Origin of Stratification in Creaming Emulsions

Abstract

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...

New possibilities of accurate particle characterisation by applying direct boundary models to analytical centrifugation

“…34 This in turn will affect the width of distributions or can even result in false peaks. Hydrodynamic non-ideality seems to play a crucial role in many AC experiments and can lead to wrong results, which can hardly be identified without complementary methods.…”

The stratified Boycott effect

“…The development of layering in a polydisperse suspension was reported by Brewer (1884), and explained in terms of the interaction of sidewall heating with the density stratification arising from the differential settling of polydisperse particles (Mendenhall & Mason 1923). Siano (1979) and Mueth et al (1996) performed further experimental investigations into the phenomenon, and a detailed numerical study was undertaken by Hosoi & Dupont (1996).…”

Instabilities in a Liquid-Fluidized Bed of Gas Bubbles

“…However, this cannot account for the observed coupling with convection. Layered structures have also been found in "creaming" emulsions [16]. There, the layers were not shock fronts, but rather convection rolls caused by a slight horizontal temperature gradient across the thin dimension of the sample.…”

“…Here, we are unable to generate convection by thermal gradients. Also, our rolls counterrotate with axes perpendicular to the sample face, whereas in [16] they all rotate in the same direction with axes parallel to the face.…”