Controlled growth of Helium nanodroplets from a pulsed source

Factors affecting the size of liquid-heliumdroplets produced by a pulsed nozzle are described. The shape of the nozzle orifice is found to be important in allowing control of the size of the droplets. With an appropriate choice of nozzle geometry, the average droplet size is shown to be continuously variable over nearly two orders of magnitude by adjustment of the helium gas stagnation pressure and/or temperature. A scaling law similar to, but not identical with, that found for heliumdroplets produced by continuous supersonic expansion sources is found for the pulsed source. The pulsed nozzle described in this article has been used to make heliumdroplets ranging in size from a few thousand atoms up to nearly 10^5 helium atoms

(K,Na)NbO₃ Nanofiber-based Self-Powered Sensors for Accurate Detection of Dynamic Strain

A self-powered active strain sensor based on well-aligned (K,Na)­NbO₃ piezoelectric nanofibers is successfully fabricated through the electrospinning and polymer packaging process. The device exhibits a fast, active response to dynamic strain by generating impulsive voltage signal that is dependent on the amplitude of the dynamic strains and the vibration frequency. When the frequency is fixed at 1 Hz, the peak to peak value of the voltage increases from ∼1 to ∼40 mV, and the strain changes from 1 to 6%. Furthermore, the output voltage is linearly increased by an order of magnitude with the frequency changing from 0.2 to 5 Hz under the same strain amplitude. The influence of frequency on the output voltage can be further enhanced at higher strain amplitude. This phenomenon is attributed to the increased generating rate of piezoelectric charges under higher strain rate of the nanofibers. By counting the pulse separation of the voltage peaks, the vibration frequency is synchronously measured during the sensing process. The accuracy of the sensing results can be improved by calibration according to the frequency-dependent sensing behavior