The spreading of thin liquid films on a water-air interface

Foda, Mohamed A.; Cox, R. G.

doi:10.1017/s0022112080001516

Cited by 145 publications

(89 citation statements)

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“…This scaling suggests a widening dynamics in agreement with the dynamics of Marangoni spreading, i.e. the spontaneous spreading of a thin film (of say oil phase) along the surface of a deep fluid layer (of say aqueous phase) of higher surface tension [24][25][26]. The driving stress of the spreading is the surface tension gradient associated with the presence of an oil drop at the interface between air and the liquid film.…”

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confidence: 56%

Bursting of Dilute Emulsion-Based Liquid Sheets Driven by a Marangoni Effect

2015

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We study the destabilization mechanism of thin liquid sheets expanding in air and show that dilute oil-in-water emulsion-based sheets disintegrate through the nucleation and growth of holes that perforate the sheet. The velocity and thickness fields of the sheet outside the holes are not perturbed by holes and hole opening follows a Taylor-Culick law. We find that a pre-hole, which widens and thins out the sheet with time, systematically precedes the hole nucleation. The growth dynamics of the pre-hole follows the law theoretically predicted for a liquid spreading on another liquid of higher surface tension due to Marangoni stresses. Classical Marangoni spreading experiments quantitatively corroborate our findings.The destabilization of free liquid films is of great importance for aerosol dispersions, and is involved in many practical situations [1,2] [6], and dilute oil-in-water emulsions [7]. However, despite its relative ubiquity, the bursting of liquid sheets through perforation events have not yet been carefully investigated nor modeled. The occurrence of perforation events in a spray directly decreases the fraction of small drops issued from the spray [8] as illustrated in the case of dilute emulsions, which are prone to induce the bursting of liquid sheets [7]. Dilute emulsions are also recognized as anti-drift adjuvants [8][9][10][11] and therefore commonly used as carrier fluids for pesticides delivery. Controlling the perforation processes would therefore allow one to finely tune the size distribution of drops issued from sprays, a goal of uttermost importance in many industrial processes. In this optics, a physical description of the mechanisms at play in perforation processes is desired. The commonly invoked conditions for perforation include a dewetting of inclusions by the fluid and an inclusion diameter equal to, or larger than, the thickness of the liquid sheet, so that inclusions cause perforation by puncturing both interfaces of the sheet [3]. But, to the best of our knowledge, those conditions have never been confronted to robust experimental facts. To unambiguously clarify the physical mechanisms at play, rationale experiments on individual perforation events are therefore required.In this Letter, we investigate the perforation mechanisms of an emulsion-based free liquid sheet issued from a single-drop experiment; resulting from the impact of one drop of fluid onto a small target [12][13][14]. During the sheet expansion, holes nucleate and grow. We show that each perforation event is preceded by the formation of a pre-hole that thins out the sheet and widens with time. We demonstrate that the pre-hole growth is governed by a Marangoni effect. The entry of emulsion oil droplets at the air/water interface leads to a spreading of the oil due to a surface tension gradient stress. This stress is counterbalanced by a viscous stress that drags the subsurface fluid, whose flow causes a local film thinning which ultimately lead to the rupture of the film. We show that the growth kinetics of pre-holes ...

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confidence: 56%

Bursting of Dilute Emulsion-Based Liquid Sheets Driven by a Marangoni Effect

2015

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“…Laminar boundary layer calculations have shown that during the late stages of an oil spill, the spreading rate is directly controlled by the balance between surface tension gradients generated at the air-liquid interface and the viscous drag generated in the water sublayer by the advancing oil film. [1][2][3][4][5][6][7][8][9] Some of the more recent studies have generalized the spreading process to include different spreading geometries and film ''feeding rates'' as well as additional forces like gravity, capillarity, and surface diffusion. These modeling efforts have not yet been extended to include evaporation or dissolution, additional mechanisms which can significantly affect the spreading rate.…”

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confidence: 99%

Dynamics of spontaneous spreading with evaporation on a deep fluid layer

Dussaud¹,

Troian²

1998

Physics of Fluids

118

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The spontaneous spreading of a thin volatile film along the surface of a deep fluid layer of higher surface tension provides a rapid and efficient transport mechanism for many technological applications. This spreading process is used, for example, as the carrier mechanism in the casting of biological and organic Langmuir-Blodgett films. We have investigated the dynamics of spontaneously spreading volatile films of different vapor pressures and spreading coefficients advancing over the surface of a deep water support. Laser shadowgraphy was used to visualize the entire surface of the film from the droplet source to the leading edge. This noninvasive technique, which is highly sensitive to the film surface curvature, clearly displays the location of several moving fronts. In this work we focus mainly on the details of the leading edge. Previous studies of the spreading dynamics of nonvolatile, immiscible thin films on a deep liquid layer have shown that the leading edge advances in time as t 3/4 as predicted by laminar boundary layer theory. We have found that the leading edge of volatile, immiscible spreading films also advances as a power law in time, t ␣ , where ␣ϳ1/2. Differences in the liquid vapor pressure or the spreading coefficient seem only to affect the speed of advance but not the value of the spreading exponent, which suggests the presence of a universal scaling law. Sideview laser shadowgraphs depicting the subsurface motion in the water reveal the presence of a single stretched convective roll right beneath the leading edge of the spreading film. This fluid circulation, likely caused by evaporation and subsequent surface cooling of the rapidly spreading film, resembles a propagating Rayleigh-Bénard convective roll. We propose that this sublayer rotational flow provides the additional dissipation responsible for the reduced spreading exponent.

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“…While early experiments in rectilinear geometry [6][7][8][9][10] suggested that the prefactor k is dependent on film constitutive behavior, later experiments by Camp and Berg 11,12 demonstrated that this constant depends only on spreading geometry. Recent calculations 13 suggest universal values of kϭ1.4150 for rectilinear spreading ͑in good agreement with Camp and Berg's 11,12 experimental value of 1.4Ϯ0.04) and kϭ1.0754 for axisymmetric spreading.…”

Section: ͑1͒mentioning

confidence: 99%

“…This specific feature of the surface velocity profile, which depends on film constitutive behavior, has not been predicted by the theoretical models developed to describe the spreading of surface active films on a deep liquid layer. 9,10,[16][17][18] Phillips has recently indicated that there do exist eigenfunctions in the spreading problem which might display this interesting jump property. 13 The theoretical models that have been proposed predict certain distinctive features in the sublayer flow.…”

Section: ͑1͒mentioning

confidence: 99%

“…In particular, we briefly describe the predicted behavior near the leading edge and near the reservoir source. 16,10,17,18,13 The unsteady laminar boundary equation describing the spreading of a surface active film on a deep layer support can be reduced to time independent form by introducing similarity variables consistent with Eq. ͑1͒ for the relevant coordinates and velocities.…”

Section: ͑1͒mentioning

confidence: 99%

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Fluorescence visualization of a convective instability which modulates the spreading of volatile surface films

1998

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The spontaneous spreading of a thin liquid film along the surface of a deep liquid layer of higher surface tension is a ubiquitous process which provides rapid and efficient surface transport of organic or biological material. For a source of constant concentration, the leading edge of a nonvolatile, immiscible film driven to spread by gradients in surface tension is known to advance as t 3/4 in time. Recent experiments using laser shadowgraphy to detect the advancing front of spreading films indicate, however, that immiscible but volatile sources of constant concentration spread with a reduced exponent according to t 1/2. Using a novel technique whereby fluorescent lines are inscribed in water, we have detected the evolution of a thermal instability beneath the leading edge of volatile films which strongly resembles a Rayleigh-Bénard roll. We propose that the increased dissipation from this rotational flow structure is likely responsible for the reduction in spreading exponent. This observation suggests a conceptual framework for coupling the effects of evaporation to the dynamics of spreading. © 1998 American Institute of Physics. ͓S1070-6631͑98͒01607-9͔ I. SPONTANEOUS SPREADING ON A DEEP LIQUID LAYERWhen a liquid substrate is contacted by a film of lower surface tension, surface traction is produced in proportion to the difference in surface tension between the liquid support and the spreading film. This traction creates rapid and spontaneous flow toward regions of higher surface tension. The speed of the spreading film is known to increase with the magnitude of the spreading coefficient defined by Sϭ␥ 1 Ϫ␥ 2 Ϫ␥ 12 , where ␥ 1 denotes the surface tension of the clean liquid support, ␥ 2 the surface tension of the spreading film, and ␥ 12 the interfacial tension between the spreading film and supporting liquid.1 This spreading coefficient is related to the local gradient in surface tension, ‫,‪x‬ץ/␥ץ‬ through the relation Sϭ͐ 0 L(t) ‫‪x‬ץ/␥ץ‬ dx, where the coordinate x denotes the horizontal or radial direction of spreading and L(t) denotes the length of the surface active film at time t after contact with the liquid support. In the examples discussed below, the gradients in surface tension are caused by variations in the surface concentration, ⌫, of the spreading film where ‫.)‪x‬ץ/⌫ץ()⌫ץ/␥ץ(‪xϭ‬ץ/␥ץ‬ The spontaneous advance of a surface film driven by this type of shear stress is called Marangoni driven spreading, 2 which occurs for positive values of the spreading coefficient. A small scale example of this spreading process occurs during the casting of Langmuir-Blodgett films. In the fabrication of these films, organic or biological molecules are dissolved in a solvent like hexane or carbon tetrachloride which spreads rapidly and spontaneously over the surface of water. The organic solvent, which has a lower surface tension than pure water, acts as the carrier liquid to transport molecules along the water surface under the action of Marangoni stresses. 3 The solvent film eventually evaporates leaving behin...

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The spreading of thin liquid films on a water-air interface

Cited by 145 publications

References 1 publication

Bursting of Dilute Emulsion-Based Liquid Sheets Driven by a Marangoni Effect

Bursting of Dilute Emulsion-Based Liquid Sheets Driven by a Marangoni Effect

Dynamics of spontaneous spreading with evaporation on a deep fluid layer

Fluorescence visualization of a convective instability which modulates the spreading of volatile surface films

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