Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session E13: Free Surface Flows III: Marangoni Flows |
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Chair: Matthieu Roché, Université Paris Diderot, CNRS Room: 201 |
Sunday, November 22, 2015 4:50PM - 5:03PM |
E13.00001: Polygonal instability of Marangoni flows Matthieu Roch\'{e}, Matthieu Labousse, Baba El Hadj Maiga, Loïc Nya, S\'{e}bastien Le Roux, Isabelle Cantat, Arnaud Saint-Jalmes The transport of pepper grains floating at the surface of a bowl of water after the release of a drop of dishwashing liquid is a classical experiment to demonstrate the Marangoni effect, i.e. the flow of a liquid layer induced by interfacial tension gradients at its surface. In this case, the interfacial tension gradient results from a surfactant interfacial concentration gradient. Recently, we showed that continuous injection of an aqueous solution of hydrosoluble surfactants at the surface of a cm-thick pure water layer induced finite-size Marangoni flows surrounded by a region characterized by the presence of several pairs of interfacial vortices arranged along the the vertices of polygons.\footnote{M. Roch\'{e} et al., \textbf{Phys. Rev. Lett.} 112, 208302 (2014)} During this talk, I will show that we can understand the flow structure induced by these Marangoni flows, in particular their tendency to have polygonal shapes. I will describe how flow features such as the number of interfacial vortices or bulk recirculation flows depend on flow geometry. Finally, I will compare these results to a model that explains similar polygonal instabilities in other flows such as the hydraulic jump.\footnote{M. Labousse and J. W. M. Bush, submitted} [Preview Abstract] |
Sunday, November 22, 2015 5:03PM - 5:16PM |
E13.00002: Vibrational instabilities of a nonisothermal liquid layer with insoluble surfactant Alexander Mikishev, Alexander Nepomnyashchy We consider an infinite horizontal layer of an incompressible liquid, the deformable upper free surface is covered by insoluble surfactant. The layer is subjected to vertical harmonic oscillations with fixed amplitude and frequency, as well as to a transverse gradient of temperature. We suppose that the surface tension of upper boundary linearly depends on temperature and surfactant concentration. Two types of waves on the surface are possible. The first one is capillary-gravity waves (transverse waves) excited by the usual Faraday instability mechanism, under the influence of the surfactant elasticity. The second type of waves is Marangoni waves (longitudinal waves) related to compressions – dilations of the surface. In this work we study the excitation of Marangoni waves by vibration and determine the existence conditions for each type of waves. The results are connected with our previous research on parametric excitation of Marangoni instability when the gradient of temperature is harmonically changed. The instability thresholds are calculated numerically using the Floquet method for disturbances with arbitrary wave numbers. [Preview Abstract] |
Sunday, November 22, 2015 5:16PM - 5:29PM |
E13.00003: The Effect of Non-Uniform Wetting Properties on Contact Line Dynamics Morgane Grivel, David Jeon, Morteza Gharib Surfaces with non-uniform wetting properties have been shown to modify contact line dynamics and induce passive displacements of shallow flows. These surfaces are patterned with alternating hydrophobic and hydrophilic stripes of a certain width, spacing and orientation. A thin rectangular wall jet impinges on the surfaces and Fourier Transform Profilometry is used to reconstruct the 3D profile of the low to medium Reynolds number flows. Our previous work reported the development of intriguing roller structures at the contact line near hydrophobic-hydrophilic interfaces and the effect of varying the stripes' dimensions and orientation on these flows. Our present work extends the study to the effects of flow rate and plate inclination angle (with respect to the horizontal). The current work also studies air entrainment by the roller structures of the modified contact line. We will also discuss potential uses of this technique for modifying contact line dynamics and bow waves near surface-piercing bodies. [Preview Abstract] |
Sunday, November 22, 2015 5:29PM - 5:42PM |
E13.00004: Enhancing Liquid Micro-volume Mixing with Wettability-Patterned Surfaces Jared Morrissette, Pallab Sinha Mahapatraa, Ranjan Ganguly, Constantine Megaridis Self-driven surface micromixers (SDSM) based on patterned wettability technology provide an elegant solution for low-cost point-of-care (POC) devices and lab-on-a-chip (LOC) applications. Our SDSMs are fabricated by strategically patterning three wettable wedge-shaped tracks onto a non-wettable surface. Current state-of-the-art micromixers require energy, however, our SDSMs utilize the inherent surface energy of liquids, coupled with wettability contrast to efficiently mix small amounts of liquids (e.g. droplets). Transport and mixing of the SDSMs is accomplished by means of Laplace pressure-driven flow and several mixing approaches, such as splitting-recombining, stretching-folding, and transversal vortices. Mixing is initiated when separate liquid micro-volumes are transported along respective, juxtaposed wettable tracks. As the liquid micro-volumes coalesce, subsequent mixing occurs during transport of the combined volume over a third separate wettable track that also features a non-wettable ``island.'' The two-dimensional island disrupts the flow of liquids, in a similar manner a three-dimensional obstacle would, thus generating the aforementioned mixing enhancement. Several SDSMs, each having different island geometries, were investigated, giving rise to a greater understanding of efficient mixing on surfaces. The study offers a design basis for developing a low-cost surface microfluidic mixing device on various substrates. [Preview Abstract] |
Sunday, November 22, 2015 5:42PM - 5:55PM |
E13.00005: A New Contact Line Structure for Surfactant-Driven Superspreading Phenomenon Hsien-Hung Wei We propose a new contact line structure capable of explaining the curious linear spreading law observed in surfactant-driven superspreading. We show that a tiny surfactant leak from the air-liquid interface to the substrate suffices to promote the motion of the contact line. This leak leads to a microscopic surfactant-depletion zone on the interface in the vicinity of the contact line. Together with pressure buildup by the Marangoni shearing, a distinctive capillary nose is then developed over the zone to drive the contact line in a surfactant-free manner at a constant wetting speed, which explains the linear superspreading law. Our study not only captures many features seen in previous experiments and simulations, but also provides renewed insights into the superspreading phenomenon. [Preview Abstract] |
Sunday, November 22, 2015 5:55PM - 6:08PM |
E13.00006: When Marangoni meets Savart: The distant interaction of a drop with a liquid sheet Baptiste N\'eel, Emmanuel Villermaux The interaction of a radially expanding water sheet (Savart) with an ethanol droplet evaporating at a short distance from it is investigated. The millimetric pendant droplet is positioned a few millimeters above a horizontal sheet, whose thickness is typically a few tens of microns. Although the droplet and the sheet are not in contact, the sheet radius is abruptly reduced downstream of the drop. We infer that the introduction of a few molecules of ethanol vapor emanating from the drop into the water sheet decreases its thickness, via a localized surface tension deficit. The corresponding Marangoni stresses induce a flow which progressively digs the sheet, hastening its rupture. A quantitative mechanism is proposed to represent all these observations, whose relevance to the puzzling problem of thin films (in the micron range) stability is underlined. [Preview Abstract] |
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