Asit K. Ray

Precision of light scattering techniques for measuring optical parameters of microspheres

This experimental and theoretical study of light scattering techniques addresses the questions of deconvolution of light scattering data (phase functions and optical resonance spectra) from droplets and the uniqueness of that deconvolution for the measurement of size and refractive index of microspheres. Theory and experiment are compared for single component and multicomponent microdroplets and for layered microspheres levitated in electrodynamic balances. Size changes of the order of 1 A have been measured by tracking resonance shifts of slowly evaporating single component droplets. For multicomponent droplet evaporation, which involves simultaneous size and refractive index changes, and for rapid evaporation, precision is reduced to approximately 1 part in 10(4) for each parameter.

Determination of size, refractive index, and dispersion of single droplets from wavelength-dependent scattering spectra

Intensities of light scattered in the planes parallel and perpendicular to the polarization plane of the incident light are used to determine the size, refractive index, and dispersion of a single droplet suspended in an electrodynamic balance. Wavelengths of TE- and TM-mode resonances are determined independently with high precision when a ring dye laser is scanned. Resonating wavelengths are matched with theoretical intensity peaks to determine the constants of a dispersion formula and the size that minimizes the difference between observed and calculated wavelengths. The procedure permits the determination of the size and refractive index with relative errors of 3 × 10(-5) and dispersion with an absolute error 2 × 10(-5) over the experimental spectral range.

Simultaneous determination of size and wavelength-dependent refractive indices of thin-layered droplets from optical resonances

A technique for determining the size and wavelength-dependent refractive indices of a droplet coated with a thin layer is presented. The existence of a layer on the droplet is identified by a procedure that involves separate alignments of independently measured TE- and TM-mode resonances with computed homogeneous-sphere resonances. The procedure also yields the mode and the order numbers associated with the measured resonances. The observed resonances are then aligned with layered-sphere resonances of the same mode and order numbers to determine the core radius, layer thickness, and constants of core and shell dispersion formulas that minimize the difference between the observed and the calculated positions of resonances. The technique has been tested with synthetic data with various levels of random errors as well as with experimental data from two droplets under identical conditions. The results show that the core radius, layer thickness, and core and layer refractive indices can be determined with relative errors of 3.5 × 10(-4), 4.5 × 10(-2), 2.3 × 10(-4), and 4.4 × 10(-3), respectively, with the technique.

Effect of inter-particle interactions on evaporation of droplets in a linear array

Effects of interparticle interactions on evaporation have been examined on droplets in a linear array generated by a modified vibrating orifice aerosol generator (VOAG). The size as well as the spacing between the droplets was varied by modulating the frequency of the generator, and the liquid flow rate through the orifice. Ethanol and methanol droplets of differing initial sizes and spacings were studied, and the instantaneous evaporation rates as well as droplet temperatures were measured using a resonance-based light scattering technique. The results show that at a fixed dimensionless spacing between the droplets (i.e., the ratio of the spacing to the droplet radius, l/a), the size and temperature versus time data are highly reproducible. The measured evaporation rates were normalized by the evaporation rates of an isolated droplet under identical conditions to obtain the interaction parameter (η) as a function of the dimensionless spacing between the droplets. The value of η was observed to decrease with a decrease in l/a as predicted by the theoretical models, and as expected, found to be the same for ethanol and methanol droplets.

Measurement of Activity Coefficients from Unsteady State Evaporation and Growth of Microdroplets

Abstract Techniques are presented for determination of activity coefficients of binary systems from unsteady state evaporation and growth of single microdroplets in controlled environments. A high-precision light scattering method based on resonances observed in light scattering by microdroplets was used to determine the size and composition of a microdroplet as functions of time. The techniques were validated through data on growth of glycerol microdroplets in slowly developing water vapor concentration fields and evaporation of microdroplets, containing volatile dimethylphthalate (DMP) and nonvolatile dioctylphthalate (DOP), in vapor-free atmospheres. When the water vapor concentration in the surrounding gas changes slowly a glycerol droplet maintains a dynamic equilibrium with the water vapor; thus the activity coefficient of water was determined from knowledge of the droplet composition and the water vapor saturation ratio in the gas phase. The activity coefficients of DMP were determined on the basis that the instantaneous evaporation rate of a DMP-DOP microdroplet in a vapor-free atmosphere is equal to the product of the activity of DMP and the evaporation rate of a pure DMP droplet. The activity coefficient values obtained from microdroplet experiments are highly reproducible and agree with data available in the literature.

Vibrating Orifice Droplet Generator for Studying Fast Processes Associated with Microdroplets

ABSTRACT A light scattering technique, based on resonances, has been applied to a linear stream of highly monodisperse droplets generated by a modified vibrating orifice aerosol generator to study evaporation of ethanol droplets. The residence time of droplets was varied by altering the distance between the orifice and a laser beam. Frequencies at which intensity peaks (i.e., resonances) appear in elastic and Raman scattered light were determined by varying the frequency of droplet generation. The absolute droplet size and refractive index were determined by matching the observed resonances with theoretical resonances that minimize the difference between the observed and calculated resonance frequencies. The size and refractive index changes between two successive residence time observations were determined from the frequency shifts of the resonances. These procedures permit determination of size and refractive index with a precision of 2 parts in 104, and their changes within 1 part in 104 of the absolut...

Measurements of collection efficiency of single, charged droplets suspended in a stream of submicron particles with an electrodynamic balance

Abstract An experimental system based on an electrodynamic balance has been developed and tested for measurements of collection efficiencies of single, stationary charged droplets in a stream of oppositely charged submicron droplets. A light scattering technique based on optical resonances has been used to obtain collector droplet size and size changes due to the deposition of droplets with high precision. The technique involves the application of Lorenz-Mie theory to high resolution experimental elastic scattering data obtained with a continuously tunable ring dye laser. The effects of collector size in the range 9–37 μm, aerosol particle size in the range 0.038–0.15 μm, collector charge varying from 3.2 × 10−14 to 3.3 × 10−13 C, and stream velocity ranging from 5.0 × 10−3 to 6 × 10−2 ms−1, on the collection efficiency have been studied. Results have been compared with a theoretical model for the collection efficiency in the presence of electrostatic forces. Experimental values of collection efficiency are found to be in good agreement with those predicted by the theory.

Analysis of time-dependent scattering spectra for studying processes associated with microdroplets.

Techniques are presented for analysis of time-dependent scattering spectra from single droplets undergoing physical changes. Times of appearance of resonances in experimental spectra are aligned with theoretical resonances, and the size and refractive index of a droplet as functions of time are determined from the minimum errors in alignment between observed and theoretical resonances. The techniques have been applied to time-dependent elastic scattering spectra obtained from single droplets evaporating under quasi-steady conditions and during unsteady growth. The results of quasi-steady evaporation data show that size and refractive index can be determined with relative errors of 1 x 10(-4). The quasi-steady evaporation data of a droplet are used to identify the resonances observed during the unsteady growth of the same droplet, and the size and refractive index at each resonance are calculated from the identity of the resonance.

Effect of optical resonances on photochemical reactions in microdroplets

We have examined photochemical reactions in microdroplets under optical resonances for situations in which photoreactive molecules dissolve from a surrounding gas phase. The link among photochemical reaction, droplet-phase diffusion, and gas-phase mass transfer rate processes produces numerous concentration distributions of reactive molecules between two limiting distributions that are characterized by the absence of reactive molecules and by the saturation concentration level. Each distribution yields a unique intensity field. The analysis shows that the reaction rate is dependent on five dimensionless parameters and is significantly enhanced by a number of combinations of parameter values.

Evaporation and growth dynamics of a layered droplet

Abstract A mathematical model has been formulated for the evaporation and growth of a two-phase isolated droplet of two partially miscible components, exposed to a stagnant gas phase. Unsteady-state transport equations of the two components in the core, shell and gas phases have been rigorously treated. The resulting mathematical model involving two moving boundaries at the core-shell interface and the droplet-gas interface has been solved numerically for various conditions. Effects of critical parameters on the droplet dynamics have been examined. In a vapor free atmosphere where both components evaporate, the results show that the core either grows or evaporates depending on the physical parameters. When the core evaporates, either the shell or core disappears first, leaving a single-phase droplet. The study shows that the volatility of the components, thermodynamic and transport parameters greatly influence the evaporation behavior of a layered droplet.