Correlation between Transient Shear Experiments and Structure Evolution of Aqueous Foams

Herzhaft, Benjamin

doi:10.1006/jcis.2001.8153

Cited by 37 publications

(35 citation statements)

References 28 publications

(46 reference statements)

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“…Besides, an analogous effect has long been observed with wormlike micellar solutions [5,6], but in that case the different flowing regions were shown to be composed of different phases [7]. For foams, shearbanding [8,9] and fracture [10] under specific conditions at low velocities, or localization of rearrangement processes beyond a critical deformation [11] under dynamic tests, were recently observed. Nevertheless, it is generally considered [10,12] that flows under other conditions are homogeneous and that these materials behave as simple yielding fluids, i.e.…”

mentioning

confidence: 75%

From “discrete” to “continuum” flow in foams

Rodts¹,

Baudez²,

Coussot³

2005

Europhys. Lett.

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Abstract. -From both NMR and conventional rheometrical data we show that a foam cannot flow steadily below a critical, apparent shear rate and a critical shear stress. At low velocities the shear localizes in a layer of thickness decreasing with the apparent shear rate. When this thickness becomes smaller than a critical value hc (about 25 bubble diameters) the continuum assumption is no longer valid and the apparent behavior in this "discrete" regime differs from the rheological behavior of the foam in the "continuum" regime (for a sheared thickness larger than hc).Concentrated emulsions, suspensions, foams or polymeric gels, of common use in civil engineering, food, cosmetic or pharmaceutical industries, are pasty materials capable to flow only when a sufficiently large stress has been applied to them, a feature associated to their "soft jammed" structure. Since these materials might constitute a new state of matter reminiscent of the glassy state, they are actively studied in physics [1]. Although the solid-liquid transition was initially assumed to be continuous, i.e. the viscosity was considered to progressively tend to infinity as the shear stress decreases towards the yield stress, various recent experimental results with pasty materials have suggested that an abrupt transition occurs at a critical shear rate [2], which leads to shear-banding at low flow rates [3]. Recent theoretical works or numerical simulations [4] providing rheophysical approaches of the behavior of softjammed systems tend to confirm the generic character of such results. Besides, an analogous effect has long been observed with wormlike micellar solutions [5,6], but in that case the different flowing regions were shown to be composed of different phases [7]. For foams, shearbanding [8,9] and fracture [10] under specific conditions at low velocities, or localization of rearrangement processes beyond a critical deformation [11] under dynamic tests, were recently observed. Nevertheless, it is generally considered [10,12] that flows under other conditions are homogeneous and that these materials behave as simple yielding fluids, i.e. with flow rate continuously decreasing to zero as the applied stress decreases to the yield stress. Here we provide a complete set of macroscopic (conventional rheometry) and local (MRI) flow data for a foam. It is shown that foam flows systematically develop shear-banding at low imposed velocities and that the corresponding sheared layer does not exhibit a consistent rheological behavior when the sheared thickness is smaller than about 25 bubble diameters.c EDP Sciences

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mentioning

confidence: 75%

From “discrete” to “continuum” flow in foams

Rodts¹,

Baudez²,

Coussot³

2005

Europhys. Lett.

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“…The most widely used rheometer types are parallelplate rheometers (Figure 2.11c) (Neff and Macosko, 1996, Herzhaft, 2002, Arif et al, 2012, Costa et al, 2013a and pipe viscometers (i.e. capillary viscometers) (Herzhaft et al, 2005, Zhao et al, 2009, Sherif et al, 2015.…”

Section: Measurement Of Foam Rheologymentioning

confidence: 99%

“…When the foam is sheared, large velocity gradients appear in this liquid film, presenting a low viscosity, and lead to apparent slip of the foam. The slip velocity will depend on the shear stress at the wall, and on the size of the liquid film, and will alter the viscosity measurements (Herzhaft, 2002). The second problem is foam aging.…”

Section: Measurement Of Foam Rheologymentioning

confidence: 99%

“…Herzhaft (2002) observed an evolution of foam structure, from a disordered state of bubbles in the gap to a state where larger bubbles exist in the centre and smaller bubbles exist either side of the gap.…”

Section: Measurement Of Foam Rheologymentioning

confidence: 99%

“…Another effect of foam aging is diffusive coarsening, which is due to the exchange of gas between bubbles and leads to an increase in the average bubble size as larger bubbles grow at the expense of smaller bubbles which eventually disappear (Durian et al, 1990, Herzhaft, 2002. As mentioned above, the bubble size is of importance in determining the foam rheology.…”

Section: Measurement Of Foam Rheologymentioning

confidence: 99%

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An investigation of flotation froth rheology

Li¹

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Flotation froth is of significant importance because it ultimately determines overall flotation performance. Froth recovery, the fraction of valuable minerals coming into the froth phase which survive and report to the concentrate, has been widely used to evaluate the efficiency of transporting the valuable minerals from the pulp-froth interface to the concentrate launder.Froth recovery is related to the time that particles spend in the froth and the rate at which bubbles burst.It is believed that froth rheology has an impact on froth recovery in flotation. A more viscous froth should resist motion towards the lip, rising vertically before overflowing into the cell launder, resulting in significantly higher froth residence times and increased probability of froth drop back as well as froth collapse. Very little work has been done to study the rheology of a highly mineralised flotation froth and how it impacts on froth recovery. Therefore, this project aims to gain an insight into the correlations between froth properties, froth rheology and flotation performance.To achieve this goal, a novel method to measure froth rheology properly was developed in this project. The method involves the use of a rotating vane surrounded by a tube. The tube is required to minimise the adverse effects of the horizontal flow on the measurement.Equations have been provided to convert the vane rotational speed and measured torque to shear rate and shear stress, depending on whether the froth within the tube is fully or partially sheared.In order to investigate the correlation between froth rheology and froth properties, flotation tests were performed in a 20 L continuous flotation cell using a synthetic ore which was a mixture of chalcopyrite and silica. The flotation tests, conducted under different flotation conditions (i.e. air rate, froth depth, impeller speed, feed grade and feed P80), resulted in significant changes in the froth properties and the viscosity of the froth. The froth was shown to be shear thinning with minor yield stress. It could be modelled using a power-law model with the consistency index of this function being able to be used to represent the degree of viscosity of the froth phase.The observed change in the froth viscosity was shown to be largely a function of the change in the bubble size and the percentage of the bubble surface covered by particles. Smaller bubbles and a greater coverage of particles on the bubble surface result in a greater resistance to froth motion. Thus changes in cell operation which reduce the froth bubble size (e.g. higher impeller speeds, higher frother dosage rates) will result in a more viscous froth. Higher feed grades and a smaller particle size which result in a greater proportion of bubble surface coverage will also result in higher froth viscosities.A model was proposed which relates the apparent viscosity of the froth to the shear rate, the froth bubble size, the gas volume fraction in the froth, the percentage of the bubble surface covered by particles and a fittabl...

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Thermorheological behaviour of a lithium lubricating grease

et al. 2006

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Thermal-induced changes in the viscous and viscoelastic responses of lubricating greases have been investigated through different rheological techniques in a temperature range of 0-175°C. Small-amplitude oscillatory shear and viscous flow measurements were carried out on a model conventional lithium lubricating grease prepared by inducing the in situ saponification reaction between 12-hydroxystearic acid and hydrated lithium hydroxide. The linear viscoelasticity functions dramatically decrease above 110°C, but not below this critical temperature, which determines the maximum recommended operating temperature in relation to its durability and resistance under working conditions. Two different regions, below and above this critical temperature, in the plateau modulus versus temperature plot have been detected. From this thermal dependence, a much larger thermal susceptibility of the lubricating grease at temperatures above 110°C is apparent. The thermo-mechanical reversibility of this material has been studied by applying different combined stress-temperature protocols. Regarding the viscous flow, a minimum in the shear stress versus shear rate plots appeared at temperatures above 60°C, more pronounced as temperature increases, resulting from material instabilities. The experimental results obtained have been explained on the basis of the thermo-mechanical degradation of the lubricating grease microstructure.

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Correlation between Transient Shear Experiments and Structure Evolution of Aqueous Foams

Cited by 37 publications

References 28 publications

From “discrete” to “continuum” flow in foams

From “discrete” to “continuum” flow in foams

An investigation of flotation froth rheology

Thermorheological behaviour of a lithium lubricating grease

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