Capillary Length in a Fluid−Fluid Demixed Colloid−Polymer Mixture

Aarts, Dirk G. A. L.

doi:10.1021/jp044312q

Cited by 44 publications

(45 citation statements)

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“…Recently, it has been demonstrated that the depletion attraction between colloids and a hard wall can also lead to interesting wetting phenomena [17][18][19]. Experiments have shown that a wetting layer of the dense colloidal phase is usually present at the wall of the container [20,21]. The exceptional feature of phase-separated colloid-polymer mixtures is the ultralow interfacial tension between the liquid and gas phases [22 -24]: it can be a million times smaller than for a typical oil-water interface, thus making the thermal roughness of the interface a thousand times bigger.…”

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Fluctuation Forces and Wetting Layers in Colloid-Polymer Mixtures

et al. 2008

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We present confocal microscopy experiments on the wetting of phase-separated colloid-polymer mixtures. We observe that an unusually thick wetting layer of the colloid-rich phase forms at the walls of the glass container that holds the mixture. Because of the ultralow interfacial tension between the colloid-rich and the polymer-rich phases, the thermally activated roughness of the interfaces becomes very big and measurable. We observe that close to the critical point the roughness of the interface between the wetting layer and the polymer-rich phase decreases with decreasing layer thickness: large excursions of the interface are confined in the wetting layer. The measured relationship between the roughness and the thickness of the wetting layer is in qualitative agreement with the predictions of renormalization group theory for short-range forces and complete wetting. DOI: 10.1103/PhysRevLett.100.178305 PACS numbers: 82.70.Dd, 68.08.Bc, 68.35.Ct, 68.37.ÿd Wetting layers are ubiquitous in both nature and industry [1,2] and occur in a wide range of systems. For two coexisting phases, Cahn first proposed an argument that explains the formation of a wetting layer of one of the two phases onto a substrate (complete wetting) when approaching the critical point [3]. One classical example is a demixed (two-phase) binary liquid mixture in which the dense phase intrudes between the lighter fluid phase and the vapor. However, once the wetting conditions are set, the remarkable density inversion can only be maintained if forces act between the two film interfaces. For ordinary liquids, these forces are generally van der Waals forces [1,2] which, given their range, invariably lead to wetting layer thicknesses of the order of 100 Å .Liquid interfaces are also rough-they are locally fluctuating due to thermally excited capillary waves [4]. Consequently, the confinement of interface fluctuations in a wetting layer can in principle create an entropic interaction that enters the force balance between surface forces and gravity [5]. The existence of such an entropic force was first established by Helfrich [6] in the case of tensionless membranes. For liquid interfaces, where surface tension dominates, the situation is more complex. From a theoretical point of view, the influence of capillary fluctuations on wetting phenomena has attracted much attention as it could lead to nonuniversal wetting behavior with critical exponents depending explicitly on temperature [7,8]. Experimentally, the observation of this entropic repulsion in typical liquid layers is challenging, but some observations have shown that a correct description of adsorption isotherms of molecularly thin films could be achieved if one takes the entropic interaction into account [9][10][11][12].Similarly to simple liquids, mixtures of colloidal particles with nonadsorbing polymer can phase separate into a dense colloidal fluid phase (colloidal liquid) and a dilute one (colloidal gas) [13]. This phase separation is due to the depletion attraction that the polymer m...

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Fluctuation Forces and Wetting Layers in Colloid-Polymer Mixtures

et al. 2008

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“…Measurement of ultralow interfacial tensions is notoriously difficult. Many available techniques, such as drop shape [1] or interfacial profile analysis [2], rely on optical visualization. However, in weakly segregated and nearcritical systems, the optical contrast between the phases is generally small.…”

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Equilibrium Capillary Forces with Atomic Force Microscopy

et al. 2007

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We present measurements of equilibrium forces resulting from capillary condensation. The results give access to the ultralow interfacial tensions between the capillary bridge and the coexisting bulk phase. We demonstrate this with solutions of associative polymers and an aqueous mixture of gelatin and dextran, with interfacial tensions around 10 N=m. The equilibrium nature of the capillary forces is attributed to the combination of a low interfacial tension and a microscopic confinement geometry, based on nucleation and growth arguments. DOI: 10.1103/PhysRevLett.99.104504 PACS numbers: 47.55.nk, 68.03.Cd, 68.37.Ps Interfaces are ubiquitous in soft matter and biological systems. The corresponding phase equilibria are often characterized by weak, tunable interactions and large length scales . As a result, the interfacial tensions, / k B T= 2 , are typically ultralow (i.e., 1 mN m ÿ1 ). Measurement of ultralow interfacial tensions is notoriously difficult. Many available techniques, such as drop shape [1] or interfacial profile analysis [2], rely on optical visualization. However, in weakly segregated and nearcritical systems, the optical contrast between the phases is generally small. Moreover, density differences are also small, because of which it is difficult to obtain accurate information from drop shape analysis under normal gravity. To induce larger deformations, the spinning drop method is commonly used [3]. However, it was recently reported that the centrifugal field that is applied in this method, significantly affects the compositions of coexisting liquid phases. Demixed systems can even be brought into the one-phase regime [4].The method that is presented in this Letter does not rely on optical contrast, density differences, or strong external fields. We use colloidal probe atomic force microscopy (CP-AFM), introduced independently by Ducker et al. [5] and Butt [6], to measure equilibrium forces originating from capillary, liquid bridges between two surfaces. We analyze the resulting force-separation profiles to extract ultralow interfacial tensions. In Fig. 1 we show a schematic representation of a capillary, liquid bridge between a sphere and a flat substrate, the characteristic configuration in the CP-AFM experiment.When a homogeneous phase, in which one of the components is near its saturation point, is confined between two surfaces, a new phase can be formed by either condensation or evaporation. It is well known that, e.g., water condenses between hydrophilic surfaces in humid air [7]. Between hydrophobic surfaces at close separation, immersed in pure water, a capillary bridge of water vapor is formed, a process known as capillary evaporation or cavitation [8]. Capillary condensation is also reported for systems that show liquid-liquid coexistence, e.g., demixed ternary polymer-solvent systems [9]. Because the curvature of the capillary bridge is negative with respect to the inner phase, a negative pressure difference is present across the interface, leading to an attractive force between the ...

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“…Numerical domain is presented on the figure 2. This equivalent size is (1800x1800x600)µm 3 , that means water and oil inlet channel have a length of 600 µm, and the equivalent mesh size is (384x384x128). Fig.…”

Section: Methodsmentioning

confidence: 99%

“…An emulsion is defined as the temporarily stable dispersion of a liquid into another one that is not miscible [2]. When the scale of the liquid-liquid flow is smaller than its capillary length [3], interfacial tension dominates over shear stress and gravity [4], making the dispersion highly reproducible in slow conditions [5]. These slow conditions ensure a highly monodisperse emulsion [6], that is usually appropriate for targeted applications like microreaction synthesis [7].…”

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Comparison between numerical and experimental water-in-oil dispersion in a microchannel

Desjonquéres

Ménard

Tarlet

et al. 2017

Proceedings ILASS–Europe 2017. 28th Conference on Liquid Atomization and Spray Systems

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The dispersion of water inside a flow of oil is investigated in a microfluidic device, producing a water-in-oil emulsion. The liquid-liquid flow mainly differs from those presented in existing literature through its high capillary number (between 3 and 14), and in the head-on collision between water and oil streams. By comparing with experimental data, numerical simulations can provide more information about the topology of the flow. A coupled Volume of Fluid and Level Set method (CLSVOF) is used to treat the interface between both phases and incompressible Navier-Stokes equations are solved. Three set of parameters, close to those in the experimental setup, are investigated to compare experimental and numerical results. The comparison between experiments and simulation provides a precise knowledge of the liquid-liquid dispersion process and the overall flow pattern within the microfluidic device. KeywordsMicrochannel, water-in-oil dispersion, liquid-liquid flow Introduction Liquid-liquid dispersion within microfluidic devices has become an important issue over the last decade [1]. An emulsion is defined as the temporarily stable dispersion of a liquid into another one that is not miscible [2]. When the scale of the liquid-liquid flow is smaller than its capillary length [3], interfacial tension dominates over shear stress and gravity [4], making the dispersion highly reproducible in slow conditions [5]. These slow conditions ensure a highly monodisperse emulsion [6], that is usually appropriate for targeted applications like microreaction synthesis [7]. However, other application like high flow-rate biofuel production [8] benefit from the considerably increased surface-to-volume ratio [9,10] of microfluidic liquid-liquid dispersion. In order to better understand the physics of microfluidic in high flow-rate liquid-liquid dispersion, experimental results of the obtained mean diameter and liquid-liquid flow photographies [8] are compared to numerical results. At the present stage, a quantitative validation of the model is not obtained, but we present a qualitative comparison of the liquid-liquid flow pattern. In a first part, experimental material and methods are presented, then numerical methods used to investigate such flows are briefly detailed. Finally, first comparisons are discussed.

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Capillary Length in a Fluid−Fluid Demixed Colloid−Polymer Mixture

Cited by 44 publications

References 43 publications

Fluctuation Forces and Wetting Layers in Colloid-Polymer Mixtures

Fluctuation Forces and Wetting Layers in Colloid-Polymer Mixtures

Equilibrium Capillary Forces with Atomic Force Microscopy

Comparison between numerical and experimental water-in-oil dispersion in a microchannel

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