Luben N. Arnaudov

Super stable foams stabilized by colloidal ethyl cellulose particles

Here we report the preparation of super stable liquid foams with various bubble sizes stabilized by colloidal ethyl cellulose (EC) particles. What is novel and different in this particle stabilized foam is that both the initial material (EC) and processes used are in principle food grade, thus it may offer scope in food applications. The particles were prepared using a conventional anti-solvent precipitation method, involving the dissolution of EC polymer into acetone, followed by fast mixing with water (anti-solvent), leading to the precipitation of EC particles, then followed by the rotary evaporation of acetone. The interfacial tension of the resulting dispersion is 36 mN m−1, indicating that particles co-exist with surface active and water soluble components, which is most likely a low molecular weight EC fraction. The average particle diameter is 0.13 μm and their zeta potential is −50mV at pH = 6, increasing to −5mV at pH = 3. This negative surface potential is attributed to adsorption of hydroxyl ions, known to occur on many hydrophobic surfaces, including oil–water, air–water and hydrophobic particle–water. As a result, there is strong electrostatic repulsion between EC particles at neutral and low ionic strength, which stabilizes EC dispersion and also significantly increases the adsorption barrier of EC particle at the air–water interface. Due to their similar origin, both inter-particle repulsion and adsorption barrier can be controlled by pH and/or ionic strength, which leads to dispersion destabilization and at the same time good foamability and extreme foam stability at acidic conditions (pH 20 mM). Foam coarsening shows an initial stage with coarsening time of approximately 1 week, followed by a plateau, where the coarsening has been arrested for a period of months. By using cryo scanning electron microscopy, we reveal that these EC foams are Pickering stabilized, where EC particles are closely packed at the air–water interface forming a single or multi-layers. We also show that super stable EC foams can be prepared using various aeration techniques, allowing us to vary the bubble diameter from a few microns to hundreds of microns.

Interfacial layers from the protein HFBII hydrophobin: Dynamic surface tension, dilatational elasticity and relaxation times

The pendant-drop method (with drop-shape analysis) and Langmuir trough are applied to investigate the characteristic relaxation times and elasticity of interfacial layers from the protein HFBII hydrophobin. Such layers undergo a transition from fluid to elastic solid films. The transition is detected as an increase in the error of the fit of the pendant-drop profile by means of the Laplace equation of capillarity. The relaxation of surface tension after interfacial expansion follows an exponential-decay law, which indicates adsorption kinetics under barrier control. The experimental data for the relaxation time suggest that the adsorption rate is determined by the balance of two opposing factors: (i) the barrier to detachment of protein molecules from bulk aggregates and (ii) the attraction of the detached molecules by the adsorption layer due to the hydrophobic surface force. The hydrophobic attraction can explain why a greater surface coverage leads to a faster adsorption. The relaxation of surface tension after interfacial compression follows a different, square-root law. Such behavior can be attributed to surface diffusion of adsorbed protein molecules that are condensing at the periphery of interfacial protein aggregates. The surface dilatational elasticity, E, is determined in experiments on quick expansion or compression of the interfacial protein layers. At lower surface pressures (<11 mN/m) the experiments on expansion, compression and oscillations give close values of E that are increasing with the rise of surface pressure. At higher surface pressures, E exhibits the opposite tendency and the data are scattered. The latter behavior can be explained with a two-dimensional condensation of adsorbed protein molecules at the higher surface pressures. The results could be important for the understanding and control of dynamic processes in foams and emulsions stabilized by hydrophobins, as well as for the modification of solid surfaces by adsorption of such proteins.

Nanoemulsions obtained via bubble-bursting at a compound interface

When a bubble bursts on reaching a surface, mass transfer from the liquid to the gas phase can occur—aerosol dispersion. Now, the inverse transport process is reported: submicrometre-sized oil droplets, formed during bubble-bursting, are zipped across the interface to the liquid phase.

Theoretical modeling of the kinetics of fibrilar aggregation of bovine β-lactoglobulin at pH 2

The authors propose a kinetic model for the heat-induced fibrilar aggregation of bovine beta-lactoglobulin at pH 2.0. The model involves a nucleation step and a simple addition reaction for the growth of the fibrils, as well as a side reaction leading to the irreversible denaturation and inactivation of a part of the protein molecules. For the early stages of the aggregation reaction, the authors obtain an analytical solution of the model. In agreement with their experimental results, the model predicts a critical protein concentration below where almost no fibrils are formed. The model agrees well with their experimental data from in situ light scattering. By fitting the experimental data with the model, the authors obtain the ionic strength dependent kinetic rate constants for beta-lactoglobulin fibrilar aggregation and the size of the critical nucleus.

Surface shear rheology of adsorption layers from the protein HFBII hydrophobin: effect of added β-casein.

The surface shear rheology of hydrophobin HFBII adsorption layers is studied in angle-ramp/relaxation regime by means of a rotational rheometer. The behavior of the system is investigated at different shear rates and concentrations of added β-casein. In angle-ramp regime, the experimental data comply with the Maxwell model of viscoelastic behavior. From the fits of the rheological curves with this model, the surface shear elasticity and viscosity, E(sh) and η(sh), are determined at various fixed shear rates. The dependence of η(sh) on the rate of strain obeys the Herschel-Bulkley law. The data indicate an increasing fluidization (softening) of the layers with the rise of the shear rate. The addition of β-casein leads to more rigid adsorption layers, which exhibit a tendency of faster fluidization at increasing shear rates. In relaxation regime, the system obeys a modified Andrades (cubic root) law, with two characteristic relaxation times. The fact that the data comply with the Maxwell model in angle-ramp regime, but follow the modified Andrades low in relaxation regime, can be explained by the different processes occurring in the viscoelastic protein adsorption layer in these two regimes: breakage and restoration of intermolecular bonds at angle-ramp vs solidification of the layer at relaxation.

Polymers at the water/air interface, surface pressure isotherms, and molecularly detailed modeling.

Surface pressure isotherms at the air/water interface are reproduced for four different polymers, poly-L-lactic acid (PLLA), poly(dimethylsiloxane) (PDMS), poly(methyl methacrylate) (PMMA), and poly(isobutylene) (PiB). The polymers have the common property that they do not dissolve in water. The four isotherms differ strongly. To unravel the underlying details that are causing these differences, we have performed molecularly detailed self-consistent field (SCF) modeling. We describe the polymers on a united atom level, taking the side groups on the monomer level into account. In line with experiments, we find that PiB spreads in a monolayer which smoothly thickens already at a very low surface pressure. PMMA has an autophobic behavior: a PMMA liquid does not spread on top of the monolayer of PMMA at the air/water interface. A thicker PMMA layer only forms after the collapse of the film at a relatively high pressure. The isotherm of PDMS has regions with extreme compressibility which are linked to a layering transition. PLLA wets the water surface and spreads homogeneously at larger areas per monomer. The classical SCF approach features only short-range nearest-neighbor interactions. For the correct positioning of the layering and for the thickening of the polymer films, we account for a power-law van der Waals contribution in the model. Two-gradient SCF computations are performed to model the interface between two coexistent PDMS films at the layering transition, and an estimation of the length of their interfacial contact is obtained, together with the associated line tension value.

Time-resolved small-angle neutron scattering during heat-induced fibril formation from bovine β-lactoglobulin

We study in situ the kinetics of heat-induced fibrilar aggregation of bovine beta-lactoglobulin at pH 2.0 and 80 degrees C for the first time by time-resolved small-angle neutron scattering. A simple model for the scattering from a mixture of monodisperse charged spheres (monomeric beta-lactoglobulin) interacting via a screened electrostatic repulsion and noninteracting long cylinders (protein fibrils) is used to describe the data. The experimental data are fitted to the model and the concentration of the monomeric protein and the protein incorporated in fibrils are obtained as adjustable parameters. Thus, a simple physical model allows the determination of realistic kinetic parameters during fibrilar protein aggregation. This result constitutes an important step in understanding the process of irreversible fibrilar aggregation of proteins.

Sonication–Microfluidics for Fabrication of Nanoparticle-Stabilized Microbubbles

An approach based upon sonication-microfluidics is presented to fabricate nanoparticle-coated microbubbles. The gas-in-liquid slug flow formed in a microchannel is subjected to ultrasound, leading to cavitation at the gas-liquid interface. Therefore, microbubbles are formed and then stabilized by the nanoparticles contained in the liquid. Compared to the conventional sonication method, this sonication-microfluidics continuous flow approach has unlimited gas nuclei for cavitation that yields continuous production of foam with shorter residence time. By controlling the flow rate ratios of the gas to the liquid, this method also achieves a higher production volume, smaller bubble size, and less waste of the nanoparticles needed to stabilize the microbubbles.

Hydrodynamic cavitation: a bottom-up approach to liquid aeration

We report the use of hydrodynamic cavitation as a novel, bottom-up method for continuous creation of foams comprising of air micro-bubbles in aqueous systems containing surface active ingredients, like proteins or particles. The hydrodynamic cavitation was created using a converging–diverging nozzle. The air bubble size obtained using this technique was found to be significantly smaller than that achieved using conventional mechanical entrapment of air via shearing or shaking routes, which are in essence top-down approaches. In addition, the technique provided the possibility of forming non-spherical bubbles due to the high elongational stresses experienced by the bubbles as they flow through the nozzle throat. We show that surface active agents with a high surface elasticity modulus can be used to stabilize the nascent air bubbles and keep their elongated shapes for prolonged periods of time. This combination of the cavitation process with appropriate surface active agents offers an opportunity for creating bubbles smaller than 10 microns, which can provide unique benefits in various applications.

A Scalable Platform for Functional Nanomaterials via Bubble‐Bursting

A continuous and scalable bubbling system to generate functional nanodroplets dispersed in a continuous phase is proposed. Scaling up of this system can be achieved by simply tuning the bubbling parameters. This new and versatile system is capable of encapsulating various functional nanomaterials to form functional nanoemulsions and nanoparticles in one step.