No Arabic abstract
Deformations of liquid interfaces by the optical radiation pressure of a focused laser wave were generally expected to display similar behavior, whatever the direction of propagation of the incident beam. Recent experiments showed that the invariance of interface deformations with respect to the direction of propagation of the incident wave is broken at high laser intensities. In the case of a beam propagating from the liquid of smaller refractive index to that of larger one, the interface remains stable, forming a nipple-like shape, while for the opposite direction of propagation, an instability occurs, leading to a long needle-like deformation emitting micro-droplets. While an analytical model successfully predicts the equilibrium shape of weakly deformed interface, very few work has been accomplished in the regime of large interface deformations. In this work, we use the Boundary Integral Element Method (BIEM) to compute the evolution of the shape of a fluid-fluid interface under the effect of a continuous laser wave, and we compare our numerical simulations to experimental data in the regime of large deformations for both upward and downward beam propagation. We confirm the invariance breakdown observed experimentally and find good agreement between predicted and experimental interface hump heights below the instability threshold.
Soap bubbles are by essence fragile and ephemeral. Depending on their composition and environment, bubble bursting can be triggered by gravity-induced drainage and/or the evaporation of the liquid and/or the presence of nuclei. In this paper, we design bubbles made of a composite liquid shell able to neutralize all these effects and keep their integrity in a standard atmosphere. This composite material is obtained in a simple way by replacing surfactants by partially-wetting microparticles and water by a water/glycerol mixture. A nonlinear model able to predict the evolution of these composite bubbles toward an equilibrium state is proposed and quantitatively compared to experimental data. This work unveils a composite liquid film with unique robustness, which can easily be manufactured to design complex objects.
The acoustic properties of liquid oxygen have been studied up to 90 T by means of the ultrasound pulse-echo technique. A monotonic decrease of the sound velocity and an asymptotic increase of the acoustic attenuation are observed by applying magnetic fields. An unusually large acoustic attenuation, that becomes 20 times as large as the zero-field value, cannot be explained by the classical theory. These results indicate strong fluctuations of antiferromagnetically coupled local structures. We point out that the observed fluctuations are a precursor of a liquid-liquid transition, from a low-susceptibility to a high-susceptibility liquid, which is characterized by a local-structure rearrangement.
We propose a simple scaling theory describing the variation of the mean first passage time (MFPT) $tau(N,M)$ of a regular block copolymer of chain length $N$ and block size $M$ which is dragged through a selective liquid-liquid interface by an external field $B$. The theory predicts a non-Arrhenian $tau$ vs. $B$ relationship which depends strongly on the size of the blocks, $M$, and rather weakly on the total polymer length, $N$. The overall behavior is strongly influenced by the degree of selectivity between the two solvents $chi$. The variation of $tau(N,M)$ with $N$ and $M$ in the regimes of weak and strong selectivity of the interface is also studied by means of computer simulations using a dynamic Monte Carlo coarse-grained model. Good qualitative agreement with theoretical predictions is found. The MFPT distribution is found to be well described by a $Gamma$ - distribution. Transition dynamics of ring- and telechelic polymers is also examined and compared to that of the linear chains. The strong sensitivity of the ``capture time $tau(N,M)$ with respect to block length $M$ suggests a possible application as a new type of chromatography designed to separate and purify complex mixtures with different block sizes of the individual macromolecules.
Although medium chain length insoluble amphiphiles are well known to form gaseous and liquid expanded phases on an air/water interface, the situation for the soluble case is less clear. We perform molecular dynamics simulations of model surfactant molecules dissolved in a bulk liquid solvent in coexistence with its vapor. Our results indicate a transition in both soluble and insoluble surfactants: a plateau in surface tension vs. surface coverage, whose instantaneous configurations display two phase coexistence, along with correlation functions indicating a transition to gaseous to liquid-like behavior.
Simulations of liquid-gas systems with extended interfaces are observed to fail to give accurate results for two reasons: the interface can get ``stuck on the lattice or a density overshoot develops around the interface. In the first case the bulk densities can take a range of values, dependent on the initial conditions. In the second case inaccurate bulk densities are found. In this communication we derive the minimum interface width required for the accurate simulation of liquid gas systems with a diffuse interface. We demonstrate this criterion for lattice Boltzmann simulations of a van der Waals gas. When combining this criterion with predictions for the bulk stability we can predict the parameter range that leads to stable and accurate simulation results. This allows us to identify parameter ranges leading to high density ratios of over 1000. This is despite the fact that lattice Boltzmann simulations of liquid-gas systems were believed to be restricted to modest density ratios of less than 20.