Direct Visual Observation of Thermal Capillary Waves

Aarts, Dirk G. A. L.; Schmidt, Matthias; Lekkerkerker, H. N. W.

doi:10.1126/science.1097116

Cited by 438 publications

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References 28 publications

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“…The goal of this work is to quantify the measured capillary length and compare to theory. Note that it is also possible to obtain the capillary length from the capillary wave spectrum as recently shown in ref 21, but this method is better suited for mixtures with larger colloids and thus even smaller interfacial tensions than presented here.…”

Section: Introductionmentioning

confidence: 86%

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

Aarts

2005

J. Phys. Chem. B

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We report measurements of the interfacial profile close to a vertical wall in a fluid-fluid demixed colloidpolymer mixture. The profile is measured by means of laser scanning confocal microscopy. It is accurately described by the interplay between the Laplace and hydrostatic pressure and from this description the capillary length is obtained. For different statepoints approaching the critical point the capillary length varies from 50 to 5 µm. These results are compared to theory. The mass density difference ∆F is calculated from the bulk phase behavior, which is described within free volume theory with polymers modeled as penetrable hard spheres. The interfacial tension γ is calculated within a squared gradient approximation. The capillary length is then given through γ/g∆F with g equal to the Earth's acceleration. Predictions from theory are in overall qualitative agreement with experiment without the use of any adjustable parameter.

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Section: Introductionmentioning

confidence: 86%

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

Aarts

2005

J. Phys. Chem. B

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“…This is typically the time we wait before doing a measurement, and we thus expect to be close to the equilibrium film thickness. In contrast, the interface fluctuations relax with a characteristic time on the order of seconds [25] and therefore adjust to the local wall thickness relatively rapidly. Further from the critical point, the large layer thicknesses observed in our experiment compared to the theoretical expectation can result from (i) direct interactions on the scale of the bulk correlation length, such as follow from the Cahn theory of wetting (see [19] for the application to colloid-polymer mixtures), (ii) residual electrostatic interactions that could lead to electrical double layer disjoining pressures-interesting charge effects have been observed recently in similar colloidal systems [33].…”

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“…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. Consequently, direct observation-in real space and time -of the thermally activated capillary waves becomes possible using optical microscopy [25]. In this Letter, we study the characteristics of wetting layers in phase-separated colloid-polymer mixtures and try to identify the contribution of the micronsized interface fluctuations to the wetting film stability.…”

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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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“…The interface may be thought of either as a continuous density profile 16 or a (step-like) local density profile dressed with thermal capillary waves 17,18 . We measure the capillary waves and thus determine the interfacial tension, σ (see the Supplementary Information).…”

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Bridging length scales in colloidal liquids and interfaces from near-critical divergence to single particles

2007

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Boiling and condensation are among the best recognized phase transitions of condensed matter. Approaching the critical point, a liquid becomes indistinguishable from its vapour, the interfacial thickness diverges and the system is dominated by long-wavelength density fluctuations 1 . Long wavelength usually means hundreds of particle diameters, but here we consider the limits of this assumption, using a mesoscopic analogue of simple liquids, a colloid-polymer mixture 2,3 . We simultaneously visualize both the colloidal particles and near-critical density fluctuations, and reveal particle-level images of the critical clusters and liquid-gas interface. Surprisingly, we find that critical scaling does not break down until the correlation length approaches the size of the constituent particles, where there is a smooth transition to non-critical classical behaviour. Our results could provide a framework for unifying the disparate particle and correlation length scales, and bring new insight into the nature of the liquid-gas interface and the limit of the critical regime.Among the simplest pictures of two different phases is that of a magnet, whose sites can only point up or down, and do so depending solely on their neighbours. This Ising model in fact describes a general class of phase transitions 1 , from the original ferromagnetic transition in magnets to phase separation in alloys, binary liquids, emulsions and polymers, and the liquidgas transition in simple liquids and colloid-polymer mixtures. Although we shall consider the latter two cases, this work has implications for many systems that belong to the same universality class. Close to the critical point, many physical properties such as the bulk correlation length, ξ B , are described by simple scaling, such as ξ B = ξ 0 B ε −ν , where ξ 0 B is the bare correlation length (or critical amplitude) and ε = (T − T C )/T C is a reduced temperature where T is the temperature and T C is the critical temperature. The critical exponent ν = 0.5 according to mean-field theory, and very close to the critical point there is a crossover to ν = 0.63, 'three-dimensional (3D) Ising universality' 4 . We shall refer to the entire scaling regime, both mean field and 3D Ising as the critical region. Although criticality is well understood, the point at which it breaks down has so far received relatively little attention. Conventional theory of critical phenomena assumes a large separation in length scales, that is, that the correlation length is decoupled from other length scales. This length-scale separation lies at the heart of critical universality, implying that the microscopic details are irrelevant. Whereas length-scale coupling has been considered in complex fluids with much longer length scales, such as polymer 5 and ionic 6 solutions, we consider here a more general limit: the case that the correlation length approaches that of the constituent particles. Although it is difficult to approach this limit experimentally in molecular systems, a colloidpolymer mixture provides...

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Direct Visual Observation of Thermal Capillary Waves

Cited by 438 publications

References 28 publications

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

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

Fluctuation Forces and Wetting Layers in Colloid-Polymer Mixtures

Bridging length scales in colloidal liquids and interfaces from near-critical divergence to single particles

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