Interfacial Dilatational Viscosity Research Articles

In vitro Digestion of Emulsions in a Single Droplet via Multi Subphase Exchange of Simulated Gastrointestinal Fluids.

Emulsions are currently being used to encapsulate and deliver nutrients and drugs to tackle different gastrointestinal conditions such as obesity, nutrient fortification, food allergies, and digestive diseases. The ability of an emulsion to provide the desired functionality, namely, reaching a specific site within the gastrointestinal tract, inhibiting/retarding lipolysis, or facilitating digestibility, ultimately depends on its susceptibility to enzymatic degradation in the gastrointestinal tract. In oil-in-water emulsions, lipid droplets are surrounded by interfacial layers, where the emulsifiers stabilize the emulsion and protect the encapsulated compound. Achieving a tailored digestibility of emulsions depends on their initial composition but also requires monitoring the evolution of those interfacial layers as they are subjected to different phases of gastrointestinal digestion. A pendant drop surface film balance implemented with a multi-subphase exchange allows for simulating the in vitro digestion of emulsions in a single aqueous droplet immersed in oil by applying a customized static digestion model. The transit through the gastrointestinal tract is mimicked by the subphase exchange of the original droplet bulk solution with artificial media, mimicking the physiological conditions of each compartment/step of the gastrointestinal tract. The dynamic evolution of the interfacial tension is recorded in situ throughout the whole simulated gastrointestinal digestion. The mechanical properties of digested interfaces, such as interfacial dilatational elasticity and viscosity, are measured after each digestion phase (oral, gastric, small intestine). The composition of each digestive media can be tuned to account for the particularities of the digestive conditions, including gastrointestinal pathologies and infant digestive media. The specific interfacial mechanisms affecting proteolysis and lipolysis are identified, providing tools to modulate digestion by the interfacial engineering of emulsions. The obtained results can be manipulated for designing novel food matrices with tailored functionalities such as low allergenicity, controlled energy intake, and decreased digestibility.

Interfacial viscosity-dictated morpho-dynamics of a compound drop in linear flows

Compound droplets are excellent analogs of complex biological entities such as vesicles or cells. Despite significant advancements toward understanding the morphological evolution of a compound droplet in an incipient flow, the specific role of interfacial rheology toward dictating the same remains unaddressed. Here, we bring out non-trivial implications of interfacial rheology on the deformation of a compound drop subject to an imposed flow. The interfacial viscosity, in effect, interacts with the flow-induced non-uniform surfactant distribution to alter the droplet morpho-dynamics in a rather engaging manner. We employ a closed-form analytical approach to delineate the relative roles of advective and diffusive transport. In the paradigm of diffusion-dominated interfacial transport, viscous interfacial stress arrests the droplet deformation, thus enhancing its stability. However, for large values of the interfacial dilatational viscosity, the drop deformation increases with the interfacial shear viscosity. On the contrary, in the paradigm of surface convection-dominated surfactant transport, the interfacial rheology does not have any significant effect on either the shape deformation or the emulsion rheology. These results may pave a way toward explaining several unique features of complex fluid–fluid interfaces encountered in nature and biology.

Complexation between flaxseed protein isolate and phenolic compounds: Effects on interfacial, emulsifying and antioxidant properties of emulsions

Interfacial and emulsifying properties of flaxseed protein isolate (FPI) and its phenolic complexes were studied, aiming to develop plant-based natural emulsifiers with improved interfacial and oxidative activity and adding value to the by-product of flaxseed oil production. Flaxseed polyphenols (FPP) and hydroxytyrosol (HT) were used as model phenolic compounds. The stability of emulsions produced using FPI, FPI-FPP and FPI-HT as emulsifiers was measured and correlated with their solubility and surface charge. The dynamic interfacial tension (DIT) and dilatational elasticity (E′) and viscosity (E”) were measured at flaxseed oil/water interface. The diffusion and penetration rate constants were calculated using DIT data. The complexation of FPI with FPP and HT significantly increased the diffusion rate constant at low FPI concentration (0.1 mg mL−1). At higher FPI concentration (from 1 to 10 mg mL−1), the DIT and penetration rate constant of FPI, FPI-FPP and FPI-HT were not different. The E’ of interfacial layer of FPI was lower than that of FPI-HT interfacial layer but was higher than that of FPI-FPP interfacial layer. The E” of FPI-FPP and FPI-HT complexes was lower than that of FPI. FPI-stabilised emulsion with higher charge density had higher physical stability compared to the emulsions stabilised by FPI-phenolic complexes which had lower charge density. The emulsions stabilised by FPI-FPP and FPI-HT complexes had higher antioxidative stability compared to that of FPI stabilised emulsion. The emulsions stabilised by FPI-FPP and FPI-HT complexes had higher red and yellow hue than the emulsion stabilised by FPI.

Influence of interfacial viscosity on the dielectrophoresis of drops

The dielectrophoresis of a Newtonian uncharged drop in the presence of an axisymmetric nonuniform DC electric field is studied analytically. The present study is focused on the effects of interfacial viscosities on the dielectrophoretic motion and shape deformation of an isolated suspended drop. The interfacial viscosities generate surface-excess viscous stress which is modeled as a two-dimensional Newtonian fluid which obeys the Boussinesq-Scriven constitutive law with constant values of interfacial tension, interfacial shear, and dilatational viscosities. In the regime of small drop deformation, we have obtained analytical solution for the drop velocity and deformed shape by neglecting surface charge convection and fluid inertia. Our study demonstrates that the drop velocity is independent of the interfacial shear viscosity, while the interfacial dilatational viscosity strongly affects the drop velocity. The interfacial viscous effects always retard the dielectrophoretic motion of a perfectly conducting/dielectric drop. Notably, the interfacial viscous effects can retard or augment the dielectrophoretic motion of a leaky dielectric drop depending on the electrohydrodynamic properties. The shape deformation of a leaky dielectric drop is found to decrease (or increase) due to interfacial shear (or dilatational) viscosity.

Viscosity of particle laden films

We perform retraction experiments on soap films where large particles bridge the two interfaces. Local velocities are measured by PIV during the unstationnary regime. The velocity variation in time and space can be described by a continuous fluid model from which effective viscosity (shear and dilatational) of particulate films is measured. The 2D effective viscosity of particulate films η 2D increases with particle surface fraction ϕ : at low ϕ , it tends to the interfacial dilatational viscosity of the liquid/air interfaces and it diverges at the critical particle surface fraction ϕc ≃ 0.84. Experimental data agree with classical viscosity laws of hard spheres suspensions adapted to the 2D geometry, assuming viscous dissipation resulting from the squeeze of the liquid/air interfaces between the particles. Finally, we show that the observed viscous dissipation in particulate films has to be considered to describe the edge velocity during a retraction experiment at large particle coverage.

Effects of ambient hydrostatic pressure on the material properties of the encapsulation of an ultrasound contrast microbubble.

Ultrasound contrast microbubbles experience widely varying ambient blood pressure in different organs, which can also change due to diseases. Pressure change can alter the material properties of the encapsulation of these microbubbles. Here the characteristic rheological parameters of contrast agent Definity are determined by varying the ambient pressure (in a physiologically relevant range 0-200 mm Hg). Four different interfacial rheological models are used to characterize the microbubbles. Effects of gas diffusion under excess ambient pressure are investigated in detail accounting for size decrease of contrast microbubbles. Definity contrast agent show a change in their interfacial dilatational viscosity (3.6 × 10(-8) Ns/m at 0 mm Hg to 4.45 × 10(-8) Ns/m at 200 mm Hg) and interfacial dilatational elasticity (0.86 N/m at 0 mm Hg to 1.06 N/m at 200 mm Hg) with ambient pressure increase. The increase results from material consolidation, similar to such enhancement in bulk properties under pressure. The model that accounts for enhancement in material properties with increasing ambient pressure matches with experimentally measured subharmonic response as a function of ambient pressure, while assuming constant material parameters does not.

Investigation on the inertial cavitation threshold and shell properties of commercialized ultrasound contrast agent microbubbles

The inertial cavitation (IC) activity of ultrasound contrast agents (UCAs) plays an important role in the development and improvement of ultrasound diagnostic and therapeutic applications. However, various diagnostic and therapeutic applications have different requirements for IC characteristics. Here through IC dose quantifications based on passive cavitation detection, IC thresholds were measured for two commercialized UCAs, albumin-shelled KangRun(®) and lipid-shelled SonoVue(®) microbubbles, at varied UCA volume concentrations (viz., 0.125 and 0.25 vol. %) and acoustic pulse lengths (viz., 5, 10, 20, 50, and 100 cycles). Shell elastic and viscous coefficients of UCAs were estimated by fitting measured acoustic attenuation spectra with Sarkar's model. The influences of sonication condition (viz., acoustic pulse length) and UCA shell properties on IC threshold were discussed based on numerical simulations. Both experimental measurements and numerical simulations indicate that IC thresholds of UCAs decrease with increasing UCA volume concentration and acoustic pulse length. The shell interfacial tension and dilatational viscosity estimated for SonoVue (0.7 ± 0.11 N/m, 6.5 ± 1.01 × 10(-8) kg/s) are smaller than those of KangRun (1.05 ± 0.18 N/m, 1.66 ± 0.38 × 10(-7) kg/s); this might result in lower IC threshold for SonoVue. The current results will be helpful for selecting and utilizing commercialized UCAs for specific clinical applications, while minimizing undesired IC-induced bioeffects.

A multi-probe non-intrusive electrical technique for monitoring emulsification of hexane-in-water with the emulsifier C 10E 5 soluble in both phases

This work measures the variation of local water fraction during emulsification of hexane-in-water by an electrical conductance technique employing multiple non-intrusive ring electrodes. The emulsifier, C 10E 5, was initially dissolved only in water although it is highly soluble in both phases. This gives birth to interesting emulsification kinetics. Experiments were conducted at four emulsifier concentrations (from 1 × 10 −4 to 1 × 10 −3 M) and three hexane-to-water ratios (from 40/60 to 5/95, v/v). Emulsification was conducted by intense mixing with a Rushton turbine. CFD calculations were employed to estimate the distribution of turbulent dissipation rate in the mixing vessel. Furthermore, measurements of interfacial tension (static and dynamic) and interfacial dilatational elasticity and viscosity were conducted. An effort was made to explain the observed phase's distribution during emulsification by invoking arguments with respect to interfacial properties and turbulent dispersivity.

Interfacial Dilatational Elasticity and Viscosity of β-Lactoglobulin at Air−Water Interface Using Pulsating Bubble Tensiometry

The ability of proteins to provide stability in foams is greatly influenced by their interfacial dilatational rheological properties. Surface tension response of a pulsatingbubble with an adsorbed layer of beta-lactoglobulin was measured for different frequencies and protein concentrations using a pulsating bubble tensiometer. A methodology, accounting for adsorption/desorption as well as variation of surface concentration due to expansion/contraction, was developed for the evaluation of surface dilatational elasticity and viscosity at different frequencies from these measurements. The adsorption rate constants were inferred from the surface pressure dynamics of protein adsorption using a Langmuir minitrough. The desorption rates were shown to be negligible for beta-lactoglobulin from the surface pressure response of a spread monolayer when subjected to compression in a Langmuir minitrough. The proposed model was employed to infer the interfacial dilatational viscosity and elasticity of an adsorbed beta-lactoglobulin layer at the air-water interface from experimental pulsating bubble data for protein concentrations in the range of 0.01-0.5 wt % at pH 7. As expected, the interfacial dilatational rheological properties were found to be higher at higher protein concentrations, this effect being less pronounced for dilatational elasticity. Heating at 80 degrees C for 30 min was found to result in higher interfacial dilatational viscosity and lower interfacial dilatational elasticity though this difference was within experimental error. The traditional approach for the inference of interfacial dilatational rheological properties is found to overpredict the interfacial dilatational elasticity whereas the viscosity values do not differ significantly from those obtained using the current analysis.

Migration of a droplet in aliquid: effect of insoluble surfactants and thermal gradient

The steady-state migration velocity of a spherical dropletplaced in a second liquid is calculated taking into accountgravity and several interfacial effects. The effect ofinsoluble surfactants at the bubble surface, surface tensiongradients, surface elasticity, interfacial dilatationalviscosity, interfacial diffusivity and convective and diffusive surface excess heat fluxes on the terminal velocity of thedroplet are included under the assumption of small Marangoni,Reynolds and capillary numbers. This work extends earlierresults, which do not take into account interfacialdiffusivity.

Measurement of the dynamic interfacial tension and interfacial dilatational viscosity at high rates of interfacial expansion using the maximum bubble pressure method. II. Liquid—liquid interface

A technique proposed previously for determining the surface dilatational viscosity at a gas-liquid surface is modified to examine liquid—liquid interfaces. Interfacial dilatational viscosities for aqueous solutions of sodium dodecyl sulfate in contact with several oil phases are reported.

Measurement of interfacial dilatational viscosity at high rates of interface expansion using the maximum bubble pressure method. I. Gas—liquid surface

A novel technique for measuring the surface dilatational viscosity using the maximum bubble pressure method (MBPM) is proposed. A significant advantage of this technique in comparison to surface wave methods is its capability of measuring the surface dilatational viscosity at surfactant concentrations well above the critical micelle concentration. In the method, bubbles are created at the tip of a capillary with a sufficiently large rate of surface expansion (kHz range) such that bubble surfaces expand essentially as insoluble monolayers. The dynamic surface tension, as determined using the MBPM, is shown to combine both thermodynamic (compositionally dependent) surface tension and surface dilatational viscosity effects; the surface dilatational viscosity is measurable at high rates of surface expansion through the gradient of dynamic surface tension with rate of surface expansion. Surface dilatational viscosity values are reported for sodium dodecyl sulfate and octanoic acid solutions; these are compared with results obtained using the longitudinal wave method. The comparison indicates that, although the qualitative behavior of surface dilatational viscosity versus surfactant concentration is similar in both sets of experimental data, the magnitudes of surface viscosity as determined by the new technique are much smaller. Part of this effect might be attributable to a dependence of surface viscosity on the rate of surface expansion (since the rate of surface expansion encountered in the new technique is as much as 10 6 times larger than the expansion rate encountered using the longitudinal wave). The discrepancy may also owe to an overprediction of the rate of surface expansion at the moment of the maximum bubble pressure (if, as suggested by the study of Garrett and Ward (37), large fluctuations in gas flow rate occur during the evolution of bubbles in the MBPM). This latter possibility is discussed at length. In addition to the above advantages, the technique is simple to construct and straightforward to operate.

Measurement of the surface dilatational viscosity of aqueous gelatin-surfactant solutions

A new method for the measurement of the interfacial dilatational viscosity has been developed and was used to determine the surface dilatational viscosity of gelatin-surfactant solutions. The mathematical analysis and correlations of the results are included. Experimental results on interfacial dilatational viscosity are very limited, and there are no systematic studies available on the surfactant concentration and temperature dependence of interfacial dilatational viscosity. The surface dilatational viscosity of the phase interface formed between gelatin-surfactant solutions and air was determined as function of surfactant concentration at various temperatures by means of a bubble growth device, designed and constructed in our laboratory. The surface dilatational viscosity of the gelatin-surfactant solutions was found to increase with temperature and exhibited a minimum at the critical micelle concentration for all temperatures investigated. Although we assume negligible surface tension variation effects which may be utilized only in certain systems, the method does provide a direct measurement of the dilatational viscosity, not coupled with the surface shear viscosity.

Effect of interfacial viscosities upon displacement in capillaries with special application to tertiary oil recovery

AbstractWashburn's equation has been generalized to explain the relative effects of the interfacial viscosities, interfacial tension, and wetting during displacement in a single, cylindrical capillary. The effect of the interfacial viscosities is to increase the resistance to displacement regardless of the wetting condition. The predictions of a prior qualitative theory for the relative effects of the interfacial viscosities and of interfacial tension during tertiary oil recovery are fully supported by this analysis. This discussion further indicates that the interfacial dilatational viscosity may be relatively more important than the interfacial shear viscosity.