V. I. Kovalchuk

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.

Studies of concentrated surfactant solutions using the maximum bubble pressure method

Abstract An analysis of the adsorption process during the deadtime period in maximum bubble pressure experiments is performed. The adsorption dynamics is calculated for the actual surface area increase of a bubble in a surfactant solution, which depends on characteristic system parameters such as the length and diameter of the capillary and the surfactants concentration and adsorption activity. Under certain conditions, the calculations yield extreme adsorption behavior. In particular, a sharp adsorption minimum is predicted in the initial period of the deadtime interval. This effect can be relevant for the interpretation of bubble pressure results for concentrated surfactant solutions, when the initial adsorption of the surfactant is significant. In this case, the experimental results should be presented as a function of the effective total time (deadtime plus lifetime) rather than the effective lifetime.

Project proposal for the investigation of particle-stabilised emulsions and foams by microgravity experiments

The utilisation of particle-surfactants nanostructures as stabilising agents represents today the technologic and scientific frontier in the stabilisation of liquid films in emulsion and foams. This topic will be addressed by the proposal STEFAN (STabilisation of Emulsions and FoAms by Nanoparticles), proposed by European groups in the framework of the ESA AO-2004 for Life and Physical Sciences and Applied Research projects Similarly to what can be observed for surfactant-stabilised emulsions and foams, microgravity provides ideal conditions for the investigation of the hierarchy of involved objects: interfacial layer, liquid film, dispersed systems foam or emulsion. Microgravity experiments are planned by refurbishing the Experimental Container FASES for the ISS Fluid Science Laboratory and the facility FASTER for the European Drawer Rack, already under development in existing research programmes. Here the scientific guidelines of the project are presented together with examples and preliminary results on the effect of nano-particle-surfactant structures adsorbed at liquid interfaces. First experimental results have been achieved for particle monolayers at the water/air interface and a thermodynamic model was derived to describe the obtained surface pressure-area isotherms.

The effect of capillary characteristics on the results of dynamic surface tension measurements using the maximum bubble pressure method

Abstract The dynamic surface tension of Triton X-100 solutions was measured using the maximum bubble pressure method (MBPM). The data obtained depended on the geometry and material of the capillaries employed. It is shown that reliable results are obtained with short capillaries of hydrophobic internal surfaces. The effect of the ratio of the bubble volume to measuring system volume on the measured dynamic surface tension is studied. Recommendations are given for the optimum choice of the capillary to be employed in the MBPM. High-speed video imaging is used for studying the hydrodynamics of the liquid meniscus after bubble separation in hydrophilic and hydrophobic capillaries.

Hydrodynamic processes in dynamic bubble pressure experiments: Part 5. The adsorption at the surface of a growing bubble

Abstract The dynamics of the adsorption process at the surface of a growing bubble is described for a diffusion-controlled adsorption mechanism. The model is linked with the measuring procedure in maximum bubble pressure experiments as it is programmed in standard tensiometers. As a result the pressure–gas flow rate dependence is obtained which is the basis of the Lauda tensiometer MPT2. The theoretical results allow to define the necessary corrections for calculation procedures used in bubble pressure tensiometry, and to estimate possible experimental errors. Moreover, the importance of particular features of commercial instruments is demonstrated, such as the requirement of a large gas reservoir and a maximum length for capillaries needed to reach short adsorption times of microseconds and below. The influence of the surfactant concentration as studied for decyl dimethyl phosphine oxide on the particular shape of all characteristic dependencies is quantitatively reproduced by the theory.

Capillary pressure studies under low gravity conditions.

For the understanding of short-time adsorption phenomena and high-frequency relaxations at liquid interfaces particular experimental techniques are needed. The most suitable method for respective studies is the capillary pressure tensiometry. However, under gravity conditions there are rather strong limitations, in particular due to convections and interfacial deformations. This manuscript provides an overview of the state of the art of experimental tools developed for short-time and high-frequency investigations of liquid drops and bubbles under microgravity. Besides the brief description of instruments, the underlying theoretical basis will be presented and limits of the applied methods under ground and microgravity conditions will be discussed. The results on the role of surfactants under highly dynamic conditions will be demonstrated by some selected examples studied in two space shuttle missions on Discovery in 1998 and Columbia in 2003.

Effect of water hardness on surface tension and dilational visco-elasticity of sodium dodecyl sulphate solutions

The complementary drop and bubble profile analysis and maximum bubble pressure tensiometry are used to measure the dynamic surface tension of aqueous SDS solutions in the presence of hardness salts (CaCl(2) and MgCl(2) in the ratio of 2:1 at concentrations of 6 and 40FH). The presence of hardness salts results in an essential increase of the SDS adsorption activity, which indicates the formation of Ca(DS)(2) and Mg(DS)(2) in the SDS solutions. The surface tension isotherms of SDS in presence of Ca(DS)(2) and Mg(DS)(2) are described using the generalised Frumkin model. The presence of hardness salts accelerates the ageing of SDS solutions as compared with the addition of 0.01 M NaCl due to a faster hydrolysis and hence formation of dodecanol. These results are used to estimate the possible concentration of dodecanol in the studied SDS solutions. The buoyant bubble profile method with harmonic surface oscillations is used to measure the dilational rheology of SDS solutions in presence of hardness salts in the frequency range between 0.005 Hz and 0.2 Hz. The visco-elasticity modulus in the presence of hardness salts is higher as compared with its values in the presence of 0.01 M NaCl additions. The ageing of SDS solutions leads to an essential increase of the visco-elastic modulus.

Hydrodynamic processes in dynamic bubble pressure experiments. 4. Calculation of magnitude and time of liquid penetration into capillaries

Abstract A numerical analysis and an approximate solution is presented to show that in bubble pressure experiments under certain conditions, a liquid meniscus can move into the capillary after bubble detachment. For wide and short capillaries this meniscus displacement is almost negligible, for narrow and long capillaries it can be much more than the capillary radius. However, the maximum displacement does not exceed 3% of the capillary length for capillary radii>10 μm. The calculations show that the excess pressure in the gas reservoir at the end of the capillary does not effect very strongly the meniscus penetration depth, but strongly affects the time of the meniscus reverse movement.

Dynamics of Rear Stagnant Cap formation at the surface of spherical bubbles rising in surfactant solutions at large Reynolds numbers under conditions of small Marangoni number and slow sorption kinetics

On the surface of bubbles rising in a surfactant solution the adsorption process proceeds and leads to the formation of a so called Rear Stagnant Cap (RSC). The larger this RSC is the stronger is the retardation of the rising velocity. The theory of a steady RSC and steady retarded rising velocity, which sets in after a transient stage, has been generally accepted. However, a non-steady process of bubble rising starting from the initial zero velocity represents an important portion of the trajectory of rising, characterized by a local velocity profile (LVP). As there is no theory of RSC growth for large Reynolds numbers Re » 1 so far, the interpretation of LVPs measured in this regime was impossible. It turned out, that an analytical theory for a quasi-steady growth of RSC is possible for small Marangoni numbers Ma « 1, i.e. when the RSC is almost completely compressed, which means a uniform surface concentration Γ(θ)=Γ(∞) within the RSC. Hence, the RSC angle ψ(t) is obtained as a function of the adsorption isotherm parameters and time t. From the steady velocity v(st)(ψ), the dependence of non-steady velocity on time is obtained by employing v(st)[ψ(t)] via a quasi-steady approximation. The measurement of LVP creates a promising new opportunity for investigation of the RSC dynamics and adsorption kinetics. While adsorption and desorption happen at the same localization in the classical methods, in rising bubble experiments desorption occurs mainly within RSC while adsorption on the mobile part of the bubble surface. The desorption flux from RSC is proportional to αΓ(∞), while it is usually αΓ. The adsorption flux at the mobile surface above RSC can be assumed proportional to βC0, while it is usually βC0(1-Γ/Γ(∞)). These simplifications may become favorable in investigations of the adsorption kinetics for larger molecules, in particular for globular proteins, which essentially stay at an interface once adsorbed.

Dilational Viscoelasticity of Adsorption Layers Measured by Drop and Bubble Profile Analysis: Reason for Different Results.

The dilational viscoelasticity of adsorption layer was measured at different frequencies of drop and bubble surface area oscillations for aqueous C12EO5 solutions. The modulus values obtained by the two experimental protocols are the same for Π < 15 mN/m, while for higher surface pressures the values from drop experiments exceed those from bubble profile analysis. The nature of this phenomenon was studied using stress deformation experiments. At high surfactant concentrations the magnitude of surface tension variations is essentially higher for drops as compared with bubbles, leading to an increased viscoelasticity modulus for oscillating drops. The observed effects are analyzed quantitatively using a diffusion controlled exchange of matter model. The viscoelasticity moduli for a number of surfactants (different CnEOm and Tritons, C13DMPO, and SDS) are reported, and it is shown that the discrepancies between the data obtained by the two methods for many surfactants agree well with the predictions made here.