Deniz Z. Gunes

Interfacial Activity and Interfacial Shear Rheology of Native β-Lactoglobulin Monomers and Their Heat-Induced Fibers

Interfacial properties of native β-lactoglobulin monomers and their heat-induced fibers, of two different lengths, were investigated at pH 2, through surface tension measurements at water-air and water-oil interfaces and interfacial shear rheology at the water-oil interface. The applied heat treatment generates a mixed system of fibers with unconverted monomers and hydrolyzed peptides. The surface tension of this system at the water-air interface decreased more rapidly than the surface tension of native monomers, especially at short times (10(-3) to 10(2) s). This behavior was not observed when the unconverted monomers and peptides were removed by dialysis. At the water-oil interface, the adsorption kinetics was much faster than at the water-air interface, with a plateau interfacial pressure value reached after 1 h of adsorption. For all the systems, interfacial shear rheology showed the formation of a highly elastic interface, with solid-like behavior at 1-10(3) s time scales. The highest modulus was observed for the long fibers and the lowest for the native monomers. Creep-compliance curves in the linear regime could be reduced to a single master curve, showing similar spectra of relaxation times for all investigated systems. Upon large deformations, the interfaces formed with long fibers showed the most rigid and fragile behavior. This rigidity was even more pronounced in the presence of unconverted monomers.

Avalanches of coalescence events and local extensional flows – Stabilisation or destabilisation due to surfactant

From two-drop collision experiments, it is known that local extensional flow favors coalescence. Recently, Bremond et al. used microfluidic methods to evidence this point. Similarly, we used specific microfluidic geometries to impose sudden extensional flow, following drop collision under controlled conditions, and coalescence events were recorded with a high-speed camera. In this study we focus on the effect of surfactant on the coalescence, or stabilisation against it, between drops flowing apart due to either imposed external flow or capillary forces related to drop shape relaxation. Coalescence can be induced even when drops are initially separated by an intersticial lubricating film by far thicker than the critical thickness for rupturing under the action of Van der Waals forces. This is particularly relevant to avalanches of coalescence events, in flowing or even quiescent emulsions or foams. When non-ionic surfactant was used, it was observed that small concentrations apparently enhance coalescence in extension. But at higher concentrations it provides stabilisation through a specific mechanism of thread formation and rupture; the stabilisation mechanism can be complex.

Microfluidic preparation and self diffusion PFG-NMR analysis of monodisperse water-in-oil-in-water double emulsions

Monodisperse water-in-oil-in-water (WOW) double emulsions have been prepared using microfluidic glass devices designed and built primarily from off the shelf components. The systems were easy to assemble and use. They were capable of producing double emulsions with an outer droplet size from 100 to 40 μm. Depending on how the devices were operated, double emulsions containing either single or multiple water droplets could be produced. Pulsed-field gradient self-diffusion NMR experiments have been performed on the monodisperse water-in-oil-in-water double emulsions to obtain information on the inner water droplet diameter and the distribution of the water in the different phases of the double emulsion. This has been achieved by applying regularization methods to the self-diffusion data. Using these methods the stability of the double emulsions to osmotic pressure imbalance has been followed by observing the change in the size of the inner water droplets over time.

Structure, diffusion, and permeability of protein-stabilized monodispersed oil in water emulsions and their gels: a self-diffusion NMR study

Self-diffusion NMR is used to investigate monodispersed oil in water emulsions and the subsequent gel formed by removing the water through evaporation. The radius of the oil droplets in the emulsions is measured using a number of diffusion methods based on the measurement of the mean squared displacement of the oil, water, and tracer molecules. The results are consistent with the known size of the emulsions. Bragg-like reflections due to the restricted diffusion of the water around the oil droplets are observed due to the low polydispersity of the emulsions and the dense packing. The resulting data are fitted to a pore glass model to give the diameter of both the pools of interstitial water and the oil droplets. In the gel, information on the residual three-dimensional structure is obtained using the short time behavior of the effective diffusion coefficient to give the surface to volume ratio of the residual protein network structure. The values for the surface to volume ratio are found to be consistent with the expected increase of the surface area of monodisperse droplets forming a gel network. At long diffusion observation times, the permeability of the network structure is investigated by diffusion NMR to give a complete picture of the colloidal system considered.

Decoupling of Mass Transport Mechanisms in the Stagewise Swelling of Multiple Emulsions

This contribution reports on the mass transport kinetics of osmotically imbalanced water-in-oil-in-water (W1/O/W2) emulsions. Although frequently studied, the control of mass transport in W1/O/W2 emulsions is still challenging. We describe a microfluidics-based method to systematically investigate the impact of various parameters, such as osmotic pressure gradient, oil phase viscosity, and temperature, on the mass transport. Combined with optical microscopy analyses, we are able to identify and decouple the various mechanisms, which control the dynamic droplet size of osmotically imbalanced W1/O/W2 emulsions. So, swelling kinetics curves with a very high accuracy are generated, giving a basis for quantifying the kinetic aspects of transport. Two sequential swelling stages, i.e., a lag stage and an osmotically dominated stage, with different mass transport mechanisms are identified. The determination and interpretation of the different stages are the prerequisite to control and trigger the swelling process. We show evidence that both mass transport mechanisms can be decoupled from each other. Rapid osmotically driven mass transport only takes place in a second stage induced by structural changes of the oil phase in a lag stage, which allow an osmotic exchange between both water phases. Such structural changes are strongly facilitated by spontaneous water-in-oil emulsification. The duration of the lag stage is pressure-independent but significantly influenced by the oil phase viscosity and temperature.

Arresting dissolution by interfacial rheology design

Significance The challenge of creating foams and emulsions with well-controlled size distribution and properties is encountered in many structured materials, such as food formulations and consumer care products. These products, like ice cream for example, must remain stable over long shelf lifetimes while their microstructure dictates product performance and consumer satisfaction. Despite the common use of particles to stabilize bubbles and emulsions, the cause of such stabilization is unknown. Here, we provide the link between the particles’ ability to impart a resistance, or “armor,” against bubble dissolution and their interfacial rheological properties. We propose a design strategy based on controlling interfacial particle interactions to arrest dissolution of small bubbles to create foam and emulsion materials with stable microstructures and controllable textures. A strategy to halt dissolution of particle-coated air bubbles in water based on interfacial rheology design is presented. Whereas previously a dense monolayer was believed to be required for such an “armored bubble” to resist dissolution, in fact engineering a 2D yield stress interface suffices to achieve such performance at submonolayer particle coverages. We use a suite of interfacial rheology techniques to characterize spherical and ellipsoidal particles at an air–water interface as a function of surface coverage. Bubbles with varying particle coverages are made and their resistance to dissolution evaluated using a microfluidic technique. Whereas a bare bubble only has a single pressure at which a given radius is stable, we find a range of pressures over which bubble dissolution is arrested for armored bubbles. The link between interfacial rheology and macroscopic dissolution of ∼ 100 μm bubbles coated with ∼ 1 μm particles is presented and discussed. The generic design rationale is confirmed by using nonspherical particles, which develop significant yield stress at even lower surface coverages. Hence, it can be applied to successfully inhibit Ostwald ripening in a multitude of foam and emulsion applications.

A study of extensional flow induced coalescence in microfluidic geometries with lateral channels

The coupled mechanisms of extensional coalescence and subsequent shape relaxation can lead to catastrophic destabilization of moderately concentrated emulsions. We demonstrate that application of local extensional flow through the use of small lateral channels allows controlled, systematic investigation of both single drop pair and propagating (avalanche) coalescence through a chain of drops. Drop–drop collisions and separations were controlled independently, and did not significantly disturb the primary flow. The probability of the first coalescence event was controlled by bulk flow parameters, allowing for systematic investigation of these phenomena. Simulations with COMSOL® were used in order to quantify and thus validate various assumptions relating to the flow characteristics of our setup. For the configurations tested, the droplet pair separation speed increased linearly with the lateral channel infusion rate. Flows were laminar and collision conditions remained stable until a first coalescence event between a pair of drops was triggered by the superposed local extensional flow field close to the lateral channels. Results are described in terms of coalescence probability versus separation capillary number (Casep). For all systems tested, an upper limit value Ca*sep was observed, above which coalescence did not occur. The probability and length of upstream coalescence propagation induced by the drop shape relaxation following the initial, triggered event are reported. Drop–drop contact times were varied by injecting fluid using different combinations of lateral channels. Ca*sep shifted to a higher value for a given system as the lubricating film drained for a longer time, which, in addition, increased the probability and length of an avalanche of events. The present results demonstrate how microfluidic tools can be used for systematically mapping the most probable behavior of complex systems with respect to coalescence under well controlled hydrodynamic conditions. In general we observe that larger drops, slower separation and higher surfactant concentration favour extensional coalescence and its propagation, in agreement with earlier published experimental studies.

Interfacial aspects of the stability of polyglycerol ester covered bubbles against coalescence

Most liquid foams are not stable, with one destabilization process being coalescence. The current study aims at better understanding the coalescence of bubbles covered by the non-ionic surfactant polyglycerol ester (PGE) in dilute solutions. Large deformations of bubble surfaces are of importance in understanding coalescence as they can induce coalescence events. Furthermore, during coalescence and the subsequent shape relaxation large deformations are imposed on the surfaces. These large deformation experiments were carried out using interfacial shear and dilatational rheology. Furthermore isolated coalescence events of bubbles were observed. The largest fraction of the interfacial shear modulus was recovered within 1 minute of halting the large deformations, with the overall recovery happening on a timescale of hours. When mimicking coalescence by reducing the surface area, a short-term increase in the elastic modulus was observed in dilatational rheology. The remarkably slow, two-mode exponential shape relaxation recorded for sufficiently covered bubbles following their coalescence hints at important interfacial structuring phenomena. As a result of the low solubility of PGE, coalescence appears to help in achieving a certain stability level at short timescales via an increase in surface coverage and the growth of multi-layer domains. Finally, coalescence patterns in two-dimensional foams were investigated, which suggested that in the absence of prior coalescence, very long adsorption times (>12 hours) are required to reach significant stability. These experiments also revealed a remarkable stability of polyglycerol ester foams against Ostwald ripening.

Tuneable thickness barriers for composite o/w and w/o capsules, films, and their decoration with particles

Oil-in-water and water-in-oil capsules, and flat membranes of tuneable thickness and composition were prepared in one single facile step, based on the interfacial complexation between chitosan and anionic phosphatidic fatty acids. The phosphatidic acid molecules were introduced via the oil phase. The thickness of the capsule shell or the membrane grows by a diffusion-controlled mechanism, hence can be tuned using e.g. concentration and formation time parameters. A mechanism is proposed to explain the observed behavior. The capsule size is set by the emulsification conditions applied. Microfluidic methods proved useful for the generation of hollow capsules of uniform size and thickness in one step. The capsules and membranes display remarkable integrity over several years in a pH window 2–14. The thickness can easily reach several micrometres within an hour for the wet capsule shell or membrane, which explains the high interfacial rheological properties measured. Hence various processes can be envisaged after their formation. The simple preparation opens the way to tailored, environment-responsive composite systems for fabricating biopolymer-based materials for various applications. The capsules could be washed from the surrounding continuous phase and placed into one of arbitrary choice. Furthermore, the surface of the w/o capsules and of the membranes could be decorated by particles that attach to the water/oil interface with a high energy. The choice of the particle functionality is left open.

Quantification of Spontaneous W/O Emulsification and its Impact on the Swelling Kinetics of Multiple W/O/W Emulsions

An osmotic imbalance between the two water phases of multiple water-in-oil-in-water (W1/O/W2) emulsions results in either emulsion swelling or shrinking due to water migration across the oil layer. Controlled mass transport is not only of importance for emulsion stability but also allows transient emulsion thickening or the controlled release of encapsulated substances, such as nutriments or simply salt. Our prior work has shown that mass transport follows two sequential stages. In the first stage, the oil-phase structure is changed in a way that allows rapid, osmotically driven water transport in the second, osmotically dominated stage. These structural changes in the oil layer are strongly facilitated by the spontaneous formation of tiny water droplets in the oil phase, induced by the oil-soluble surfactant, i.e., polyglycerol polyricinoleate (PGPR). This study provides a simple method based on microscopy image analysis, allowing a detailed investigation of spontaneous W/O emulsification. It quantitatively describes the volume of droplets generated and the rate of droplet creation. Moreover, it describes the effect of spontaneous W/O emulsification on the swelling kinetics of microfluidic processed W1/O/W2 emulsions. Two different concentration regimes of the oil-soluble surfactant are identified: below a critical concentration the overall water transport rate increases, and above a critical concentration water transport stagnates because of maximized structure formation.