Immersion liquid techniques in solid particle characterization: A review

Determination of refractive index, size, and solid content of monodisperse polystyrene microsphere suspensions for the characterization of optical phantoms

Monodisperse polystyrene microspheres are often utilized in optical phantoms since optical properties such as the scattering coefficient and the scattering phase function can be calculated using the Mie theory. However, the calculated values depend on the inherent physical parameters of the microspheres which include the size, refractive index, and solid content. These parameters are often provided only approximately or can be affected by long shelf times. We propose a simple method to obtain the values of these parameters by measuring the collimated transmission of polystyrene microsphere suspensions from which the wavelength-dependent scattering coefficient can be calculated using the Beer-Lambert law. Since a wavelength-dependent scattering coefficient of a single suspension is insufficient to uniquely derive the size, refractive index and solid content by the Mie theory, the crucial and novel step involves suspending the polystyrene microspheres in aqueous sucrose solutions with different sucrose concentrations that modulates the refractive index of the medium and yields several wavelength-dependent scattering coefficients. With the proposed method, we are able to obtain the refractive index within 0.2% in the wavelength range from 500 to 800 nm, the microsphere size to approximately 15 nm and solid content within 2% of their respective reference values.

Measurement of Wavelength and Temperature-Dependent Optical Properties of Thermochromic Pigments

Thermochromic material is a substance that is capable of changing reversibly the color as the temperature rises. Therefore, the optical spectrum of thermochromic material is responsive to the environmental temperature. In this study, the temperature-dependent optical constants of thermochromic pigments over the wavelength of 350-1800 nm were investigated. Three kinds of thermochromic pigments featured with black, blue, and red colors at room temperature were suspended in water and the light reflection and transmission of the suspensions at different temperatures were measured by a multifunctional spectrophotometer. It was found that below the transition temperature of thermochromic material, the refractive index was 2.1-2.5, 2.2-2.6, and 2.0-2.4 over the wavelength range of 350-1800 nm for black, blue, and red thermochromic pigment, respectively, while above the transition temperature it reached 2.3-2.7, 2.4-2.9, and 2.4-2.7, respectively. It was also observed that the relationship between refractive index of thermochromic pigment and wavelength follows the cubic polynomial function. Furthermore, the extinction coefficient is in the range of 1 × 10^-5-1.2 × 10^-4 for all thermochromic pigments and remains approximately stable at different temperatures. The determination of optical constants of thermochromic pigments provides essential parameters in the modeling of light scattering and absorption by pigment particles to further fine-tune the optical properties of thermochromic coating.

On the Immersion Liquid Evaporation Method Based on the Dynamic Sweep of Magnitude of the Refractive Index of a Binary Liquid Mixture: A Case Study on Determining Mineral Particle Light Dispersion

This is a feasibility study of a modified immersion liquid technique for determining the refractive index of micro-sized particles. The practical challenge of the traditional liquid immersion method is to find or produce a suitable host liquid whose refractive index equals that of a solid particle. Usually, the immersion liquid method uses a set of immersion liquids with different refractive indices or continuously mixes two liquids with different refractive indices, e.g., using a pumping system. Here, the phenomenon of liquid evaporation has been utilized in defining the time-dependent refractive index variation of the host liquid. From the spectral transmittance data measured during the evaporation process, the refractive index of a solid particle in the host liquid can be determined as a function of the wavelength. The method was tested using calcium fluoride (CaF₂) particles with an immersion liquid mixed from diethyl ether and diffusion pump fluid. The dispersion data obtained were consistent with the literature values thus indicating the proper functioning of the proposed procedure.