Third order dielectric time dependent susceptibilities and the "box model".

We present here the calculation of the "time dependent" moduli of the third order, dielectric, susceptibilities measured at the frequency ω of the applied electric field and at the frequency 3ω. The calculation is performed within the frame work of the "box model," both in the ideal case of a pure third order polarization and in the practical case studied by Samanta and Richert [J. Chem. Phys. 140, 247101 (2014)] where a first order polarization contribution is also detected. We show that, in the two cases, those two modules have a largely different dynamics and that the results can be easily compared with experiments thanks to the data gathered by Samanta and Richert [J. Chem. Phys. 140, 247101 (2014)]. This should provide a new test on the validity of the "box model."

Third order susceptibilities in supercooled liquids and the "box model" theory versus experiments.

We give here the exact expression of the time dependent third order polarization predicted by the "box model" when a supercooled liquid is permanently subjected since time t = 0 to a strong electric field E(t) = E(0) sin(ωt), Dielectric Hole Burning experiment. We derive different forms of the time dependent susceptibilities, both at frequencies ω and 3ω, and examine particularly one of them. We also show that its susceptibility at ω should exhibit, at short times, presently undetected oscillations at frequency 2ω that are a signature of the "box model." We finally compare, for a large frequency range, the ω and 3ω time independent susceptibilities with the corresponding measurements on glycerol at 204.7 K. The agreement is good at frequency ω but, as already shown in Ladieu et al. [J. Chem. Phys. 134, 194507 (2011)], it is not the case at 3ω.

Observation of nanophase segregation in LiCl aqueous solutions from transient grating experiments.

Transient grating experiments performed on supercooled LiCl, RH2O solutions with R > 6 reveal the existence of well resolved, short time, extra signal which superposes to the normal signal observed for the R = 6 solution and for homogenous glass forming systems. This extra signal shows up below 190 K, its shape and the associated timescale depend only on temperature, while its intensity increases with R. We show that the origin of this signal is a phase separation between clusters with a low solute concentration and the remaining, more concentrated, solution. Our analysis demonstrates that these clusters have a nanometer size and a composition which are rather temperature independent, while increasing R simply increases the density of these clusters.

The low frequency dynamics of supercooled LiBr, 6H2O.

We present results of a series of experiments performed on LiBr, 6H(2)0 from room temperature down to 172 K ≈ 1.2T(g). These ultrasound, Brillouin and depolarized light scattering, and transient grating experiments show that, above 215 K, this solution behaves like supercooled water: its zero frequency sound velocity C(0) continuously decreases with decreasing temperature, and the reorientational dynamics of the water molecules can be directly detected at some temperatures of this domain. Conversely, below 215 K, a new regime sets in, where the apparent C(0) is practically temperature independent and where a ß, Arrenhius like, relaxation process coexists with the usual, Vogel-Fulcher like, α relaxation process of the supercooled liquid. These results are similar to those recently obtained in LiCl, 6H(2)O. The onset of the new regime is possibly due to an increase of the interaction of the water molecules with a neighboring Li(+) ion when lowering the temperature. We also compare our results with published dielectric data on water solutions of glass forming polyalcohols. Some of them present a low temperature splitting of their relaxation time similar to what is found in LiBr, 6H(2)O.

The low frequency phonons dynamics in supercooled LiCl, 6 H2O.

We report the results of a series of ultrasound, Brillouin scattering, and optical heterodyne detected transient grating experiments performed on a LiCl, 6H(2)O solution from room temperature down to the vicinity of its liquid-glass transition, T(g) approximately 138 K. Down to T approximately 215 K, the supercooled liquid has a behavior similar to what is expected for supercooled water: its zero frequency sound velocity, C(0), continuously decreases while the corresponding infinite frequency velocity, C(infinity), sharply increases, reflecting the increasing importance of H bonding when temperature is lowered. Below 215 K, specific aspects of the solution, presumably related to the role of the Li(+) and Cl(-) ions, modify the thermal behavior of C(0), while a beta relaxation process also appears and couples to the sound propagation. The origin of those two effects is briefly discussed.

The Prigogine-Defay ratio and the microscopic theory of supercooled liquids.

Prigogine-Defay ratios and, more recently, their frequency extension have been proposed to be a measure of the number of nonmacroscopic processes involved in the relaxation dynamics of supercooled liquids. We show that the microscopic theory of the Navier-Stokes equations of those liquids provides a consistent thermodynamic framework in which all possible dynamical Prigogine-Defay ratios can be expressed in terms of the same relaxation functions and that these ratios provide less information than the microscopic theory itself. The latter shows that more than one relaxation process is certainly always involved in this relaxation dynamics, whatever is the molecular dynamics, or experimental, technique used to determine the latter.

Scaling of the structural relaxation in simulated liquid silica.

A scaling law for the alpha relaxation time tau , involving the ratio of a density-dependent energy to the thermal energy, has been found to hold in many fragile glass-forming liquids. This scaling has been recently linked to a local quantity n{loc}(rho,T) relating the variation of tau with density to its variation with temperature. In many fragile liquids, the variation of n{loc}(rho,T) is weak enough to take it as constant over the experimental temperature and density domain. We show that simulated liquid silica has an opposite behavior; close to T{g}, n{loc} is negative for moderate densities and exhibits a significant variation with rho and T, reaching positive values for higher temperature and/or densities. Moreover, those variations linearly correlate to a measure of the bonding properties of the liquid.

Analysis of a heterodyne-detected transient-grating experiment on a molecular supercooled liquid. II. Application to m-toluidine.

This paper reports the first detailed analysis of a transient grating (TG) experiment on a supercooled molecular liquid, m-toluidine, from 330K (1.75Tg) to 190K (1.01Tg) based on the theoretical model presented in Paper I of this series. This method allows one to give a precise description, over a wide dynamical range, of the different physical phenomena giving rise to the signals. Disentangling the isotropic and the anisotropic parts of the TG response, a careful fitting analysis yields detailed information on the rotation-translation coupling function. We also extract the structural relaxation times related to the "longitudinal" viscosity over almost 10 decades in time and the corresponding stretching coefficient. The value of some other parameters and information on their thermal behavior is also reported.

Analysis of a heterodyne-detected transient-grating experiment on a molecular supercooled liquid. I. Basic formulation of the problem.

We present the basic equations necessary to interpret heterodyne-detected transient-grating experiments performed on a supercooled liquid composed of anisotropic molecules. The final expressions are given under a form suitable for their direct application to a test case.

Brillouin scattering study of salol: exploring the effects of rotation-translation coupling.

Brillouin scattering in liquids composed of optically and mechanically anisotropic molecules is affected by coupling between rotational and translational dynamics. While this effect has been extensively studied in depolarized (VH) scattering where it produces the "Rytov dip," recent theoretical analyses by Pick, Franosch show that it should also produce observable effects in polarized (VV) scattering [Eur. Phys. J. B 31, 217 (2003)]; 31, 229 (2003)]]. To test this theory, we carried out Brillouin scattering studies of the molecular glassformer salol in the temperature range 210-380 K, including VH-backscattering, VH-90 degrees, and VV-90 degrees spectra. The data were analyzed consistently to determine the effects of rotation-translation coupling on both the polarized and depolarized spectra. A previously unanticipated feature predicted by these authors was observed: a narrow negative region in the q -dependent part of the 90 degrees VV spectra, which we designate as the "VV dip." It is an analog of the Rytov dip observed at high temperatures in the 90 degrees VH spectra, which is also accurately described by this theory. Analysis of the 90 degrees VV spectra was carried out both with and without inclusion of translation-rotation coupling in order to determine quantitatively the role this coupling plays.

Temperature and pressure study of Brillouin transverse modes in the organic glass-forming liquid orthoterphenyl.

Transverse Brillouin spectra of orthoterphenyl are measured in the (250-305 K; 0.1-100 MPa) temperature-pressure range, which corresponds to the supercooled phase of this organic glass former. We show that the analysis of these spectra combined with an extrapolation of the reorientation times under pressure leads to an estimate of the static shear viscosity in a pressure range whose validity extends beyond the range of the Brillouin measurements. The relative contributions of temperature and of density to the change of this reorientation time measured along an isobar are extracted from our results in a large temperature range extending from the liquid to the low temperature supercooled state. They appear to be always of the same order of magnitude. It is also shown that in the range of the experiment, the orientational time is depending on a unique parameter built on temperature and density.

Brillouin scattering study of molten zinc chloride.

Polarized and depolarized Brillouin scattering experiments on molten ZnCl2 were performed between 300 and 600 degrees C in different geometries. VV spectra measured in backscattering and small angle scattering were analyzed with conventional viscoelastic theory using either a Debye or a Cole-Davidson model for the memory function. We also analyzed in the same way the temperature dependence of the transverse Brillouin lines detected in a 90 degrees VH geometry. We show that the Cole-Davidson memory function yields a consistent interpretation of all the spectra. The resulting shear and longitudinal relaxation times are equal within their error bars, and are about 2.5 times smaller than the alpha relaxation time previously determined. The static shear viscosity values deduced from the analysis of the propagating transverse waves agree, at all temperatures, with the measured viscosity values.

Current status of lasers in soft tissue dental surgery.

The aims of this paper are to briefly describe laser physics, the types of lasers currently available for use on soft tissues focusing primarily on CO2 and Nd:YAG laser energies, the histological effects of lasers on oral tissues, laser safety, the clinical applications of lasers on oral soft tissues, and future directions. Of the two types of lasers currently available for dental applications, both the CO2 and Nd:YAG lasers can be used for frenectomies, ablation of lesions, incisional and excisional biopsies, gingivectomies, gingivoplasties, soft tissue tuberosity reductions, operculum removal, coagulation of graft donor sites, and certain crown lengthening procedures. The advantages of lasers include a relatively bloodless surgical and post-surgical course, minimal swelling and scarring, coagulation, vaporization, and cutting, minimal or no suturing, reduction in surgical time, and, in a majority of cases, much less or no post-surgical pain. CO2 lasers, compared to Nd:YAG are faster for most procedures, with less depth of tissue penetration and a well-documented history. There have been recent reports on the use of the Nd:YAG laser for periodontal scaling, gingival curettage, and root desensitization, but further research needs to be conducted. Both the CO2 and the Nd:YAG laser have limited use in conventional flap therapy.

Laser in dentistry. Soft-tissue procedures.

Dentistry has entered the 1990s, an era of high technology. The dental laser offers the dentist not only a window, but a door into this high-tech arena. All of the advantages lasers offer, from bloodless procedures to minimal postoperative pain and from reduction of operative time to high patient acceptance, indicate that lasers are great dental instruments for today and the future. As a futuristic idea, the ideal laser for dentistry would be able to work well, not only in soft tissue, but in hard tissue. Envision a laser with multiple wavelengths in the same unit: one for incision, another for removal of hard tissue, and yet still another for making tissue "sticky" for flap placement and tissue welding. So often dentists are associated with pain, fear, and the noise of the high-speed drill. The laser certainly helps to dispel these stereotypes and apprehensions, and to bring dentistry into a new era.