Valentina Preziosi

Phase inversion emulsification: Current understanding and applications

This review is addressed to the phase inversion process, which is not only a common, low-energy route to make stable emulsions for a variety of industrial products spanning from food to pharmaceuticals, but can also be an undesired effect in some applications, such as crude oil transportation in pipelines. Two main ways to induce phase inversion are described in the literature, i.e., phase inversion composition (PIC or catastrophic) and phase inversion temperature (PIT or transitional). In the former, starting from one phase (oil or water) with surfactants, the other phase is more or less gradually added until it reverts to the continuous phase. In PIT, phase inversion is driven by a temperature change without varying system composition. Given its industrial relevance and scientific challenge, phase inversion has been the subject of a number of papers in the literature, including extensive reviews. Due to the variety of applications and the complexity of the problem, most of the publications have been focused either on the phase behavior or the interfacial properties or the mixing process of the two phases. Although all these aspects are quite important in studying phase inversion and much progress has been done on this topic, a comprehensive picture is still lacking. In particular, the general mechanisms governing the inversion phenomenon have not been completely elucidated and quantitative predictions of the phase inversion point are limited to specific systems and experimental conditions. Here, we review the different approaches on phase inversion and highlight some related applications, including future and emerging perspectives.

Droplet deformation under confined Poiseuille flow.

The flow behavior of droplet-based liquid-liquid systems, such as emulsions, polymer blends, and foodstuff, which are ubiquitous in everyday life, has attracted scientific interest in different disciplines. In this review, we focus on the pressure-driven confined flow behavior of isolated droplets in circular and rectangular cross-section channels, which are valuable model geometries to gain insight into more complex flow conditions found in industrial applications. The effect of the relevant nondimensional parameters governing droplet deformation and breakup, such as viscosity ratio, capillary number, and ratio of droplet to tube radius, is presented both for axisymmetric and off-axis droplets, including cross-stream migration. The role of surfactants is also discussed. Ongoing research directions include the field of microfluidics techniques, where confined flow geometries can be exploited to manipulate droplets with a variety of possible applications.

Monitoring emulsion microstructure by using organic electrochemical transistors

Organic electrochemical transistors (OECTs) are powerful amplifying transducers of chemical signals allowing us to measure ionic transport between an electrolyte solution and an organic semiconductor film with nanosensing capabilities and low cost of fabrication. Here, we report how OECTs can also be exploited to detect microstructural features of complex soft materials, such as oil/water emulsions. To this purpose, the response of OECTs is investigated for samples obtained at different stages of a nano-emulsification process carried out by gradually adding water to a mixture of oil and two non-ionic surfactants. Our results demonstrate that, above a critical water volume fraction, OECTs are able to work as depletion-mode transistors displaying specific features in terms of the final current modulation capability and the transient time response. In particular, the kinetics of the device current upon the application of step-like probing gate voltages is successfully modelled by using a double exponential law with characteristic time constants. We relate the OECT behavior to the clustering and percolation of water droplets as detected by confocal laser scanning microscopy (CLSM) and rheometrical measurements. Our results lay the foundation for the quantitative application of OECTs to identify the phase behaviour and microstructure in complex soft materials, a relevant issue in industrial processing and material characterization.

Confined tube flow of low viscosity emulsions: Effect of matrix elasticity

This study is addressed to the effect of the elasticity of the continuous phase on confined flow of low viscosity emulsions, a topic of interest in several applications. A Newtonian silicone oil was used as the dispersed phase, while three different water-based fluids, a Newtonian and two Boger fluids, were used as the continuous phase. All the fluids have the same viscosity (ca. 0.05 Pa s). Emulsions were prepared by feeding the two phases to a T-junction followed by a membrane. When the continuous phase was a Boger fluid, the average drop size of the so obtained emulsions was higher (as compared to the Newtonian continuous phase) and a bimodal distribution was observed. Such lower mixing efficiency for a Boger continuous phase is in line with the result for an isolated droplet that breakup is hindered by matrix elasticity. High-speed microscopy visualization of the emulsions flowing through a microcapillary allowed one to determine the velocity profiles and the droplet spatial distribution as a function...

Measuring Interfacial Tension of Emulsions in Situ by Microfluidics

Interfacial tension is a key parameter affecting industrially relevant properties of emulsions, such as morphology and stability. Although several methods are available to measure interfacial tension, they are based on generation of droplets starting from separate emulsion components and cannot directly probe the interfacial tension of an emulsion as such. Here, a novel microfluidic tensiometry device to measure interfacial tension of a water-in-oil emulsion in situ as a function of surfactant concentration is presented. In our approach, interfacial tension is obtained from a quantitative analysis of the deformation of individual emulsion droplets under steady state shear flow in microfluidic channels. The technique is validated by comparing the results with experimental data obtained by the pendant drop method in a broad range of interfacial tension values. A very good agreement is found, and an estimate of the surfactant critical micellar concentration (CMC) is also obtained. The proposed microfluidic setup can be used even at high surfactant concentrations, where the measurement is made more challenging by sample viscoelasticity, thus providing a powerful tool to determine the interfacial tension of complex systems in an extended concentration range. The technique could be also used for in-line monitoring of emulsion processing.

Emulsions in porous media: From single droplet behavior to applications for oil recovery

Emulsions are suspensions of droplets ubiquitous in oil recovery from underground reservoirs. Oil is typically trapped in geological porous media where emulsions are either formed in situ or injected to elicit oil mobilization and thus enhance the amount of oil recovered. Here, we briefly review basic concepts on geometrical and wetting features of porous media, including thin film stability and fluids penetration modes, which are more relevant for oil recovery and oil-contaminated aquifers. Then, we focus on the description of emulsion flow in porous media spanning from the behaviour of single droplets to the collective flow of a suspension of droplets, including the effect of bulk and interfacial rheology, hydrodynamic and physico-chemical interactions. Finally, we describe the particular case of emulsions used in underground porous media for enhanced oil recovery, thereby discussing some perspectives of future work. Although focused on oil recovery related topics, most of the insights we provide are useful towards remediation of oil-contaminated aquifers and for a basic understanding of emulsion flow in any kind of porous media, such as biological tissues.

Flow-induced concentration gradients in shear-banding of branched wormlike micellar solutions

HYPOTHESIS Shear-banding of branched wormlike solutions is a topic of active investigation which has not been fully elucidated. Here, we surmise that flow-induced microstructuring in the shear banding regime is associated with spatial concentration gradients. EXPERIMENTS The experiments focus on the flow-induced behavior of a CTAB/NaSal wormlike micellar system. A unique approach based on a microfluidic-spitter geometry, combined with particle-image velocimetry and high-speed video microscopy, is used to separate the streams flowing out from the core and the near wall zones of the microchannel. FINDINGS Here, we present the first direct experimental evidence of the correlation between phase separation and shear banding. By increasing the pressure-drop across a microcapillary, the onset of a grainy texture close to the wall, showing a flow-induced demixing effect, is observed. We use a splitter to measure effluent streams from the center and the near-wall zones in terms of viscosity, conductance and dry mass. We observe that phase-separation induced by the flow correlates with chemical concentration gradients. This confirms our hypothesis that shear-induced local de-mixing of the system is strongly related to chemical concentration gradients.

Catastrophic Phase Inversion Techniques for Nanoemulsification

Abstract Nanoemulsions can be fabricated using the catastrophic phase inversion (CPI) method by exploiting either surfactants or solid particles. This emulsification method is also sometimes known as the phase inversion composition or emulsion inversion point method and is based on gradually diluting, under mild flow conditions, one liquid (such as water) with another immiscible liquid (such as oil) until phase inversion occurs and a nanoemulsion is formed (such as oil in water). Usually, the inversion of the phases is signaled by the appearance of an intermediate phase with specific physicochemical properties that is broken down during the emulsification process, thus giving rise to small droplets. The name CPI was termed because researchers thought that this process could be described using catastrophe theory. However, the physicochemical mechanisms associated with phase inversion cannot always be described using this theory because there are too many hydrodynamic and physicochemical variables to account for. Despite the lack of an analytic model capable of including both the hydrodynamics and physical chemistry of the system and of describing the complex morphological changes associated with this phenomenon, CPI can still be used successfully to encapsulate many active compounds (such as pharmaceuticals, vitamins, nutraceuticals, antimicrobials, colors, and flavors) in nanoemulsions. In addition, our current mechanistic understanding allows nanosized droplet dimensions to be roughly predicted for systems prepared using pure single surfactants. This chapter provides an overview of the current understanding of the theory and practice of nanoemulsion production using the CPI method.

The effect of flow on viscoelastic emulsion microstructure

Abstract.Emulsions made of oil, water and surfactants are widespread soft materials with complex structures depending on composition and temperature. Emulsion phase behavior at rest has been widely investigated but flow-induced effects, which are very relevant in many applications, can still be further explored towards improved emulsion microstructural design. In this work, we use low energy emulsification processing to create small-sized emulsions. In a previous report, we showed the emulsion morphology development and the effect of flow on the microstructure of a highly viscoelastic attractive emulsion which result in a concentrated nanoemulsion after viscoelastic droplet filaments are disrupted. Here, we show that upon stopping the flow, the filaments slowly buckle, recoil and finally form clusters of randomly flocculated droplets. We thus obtain two completely different emulsion morphologies simply induced by the action of flow, where in both cases attractive interactions play a key role. The emulsion high interfacial area represents a valuable feature for several applications such as upstream operations, microreaction media and drug delivery.Graphical abstract