Layering Phenomena in Colloidal Suspensions

Merchant, S. R.

doi:10.1346/ccmn.1969.0170506

Cited by 7 publications

(2 citation statements)

References 8 publications

(8 reference statements)

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“…Such layer formation already was well known by 1884 when Brewer [1] described his observations on sedimenting clay suspensions. Subsequent reports over the past century have described stratification in monodisperse [2][3][4] and polydisperse [1,[5][6][7][8][9] suspensions of spherical and irregularly shaped [8,9] colloidal particles. Stratification has been seen for bouyant particles which cream to the top of their suspension as well as for dense particles which settle to the bottom.…”

Section: Origin Of Stratification In Creaming Emulsionsmentioning

confidence: 99%

Origin of Stratification in Creaming Emulsions

et al. 1996

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Colloidal suspensions aging under gravity have long been known to spontaneously separate into layers. We demonstrate through flow visualization experiments that a lateral temperature gradient as small as 10 mK͞cm can be responsible for such stratification and that strata correspond to a stack of convection rolls. A linear stability analysis suggests that the onset of stratification results from a convective instability peculiar to fluids with inhomogeneous mass densities subjected to lateral thermal gradients. [S0031-9007(96)00672-2] PACS numbers: 82.70. Kj, 47.20.Bp, 47.55.Hd Initially homogeneous colloidal suspensions often stratify as they settle under gravity with a stratified suspension's density changing abruptly at sharp interfaces between horizontal layers. The discrete layers can be observed easily using the light scattered by the colloidal particles. Photographs in Fig. 1 show six stages in the spontaneous stratification of a column of decane emulsion. Such layer formation already was well known by 1884 when Brewer [1] described his observations on sedimenting clay suspensions. Subsequent reports over the past century have described stratification in monodisperse [2-4] and polydisperse [1,5-9] suspensions of spherical and irregularly shaped [8,9] colloidal particles. Stratification has been seen for bouyant particles which cream to the top of their suspension as well as for dense particles which settle to the bottom.Several mechanisms have been proposed to explain these observations, including spinodal decomposition in the fluid ensemble of particles [2], the formation of vertical streaming flows [10], and the generation of a sequence of Burgers shocks [11]. We describe experiments on creaming emulsions which demonstrate that each layer in a stratified suspension is a discrete convention roll and that the stack of co-rotating convection rolls is driven by a small horizontal thermal gradient. We suggest that these rolls are created by a convective instability due to a coupling between a vertical gradient in the concentration of suspended particles and the horizontal thermal gradient.The decane-in-water emulsions used in this study were prepared by inversion emulsification using Pluronic F88 surfactant to stabilize the resulting decane droplets against recombination. The droplet diameters were determined by digital video microscopy to follow a log-normal distribution peaked at 0.6 mm with most droplet diameters falling between 0.3 and 1.2 mm. Repeated observations on well-mixed samples suggest that the emulsion is stable against aggregation over the duration of these experiments.The emulsions were diluted with deionized water to decane volume fractions, f, ranging from 10 23 to 10 24 and were placed in glass tubes 30 cm tall with inner diameters ranging between 8 and 22 mm. As suggested by the literature, we strove to isolate the tubes from thermal gradients by placing them in a dark, vacant, and windowless room. Under these conditions, we would expect the decane droplets to rise in the column with the...

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Section: Origin Of Stratification In Creaming Emulsionsmentioning

confidence: 99%

Origin of Stratification in Creaming Emulsions

et al. 1996

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“…Yet, experimentally observed behavior is a stable, traveling staircase pattern, appearing to the eye as several distinct bands of differing concentrations just below the primary interface between sediment and clear fluid, a phenomenon known as "layering". This is most familiar in the context of sedimentation of clay solutions, [13,21].…”

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confidence: 99%

On a nonlocal dispersive equation modeling particle suspensions

Zumbrun¹

1999

Quart. Appl. Math.

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Abstract.We study a nonlocal, scalar conservation law Ut + ((Ka * u)u)x = 0, modeling sedimentation of particles in a dilute fluid suspension, where Ka{x) = a-1 K(x/a) is a symmetric smoothing kernel, and * represents convolution. We show this to be a dispersive regularization of the Hopf equation, Ut + (u2)x = 0, analogous to KdV and certain dispersive difference schemes. Using the smoothing property of convolution and the physical principle of conservation of mass, we establish the global existence of smooth solutions.In physical applications, boundary considerations impose initial data of shock type. Up to shock formation time, we show that solutions remain close to solutions of the Hopf equation. After shock formation time, we give evidence that dispersive, hydrodynamic effects, interacting with Brownian diffusion, can generate oscillatory interface patterns in place of shocks. We discuss possible relevance to the physical phenomenon of layering.

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Pharmaceutical Sciences—1968. A Literature Review of Pharmaceutics

Bernardo¹,

Russo²

1969

Journal of Pharmaceutical Sciences

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Layering Phenomena in Colloidal Suspensions

Cited by 7 publications

References 8 publications

Origin of Stratification in Creaming Emulsions

Origin of Stratification in Creaming Emulsions

On a nonlocal dispersive equation modeling particle suspensions

Pharmaceutical Sciences—1968. A Literature Review of Pharmaceutics

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