Collinear Mirage Research Articles

Novel Results on Collinear Mirage Deflection

In this paper we report on the strong influence of the polarization state of the probe beam on the amplitude and phase of the collinear mirage deflection. It is shown that the photoelastic effect, i.e. the change of the refractive index with stresses, is the responsible of this influence. Theoretical and experimental studies on isotropic materials have been performed. The influence on the measurement of the thermal diffusivity of these materials is analyzed.

Investigation of the critical behavior at lyotropic systems by collinear mirage technique

We investigate the behavior of the thermal diffusivity (D) in the vicinity of the nematic to isotropic (N - I) and uniaxial to biaxial (N - N) phase transitions for different lyotropic liquid crystals. For the first time, collinear mirage effect technique is used to observe that the presence of a peak in the thermal diffusivity is accomplished by a phase transition due to the variation at the sample's temperature. Furthermore, we show that our experimental setup is sensitive to both first and second order transitions in lyotropic systems. We also propose a critical exponent related to the thermal diffusivity.

Thermodiffusion study in ferrofluids through collinear mirage effect

We studied some thermodiffusion effects for ferrofluid samples with two different kinds of surfactant by mirage effect technique. We focused a laser beam in the sample introducing a heat gradient that provoked ferroparticles mobility (thermodiffusion) and analyzed the behavior of the mirage signal as a function of the pump laser beam intensity, for a fixed frequency. The main results showed that for the ferrofluid with anionic surfactant no thermodiffusion variation was verified, even for the most concentrated sample. On the other hand, for the cationic one we observed an increase in the thermodiffusion with greater ferroparticles concentration. Therefore, we showed that collinear mirage effect technique can be used for detecting Soret effect generated by the thermal interactions between the laser radiation and the highly absorbing material.

Photoelastic effect and mirage deflection in anisotropic materials

The collinear mirage technique is widely used to measure the thermal diffusivity of semi-transparent materials. However, in a recent paper [A. Salazar, M. Gateshki and A. Sanchez-Lavega: Appl. Phys. Lett. 76, 2665 (2000)], it was shown that for isotropic materials, because of the influence of photoelastic effect, the method was sensitive to the polarization state of the probe beam. The present paper extends the previous work to include anisotropic materials. In particular, we focus on the experimental conditions under which the thermal diffusivity of each crystal system can be measured using the phase method. Our theoretical model indicates that while the thermal diffusivity of isotropic materials can be measured using an unpolarized probe beam, for anisotropic materials, even the use of an unpolarized probe beam does not guarantee the validity of the method in all crystal systems. Experimental measurements performed on cubic, hexagonal and monoclinic crystals confirm the validity of the model.

The effect of the ferrofluid magnetic particle concentration on the thermal diffusivity

Thermal diffusivity results obtained with the collinear mirage technique, are reported for different concentrations surfacted ferrofluid and for a set of acid ferrofluids with variable pH values. The collinear mirage technique was used because the materials are semitransparent and have low thermal diffusivities. The results show that, as the ferrofluid magnetic particles concentration increases, the thermal diffusivity also increases. Their behaviour differs from that reported for ferronematics [1]. The different geometry of thermal waves propagating through the material can account for the difference.

On the strong influence of the photoelastic effect in the collinear mirage deflection

We report in this letter on the strong influence of the polarization state of the probe beam in the amplitude and phase of the collinear mirage deflection. A model of the collinear mirage deflection that takes into account the photoelastic effect in the sample has been developed. The agreement between the model predictions and the experimental results is excellent.

Collinear mirage effect measurement of the thermal diffusivity in ferrofluids

Thermal diffusivity results obtained for different concentrations of the same ferrofluid are reported. The collinear mirage technique was used for its adequacy in obtaining the thermal properties of semitransparent materials with low thermal diffusivities. The results show that, as the ferrofluid concentration increases, the thermal diffusivity also increases, leading to a different trend compared to those reported for ferronematics. The different behaviors observed can be related to the different geometries of thermal waves propagating throught the material.

Collinear mirage effect measurement of the thermal diffusivity in Ferronematics

Thermal diffusivity results obtained for a lyotropic nematic liquid crystal sample doped with different amounts of ferrofluid (ferronematics) are reported. The collinear mirage technique was used for measurements along axes parallel and perpendicular to the director of this anisotropic material. The results show that as the ferrofluid concentration increases the thermal diffusivity decreases, leading to values even lower than those already reported for plain lyotropic nematic liquid crystals. The ratio of the two diffusivities increases as the ferrofluid concentration increases, reaching the exact value theoretically expected, though not achieved experimentally, for plain lyotropic nematic liquid crystals.

Thermal diffusivity of anisotropic materials by photothermal methods

In this paper we analyze the possibility of extending the photothermal methods developed for thermal diffusivity measurements of isotropic materials [A. Salazar and A. Sánchez-Lavega, Rev. Sci. Instrum. 65, 2896 (1994)] to the case of anisotropic specimens. A full theoretical treatment of the photothermal signal generation for the case of an anisotropic sample with its principal axes aligned with the sample surface is presented. Three photothermal detection schemes have been studied: infrared radiometry, photothermal reflectance, and optical beam deflection (mirage effect). The fundamental result we have obtained is that when using infrared radiometry, photothermal reflectance and collinear mirage setups, the thermal magnitude retrieved is the resistivity to heat diffusion (a tensor defined as the inverse of the thermal diffusivity tensor). Only the perpendicular mirage experiment allows one to directly retrieve the thermal diffusivity along any direction of the material. Experimental measurements performed on two highly anisotropic materials confirm the above-mentioned results.

Collinear mirage effect at a microscopic scale

This demonstration of mirage effect under a microscope provides a new possibility to measure the thermal diffusivity of a transparent sample as small as a few microns, with micronic spatial resolution.

Application of collinear mirage detection for thermal diffusivity measurements of solids at high temperatures

Application of collinear mirage detection for thermal diffusivity measurements of solids at high temperatures

Thermal diffusivity measurements on solids using collinear mirage detection

A report on the adequacy of the collinear mirage technique for thermal diffusivity measurements on bulk homogeneous solids is presented. A 3D theoretical model for collinear deflection has been developed from which two simple linear relations between measurable parameters and the thermal diffusivity have been obtained. Two methods, the so-called zero-crossing and phase methods, are discussed in detail. The second one seems to be a promising tool for thermal diffusivity determination. It has been validated by means of experimental measurements on a set of samples with known thermal diffusivities. The technique is restricted to semitransparent solids but is also valid for materials with either high or low thermal diffusivities, being specially useful for this last group.