Dilatational Viscosity Research Articles

Domain-Growth Kinetic Origin of Nonhorizontal Phase Coexistence Plateaux in Langmuir Monolayers: Compression Rigidity of a Raft-Like Lipid Distribution

The present work addresses the question of a nonhorizontal coexistence plateau found in the liquid-expanded (LE) to liquid-condensed (LC) transition of Langmuir monolayers of lipid amphiphiles, which is apparently incongruent with the first-order character of this main LE/LC phase transition. This pathology is understood in a mechanical context as a resistance of the monolayer against compression giving rise to a nonzero rigidity in the coexistence region. Surface rheology has allowed for a quantitative determination of the compression parameters, namely, dilational elasticity epsilon and viscosity eta. Data for the phase coexistence region reveal dynamical stiffening at faster deformation, which points out a chief control of lipid diffusion on monolayer rigidity. Monolayer viscosity remains however low at the value corresponding to the continuous fluid phase. The presence of coexistence domains is then invoked as the structural element responsible for such a nontrivial rheology, the finite domain growth rate imposing a kinetic limit for equilibrium compression along a quasi-static path. Brewster angle microscopy has allowed for studying the kinetic mechanism for domain growth. The finite rigidity observed at the coexistence region is related to the resistance of LC domains to grow at the expense of the LE phase. A reconciliation of the nonhorizontal plateau observed at finite compression rates with the first-order character of the LE/LC transition emerges then naturally from this kinetic scenario. New mechanical features are consequently assigned to the phase-separated monolayers made of stiff grains rafting in a fluid matrix. Particularly, for a raft-like lipid distribution, we hypothesize a finite rigidity kinetically controlled by the rate of domain growth and a high fluidity controlled by the continuous phase. We have depicted a minimal model of membrane mechanics that accounts for the elasticity of such a heterogeneous composite medium. This "Plum-cake" model is able to qualitatively predict the observed mechanical features and is suggested to describe raft-like membranes as compliant elastic media where lipid domains work as reservoirs able to exchange material with the surrounding fluid phase.

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.

The surface dilational viscosity of polypropylene glycol solutions and its influence on water flow and foam behavior

The non-equilibrium interfacial properties of a homologous series of aqueous polypropylene glycol solutions (400–2000 g/mol) were investigated as a function of concentration to understand their role in determining foam behavior. The overflowing cylinder technique was used to generate a continually expanding surface. Static and dynamic surface tensions, as well as the surface velocity of the expanding surface, were characterized. The surface dilational viscosity was compared to water flow rate and foam retention time of the polypropylene glycol solutions reported in the literature. This study suggests that the surface dilational viscosity is a key parameter in the Marangoni effect which retards water drainage, resulting in wetter foam and increased water recovery across flowing foams.

Equilibrium and Surface Rheology of Monolayers of Insoluble Polycations with Side Chains

We have studied monolayers of poly(n-tetradecyl 4-vinylpyridinium-co-4-vinylpyridine) bromide with different degrees of quaternization at the air-water interface. The isotherms (surface pressure vs area) present several phase transitions: at low monolayer coverage, there is a phase transition over a characteristic area that increases on increasing the quaternization degree. This behavior can be rationalized in terms of a mean-field theory of 2D semiflexible polymeric chains and could be an indication of a disorder-order transition from a 2D isotropic liquid (IL) at low surface concentration to a 2D nematic phase (N) at higher concentrations. Low-frequency oscillatory strain experiments show that at low surface coverage the monolayers exhibit highly nonlinear behavior, even for low strain amplitude, whereas at higher surface coverage the response is linear for strains higher than 20%. In addition, stress relaxation experiments show a minimum in the characteristic times that coincide with the transition area. These unexpected results at low surface coverage might be characteristic of the system or related to the fact that the oscillatory experiments do not strictly correspond to constant surface-coverage conditions. However, they are in agreement with high-frequency viscoelasticity, obtained by surface quasielastic light scattering, that shows that the dilational viscosity is higher at low surface concentration than for concentrations beyond the surface phase transition. At higher coverage, there is a second phase transition, after which the isotherms present hysteresis, which is not observed below. Ellipsometry indicates that, after this transition, the monolayer thicken, which may be related to 3D growth into a multilayer.

Effective surface viscosities of a particle-laden fluid interface

The Einstein formula for the effective shear viscosity of low Reynolds number suspension flows is generalized to the case of flat, low-concentration, particle-laden interfaces separating two immiscible fluids. The effective surface shear and dilational viscosities of this system is found to be eta{s}=5/3(eta{1}+eta{2})R phi and zeta{s}=5(eta_{1}+eta_{2})R phi , correspondingly, where eta{1} and eta{2} are the shear viscosities of two bulk fluids and phi is the surface concentration of spherical particles of radius R . The formula is found to be in excellent agreement with data obtained using multicomponent lattice Boltzmann equation simulation.

Evaluation of Rayleigh–Plesset Equation Based Cavitation Models for Squeeze Film Dampers

This work is intended to evaluate a cavitation model based on the complete Rayleigh–Plesset (RP) equation for use in squeeze film damper calculations. The RP equation governs the variation in the radius of the cavitation bubbles at rest, surrounded by an infinite incompressible fluid and subjected to an external pressure. This equation is obtained from the momentum equation and it takes into account the ensemble of the phenomena related to the dynamics of the bubbles (surface tension, damping, and inertia). All the terms in the RP equation will be taken into account in the present work plus a dilatation viscosity introduced by Someya in 2003. Numerical results will be compared with experimental data obtained by Adiletta and Pietra in 2006. The results underline the influence of the effects contained in the RP equation on the pressure field.

Surface tension isotherms, adsorption dynamics and dilational visco-elasticity of sodium dodecyl sulphate solutions

The adsorption kinetics, adsorption isotherm and the dilational surface rheology at low oscillation frequencies of sodium dodecyl sulphate (SDS) were studied by drop and bubble profile, and bubble pressure tensiometry. The effect of the inevitably present dodecanol in any SDS solution is taken into account by interpreting the experimental data with theoretical models valid for mixed surfactant solutions. In particular the dilational rheology method appears to be very suitable for demonstrating the continuously on-going hydrolysis reaction in a SDS solution, as the resulting dilational elasticities and viscosities change from a frequency dependencies characteristic to SDS to those expected for dodecanol adsorption layers. For surfactants as active as dodecanol, it turns out that all adsorption and rheological quantities depend significantly on the use of drop or bubble profile tensiometry, i.e. the small reservoir of available surface active molecules in a single droplet has to be considered quantitatively to obtain reliable data.

Dilation and Shear Rheology of Mixed β-Casein/Surfactant Adsorption Layers

The present study deals with dilational and shear rheological properties of adsorption layers of the milk protein beta-casein (BCS) mixed with the nonionic dodecyl dimethyl phosphine oxide (C12DMPO) and the positively charged dodecyl trimethyl ammonium bromide (DoTAB), respectively. The drop profile analysis tensiometer PAT-1 was applied for the dilational rheological studies at low frequency harmonic relaxations. A special modification of the setup, consisting of a coaxial capillary combined with a double dosing system, provides exchange of the drop volume during experiments. This arrangement offers a unique protocol for studies of mixed surface layers, formed by sequential adsorption of the individual compounds. The dilational viscoelastic modulus and the dilational viscosity of the mixed layers, built-up in the two different ways, were investigated and compared. The features of the mixed surface layers drawn from the dilational rheology are qualitatively confirmed by the shear rheological parameters measured by torsion shear rheometry ISR-1. Recently derived theoretical models were used for a quantitative description of the equilibrium state and dilational rheology of the surface layers formed by the single components and their mixtures.

Stability of thin emulsion film between two oil phases with a viscoelastic liquid–liquid interface

The viscoelastic properties of adsorbed protein layer in food emulsions and foams are important in providing stability to such systems. Linear stability analysis for a protein stabilized aqueous film sandwiched between two semi-infinite oil phases with a viscoelastic liquid–liquid interface is presented. The interfacial dilatational and shear viscoelastic properties are described by Maxwell models. The aqueous film is found to be more stable for smaller values of dilatational (shear) relaxation times and larger values of interfacial dilatational (shear) viscosities. The asymptotic values of maximum growth coefficient for very large and very small values of interfacial dilatational (shear) viscosities were found to be independent of relaxation times and correspond to those for immobile and fully mobile liquid–liquid interfaces respectively. The aqueous film is shown to be more stable for larger viscosities of the oil phase with the maximum growth coefficient approaching zero as the ratio of viscosities of oil and aqueous phases approach very large values and an asymptotic value corresponding to that for a foam film for very small viscosity ratios.

Interfacial dilational properties of tri-substituted alkyl benzene sulfonates at air/water and decane/water interfaces

The dilational rheological properties of absorbed film of three pairs of structural isomers, tri-substituted alkyl benzene sulfonates, at the air–water and decane–water interfaces have been investigated by drop shape analysis method. The influences of bulk concentration on dilational elasticity and viscosity were expounded. Interfacial tension relaxation method was employed to obtain dilational parameters in a reasonably broad frequency range. The experimental results showed that the meta-alkyl to sulfonate group plays a crucial role in the interfacial dilational properties: the longer meta-alkyl will lead to higher dilational parameters for air–water interface and lower ones for decane–water interface when the total alkyl carbon numbers are equal. For alkyl benzene sulfonates with shorter meta-alkyl, the surface dilational properties are similar to interfacial dilational properties, whereas the surface dilational parameters are obviously higher than the interfacial dilational parameters for alkyl benzene sulfonates with longer meta-alkyl in general. The possible mechanism has been proposed and ensured by Cole–Cole plots.

Bubble stability in the presence of oil-in-water emulsion droplets: Influence of surface shear versus dilatational rheology

We discuss the stability of bubbles to coalescence when undergoing a pressure drop and their stability to disproportionation under quiescent conditions, studied using previously established ‘single bubble layer experimental’ techniques, focussing on the effects on stability of the inclusion of a low volume fraction (0.25%) of stable oil droplets. Detailed measurements of the surface dilatational elasticity ( ɛ s) and surface shear viscosity ( η s) of systems in the presence and absence of oil droplets have been performed. The surface rheology and stability have been measured as a function of adsorption time and pH, between pH 4.5 and 7, by including glucono-δ-lactone (GDL) as an acidification agent. Commercial sodium caseinate (SC) and purified β-lactoglobulin (β-L) were used at 1 wt% bulk concentration as bubble stabilizing agents. The emulsion oil droplet phase was n-tetradecane (TD) or 1-bromohexadecane (BHD), with a mean droplet size of ( d 43) = 0.59 and 0.67 μm, respectively. The emulsion droplets were completely stable to coalescence and did not enter or spread at the air–water (A–W) interface. With SC at all pH values the values of η s were markedly higher in the presence of TD droplets than in their absence, but particularly when the pH was lowered to pH ≤ 5.5 such that the SC started to aggregate. The increase in η s correlated with the increase in coalescence stability under the same conditions. With neutrally buoyant BHD droplets the increase in η s was not as great, but η s was still significantly higher than in the absence of droplets, indicating that the rise to, and packing of, TD droplets at the A–W interface due to gravity was not solely responsible for their observed effects. The values of ɛ s did not increase much at all for either β-L or SC as the pH was lowered and/or TD droplets were added, except at very low pH values, when the effects with SC were obscured by the tendency for the bulk SC to gel. In agreement with the relatively insignificant changes in ɛ s as the pH was lowered or droplets were added, the resistance to disproportionation of bubbles did not change very much either.

Interfacial dilational properties of partly hydrolyzed polyacrylamide and gemini surfactant at the decane–water interface

The dilational viscoelastic properties of partly hydrolyzed polyacrylamide (HPAM) and surfactant (C12COONa-p-C9SO3Na) in the absence or presence of electrolyte were investigated at the decane–water interface by means of longitudinal method and the interfacial tension relaxation method. The polymer plays different roles in influencing the structure of HPAM–surfactant mix-adsorbed layer at different surfactant concentration. At low surfactant concentration, the addition of polymer could sharply decrease the dilational elasticity mainly due to the weakening of the “entanglement” among long alkyl chains in surfactant molecules, while the addition of the polymer may enhance the dilational elasticity due to the slow diffusivity of the polymer chains at higher surfactant concentration. And the added electrolyte, which results in screening of electrostatic interactions between the ionized groups, generally decreases the interfacial dilational elasticity and increases the dilational viscosity. The data obtained on the relaxation processes via interfacial tension relaxation measurement can explain the results from dilational viscoelasticity measurements very well.

Stability and simulations of pulsating contrast agents

The encapsulating membrane of ultrasound contrast agents, UCA's, is treated as a viscoelastic thin shell whose deviation from linear Hookean behavior is modeled as a strain softening or strain hardening effect via a parameter measuring the degree of membrane softness. As the amplitude of sound increases it controls the shift in resonance frequency until it hits the forcing frequency in which case an abrupt increase in the microbubble response takes place. Only strain softening shells exhibit this behavior. Deviations from sphericity are modeled via an additional parameter, namely the scalar bending modulus. This parameter controls static buckling of the shell, the onset of parametric instability and dynamic buckling. In this fashion phase diagrams can be constructed for a specific UCA that map regions of subharmonic growth of shape modes. Stability analysis and numerical simulations are employed in order to capture the onset, growth and break-up or saturation of shape modes. The above two parameters are added to the area dilatation modulus and viscosity of the membrane in order to construct a model that can be used to design new agents that behave optimally in different diagnostic or therapeutic modalities.

Non-linear extensions to shell models of ultrasound contrast agents: theory and experiment

The standard UCA models, characterized by the area dilatation modulus and viscosity of the encapsulating membrane, are extended in order to account for deviations from linear material behaviour and for non-spherical pulsations. The former effect is captured by introducing a parameter that measures the degree of membrane softness, whereas the latter is modelled by introducing a bending elasticity. Lipid shells exhibit the phenomenon of 'thresholding behavior' where excursions in the relative area dilatation are measured only beyond a critical sound amplitude. Treating the lipid shell as strain softening reproduces this non-linear behaviour which can be further optimized by controlling the shell softness parameter. Such a tuning can be used to estimate the degree of softness for the contrast agent BR-14. Stability analysis allowing for shape distortions provides an estimate for the bending elasticity that reproduces the experimentally observed threshold for growth of surface modes. Phase diagrams are constructed for BR-14, based on high-speed optical observations and stability analysis. Both experiment and numerical analysis exhibit a smaller variation between the stability threshold of subsequent shape modes in the case of UCA's, as compared to free bubbles. Numerical simulations reproduce the experimentally observed saturation of growing shape modes for pre-stressed UCA's.

Surface Rheology and Phase Transitions of Monolayers of Phospholipid/Cholesterol Mixtures

The dynamic surface elasticity and the surface dilational viscosity of three binary phospholipid/cholesterol mixtures were determined with axisymmetric drop shape analysis on a harmonically oscillating pendent drop. Dipalmitoylphosphatidylcholine, dimyristoylphosphatidylcholine, and dioleoylphosphatidylcholine were used to explore the rheological properties and phase transitions of mixtures of saturated and unsaturated phospholipids with cholesterol. The growth rates for surface dilational viscosity and dynamic elasticity are parallel for all film pressures studied. Characteristic breaks and plateaus could be found for these growth rates, indicating phase transitions. For dipalmitoylphosphatidylcholine/cholesterol and dimyristoylphosphatidylcholine/cholesterol mixtures, phase diagrams with six regions separated by phase boundaries were found, which are in good agreement with phase transitions reported in the literature for static measurements of isotherms and isobars on a Langmuir film balance and from fluorescence microscopy. Some phase boundaries were only found by dynamic, but not by static, elasticity measurements. Imaging methods revealed phase separations produced by the formation of condensed stoichiometric complexes leading to micron-sized and mostly circular domains. The effects of these complexes on monolayer rheology in liquid/liquid phases is described. Furthermore, liquid/solid and solid phase transitions are discussed.

Synergy between Hofmeister effect and coupled water in proteins: Unusual dilational moduli of BSA at air/solution interface

This study relates interfacial interactions of bovine serum albumin (BSA) molecules in dilute solutions with its dilatational rheology. Dynamic surface tension and the associated dilational elastic modulus and viscosity for BSA and mixtures of BSA with Hofmeister electrolytes—NaCl, NaClO 4, Na 2SO 4, NaF and Na 2HPO 4 have been studied using a sinusoidal surface compression and expansion for frequencies ranging from 0.01 to 0.4 Hz. at solution/air interface. In all the BSA + electrolyte systems, both the elastic modulus and viscosity show unusually high values compared with pure BSA or pure electrolytes. In the presence of NaF and Na 2SO 4 the viscosity of protein increases almost by 50–80-fold and the corresponding elastic modulus also changes by 30–50-fold. Hydrated Hofmeister ions surely influence the measured rheological properties. In addition, the synergistic effect of the hydrated protein and the vicinal hydrated electrolytes possibly contribute to the high viscosity and elasticity due to change in dynamics of these assemblies. Thus the behavior of BSA is effected by salts in different ways, especially due to the dynamics and strength of the water molecules in the assembly.

Real-Time Measurements and Modelling on Dynamic Behaviour of SonoVue Bubbles Based on Light Scattering Technology

The dynamic behaviour of SonoVue microbubbles, a new generation ultrasound contrast agent, is investigated in real time with light scattering method. Highly diluted SonoVue microbubbles are injected into a diluted gel made of xanthan gum and water. The responses of individual SonoVue bubbles to driven ultrasound pulses are measured. Both linear and nonlinear bubble oscillations are observed and the results suggest that SonoVue microbubbles can generate strong nonlinear responses. By fitting the experimental data of individual bubble responses with Sarkar's model, the shell coating parameter of the bubbles and dilatational viscosity is estimated to be 7.0 nm·s·Pa.

Effect of the Addition of Polyelectrolytes on Monolayers of Carboxybetaines

We have studied the effect of the addition of sodium poly(styrene sulfonate) polymers on the properties of monolayers formed by the adsorption of two carboxybetaines with different number of separation methylenes between their charged groups. Fluorescence and surface tension measurements indicate that above the critical aggregation concentration a surfactant-polymer complex of electrostatic origin is formed in bulk. The complexes have a negative charge that is repelled by the negative charge of the surfactant adsorbed at the interface; consequently, the monolayer seems to be exclusively formed by surfactant carboxybetaines. The high-frequency surface viscoelasticity of the monolayers was studied by surface dynamic light-scattering measurements. The behavior of the dilational elasticity and viscosity is explained by relaxation involving molecular reorientation within the adsorbed layer.

Aggregation Behavior of Cationic Copolymer Methacryloxyethyl Trimethyl Ammonium Chloride‐Butyl Acrylate‐Acrylamide in Aqueous Solution

The aggregation behavior of cationic copolymer methacryloxyethyl trimethyl ammonium chloride‐butyl acrylate‐acrylamide (MTAC‐BA‐AM) was investigated via surface property and fluorescence spectroscopy measurements, as well as Mesodyn simulation, etc. The experimental results indicate that MTAC‐BA‐AM has the ability for decreasing surface tension of water and there are two break points in the surface tension isotherm; and its surface dilational viscoelasticity and apparent viscosity in aqueous solution depend upon aggregate structure. The Mesodyn simulation results show that spherical, cylindrical aggregates, and network structures form in aqueous solution with the concentration increasing. The aggregation process is driven by enthalpy and can be divided into two stages. The first stage is controlled by diffusion, while the second one is controlled by hydrophobic interaction.

Surface Dilatational Elasticity of Poly(oxy ethylene)-Based Surfactants by Oscillation and Relaxation Measurements of Sessile Bubbles

Surface dilatational elasticities and viscosities have been measured by means of the axisymmetric bubble shape method. Two different techniques using sinusoidal oscillations and step relaxations have been used, and the results are treated by means of the bulk/surface diffusional exchange model. Three different nonionic surfactants based on poly(oxy ethylene) as the hydrophilic group have been used: one simple surfactant, Brij 35, and two block copolymers, Pluronic F68 and P9400. Step relaxation and oscillation give mostly the same limiting surface dilatational elasticities, but step relaxation is a more model-dependent method. In the cases where the bulk/surface diffusion model is correct, the two methods give the same results, but otherwise step relaxation gives average values of the limiting elasticity E0 and the typical relaxation frequency omega0. Limiting elasticities up to ca. 25 mN m(-1)have been found for these substances. The surface/bulk diffusion model describes quite well the two relatively hydrophilic substances Brij 35 and F68, especially at low concentrations, but less so for the more hydrophobic P9400. The surface dilatational elasticity as a function of the surface pressure of the surface-active polymers goes through at least one maximum as a result of surface conformational changes.