Inertial cavitation of lyophilized and rehydrated nanoparticles of poly(L-lactic acid) at 835 kHz and 1.8 MPa ultrasound

Nanoparticles of poly-L-lactic acid dispersed in water and of approximately 120 nm diameter were prepared by a nanoprecipitation method followed by lyophilization together with trehalose. After rehydration, the nanodispersion was exposed to ultrasound at 835 kHz frequency and 1.8 MPa peak negative sound pressure. Substantial levels of broadband noise were surprisingly detected which are attributed to the occurance of inertial cavitation of bubbles present in the dispersion. Inertial cavitation encompasses the formation and growth of gas cavities in the rarefaction pressure cycle which collapse in the compression cycle because of the inwardly-acting inertia of the contracting gas-liquid interface. The intensity of this inertial cavitation over 600 s was similar to that produced by Optison microbubbles used as contrast agents for diagnostic ultrasound. Non-lyophilized nanodispersions produced negligible broadband noise showing that lyophilization and rehydration are requirements for broadband activity of the nanoparticles. Photon correlation spectroscopy indicates that the nanoparticles are not highly aggregated in the nanodispersion and this is supported by scanning (SEM) and transmission (TEM) electron micrographs. TEM visualized non-spherical nanoparticles with a degree of irregular, non-smooth surfaces. Although the presence of small aggregates with inter-particulate gas pockets cannot be ruled out, the inertial cavitation activity can be explained by incomplete wetting of the nanoparticle surface during rehydration of the lyophilizate. Nano-scale gas pockets may be trapped in the surface roughness of the nanoparticles and may be released and coalesce to the size required to nucleate inertial cavitation on insonation at 835 kHz/1.8 MPa

Der Plasma-Lautsprecher als Lehrversuch

In Lehrveranstaltungen lassen sich physikalische Effekte und deren zugrunde liegende Theorie anschaulich anhand von Experimenten verdeutlichen. Besonderes Interesse wird hierbei durch optische, akustische und olfaktorische Sinneswahrnehmungen und vor allem durch die Kombination dieser Reize geweckt. Plasma-Lautsprecher stellen vor diesem Hintergrund ein hervorragendes Anschauungsobjekt für die Physikdidaktik im Fachgebiet „Akustik“ mit hohem motivierendem Potenzial dar.Im Rahmen dieses Betrags wird ein einfaches, verständliches und kostengünstiges Schaltungskonzept für einen Plasma-Lautsprecher vorgestellt und der zugrundeliegende Schallentstehungsmechanismus anschaulich erläutert. Anhand der vorgestellten Experimente können die Lernenden die akustischen Eigenschaften des Lautsprechers qualitativ und quantitativ untersuchen. Abschließend wird ein didaktisches Konzept für den Versuch gegeben. Der didaktische Schwerpunkt des Versuchs kann nach Vorwissen der Lernenden an die Zielgruppe angepasst werden

Finite element based system simulation for piezoelectric vibration energy harvesting devices

We present a system simulation approach for piezoelectric vibration energy harvesting devices. Accurate modeling of the electromechanical structure is achieved by the finite element method. For consideration of power electronic circuits as a means of energy extraction, the finite element model is iteratively coupled to electric circuits via Simulink. The high computational cost of conventional finite element calculations is overcome by a specialized modal truncation method for general linear piezoelectric structures. In doing so, the simulation approach allows efficient prediction of mechanical quantities (e.g. displacements, stresses) as well as electric potentials in the continuum under the influence of arbitrary electrical circuits. Several examples are studied to validate the truncation approach against analytical models and full finite element models. The applicability of the method is demonstrated for a piezoelectric vibration energy harvester in conjunction with a power electronic circuit

Experimental characterisation of a piezoelectrically actuated force amplifier using optical and self-sensing methods

In this work, we characterise a flexural mechanical amplifier, which is used for the realisation of a miniaturised piezoelectric inchworm motor designed for large force (some N) and stroke (tens of mm) operation as it is required e.g., for medical implants. The characterisation is based on high precision optical displacement measurements and a force self-sensing approach. An optically measured displacement of 292 nm in lateral direction and 910 nm in vertical direction of the flexural mechanical amplifier has been obtained, corresponding to a deflection attenuation factor of 3.1. Piezoelectric self-sensing of force was used to determine a force amplification factor of 3.43 from the mechanical oval structure

Theoretical Model and Simulation of a 3D Printed Multi-Material Vibration Harvester

In this contribution, we present a novel 3D printed multi-material, electromagnetic vibration harvester. The harvester is based on a cantilever design and utilizes an embedded constantan wire within a matrix of polyethylene terephthalate glycol (PETG). A prototype has been manufactured with a combination of a fused filament fabrication (FFF) printer and a robot with a custom-made tool

Study on Sensitivity and Accuracy of Piezoelectric Stack Actuators for Charge Self-Sensing

The charge response of a force applied to piezoelectric stack actuators was characterized in the range of 0 N – 20 N for application in piezoelectric self-sensing. Results show linear behavior between ap-plied force and collected charge for both actuators tested in this study. One actuator exhibits a 3.55 times higher sensitivity slope than the other related to its larger capacitance. An error analysis reveals a reduction of relative error in charge measurement with rising forces applied to the actuators