Comparison of Refractive Index Matching Techniques and PLIF40 Measurements in Annular Flow.

This paper investigates non-invasive techniques for annular two-phase flow analysis, focusing on liquid film characterization to understand the interfacial phenomena that are crucial for heat and mass transfer. Limited methods allow the study of the temporal and spatial evolution of liquid film, such as Planar Laser-Induced Fluorescence (PLIF). However, this method possesses optical challenges, leading to the need for improved techniques to mitigate refraction and reflection, such as Refractive Index Matching (RIM). This study utilizes an experimental annular flow facility to analyze both RIM and non-RIM PLIF over a range of liquid Reynolds numbers from 4200 to 10,400. Three configurations-PLIF RIM90, PLIF RIM40, and PLIF nRIM40-are compared from both qualitative and quantitative perspectives. In the quantitative analysis, key variables of the liquid film are measured, namely mean film thickness, disturbance wave height, and frequency. Variations in the analyzed variables indicate minor deviations, which are not likely to be caused by the technique used. However, all three methodologies exhibited errors that are estimated to be within a maximum of 10%, with a mean value of approximately 8%.

Unveiling the multi-step solubilization mechanism of sub-micron size vesicles by detergents.

The solubilization of membranes by detergents is critical for many technological applications and has become widely used in biochemistry research to induce cell rupture, extract cell constituents, and to purify, reconstitute and crystallize membrane proteins. The thermodynamic details of solubilization have been extensively investigated, but the kinetic aspects remain poorly understood. Here we used a combination of single-vesicle Förster resonance energy transfer (svFRET), fluorescence correlation spectroscopy and quartz-crystal microbalance with dissipation monitoring to access the real-time kinetics and elementary solubilization steps of sub-micron sized vesicles, which are inaccessible by conventional diffraction-limited optical methods. Real-time injection of a non-ionic detergent, Triton X, induced biphasic solubilization kinetics of surface-immobilized vesicles labelled with the Dil/DiD FRET pair. The nanoscale sensitivity accessible by svFRET allowed us to unambiguously assign each kinetic step to distortions of the vesicle structure comprising an initial fast vesicle-swelling event followed by slow lipid loss and micellization. We expect the svFRET platform to be applicable beyond the sub-micron sizes studied here and become a unique tool to unravel the complex kinetics of detergent-lipid interactions.

Fluorescence emission of pyrene in surfactant solutions.

The systematic description of the complex photophysical behaviour of pyrene in surfactant solutions in combination with a quantitative model for the surfactant concentrations reproduces with high accuracy the steady-state and the time resolved fluorescence intensity of pyrene in surfactant solutions near the cmc, both in the monomer and in the excimer emission bands. We present concise model equations that can be used for the analysis of the pyrene fluorescence intensity in order to estimate fundamental parameters of the pyrene-surfactant system, such as the binding equilibrium constant K of pyrene to a given surfactant micelle, the rate constant of excimer formation in micelles, and the equilibrium constant of pyrene-surfactant quenching. The values of the binding equilibrium constant K(TX100)=3300·10³ M⁻¹ and K(SDS)=190·10³ M⁻¹ for Triton X-100 (TX100) and SDS micelles, respectively, show that the partition of pyrene between bulk water and micelles cannot be ignored, even at relatively high surfactant concentrations above the cmc. We apply the model to the determination of the cmc from the pyrene fluorescence intensity, especially from the intensity ratio at two vibronic bands in the monomer emission or from the ratio of excimer to monomer emission intensity. We relate the finite width of the transition region below and above the cmc with the observed changes in the pyrene fluorescence in this region.

A model for monomer and micellar concentrations in surfactant solutions: application to conductivity, NMR, diffusion, and surface tension data.

An empirical model for the concentrations of monomeric and micellized surfactants in solution is presented as a consistent approach for the quantitative analysis of data obtained with different experimental techniques from surfactant solutions. The concentration model provides an objective definition of the critical micelle concentration (cmc) and yields precise and well defined values of derived physical parameters. The use of a general concentration model eliminates subjective graphical procedures, reduces methodological differences, and thus allows one to compare directly the results of different techniques or to perform global fits. The application and validity of the model are demonstrated with electrical conductivity, surface tension, NMR chemical shift, and self-diffusion coefficient data for the surfactants SDS, CTAB, DTAB, and LAS. In all cases, the derived models yield excellent fits of the data. It is also shown that there is no need to assume the existence of different premicellar species in order to explain the chemical shifts and self-diffusion coefficients of SDS as claimed recently by some authors.