P. K. Srirama

Particle Size and Charge Distribution Analysis of Pharmaceutical Aerosols Generated by Inhalers

Aerosol particles generated by inhalers for respiratory drug delivery acquire electrostatic charge during the dispersion process. The electrostatic charge distribution of the particles can affect the efficiency of drug delivery by influencing both the transport and deposition of inhaled particles in the human lung. To analyze the electrostatic charge acquired by the aerosol particles, two sets of metered-dose inhaler (MDI) and dry powder inhaler (DPI) devices were investigated. Both the particle size and charge distributions were measured simultaneously by using an electrical single-particle aerodynamic relaxation time (E-SPART) analyzer. The analyzer was calibrated with particles of known size, which were generated by a vibrating orifice aerosol generator (TSI Inc.) and charge using the Faraday cup method. The charge distributions of the pharmaceutical aerosols from both the DPI and MDI devices were bipolar in nature. Although the net charge-to-mass ratio was less than 0.2 μC/g, the individual particles were charged with a relatively high charge: −2 to + 2 μC/g. The count mean aerodynamic diameter of the aerosols generated from these devices was 3–5 μm.

Tribocharging Characteristics of the Mars Dust Simulant (JSC Mars-1)

The determination of triboelectric properties of the Martian dust would be critical for future human and robotic missions to Mars. Several major studies relating to future missions to Mars have highlighted the significance of electrostatic charge accumulation and its impact on various aspects of the mission due to tribocharging of the Mars dust. The tribocharging properties of the Mars dust simulant (JSC Mars-1) against stainless steel and Teflon were characterized. An electronic single particle aerodynamic relaxation time (ESPART) analyzer was used to measure the aerodynamic size and charge distributions of the dust particles. Faraday cup measurements were used for calculating average charge-to-mass ratio. The average charge-to-mass ratio of dust tribocharged against stainless steel (SS) was -1.19 muC/g, whereas it was found to be +1.21 muC/g against Teflon (PTFE). These results indicated that the work function of the JSC Mars-1 lies between that of the SS and Teflon.

Portable Free-Fall Electrostatic Separator for Beneficiation of Charged Particulate Materials

A portable free-fall electrostatic separator capable of analyzing gram quantities of charged powders is presented. Unlike a Faraday pail, in which only the net average charge-to-mass (Q/M) ratio of the particles sampled by the instrument is measured, an electrostatic separator is capable of separately measuring the charge-to-mass ratios of positively and negatively charged sampled powders. Thus, with an electrostatic separator it is possible to measure the mass fractions of powders that are charged with different polarities and the respective charge-to-mass ratios, along with the mass fraction of particles that are uncharged or charged below a threshold level. We describe a method of measuring the total charge of the collected particles in real time by incorporating an electrometer to integrate the current flowing through the collecting electrode to the high voltage power supply. In this manner, both the total charge and total mass of powder deposited on the two electrodes are measured in near real time, providing information on charge-to-mass ratio of the aerosol cloud sampled. Such real time measurements are often needed to analyze the electrostatic charging properties of small quantities of dispersed powder, particularly in such applications where the charge characteristics are of high importance.

E-SPART Analyzer for Mars Mission: A New Approach in Signal Processing and Sampling

This paper introduces a novel E-SPART analyzer for the dust particle analysis for Mars mission. The new E-SPART analyzer is based on a stationary-fringe LDV which can simplify the system hardware composition and reduce both the physical size and weight of the system. The paper presents the new operational principle, the signal processing method and the algorithm. It also discusses the requirement of the size of sensing volume. Experimental results are given for the verification of the principle and algorithms.

Non-contact Measurements of Size and Charge Distributions of Submicron Particles using an ESPART Analyzer

Characterization of size and electrostatic charge distributions of submicron particles with an instrument capable of in-situ and non-contact measurements is often necessary for many industrial and medical applications. These applications are related to thin film coatings, nanostructural assembly particles for energy conversion research and processing pharmaceutical fine powders, for example. The ESPART analyzer, which is used widely in the electro-photography industries for quality control of toner particles and, recently, in the lung deposition studies of inhaled particles for respiratory drug delivery, operates well in the size range of 1.0 to 20 mum in aerodynamic diameter and for an electrostatic charge range from zero to the saturation charge limit of individual particles with either positive or negative polarity. For measurement of submicron particles, however, several design changes in the instrument are necessary since both optical scattering and electrodynamic properties are significantly different with respect to those of larger particles. Advancement of the ESPART Analyzer with (1) a new optical geometry of Laser Doppler Velocimeter (LDV) for an efficient collection of scattered light from submicron particles, (2) a high frequency simultaneous excitation of the particles, as they cross the sensing volume of the LDV, with acoustic and electric drives and (3) an electronic signal processing system capable of analyzing LDV signals for determining relative phase shift and amplitude of the individual particle motion with respect to the excitation drives is reported. A description of the experimental design is presented. The instrumentation development is supported by a grant from the National Science Foundation.

Electrohydrodynamic Dispersion of Dry Fine Powders

Dispersion of fine dry powders in the size range 1.0 to 10.0 mum in aerodynamic diameters requires a high energy dispersive force since the powders in this size range are very cohesive and their dispersion by vibrational, centrifugal or by aerodynamic force (using air currents) is inefficient in most cases. The adhesive force, that includes van der Waals, electrostatic, and liquid bridge forces, is, in general, proportional to the diameter d of the particles, whereas, the vibrational and centrifugal detachment forces are proportional to d3, and the aerodynamic shear force for detachment is proportional to d2. Thus smaller the diameter d the higher is the dispersion force needed for efficient dispersion for forming aerosol from a sample of powder that will contain the primary particles present in the powder. We report here dispersion of fine powders deposited on a dielectric substrate by applying a traveling electric field using embedded electrodes. The dispersed particles are aerosolized by passing a gentle air current over the substrate. The dispersion of the powder forming an aerosol particle is achieved here by both electrodynamic force and air current. A brief discussion of van der Waals, electrostatic, and capillary forces between individual particles within a bulk powder and the dispersive forces applied by a using a low frequency three-phase electric field are reported. The electrostatic force of dispersion is proportional to qE, where q is proportional to d2 and E is the applied electric field. The charged particle cloud produced by the combination of electrohydrodynamic and aerodynamic shear forces were characterized with respect their size and electrostatic charge distributions by using an ESPART analyzer. The dispersive properties of the electrodynamic forces are analyzed for powders of different size distributions and conductivities. The traveling wave is produced by placing a series of electrodes, embedded, just beneath the surface of the substrate. The required power consumption needed for dispersion is less than 5 mW and requires only a few seconds for dispersion. The possible applications of breath actuated air flow in conjunction with electro-hydrodynamic dispersion for respiratory drug delivery are discussed.

In-Situ Measurements of Particle Size and Charge Distributions for Mars and Lunar Missions

E-SPART analyzer is an instrument capable of measuring aerodynamic diameter and electrostatic charge distributions on a single particle basis in real time. A frequency biased laser Doppler velocimeter (LDV) is used to make non- contact in-situ analysis of the particles under ambient conditions, with respect to the temperature, pressure, and composition of the gaseous medium in which the particles are suspended. We report here new developments related to LDV optics, signal processor, and miniaturization of the E-SPART analyzer so that it can be applied on the surface of Mars and the Moon for characterization of dust particles. The conventional LDV was replaced with a diode laser based MiniLDV and we eliminated the frequency biasing device that requires either use of Bragg Cells or rotating diffraction grating with a new LDV beam geometry. The new optical system, a Stationary Fringe LDV (SFLDV), in which the fringe pattern in the sensing volume is stationary, is able to resolve directional ambiguity of an oscillating particle motion without a frequency bias between the two laser beams. Thus the complexity of the frequency biasing system is absent allowing both stability and robustness of the miniature E-SPART analyzer. For comparison between the conventional E-SPART analyzer and the new miniaturized instrument, experimental results on size and charge distributions for different test particle clouds are presented. The advantages of the new instrument for in-situ particle characterization for different industrial, pharmaceutical, and space explorations applications are discussed.