Dynamic Surface Pressure Research Articles

Reconsidering the importance of interfacial properties in foam stability

In food industry, protein isolates are often used to help in the formation and stabilisation of food foams. Subsequently there is great interest in (1) understanding the effect of processing parameters on the functional properties of the isolate, and (2) methods and techniques that can help to predict the foam properties. This article describes the foaming properties of proteins that were modified in the Maillard reaction. From these relatively simple experiments results were obtained that indicate that for certain protein solutions the foam properties can vary significantly, while the interfacial properties are constant. Commercial protein isolates originate from only a few sources, mainly egg white and whey, and sometimes plant proteins (e.g. soy). Despite these limited sources a large variety of isolates with a wide range of properties is produced. One source of variation is the isolation procedure, but at least equally important are the conditions used before, during and after drying the protein solution to form the dry powder. From the literature it was found that one of the major changes to the protein during processing of the isolates is the covalent coupling of sugars via the Maillard reaction. To study the effects of these reactions, a model system was produced that consists of proteins that were glycated to different degrees using Maillard reaction. For each sample, interfacial properties (e.g. surface pressure, dilatational modulus) were determined, and foam experiments were performed. The results show that at constant concentration of both the protein (0.5 g/L) and sugar (0.7 g/L), the foam-ability and stability could be significantly improved (e.g. non-modified lysozyme does not foam, the highest modification is easily foamed and the foam has a half-life time of 200 s). The results confirm other observations that Maillard reactions improve foaming properties. Moreover, strong indications were found that to predict foam stability we need more than the traditional parameters (i.e. (dynamic) surface pressure, interfacial reology, and disjoining pressure).

Interactions in the aqueous phase and adsorption at the air–water interface of caseinoglycomacropeptide (GMP) and β-lactoglobulin mixed systems

The aim of this work was to study the interactions and adsorption of caseinoglycomacropeptide (GMP) and GMP:β-lactoglobulin (β-lg) mixed system in the aqueous phase and at the air–water interface. The existence of associative interactions between GMP and β-lg in the aqueous phase was investigated by dynamic light scattering, differential scanning calorimetry (DSC), fluorometry and native PAGE-electrophoresis. The surface pressure isotherm and the static and dynamic surface pressure were determined by tensiometry and surface dilatational properties. The results showed that GMP presented higher surface activity than β-lg at a concentration of 4% wt but β-lg showed higher film forming ability. In the mixed systems β-lg dominated the static and dynamic surface pressure and the rheological properties of interfacial films suggesting that β-lg hinders GMP adsorption because, in simple competition, GMP should dominate because of its higher surface activity. The surface predominance of β-lg can be attributed to binding of GMP to β-lg in the aqueous phase that prevents GMP adsorption on its own.

Interfacial dynamic properties of whey protein concentrate/polysaccharide mixtures at neutral pH

The effect of two non-surface active polysaccharides (sodium alginate, SA, and λ-carrageenan, λ-C) in the aqueous phase on the surface dynamic properties (dynamic surface pressure and surface dilatational properties) of a commercial milk whey protein concentrate (WPC) adsorbed film at the air–water interface has been studied. A whey protein isolate (WPI) was used as reference. The WPC and WPI concentration (at 1.0% wt), temperature (at 20 °C), pH (7), and ionic strength (at 0.05 M) were maintained constant, while the effect of polysaccharide (PS) was evaluated within the concentration range 0.0–1.0% wt. The surface dynamic properties of the adsorbed films were measured in an automatic pendant drop tensiometer. At short adsorption time and in the presence of PS, the rate of diffusion of WPC to the interface was affected by the interactions with PS in the aqueous phase, which could limit protein availability for the adsorption. On the other hand, at long-term adsorption, the magnitudes of the molecular penetration and configurational rearrangement rates of WPC in mixed systems (WPC/PS) reflected the viscoelastic characteristics of the adsorbed films. The attractive interactions between WPC and PS and/or the WPC aggregation in the presence of PS, which depend on the proper polysaccharide and its concentration in the aqueous phase, have an effect on the adsorption kinetic parameters, the amount of WPC adsorbed at the air–water interface, and the dilatational viscoelastic characteristics of WPC/PS mixed systems.

Effect of Sucrose on Functional Properties of Soy Globulins: Adsorption and Foam Characteristics

In this contribution, we have analyzed the effect of sucrose on dynamic interfacial (dynamic surface pressure and surface dilatational properties) and foaming (foam capacity and foam stability) characteristics of soy globulins (7S and 11S). The protein (at 1 x 10(-3), 1 x 10(-2), 0.1, and 1 wt %) and sucrose (at 0, 0.25, 0.5, and 1.0 M) concentrations in aqueous solution and the pH (at 5 and 7), and ionic strength (at 0.05 and 0.5 M) were analyzed as variables. The temperature was maintained constant at 20 degrees C. We have observed the following. (i) The dynamics of adsorption (presence of a lag period, diffusion, and penetration at the air-water interface) of soy globulins depend on the peculiar molecular features of proteins (7S or 11S soy globulin) and the level of association/dissociation of these proteins by varying the pH and ionic strength, as well as the effect of sucrose in the aqueous phase on the unfolding of the protein. The rate of adsorption increases with the protein concentration in solution, at pH 7 compared to pH 5, at high ionic strength, and in the absence of sucrose. (ii) The surface dilatational properties reflect the fact that soy globulin adsorbed films exhibit viscoelastic behavior. The surface dilatational modulus increases at pH 7 compared to pH 5, but decreases with the addition of sucrose into the aqueous phase. (iii) The rate of adsorption and surface dilatational properties (surface dilatational modulus and phase angle) during adsorption at the air-water interface play an important role in the formation of foams generated from aqueous solutions of soy globulins. (iv) The increased interfacial adsorption (at high surface pressures) and the combined effects of interfacial adsorption and interfacial interactions between adsorbed soy globulin molecules (at high surface dilatational modulus) can explain the higher stability of the foam, with few exceptions.

Noise radiation of aircraft panels subjected to boundary layer pressure fluctuations

In this paper, a method which predicts the sound radiation of aircraft panels subjected to turbulent boundary layer excitation is described. The method is the extension of an earlier deterministic approach, where the modal expansion and modal receptance methods were used to predict random noise transmission through curved aircraft panels with stringer and ring frame attachments. Here, with implementation of the Corcos and Efimtsov models to characterize the dynamic surface pressure cross-spectra, closed-form solutions for the panel displacements, radiation and transmission pressures are derived. Numerical examples are presented to illustrate the effects of the stringers, ring frames, hydrodynamic coincidence, curvature, in-plane tension, structural dissipation and composite material on the structural and acoustic response of the panel.

Ground-Based Simulation of Complex Maneuvers of a Delta-Wing Aircraft

Ground-Based Simulation of Complex Maneuvers of a Delta-Wing Aircraft

Formulation Engineering Can Improve the Interfacial and Foaming Properties of Soy Globulins

In this contribution, we have analyzed the effect of different strategies, such as change of pH (5 or 7) or ionic strength (at 0.05 and 0.5 M), and addition of sucrose (at 1 M) and Tween 20 (at 1 x 10(-4) M) on interfacial characteristics (adsorption, structure, dynamics of adsorption, and surface dilatational properties) and foam properties (foam capacity and stability) of soy globulins (7S and 11S at 0.1 wt %). We have observed that (1) the adsorption (presence of a lag period, diffusion, and penetration at the air-water interface) of soy globulins depends on the modification in the 11S/7S ratio and on the level of association/dissociation of these proteins by varying the pH and ionic strength (I), the effect of sucrose on the unfolding of the protein, and the competitive adsorption between protein and Tween 20 in the aqueous phase. The rate of adsorption increases at pH 7, at high ionic strength, and in the presence of sucrose. (2) The surface dilatational properties reflect the fact that soy globulin adsorbed films exhibit viscoelastic behavior but do not have the capacity to form a gel-like elastic film. The surface dilatational modulus increases at pH 7 and at high ionic strength but decreases with the addition of sucrose or Tween 20 into the aqueous phase. (3) The rate of adsorption and surface dilatational properties (surface dilatational modulus and phase angle) during adsorption at the air-water interface plays an important role in the formation of foams generated from aqueous solutions of soy globulins. However, the dynamic surface pressure and dilatational modulus are not enough to explain the stability of the foam.

The role of static and dynamic characteristics of diglycerol esters and β-lactoglobulin mixed films foaming. 1. Dynamic phenomena at the air–water interface

In this work, we have studied dynamic surface pressure and surface dilatational characteristics of diglycerol esters (diglycerol monolaurate and diglycerol monostearate) and β-lactoglobulin mixed films adsorbed at the air–water interface. The concentrations of protein and lipid and the protein/lipid ratio in the aqueous bulk phase were studied as variables, while pH (7), ionic strength (0.05 M) and temperature (20 °C) were maintained constant. We have observed competitive adsorption from aqueous solutions of mixed emulsifiers ( β-lactoglobulin and diglycerol esters) and/or the displacement between emulsifiers at the air–water interface depending on the chemistry of the lipid and on the protein/lipid ratio. For the mixed systems a lag period was not observed, even for low concentrations of emulsifiers in solution. During the initial period the kinetics of adsorption of mixed emulsifiers at the air–water interface are controlled by a diffusion mechanism. At long-term adsorption the rate of penetration controls the adsorption of mixed emulsifiers at the interface, which increases with the lipid concentration in solution and is faster for diglycerol monostearate than for diglycerol monolaurate. From a rheological point of view, mixed films behave as viscoelastic (practically elastic) with a loss angle tangent higher than zero. The evolution of surface dilatational modulus ( E) with surface pressure ( π) for β-lactoglobulin–diglycerol ester mixed films reflects the amount of emulsifier adsorbed at the interface (giving high values of E at the higher π) and confirms the idea that the protein–lipid interactions at the air–water interface are somewhat weak, there even being the possibility of phase separation.

Feedback control of subsonic cavity flows using reduced-order models

Development, experimental implementation, and the results of reduced-order model based feedback control of subsonic shallow cavity flows are presented and discussed. Particle image velocimetry (PIV) data and the proper orthogonal decomposition (POD) technique are used to extract the most energetic flow features or POD eigenmodes. The Galerkin projection of the Navier–Stokes equations onto these modes is used to derive a set of nonlinear ordinary differential equations, which govern the time evolution of the eigenmodes, for the controller design. Stochastic estimation is used to correlate surface pressure data with flow-field data and dynamic surface pressure measurements are used to estimate the state of the flow. Five sets of PIV snapshots of a Mach 0.3 cavity flow with a Reynolds number of 105 based on the cavity depth are used to derive five different reduced-order models for the controller design. One model uses only the snapshots from the baseline (unforced) flow while the other four models each use snapshots from the baseline flow combined with snapshots from an open-loop sinusoidal forcing case. Linear-quadratic optimal controllers based on these models are designed to reduce cavity flow resonance and are evaluated experimentally. The results obtained with feedback control show a significant attenuation of the resonant tone and a redistribution of the energy into other modes with smaller energy levels in both the flow and surface pressure spectra. This constitutes a significant improvement in comparison with the results obtained using open-loop forcing. These results affirm that reduced-order model based feedback control represents a formidable alternative to open-loop strategies in cavity flow control problems even in its current state of infancy.

Viscoelastic Properties of Diglycerol Ester and Protein Adsorbed Films at the Air−Water Interface

The dynamic surface pressure (π) and surface viscoelastic properties (surface dilatational modulus, E, its elastic, Ed, and viscous, Ev, components, and loss angle tangent, tan φ) of lipids (diglycerol-monostearate and diglycerol-monolaurate) and proteins (sodium caseinate and β-lactoglobulin) at different concentrations in the aqueous phase were measured using a dynamic drop tensiometer. Temperature, pH, and, ionic strength were maintained constant at 20 °C, 7, and 0.05 M, respectively. The surface dilatational properties depend on the chemistry of the molecule (length of the hydrocarbon chain in lipids and disordered or globular characteristics of the protein). Lipid films are essentially elastic, but protein films are viscoelastic. The values of Ed increase with the emulsifier (lipid or protein) concentration in solution and were higher at the critical micellar concentration (CMC) for lipids or at the adsorption efficiency (AE) for proteins. The values of E reflect not only the amount of emulsifier adsorbed at the interface but also the degree of interaction between adsorbed emulsifier molecules. E increases with the van der Waals interactions between lipid hydrocarbon chains, which are higher for diglycerol-monostearate than for diglycerol-monolaurate. The values of E for caseinate adsorbed films were lower than those for β-lactoglobulin adsorbed films. These differences are due mainly to differences in the looping of amino acid residues for adsorbed films of random coil (caseinate) and globular (β-lactoglobulin) proteins at the air−water interface. The values of E are lower for protein compare to lipid adsorbed films. The role of surface dynamic properties of lipid and protein adsorbed films on foam formation and stabilization is discussed.

Interfacial Properties of Diglycerol Esters and Caseinate Mixed Films at the Air−Water Interface

The surface pressure (π) at equilibrium (surface pressure isotherm) and surface dynamic properties (dynamic surface pressure and surface dilatational characteristics) of diglycerol ester (diglycerol-monocaprinate, diglycerol-monolaurate, diglycerol-monostearate, and diglycerol-monooleate) and protein (sodium caseinate) as emulsifiers, at different concentrations in the aqueous phase, were measured using tensiometry and a dynamic drop tensiometer, respectively. We have observed that (1) at equilibrium the value of critical micelle concentration (CMC) decreases and the maximum surface excess (Γmax) increases as the hydrocarbon chain increases because the hydrophobic character of the lipid also increases. The presence of a double bond in the hydrocarbon chain also increases the value of CMC and decreases those of Γmax. Caseinate presents higher adsorption efficiency but the surface activity is between those for lipids. The surface pressure isotherm of mixed systems is dependent on the emulsifier concentration and the protein/lipid ratio in the mixture. (2) The adsorption of pure emulsifiers at the air−water interface increases with emulsifier concentration in the aqueous phase via diffusion and penetration of the emulsifier at the interface. For mixed films, the rate of adsorption depends on the concentration and composition of the mixture. Competitive or cooperative phenomena were observed during the adsorption of both emulsifiers at the interface. (3) The surface dilatational characteristics of mixed films are viscoelastic. The surface dilatational modulus reflects the amount of emulsifier adsorbed at the interface and confirms the idea that the protein−lipid interactions at the air−water interface are somewhat weak, there even being the possibility of phase separation.

Soy protein–polysaccharides interactions at the air–water interface

This work studies the interfacial behavior of mixed soy protein (SP) + polysaccharide (PS) systems to gain knowledge on the interactions between these biopolymers at the air–water interface under dynamic conditions at neutral pH where a limited incompatibility between macromolecules can occur. The PSs used were: hydroxypropylmethylcellulose (HPMC) as surface-active PS; lambda carrageenan ( λ C ) and locust bean (LB) gum as non-surface-active PSs. Protein and PS concentration in the mixed systems were 2% and 0.25%, respectively. The dynamic surface pressure and rheological properties of films were evaluated with a drop tensiometer at 20 ∘ C , pH 7 and ionic strength 0.05 M. The presence of HPMC and λ C greatly increased the surface pressure, surface dilatational elasticity and relative viscoelasticity on the basis of different mechanisms. HPMC competed for the interface with SP, but due to its unusual strong surface activity it could dominate the surface pressure and improve film viscoelasticity. The modification of surface pressure and rheological properties of adsorbed SP films in the presence of λ C necessarily suggests the participation of λ C + contaminants at the interface. Pure λ C could influence the interface by a complexation mechanism, or indirectly by a depletion mechanism in the vicinity of the interface. In addition surface-active contaminant of λ C if strongly bound to the PS could bring some PS molecules at the interface. LB little affected the surface pressure and rheological properties of SP films even if surface-active contaminants were present in the commercial preparation. Differences in the interaction of λ C and LB gum with the protein should be mainly ascribed to different degrees of incompatibility and to the fact that LB is not charged.

Adsorption of water-soluble polymers with surfactant character.: Adsorption kinetics and equilibrium properties

A comparative study between Langmuir and Gibbs monolayers of a hyperbranched polyol, poly(propylene glycol) homopolymers, and poly(propylene glycol)–poly(ethylene glycol) copolymers with different structure and molecular weight, is reported. Dynamic surface tension (DST) and surface pressure measurements have been carried out to characterize these amphiphilic water-soluble polymers. The adsorption kinetics results are consistent with a rapid diffusion stage followed by a slow reorganization at the air–water interface. The characteristic times of these steps, calculated by the Joos model, point out differences among the polymers in the diffusion rate and rearrangement mechanisms for diluted solutions. Short time analysis of DST data leads to diffusion coefficients in qualitative agreement with the diffusion times calculated with Joos' model. Spread monolayers remain stable for long periods of time. The desorption process seems quite inoperative. As a consequence, the surface pressure of the spread monolayers can be studied over a broad surface concentration range. 2D first-order phase transitions have been evidenced from plateaux observed in Langmuir and Gibbs isotherms. It has been found that Gibbs monolayers lead to lower surface tension states than the Langmuir ones.

Surface Rheology Parameters of Source-Specific Surfactant Films as Indicators of Organic Matter Dynamics

The paper contains the results of natural film experiments carried out on inland and coastal waters in the Dead Vistula catchment area and mouth during 2000–2002, using the integrated Langmuir trough–Wilhelmy plate system. The static film parameters result from the generalized scaling procedures applied to the surface pressure–area isotherms. They appear to reflect in a quantitative and sensitive way the film composition (A lim, M w, E isoth), film solubility and the miscibility of its components (via R, ΔS c and y factors), and surface concentration (πeq, Γeq). The adsorption kinetics parameters: effective diffusion coefficient D eff/D and activation energy barrier E a/RT are derived from dynamic surface pressure. There is a reason to suggest that certain classes of film-forming components or ‘end-members’ may dominate the static and dynamic surface properties. Variation in the surface rheological parameters of source-specific biosurfactants is postulated to reflect organic matter dynamics in natural waters and were measured for the Dead Vistula river, its tributaries and the adjacent coastal area.

Simple Sequential Injection Analysis Systems with a Dynamic Surface Tension Detector

Simple sequential injection analysis systems with DSTD (SIA/DSTD) have been developed. One was employed for the study of the effects of the ion contents in solutions to the dynamic surface pressure of ionic surfactants. The results from the studies show the possibility for an alternative simple fast screening, but also a sensitive procedure for water quality determination. Another simple SIA/DSTD system has been demonstrated for the quantification of an anionic surfactant using a single standard calibration.

Dynamic surface pressure measurements on a square cylinder with pressure sensitive paint

The dynamic and static surface pressure on a square cylinder during vortex shedding was measured with pressure sensitive paints (PSPs) at three angles of incidence and a Reynolds number of 8.9×104. Oscillations in the phosphorescence intensity of the PSP that occurred at the vortex shedding frequency were observed. From these phosphorescent oscillations, the time-dependent changes in pressure distribution were calculated. This work extends PSP’s useful range to dynamic systems where oscillating pressure changes are on the order of 230 Pa and occur at frequencies in the range of 95–125 Hz.

Viscoelastic properties of insoluble amphiphiles at the air/water interface

The effects of the presence of a molecular monolayer on the dilatational properties of the air/water interface have been investigated. Two water insoluble amphiphiles, dipalmitoyl phosphatidyl choline and quercetin 3- O-palmitate, were spread onto a pendant drop and the dynamic surface pressure was measured by means of drop shape analysis. The surface dilatational elasticity and viscosity of the spread monolayers were also determined by the oscillating drop technique. Constraints on the range of measuring conditions were investigated and we demonstrated that the pressure–area isotherms derived from oscillatory dynamic measurements display phase behaviour similar to that found in equilibrium measurements, albeit at reduced resolution. Both the amphiphiles formed purely elastic films that were characterised by a dilatational modulus that depended on the surface concentration and obeyed a power scaling law. The exponent of the relationship could be related to the thermodynamic conditions prevailing at the interface. The phospholipid monolayer scaling exponent was 2.8 in a temperature range of 20–26 °C indicates a favourable solvency of molecules in the bidimensional matrix. A very high scaling exponent (11.8 at 7 °C) for quercetin palmitate was interpreted assuming that molecules self-organise in fibre-like structures. This interface structure and the phase behaviour was found consistent with observations of the surface film obtained by Brewster angle microscopy. The structured quercetin 3- O-palmitate monolayers are disrupted by temperature increase or by adding a 0.2 molar fraction of the immiscible dipalmitoyl phosphatidyl choline.

Ultra-small microphone scales new heights of surface dynamic pressure measurement performance

Ultra-small microphone scales new heights of surface dynamic pressure measurement performance

Effect of solution viscosity on dynamic surface tension detection

A dynamic surface tension detector (DSTD) was used to examine the molecular diffusion and surface adsorption characteristics of surface-active analytes as a function of solution viscosity. Dynamic surface tension is determined by measuring the differential pressure across the air/liquid interface of repeatedly growing and detaching drops. Continuous surface tension measurement throughout the entire drop growth is achieved for each eluting drop (at a rate of 30 drops/min for 2 μl drops), providing insight into the kinetic behavior of molecular diffusion and orientation processes at the air/liquid interface. Three-dimensional data are obtained through a calibration procedure previously developed, but extended herein for viscous solutions, with surface tension first converted to surface pressure, which is plotted as a function of elution time axis versus drop time axis. Thus, an analyte that lowers the surface tension results in an increase in surface pressure. The calibration procedure derived for the pressure-based DSTD was successfully extended and implemented in this report to experimentally determine standard surface pressures in solutions of varied viscosity. Analysis of analytes in viscous solution was performed at low analyte concentration, where the observed analyte surface activity indicates that the surface concentration is at or near equilibrium when in a water mobile phase (viscosity of 1.0 Cp). Two surface-active analytes, sodium dodecyl sulfate (SDS) and polyethylene glycol (MW 1470 g/mol, PEG 1470), were analyzed in solutions ranging from 0 to 60% (v/v) glycerol in water, corresponding to a viscosity range of 1.0–15.0 Cp. Finally, the diffusion-limited surface activity of SDS and PEG 1470 were observed in viscous solution, whereby an increase in viscosity resulted in a decreased surface pressure early in drop growth. The dynamic surface pressure results reported for SDS and PEG 1470 are found to correlate with solution viscosity and analyte diffusion coefficient via the Stokes–Einstein equation.

Active Flow Control on the Stingray Uninhabited Air Vehicle: Transient Behaviour

The application of leading-edge separation control on an uninhabited air vehicle (UAV) with 50-deg leading-edge sweep is investigated experimentally in a full-scale close-return wind tunnel using arrays of synthetic jet actuators. Active flow control is used to enhance vehicle control at moderate and high angles of attack for takeoff and landing activities or gust load alleviation. The surface-mounted synthetic jet actuators are operated in various waveforms where the carrier frequency is at least an order of magnitude higher than the characteristic shedding frequency of the UAV. Actuation yields a suction peak near the leading edge, however, while excitation with a sinusoidal waveform results in a sharp suction peak near the leading edge; pulse modulation yields a larger and wider suction peak. The flow transients associated with controlled reattachment and separation of the flow over the UAV are investigated using amplitude modulation of the actuation waveform by measuring the dynamic surface pressure at different locations on the upper surface of the UAV’s wing. Phase-locked measurements show that the transients, associated with the onset of reattachment and separation, at high angles of attack are accompanied by the shedding of large-scale vortical structures and oscillations of the surface pressure. However, no oscillations are observed when the baseline flow is attached.