A.I. Bailey

Mechanism of stable jet formation in electrohydrodynamic atomization

The disruption of a liquid into a spray of charged droplets, when subjected to an intense electric field, has attracted considerable interest from both a fundamental and applied point of view. On the applied side, the process has been used in paint spraying, electrostatic printing, electrostatic emulsification, fuel atomization in combustion systems and in space vehicle propulsion systems. More recently, the phenomenon has found potential application in crop spraying because of the low energies required, the ability to produce fine sprays within a narrow size distribution and the preferential deposition on the target surfaces. Despite its wide range of applications, the mechanism of liquid disruption is only poorly understood. At a critical potential, the so-called Taylor1 cone develops, with a fine stable jet issuing from its tip. The formation of such cones and jets is essential for the production of an electrohydrodynamic spray. Most previous work aimed at understanding jet formation has been based on high-speed photographic techniques. We have made direct observations of jet formation and our results, presented here, demonstrate the role of electrical shear effects in this process, and invalidate those theories that assume a uniform velocity profile2 for the liquid in the conical base of the jet.

Dewatering of crude oil emulsions 2. Interfacial properties of the asphaltic constituents of crude oil

The importance of the interfacial rheology in determining the stability of water-in-Buchan crude oil emulsions has been demonstrated in part 1 of this series of papers (R.A. Mohammed, A.I. Bailey, P.F. Luckham and S.E. Taylor, Colloids Surfaces A: Physicochem. Eng. Aspects, 80(1993)223). In part 2, interfacial tensions of crude oil, and solutions of asphaltenes and resins in a model oil have been investigated. Surface pressure vs. area (Π—A) curves of monolayers of asphaltenes, resins and their mixtures have been established. In its dependence on the ratio of resins to asphaltenes, the pseudostatic dilatational modulus has high values for low resin-to-asphaltene ratios and low values for high resin-to-asphaltene ratios. This is expected to throw light on the cause of the enhanced stability of water-in-crude oil emulsions.

Dewatering of crude oil emulsions 1. Rheological behaviour of the crude oil—water interface

Abstract Rheological properties of Buchan crude oil—water interfaces have been determined using a biconical bob rheometer. The effects of temperature and the presence of surface-active demulsifiers on film characteristics have been evaluated. Free oscillation and creep modes have been used, depending on the relative viscoelastic behaviour of the films. Film ageing is particularly significant, whilst temperature and the nature and concentration of any demulsification chemicals also contribute significantly. Further detailed understanding of interfacial film rheology is considered to be desirable for a complete appreciation of the stability of water-in-crude oil emulsions.

Dewatering of crude oil emulsions 3. Emulsion resolution by chemical means

Abstract In Parts 1 and 2 of this series of papers, the importance of interfacial properties in determining the stability of water-in-Buchan crude oil emulsions has been demonstrated. In this third part of the series, we report on the chemical demulsification of water-in-crude oil emulsions. Two classes of experiment were carried out; bottle tests in which emulsion resolution was followed by the appearance of a separated aqueous layer in the bottom of the tube, and microscopic examinations, where resolution was directly followed by observing coalescence of the droplets. Demulsification was achieved by adding one of the commercial demulsifiers Unidem120 and BJI8. Unidem120 is a blend of non-ionic and anionic surfactants, whereas BJ18 was specially developed for the demulsification of Buchan crude. Some of the pure constituents of these blended surfactants were also used for resolving water-in-Buchan crude oil emulsions. Two types of behaviour were observed: type I behaviour, showing constant water separation with increased demulsifier concentration, and type II, showing reduced water separation with increased demulsifier concentration. To a small extent, there exists a degree of functional specificity in the demulsifier molecules towards interaction with the constituents of the interfacial film, and to a larger extent good emulsion resolution is governed by the interfacial properties of the demulsifier blend.

Dewatering of crude oil emulsions 4. Emulsion resolution by the application of an electric field

Abstract The effect of the application of an a.c. electric field on the structure of a water-in-crude oil emulsion has been investigated both experimentally and using a molecular dynamics simulation. In both cases, long chains of droplets are seen to grow between the electrodes as a function of time. This results from an induced dipole on the water droplets in the presence of an external electric field, similar to that seen in electrorheological fluids. In the presence of a rigid interfacial film, resulting from adsorbed crude oil components such as asphaltenes, no coalescence of the drops was apparent. Simulations were also carried out to study the coalescence of water droplets in our emulsion, typical of those in which the asphaltene film was disrupted by the presence of added demulsifier. Coalescence was observed in the microscopic analysis. The results show that the solid interfacial film is a key factor for the prevention of coalescence between droplets in the electric field.

The interfacial rheological behaviour of monolayers of PEO/PMMA graft copolymers spread at the air/water and oil/water interfaces

Abstract Dilatational and shear rheological properties of spread films of PEO/PMMA graft (comb) copolymers were examined at the air/water, toluene/water and toluene— n -heptane/water interfaces. The interfacial dilatational studies were carried out using the stress relaxation Fourier transformation method. The experimental measurement of the interfacial tension decay was performed by means of the Langmuir trough—Wilhelmy plate technique. The high values obtained for the characteristic times and the shape of the Cole—Cole plots indicate that the relaxation processes involve mainly rearrangement in the conformation of the molecules. The interfacial shear viscosity was measured by observing the damped oscillation of a torsion pendulum. The largest interfacial viscosity was exhibited by the monolayer spread at the toluene— n -heptane/water interface. Emulsions prepared with this system, in a separate study to be published, showed the lowest coalescence rate indicating that at this interface the graft copolymer forms a coherent film which retards interdroplet film drainage. The results show that films which exhibit larger values of the interfacial rheological parameters produced a more stable emulsion owing to an increase in the mechanical strength of the interfacial film and its ability to respond to local thickness variations.

The effect of demulsifiers on the interfacial rheology and emulsion stability of water-in-crude oil emulsions

Abstract The interfacial rheological properties of the crude oil-water interface have been measured. It is found that the rheological properties are time dependent and the film takes at least 8 h to attain equilibrium. During this period the surface elasticity and viscosity increase markedly with time, with the surface viscosity increasing from 3 × 10 −2 mN s m −1 after 2 h of ageing to 3 × 10 3 mN s m −1 after 8 h of ageing, and the surface elasticity increasing from zero (i.e. the film was simply viscous) to 2 mN m −1 . The effect of the addition of the demulsifier Unidem 120 to an 8 h aged film was to decrease the surface viscosity by an order of magnitude. In the demulsification experiments, this corresponded to a high resolution of the emulsion. The addition of the demulsifier BJ18, however, produced markedly different results. If the demulsifier was added to a pre-aged film, there was little change in the interfacial rheology of the film; indeed the interfacial rheological properties seemed to increase. However, if the demulsifier was added to the crude oil before the interface between the crude oil and water had formed, the interfacial properties of that film remained constant with time, i.e. no thick viscoelastic film typical of the crude oil-water interface was formed. This result indicates that this demulsifier is poor at displacing the naturally occurring asphaltene surfactants from the crude oil-water interface, but if they adsorb at that interface first they prevent the formation of the stable, rigid asphaltene films. Despite this, however, BJ18 was effective in demulsifying freshly formed water in crude oil emulsions.

Graft copolymers as stabilizers for oil-in-water emulsions Part 2. Preparation of the emulsions and the factors affecting their stability

Abstract Oil-in-water emulsions were prepared using a series of synthetic graft copolymers with a backbone of poly(methyl methacrylate) and a number of side-chains of poly(ethylene glycol). The properties of such preparations are reported in a separate paper (Part 1 of this communication; A.E. Cardenas-Valera, A.I. Bailey and A. Doroszkowski, Colloids Surfaces A: Physicochem. Eng. Aspects 96(1995)53–57). The organic phase was toluene, n-heptane or a mixture of the two materials. The emulsions were characterized for mean droplet diameter by means of a laser scattering technique. The stability of the emulsions was monitored as a function of time by the observation of changes in the mean droplet size distribution. Drainage tests were also carried out. The results were correlated with the structure of the molecules, their conformation at the interface, and formulation variables such as the water/oil ratio and surfactant concentration. The results show that stability is enhanced by increasing the thickness and density of the sheath formed around the oil droplet. For copolymers containing a large proportion of the stabilizing moiety (>40%), the addition of heptane to the organic phase leads to a more stable emulsion. This is due to the reduction of the solvency of the backbone in the oil phase which pushes it towards the plane of the interface, thus achieving better interfacial coverage.

Graft copolymers as stabilizers for oil-in-water emulsions Part 1. Synthesis of the copolymers and their behaviour as monolayers spread at the air-water and oil-water interfaces

Abstract A series of graft (comb) copolymers having a range of emulsifying properties has been prepared. Each molecule has a backbone of poly(methyl methacrylate) (PMMA) and a number of side-chains of methoxy-poly(ethylene oxide) (MeOPEG). The intermediate PEG methacrylate was synthesized by the ester interchange method. The copolymerization was carried out by means of the free radical method. The molecules were prepared having different number. length and spacing of the PEG side-chains and their overall molecular weight was varied between 7000 and 20 000. The variation of surface interfacial pressure, π, with compression of spread monofilms of these copolymers was studied. For the interfacial studies the influence of the solvency conditions of the anchor (PMMA) on the π A curves and on the resulting conformational changes occuring at such interfaces was also investigated.

Non-specific interactions between heparin and poly-L-lysine surfaces

Abstract The surface force apparatus has been used to study the force law between mica surfaces modified with the biopolymers heparin and poly- l -lysine by direct adsorption from solution. Between identical surfaces the interactions were well described by the DLVO theory for repulsive double-layer overlap. Between dissimilar surfaces, strong attractive forces were measured which were consistent with the Hogg—Healy—Fuerstenau approximation to the Poisson—Boltzmann distribution for asymmetric double layers, suggesting a non-specific electrostatic basis for the measured long-range forces.