Excitation of Capillary Waves in Strongly Absorbing Liquids by a Modulated Laser Beam (original) (raw)
Related papers
Journal of Fluid Mechanics, 1993
The disturbances on the free surface of dielectric fluids resulting from intense laser heating of their boundary layer are studied theoretically and experimentally. The heating is accompanied by pronounced evaporation from the surface and thereby leads to a recoil pressure momentum applied to the surface. For small values of total momentum transferred to the fluid, the low-amplitude initially hollow-like displacement of the surface in the impact zone decays to produce linear gravity-capillary waves (GCW) spreading out on the surface. This regime is treated analytically and the results obtained are compared with experiments involving weakly viscous (water, ethanol) and highly viscous (glycerol) liquids. An experimental arrangement for remote generation and subsequent detection of probe GCW-packets is given. The evolution of broadband GCW-disturbances on clean and surfactant-contaminated water surfaces are described. Results of GCW-attenuation spectrum measurements on clean water surfaces and on film-covered surfaces are presented.
Nonlinear Laser-Induced Deformations of Fluid-Fluid Interfaces
2008
Experimentally, it turns out that radiation forces from a cw-laser on a liquid-liquid interface are able to produce giant deformations (up to about 100 µm), if the system is close to the critical point where the surface tension becomes small. We present a new model for such a fingerlike deformation, implying that the system is described as an optical fiber. One reason for introducing such a model is that the refractive index difference in modern experiments, such as those of the Bordeaux group, is small, of the same order as in practical fibers in optics. It is natural therefore, to adopt the hybrid HE 11 mode, known from fiber theory, as the fundamental mode for the liquid system. We show how the balance between hydrodynamical and radiation forces leads to a stable equilibrium point for the liquid column. Also, we calculate the narrowing of the column radius as the depth increases. Comparison with experimental results of the Bordeaux group yields quite satisfactory agreement as regards the column width.
Theoretical considerations of laser-induced liquid–liquid interface deformation
Physica Scripta, 2013
In the increasingly active field of optofluidics, a series of experiments involving near-critical two-fluid interfaces have shown a number of interesting nonlinear effects. We here offer, for the first time to our knowledge, an explanation for one such feature, observed in experiments by Casner and Delville (2003 Phys. Rev. Lett. 90 144503), namely the sudden formation of 'shoulder'-like shapes in a laser-induced deformation of the liquid-liquid interface at high laser power. Two candidate explanations are the following. Firstly, the shape can be explained by balancing forces of buoyancy, laser pull and surface tension only, and the observed change of deformation shape is the sudden jump from one solution of the strongly nonlinear governing differential equation to another. Secondly, it might be that the non-trivial shape observed could be the result of temperature gradients due to local absorptive heating of the liquid. We report that a systematic search for solutions of the governing equation in the first case yields no trace of solutions containing such features. By contrast, an investigation of the second option shows that the narrow shape of the tip of the deformation can be explained by a slight heating of the liquids. The local heating amounts to a few kelvins, with the parameters given, although there are uncertainties here. Our investigations suggest that local temperature variations are the crucial elements behind the instability and the shoulder-like deformation.
An experimental technique for measurements of capillary waves
Review of Scientific Instruments, 1989
An experimental method has been developed for the measurements of wave profiles at liquid interfaces. An oscillating string was used to generate interface waves, which gave rise to oscillations of a laser beam reflected from the interface. These oscillations were detected using a lock-in amplifier technique, which allowed us to measure the derivative of the surface wave profile at any given phase in the wave period. We discuss in detail experiments on the water-air interface. From the observed form of the capillary waves, we deduce the surface tension and the viscosity. The dispersion relation of capillary waves has been observed, and we found a spatial damping coefficient and wave number consistent with results obtained by dynamic light scattering and mechanical methods. Our technique is to a large extent insensitive to mechanical and optical noise and determines the capillary waveform reliably and reproducibly.
Journal De Physique, 1982
2014 La surface libre d'un échantillon liquide (huile lourde) est chauffée au moyen d'un faisceau laser Gaussien. On présente des enregistrements photographiques de la forme de la surface et de la distribution des vitesses. La surface passe par une étape initiale de dilatation, suivie de la formation d'une dépression. Une interprétation théorique en est donnée dans le cadre de solutions stationnaires de l'équation de Navier-Stokes, considérant la dépendance de la masse volumique et de la tension superficielle vis-à-vis de la température. On prouve que le signe des variations de la hauteur de la surface dans la région chaude dépend de façon critique des paramètres expérimentaux. Les expériences et les résultats théoriques montrent que le phénomène est dû à la compétition entre les variations de la masse volumique et de la tension superficielle. Abstract. 2014 The free-surface of a liquid sample (heavy oil) is locally heated by irradiation with a Gaussian laser beam. Photographic records of the surface profile and flow pattern are presented. The free-surface passes by an earlier dilatation stage, followed by the formation of a pit in the top of the expanded region. A theoretical analysis based on the stationary solution of Navier-Stokes equation with temperature-dependent density and surfacetension is made. It is shown that the sign of the surface-height variations in the hot region depends critically on the values of the dimensionless quantities characterizing the experiment. The surface profile and the velocity distribution are calculated and compared with the experimental results. We conclude that the phenomena observed are due to competition between the density-and surface-tension gradients.
Nonlinear laser-induced deformations of liquid-liquid interfaces: An optical fiber model
Physical Review E, 2008
Experimentally, it turns out that radiation forces from a cw-laser on a liquid-liquid interface are able to produce giant deformations (up to about 100 µm), if the system is close to the critical point where the surface tension becomes small. We present a new model for such a fingerlike deformation, implying that the system is described as an optical fiber. One reason for introducing such a model is that the refractive index difference in modern experiments, such as those of the Bordeaux group, is small, of the same order as in practical fibers in optics. It is natural therefore, to adopt the hybrid HE 11 mode, known from fiber theory, as the fundamental mode for the liquid system. We show how the balance between hydrodynamical and radiation forces leads to a stable equilibrium point for the liquid column. Also, we calculate the narrowing of the column radius as the depth increases. Comparison with experimental results of the Bordeaux group yields quite satisfactory agreement as regards the column width.
Transient deformation of liquid surfaces by laser-induced thermocapillarity
Applied Optics, 1979
Heavy hydrocarbons are irradiated with a Gaussian laser beam. The time-varying temperature distribution induced in the fluid gives rise to a surface-tension distribution. The later gives rise in turn to an accompanying liquid flow and surface-height distribution. In this paper, the time-varying shape of the induced depression is calculated as a function of the power distribution in the laser beam and of the thermal and mechanical properties of the material. The intensity distribution in the laser beam reflected from the depression is also calculated. Good agreement is found with previous experimental results.
Laser-Heating-Induced Displacement of Surfactants on the Water Surface
The Journal of Physical Chemistry B, 2012
We report a combined vibrational sum-frequency generation (SFG) spectroscopy, Brewster angle microscopy (BAM), and ellipsometry study of different surfactants on water as a function of surfactant density. Vibrational SFG spectra of surfactants on the water surface in a Langmuir trough have been measured in both the surfactant CH and the water OH stretch regions. At low densities, the SFG signal generated at the surface in the presence of the surfactant is indistinguishable from the SFG signal generated at the clean water−air interface. When the surfactant density increases, i.e., upon compressing the monolayer, a very sudden increase in the SFG signal in both the CH and OH spectral regions is observed. For higher laser fluences, this stepwise increase occurs at increasingly higher surfactant densities. Since BAM shows that surfactant molecules are clearly present at these low densities, we conclude that at low surfactant density the laser beam displaces relatively high-density domains with surfactants in the liquid expanded phase out of the region of the laser focus. This is a consequence of the thermal gradient induced by local heating of the water phase with the monolayer on top due to repetitive laser excitation at 1 kHz. It can be circumvented by using a rotating trough. In this manner, the sampled surface area can be refreshed, allowing artifact-free vibrational SFG spectra to be measured down to the very lowest surfactant densities. In ellipsometry experiments, a similar step can be noticed, which, however, is of a different nature; i.e., it is not related to heating (the laser fluence is very low and the light nonresonant) but to a molecular transition. The occurrence of the step in ellipsometry as a function of area per molecule depends critically on the preparation of the monolayer. By giving the molecules time and space to relax during the preparation of the monolayer, this step could also be eliminated.