Influence of dynamic interfacial properties on droplet breakup in simple shear flow

The breakup of drops in an inhomogeneous flow is the key to emulsification. Frequently, the local flow experienced by the drops is a (quasi-) simple shear flow. The breakup of drops in a steady, simple shear flow in the absence of emulsifiers has been studied extensively. In the presence of emulsifiers, the drop interface may acquire viscoelastic properties, which are important in the prevention of coalescence, but their influence on drop breakup has not been established solidly. This article reports on a phenomenol. approach, which links the drop breakup dynamics to the interfacial viscoelasticity, identifying the latter with the elasticity modulus of a deforming planar interface. Over a viscosity ratio range of three decades, the results have been found to agree with the model, which uses only independently known quantities derived from equil. interfacial tension properties. [on SciFinder (R)

Coalescence in semi-concentrated emulsions in simple shear flow

The coalescence frequency in emulsions containing droplets with a low viscosity (viscosity ratio ∼ 0.005) in simple shear flow has been investigated experimentally at several volume fractions of the dispersed phase (2%–14%) and several values of the shear rate (0.1–10 s−1). The evolution of the size distribution was monitored to determine the average coalescence probability from the decay of the total number of droplets. Theoretically models for two-droplet coalescence are considered, where the probability is given by Pc = exp(−τdr/τint). Since the drainage time τdr depends on the size of the two colliding droplets, and the collision time τint depends on the initial orientation of the colliding droplets, the calculated coalescence probability was averaged over the initial orientation distribution and the experimental size distribution. This averaged probability was compared to the experimentally obtained coalescence frequency. The experimental results indicate that (1) to predict the average coalescence probability one has to take into account the full size distribution of the droplets; (2) the coalescence process is best described by the “partially mobile deformable interface” model or the “fully immobile deformable interface” model of Chesters [ A. K. Chesters, Chem. Eng. Res. Des. 69, 259 (1991) ]; and (3) independent of the models used it was concluded that the ratio τdr/τint scales with the coalescence radius to a power (2±1) and with the rate of shear to a power (1.5±1). The critical coalescence radius Ro, above which hardly any coalescence occurs is about 10 μm

Dispergeren van pigmenten in watergedragen verf

Measurements show that TiO2 aggregates have a fractal structure. Therefore, their dispersibility can be studied based on their morphol. This is best carried out in laminar Couette flow, as its flow profile has been well-established. This permits the investigation of the effect of process parameters on the break-up of the aggregate, and, via (micro)rheol. models, the switch to complex industrial size redn. app. [on SciFinder (R)

Viscosity of surfactant stabilized emulsions

A new scaling parameter for the viscosity of surfactant stabilized emulsions is proposed. We suggest that the attractive force between emulsion droplets is caused by the small surfactant micelles in the continuous phase of an emulsion. The new scaling parameter will be referred to as the depletion flow number, Fld=4πηsa2am/kTϕm, and is defined as the ratio between the viscous energy needed to separate the droplets and the depletion energy that opposes this separation. Here ηs, a, am, and ϕm are the solvent viscosity, dispersed phase droplet radius, micelle radius, and micelle volume fraction, respectively. Fld is of the order of unity at the onset of shear thinning and is capable of explaining all previously observed effects of drop size, solvent viscosity, and surfactant concentration. With master curves which are obtained by using Fld as the running parameter, a relatively simple empirical model is constructed which can reproduce the viscosity curves of many previously reported in the literature

Droplet breakup in concentrated emulsions

In this paper we report an experimental study on the conditions for droplet breakup in concentrated emulsions under simple shear flow. We present a set of experiments where the ratio between drop and matrix viscosity was varied from 0.1 to 22 and the volume fraction ranged from 0% to 70%. It was observed that the critical shear rate for breakup decreased by more than an order of magnitude for the most concentrated emulsions. Further, drops with viscosity ratio of 22 were seen to rupture in simple shear as soon as the emulsion concentration was raised to 40%. All these effects were conveniently explained by means of a mean field model which assumes simply that breakup of a droplet in a concentrated emulsion is determined by the average emulsion viscosity rather than the continuous phase viscosity

Influence of dynamic interfacial properties on droplet breakup in simple shear flow

The breakup of drops in an inhomogeneous flow is the key to emulsification. Frequently, the local flow experienced by the drops is a (quasi-) simple shear flow. The breakup of drops in a steady, simple shear flow in the absence of emulsifiers has been studied extensively. In the presence of emulsifiers, the drop interface may acquire viscoelastic properties, which are important in the prevention of coalescence, but their influence on drop breakup has not been established solidly. This article reports on a phenomenol. approach, which links the drop breakup dynamics to the interfacial viscoelasticity, identifying the latter with the elasticity modulus of a deforming planar interface. Over a viscosity ratio range of three decades, the results have been found to agree with the model, which uses only independently known quantities derived from equil. interfacial tension properties. [on SciFinder (R)

Influence of dynamic interfacial properties on droplet breakup in simple shear flow

The breakup of drops in an inhomogeneous flow is the key to emulsification. Frequently, the local flow experienced by the drops is a (quasi-) simple shear flow. The breakup of drops in a steady, simple shear flow in the absence of emulsifiers has been studied extensively. In the presence of emulsifiers, the drop interface may acquire viscoelastic properties, which are important in the prevention of coalescence, but their influence on drop breakup has not been established solidly. This article reports on a phenomenol. approach, which links the drop breakup dynamics to the interfacial viscoelasticity, identifying the latter with the elasticity modulus of a deforming planar interface. Over a viscosity ratio range of three decades, the results have been found to agree with the model, which uses only independently known quantities derived from equil. interfacial tension properties. [on SciFinder (R)

Coalescence in emulsions containing inviscid drops with high interfacial mobility

The drainage process of a thin liq. films between 2 droplets of low viscosity is analyzed to specify conditions for coalescence. The time available for film drainage is detd. by the duration of a collision. A. K. Chesters (1975) considered a similar problem and obtained an expression for the rate of drainage which neither contained the extent of flattening of the droplets nor the strength of the flow field. The same hydrodynamic anal. was used to evaluate the rate of drainage in the case of fully mobile interfaces at a vanishing viscosity ratio for the dispersed and continuous phase, from the balance of the driving and resistance forces of the film drainage. In the 1st stage of film drainage, the droplets remain perfect spheres. Once the pressure in the film exceeds the Laplace pressure, deformation sets in. The extent of flattening follows from an energy equation. The drainage process is described by 2 dimensionless parameters, the Capillary no., which gives the strength of the flow field, and the Flow no., which is the ratio of the hydrodynamic and van der Waals forces. The time available for film drainage is obtained from the duration of a collision, which is detd. by the rotation of the collision doublet. Flattening of the droplets retards drainage and reduces the coalescence probability considerably above a value of 0.02 for the Capillary no. Generally, drops with fully mobile interfaces have a much higher coalescence probability than those having rigid interfaces. [on SciFinder (R)