networks Richard Mcnair, Fernando TempranoColeto, Frederic Gibou, Paolo LuzzattoFegiz, François J Peaudecerf, Oliver E Jensen, Julien R Landel

Monday, November 20, 2023 5:30PM  5:43PM 
T39.00006: The role of monolayer viscosity in Langmuir film hole closure dynamics Leroy Jia, Michael J Shelley We reexamine the model proposed by Alexander et al. (Phys. Fluids, vol. 18, 2006, 062103) for the closing of a circular hole in a molecularly thin incompressible Langmuir film situated on a Stokesian subfluid. For simplicity their model assumes that the surface phase is inviscid which leads to the result that the cavity area decreases at a constant rate determined by the ratio of edge tension to subfluid viscosity. We reformulate the problem, allowing for a regularizing monolayer viscosity. The viscositydependent corrections to the hole dynamics are analysed and found to be nontrivial, even when the monolayer viscosity is small; these corrections may explain the departure of experimental data from the theoretical prediction when the hole radius becomes comparable to the Saffman–Delbrück length. Through fitting, under these relaxed assumptions, we find the edge tension could be as much as six times larger (∼4.0 pN) than reported previously. 
Monday, November 20, 2023 5:43PM  5:56PM 
T39.00007: Remobilizing a stagnant cap of surfactant through fast kinetic exchange and surface diffusion Anna E Curran, Darren G Crowdy, Demetrios T Papageorgiou Explicit solutions are presented describing the steady remobilization of a planar interface between two viscous fluids laden with surfactant soluble to the upper fluid. In the presence of a linear extensional flow, steady stagnant caps of surfactant form resulting in an immobilized interface. We show analytically that both fast kinetic exchange with the bulk fluid and surface diffusion can mollify these sharp surfactant gradients, thereby reducing the interfacial Marangoni stresses. The case of insoluble surfactant on an interface between a viscous fluid and a constant pressure region is also treated equivalently. 
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