Antonio Perazzo

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.

A microfluidic approach for flexible and efficient operation of a cross-coupling reactive flow

In this work, a flexible and efficient flow microreactor has been developed for the synthesis of an aromatic amine via a Buchwald–Hartwig reaction, a key bond-forming reaction in the synthesis of a wide range of naturally-occurring and pharmaceutically active targets. The microreactor, coupled with a highly active palladium N-heterocyclic carbene (NHC) catalyst, enabled the full conversion of the reagents within twenty minutes, even at very low catalyst concentrations. In addition to the classical two-feed design, a novel, more flexible four-feed flow system was developed with a configuration optimised to avoid clogging, which is one of the main problems in microreactors and in continuous flow reactive systems in general. We report the effect of flow rate, temperature and catalyst loading on conversion degree. In particular, a slight increase in temperature allowed faster conversion even at low catalyst loadings, likely due to the highly efficient heat transfer provided by the confined microreactor geometry.

Flow-induced gelation of microfiber suspensions

Significance Suspensions of flexible fibers usually behave as shear thinning fluids; that is, their effective viscosity, or resistance to flow, decreases as they are exposed to higher shear stresses. Here we demonstrate that for suspensions created with very-high-aspect-ratio fibers, which are highly flexible, shear thickening behavior of a fiber suspension is obtained. Such a property can be exploited to produce a biocompatible hydrogel by injecting the suspension from a standard needle and syringe without any chemical reactions (unlike a chemically cross-linked hydrogel) or chemical interactions (unlike a traditional physical hydrogel). Once extruded, the hydrogel is a yield-stress material with potentially useful mechanical properties for bioengineering and biomedical applications. The flow behavior of fiber suspensions has been studied extensively, especially in the limit of dilute concentrations and rigid fibers; at the other extreme, however, where the suspensions are concentrated and the fibers are highly flexible, much less is understood about the flow properties. We use a microfluidic method to produce uniform concentrated suspensions of high aspect ratio, flexible microfibers, and we demonstrate the shear thickening and gelling behavior of such microfiber suspensions, which, to the best of our knowledge, has not been reported previously. By rheological means, we show that flowing the suspension triggers the irreversible formation of topological entanglements of the fibers resulting in an entangled water-filled network. This phenomenon suggests that flexible fiber suspensions can be exploited to produce a new family of flow-induced gelled materials, such as porous hydrogels. A significant consequence of these flow properties is that the microfiber suspension is injectable through a needle, from which it can be extruded directly as a hydrogel without any chemical reactions or further treatments. Additionally, we show that this fiber hydrogel is a soft, viscoelastic, yield-stress material.

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.

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.

Viscoplastic Matrix Materials for Embedded 3D Printing

Embedded three-dimensional (EMB3D) printing is an emerging technique that enables free-form fabrication of complex architectures. In this approach, a nozzle is translated omnidirectionally within a soft matrix that surrounds and supports the patterned material. To optimize print fidelity, we have investigated the effects of matrix viscoplasticity on the EMB3D printing process. Specifically, we determine how matrix composition, print path and speed, and nozzle diameter affect the yielded region within the matrix. By characterizing the velocity and strain fields and analyzing the dimensions of the yielded regions, we determine that scaling relationships based on the Oldroyd number, Od, exist between these dimensions and the rheological properties of the matrix materials and printing parameters. Finally, we use EMB3D printing to create complex architectures within an elastomeric silicone matrix. Our methods and findings will both facilitate future characterization of viscoplastic matrices and motivate the development of new materials for EMB3D printing.

Salt type and concentration affect the viscoelasticity of polyelectrolyte solutions

The addition of small amounts of xanthan gum to water yields viscoelastic solutions. In this letter, we show that the viscoelasticity of aqueous xanthan gum solutions can be tuned by different types of salts. In particular, we find that the decrease in viscoelasticity not only depends, as is known, on the salt concentration, but also is affected by the counterion ionic radius and the valence of the salt.The addition of small amounts of xanthan gum to water yields viscoelastic solutions. In this letter, we show that the viscoelasticity of aqueous xanthan gum solutions can be tuned by different types of salts. In particular, we find that the decrease in viscoelasticity not only depends, as is known, on the salt concentration, but also is affected by the counterion ionic radius and the valence of the salt.

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.

Foam-driven fracture

Significance Hydraulic fracturing plays an important role in meeting today’s energy demands. However, the substantial use of fresh water in fracturing and wastewater returning to the surface pose risks to the environment. Alternative technology has been developed that reduces the water-related risks by injecting aqueous foam instead of water to fracture shale formations, but the mechanism is poorly understood. Here, we show, using laboratory experiments, that the injection of foam instead of water dramatically changes the fracture dynamics when the foam compressibility is important. We develop a scaling argument for the fracture dynamics that exhibits excellent agreement with the experimental results. Our findings extend to other systems involving compressible foams, including fire-fighting, energy storage using compressed foams, and enhanced oil recovery. In hydraulic fracturing, water is injected at high pressure to crack shale formations. More sustainable techniques use aqueous foams as injection fluids to reduce the water use and wastewater treatment of conventional hydrofractures. However, the physical mechanism of foam fracturing remains poorly understood, and this lack of understanding extends to other applications of compressible foams such as fire-fighting, energy storage, and enhanced oil recovery. Here we show that the injection of foam is much different from the injection of incompressible fluids and results in striking dynamics of fracture propagation that are tied to the compressibility of the foam. An understanding of bubble-scale dynamics is used to develop a model for macroscopic, compressible flow of the foam, from which a scaling law for the fracture length as a function of time is identified and exhibits excellent agreement with our experimental results.

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.