The role of fluid dynamics on compressed/expanded surfactant monolayers

Abstract

A typical experiment to measure monolayer surface rheological properties consists of two parallel, slightly immersed, moving solid barriers that compress and expand a shallow liquid layer that contains the surfactant monolayer in its free surface. The area between the barriers controls the surfactant concentration, which is frequently assumed as spatially constant. In order to minimize the fluid dynamics and other non-equilibrium effects, the barriers motion is very slow. Nevertheless, the surfactant concentration dynamics exhibit some unexpected features such as irreversibility, suggesting that the motion is not slow enough. We present a long wave theory that takes into account the fluid dynamics in the bulk phase coupled to the free surface elevation. In addition, apparent irreversibility is also discussed that may result from artifacts associated with the menisci dynamics when surface tension is measured using a Wilhelmy plate. Instead, additional, purely chemical, non-equilibrium effects are ignored. Results from this theory are discussed for varying values of the parameters, which permit establishing specific predictions on experiments. On the other hand, these results compare fairly well with the available experimental observations, at least qualitatively. The overall conclusion is that the fluid dynamics should not be ignored in the analysis of these experimental devices.

Simulation of dense non-Brownian suspensions with the lattice Boltzmann method: shear jammed and fragile states

“…The second reason to use the configurations is the easiness of the visualization of the microstructure of particles. Furthermore, monolayered suspensions have also been studied in some experiments on lipid domains 58 and on fluid surfaces [59][60][61][62] .…”

Effect of the Incorporation of Nanosized Titanium Dioxide on the Interfacial Properties of 1,2-Dipalmitoyl-sn-glycerol-3-phosphocholine Langmuir Monolayers

“…As expected, the increase of the amplitude of the dilatational deformation leads to the increase of the THD. 30,58,64 Furthermore, the penetration of nanoparticles also leads to the increase of the THD. The increase of the nonlinearity in the presence of nanoparticles is due to the larger inhomogeneity of the monolayers that leads to the hindering of the lateral redistribution of the material at the interface.…”

“…These studies were carried out at a fixed value of surface pressure (∼40 mN/m) that is considered a reference state for the study of biosystems such as lung surfactant or biomembranes. − The increase of the amplitude for the area deformation is expected to induce nonlinearity in the response. This makes it impossible to use the classical approach based on the fitting of the response to a sinusoidal function for the analysis of the experimental results, being necessary to use a description based on a complex sum of trigonometric functions which makes difficult the data analysis . This can be solved in part using a fast Fourier transform (FFT) algorithm, implemented in the software package Origin (Origin Lab Corp., USA) as was described elsewhere. ,,,− From the analysis of the surface pressure using the FFT algorithm, it is possible to obtain information on the response as a spectrum in the frequency domain, which provides information about the nature of the signal.…”

Influence of Semifluorinated Alkane Surface Domains on Phase Behavior and Linear and Nonlinear Viscoelasticity of Phospholipid Monolayers

“…It should be noted that this is characteristic for dilational rheology measurements, as the strain is perfectly symmetric in the case of shear rheology experiments. Actually, recent theoretical simulations suggested potential artifacts caused by hydrodynamics, such as the fluid dynamics in the water phase coupled to the elevation of the meniscus . Therefore, the nonlinear response could be attributed to the phase transition of lipids.…”

“…Actually, recent theoretical simulations suggested potential artifacts caused by hydrodynamics, such as the fluid dynamics in the water phase coupled to the elevation of the meniscus. 46 Therefore, the nonlinear response could be attributed to the phase transition of lipids. The Fourier mode signal from both DPPC (Figure 6a) and DPPC/F8H16 monolayers (Figure 6b) collected at a higher strain amplitude (u 0 = 0.05) exhibited a more pronounced signal from the second mode, while it was close to the noise at u 0 = 0.02 (Figure 5a,c).…”