Jyrki Stor-Pellinen

Photoacoustic evaluation of elasticity and integrity of pharmaceutical tablets

A nondestructive method based on pulse photoacoustics was applied for evaluation of elasticity and integrity of pharmaceutical tablets. Variations in porosity, density and sodium chloride content of microcrystalline cellulose tablets were found to be related to parameters extracted from the through-transmitted ultrasonic wave forms. By using the amplitudes and ultrasonic velocities of these wave forms, it was possible to obtain values of a transverse to longitudinal amplitude ratio, and also elastic parameters, such as Youngs and shear moduli, for the tablets. Poissons ratio was calculated from the elastic moduli as well as from the amplitudes. An exponential relationship between tablet porosity and the attenuation of longitudinal wave form was noticed. The transverse to longitudinal amplitude ratio and the amplitudinal Poissons ratio were indicative of structural variations, e.g., changes in the porosity and the sodium chloride content of tablets. Youngs and shear moduli of microcrystalline cellulose tablets were found to follow similar porosity trends to those in previously published beam bending and twisting studies, although the absolute values and the values extrapolated to zero porosity were slightly smaller. The Poissons ratio calculated from the experimental Youngs and shear modulus values was also in agreement with earlier studies, but the values extrapolated to zero porosity differed significantly. The method is a promising tool for evaluating the elastic properties of tableting materials and the structural variations in tablets.

Factors affecting the sensitivity of electrostatic ultrasonic transducers

The sensitivity of an electrostatic transducer determines the range of pulse-echo detection. Maximum sensitivity can be achieved by increasing the bias voltage within certain limits and by using a light membrane. The tension of the membrane was not found to affect the transducers sensitivity. By using a uniformly grooved backplate the acoustic properties of the transducer can be optimized. The different sensitivity factors of the electrostatic ultrasonic transducer were confirmed with experimental results. High sensitivity can be achieved by using well manufactured backplates with shallow grooves and a small ridge angle.

A model for an electrostatic ultrasonic transducer with a grooved backplate

A model has been constructed for an electrostatic ultrasonic transducer with a uniformly grooved backplate. The membrane and the groove pattern of the transducer was divided into individual elements, and each element was treated as a Helmholtz resonator. The resonant frequency of the transducer can be calculated using one single resonator element. The calculated resonant frequencies were compared with experimental results.

Paper roughness measurement using airborne ultrasound

Abstract Paper roughness has been studied using high-frequency airborne ultrasound. The measurement principle is based on the attenuation that occurs when ultrasound is reflected from a rough surface. Paper samples have been tested and results compared with data from conventional air-leak measurements. The proposed method is non-contacting and it allows the possibility of developing fast and rugged applications for the laboratory or for on-location product testing.

Measuring paper wetting processes with laser transmission

We introduce a method to measure paper wetting based on the measurement of monochromatic (670 nm) light transmission through paper. The samples were 49.8–51.7 g m−2 handsheets, the hydrophobicity of which was tuned by the fibre content of the furnish. In addition, one of the samples was internally sized. Impact wetting with isopropanol and water mist pulses was applied. The duration of a pulse was 100 ms, and the amount of liquid per pulse was 12.6 ± 1.2 g m−2. Optical changes in a paper sample were detected by measuring the average intensity of transmitted light through a selected area. The results showed that different wetting processes (pure capillary and combined capillary and diffusion wetting) could be distinguished from each other. With the presented method, differences in the individual behaviour of the samples during wetting could also be seen.

Measurement of paper-wetting processes by ultrasound transmission

Understanding of wetting processes for papers of various qualities is important for the analysis and the adjustment of paper-production processes as well as for designing properties of paper for printing. In this investigation, papers of one kind with three different sizing-content values were examined by time-dependent attenuation of ultrasound transmission in mixtures of isopropanol and water. The attenuation depends on the stage of absorption of liquid into the paper sample, which furnishes the signature of the paper type and quality. Papers that have contact angles of 40° (without size), 70° (low size content) and 110° (high size content) were tested and a measurement procedure was developed. The measurement results showed that the development of attenuation with time differs at different frequencies, indicating the possibility of detecting various phases of wetting phenomena.

Air-coupled ultrasonic measurement of the change in roughness of paper during wetting

We investigated the change in surface roughness of paper during wetting. Four kinds of paper were examined by time-dependent attenuation of focused 1.2 MHz airborne ultrasound reflected from a paper sample. The samples were wetted with fog from mixtures of water and isopropanol and the transmitted ultrasonic signal was measured at 15 ms intervals for 16 s after the application of the mixture. The results showed that the attenuation of the reflected sound by time differs for various kinds of paper, indicating that there are differences in the development of wetting.

Analog pulse compression system for real-time ultrasonic non-destructive testing

An analog pulse compression system based on the use of a magnetoacoustic storage correlator as a pulse compressor is proposed, and a real-time operation of the system is demonstrated. The system offers good flexibility; it is not limited to any particular type of large time-bandwidth signal.

A Helmholtz resonator model for an electrostatic ultrasonic air transducer with a V-grooved backplate

Abstract A model for the resonance frequency of an electrostatic ultrasonic air transducer with a uniformly grooved backplate is described based on the Helmholtz resonator theory. The resonance frequency of the transducer can be determined from the shape and dimensions of the backplates grooves and the membranes characteristics. In the case of a V-grooved backplate, the depth of the groove and the mass per unit area of the membrane are the parameters that determinate the resonance frequency. Experimental results suggest that the tension of the membrane seems to have only a very weak influence on the resonance frequency.

Measurement of the effect of high-power ultrasound on wetting of paper

High-power ultrasound has been known to promote penetration of liquids into porous materials. This work presents results of experimental investigations of the influence of 40 kHz high-power ultrasound on wetting processes of papers. Wetting was monitored by the measurement of the attenuation of 0.5-9 MHz ultrasound transmitted through the immersed sample. The samples were sized papers with contact angles 40 degrees, 70 degrees and 110 degrees, and the immersion liquids were isopropanol-water mixtures of 0-100% isopropanol concentration. The investigation showed that application of high-power ultrasound resulted in faster sorption processes. However, the absolute reduction in time to reach the stationary state was not very great.