Abstract:Spreading on the free surface of a complex fluid is ubiquitous in nature and industry, owing to the wide existence of complex fluids. Here we report on a fingering instability that develops during Marangoni spreading on a deep layer of polymer solution. In particular, the wavelength depends on molecular weight and concentration of the polymer solution. We use the Transmission Lattice Method to characterize the finger height at the micron scale. We model the evolution of spreading radius, involving viscoelastic… Show more
“…This observation thus allows us to disregard the thermomechanical mechanism in the following. Once the droplets reach the T sep isotherm, their migration ceases and they sit on the isotherm circular line forming a flower-like pattern, recalling similar experiments involving Marangoni effects [48][49][50][51][52]. This crown of non-wetting water-rich drops delimits a central wetting drop enriched in Ionic Liquid.…”
Section: Description Of the Phase Separation Patternsupporting
Ionic liquids have remarkable properties and are commonly harnessed for green chemistry, lubrication and energy applications. In this paper, we study a thermo-responsive Ionic Liquid (IL) solution which has the property of phase separating above a critical temperature, an interesting feature for the recovery of the IL-rich phase. For this purpose, we generate a temperature gradient in a microfluidic cavity where the confinement strengthens wetting effects and enhances the demixing. In this experimental configuration, we report the separation patterns along the phase diagram of the binary mixture composition. Three separation dynamics regime are identified that may display complex three-dimensional flows. In spite of this complexity, we rationalize all the observed regimes. Only two regimes lead to a complete spatial separation of the two phases. Interestingly, one is reminiscent of a Marangoni instability in radial geometry, even at confinement below 100 µm. We believe this work will find applications in the recycling of ionic liquids.
“…This observation thus allows us to disregard the thermomechanical mechanism in the following. Once the droplets reach the T sep isotherm, their migration ceases and they sit on the isotherm circular line forming a flower-like pattern, recalling similar experiments involving Marangoni effects [48][49][50][51][52]. This crown of non-wetting water-rich drops delimits a central wetting drop enriched in Ionic Liquid.…”
Section: Description Of the Phase Separation Patternsupporting
Ionic liquids have remarkable properties and are commonly harnessed for green chemistry, lubrication and energy applications. In this paper, we study a thermo-responsive Ionic Liquid (IL) solution which has the property of phase separating above a critical temperature, an interesting feature for the recovery of the IL-rich phase. For this purpose, we generate a temperature gradient in a microfluidic cavity where the confinement strengthens wetting effects and enhances the demixing. In this experimental configuration, we report the separation patterns along the phase diagram of the binary mixture composition. Three separation dynamics regime are identified that may display complex three-dimensional flows. In spite of this complexity, we rationalize all the observed regimes. Only two regimes lead to a complete spatial separation of the two phases. Interestingly, one is reminiscent of a Marangoni instability in radial geometry, even at confinement below 100 µm. We believe this work will find applications in the recycling of ionic liquids.
“…The above studies were focused on axisymmetric spreading. Some experiments involving films with stronger viscoelastic properties reported the formation of finger-like, non-axisymmetric modulations at the contact line between the droplet, substrate and air (Motaghian et al 2022;Ma et al 2020). These observations suggested the existence of a new class of interfacial elastic instability similar to the one observed by Grillet, Lee & Shaqfeh (1999) in their experiments with viscoelastic fluids confined between eccentric cylinders displacing the less viscous fluid (the air).…”
Marangoni spreading on thin films is widely observed in nature and applied in industry. It has serious implications for airway drug delivery, especially in surfactant displacement therapy. This paper reports the results of experimental investigations of a surfactant-laden droplet spreading on films made of more viscous Newtonian fluids as well as on films made of viscoelastic fluids. The experiments used particle seeding, the transmission-speckle method and particle tracking velocimetry (PTV) to determine the deformation of the film–droplet interface and to measure velocity fields. Radially aligned patterns were observed on Newtonian films. Similar patterns, but with much smaller wavenumber, were observed on viscoelastic films in combination with rapid azimuthal variations of the film thickness. The Saffman–Taylor instability at the film–droplet interface explains the formation of patterns on a more viscous Newtonian film, and their onset requires exceeding the critical capillary number. The pattern formation on viscoelastic films is correlated with an instability at the film–droplet–air contact line when the liquid is expelled radially by the spreading droplet. PTV revealed azimuthal variations of the velocity field in the vicinity of the contact line. The observed contact line instability is different from previously reported fingering instabilities of Newtonian thin films. A simple scaling law accounting for the Marangoni-stress-induced elastic shear deformation is proposed to describe the flow field in the patterns formed in the viscoelastic films.
“…This is a spontaneous isothermal process 5 . The solutal Marangoni effect has been widely used for mass transfer on liquid surfaces, to form different kinds of patterns, such as fingering 6 , 7 , schlieren patterns 8 , and flower-like patterns 9 . Those patterns are usually liquid films generated by the Marangoni-driven spreading of a droplet on an aqueous substrate 6 – 10 , which has been applied in the fabrication of ultra-thin hydrogel films 11 , organic thin film transistors 12 , and organic solar cells 13 .…”
Section: Introductionmentioning
confidence: 99%
“…While the release of surfactant from the deposited droplet or solid particle reduces the SFT efficiently, the release tends to be asymmetric, generating self-motion of the droplet/particle that disturbs the transport process 15 . Moreover, the surfactant tends to remain at the surface and to prevent further decrease of the SFT, making the transport process difficult to control 6 – 9 . Fig.…”
The solutal Marangoni effect is attracting increasing interest because of its fundamental role in many isothermal directional transport processes in fluids, including the Marangoni-driven spreading on liquid surfaces or Marangoni convection within a liquid. Here we report a type of continuous Marangoni transport process resulting from Marangoni-driven spreading and Marangoni convection in an aqueous two-phase system. The interaction between a salt (CaCl2) and an anionic surfactant (sodium dodecylbenzenesulfonate) generates surface tension gradients, which drive the transport process. This Marangoni transport consists of the upward transfer of a filament from a droplet located at the bottom of a bulk solution, coiling of the filament near the surface, and formation of Fermat’s spiral patterns on the surface. The bottom-up coiling of the filament, driven by Marangoni convection, may inspire automatic fiber fabrication.
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