Acoustic black holes offer superior capabilities for slowing down and trapping acoustic waves for various applications such as metastructures, energy harvesting, and vibration and noise control. However, no studies have considered the linear and nonlinear effects of acoustic black holes on micro/nanoparticles in fluids. This study presents acoustofluidic black holes (AFBHs) that leverage controlled interactions between AFBH-trapped acoustic wave energy and particles in droplets to enable versatile particle manipulation functionalities, such as translation, concentration, and patterning of particles. We investigated the AFBH-enabled wave energy trapping and wavelength shrinking effects, as well as the trapped wave energy–induced acoustic radiation forces on particles and acoustic streaming in droplets. This study not only fills the gap between the emerging fields of acoustofluidics and acoustic black holes but also leads to a class of AFBH-based in-droplet particle manipulation toolsets with great potential for many applications, such as biosensing, point-of-care testing, and drug screening.

In this paper, the transient and steady-state temperature field of a traveling-wave rotary ultrasonic motor is analyzed by the finite element method, based on a theoretical model of power loss of this motor in rated operation. Using this model, the temperature field of this motor is calculated and the effects of the heat conductivity of friction material, motor size, ambient temperature, and pressure on the temperature field are estimated. The calculated temperature distribution and transient temperature change agree with the experimental results. The variation of heat conductivity of the friction material has little effect on the minimum temperature in the motor but this variation seriously affects the maximum temperature in the motor when the heat conductivity of the friction material is lower than 0.5 W/(m°C). Two indices are defined to express the non-uniformity of temperature field and how quickly the temperature field reaches its steady state for traveling-wave ultrasonic motors of different sizes. It is found that traveling-wave ultrasonic motors with different sizes have different nonuniformity of temperature field and take different amounts of time to reach thermal steady state. The maximum temperature rise is lower when the ambient temperature is higher; the maximum temperature increases as the vacuum degree increases and it is not affected by the vacuum degree when the vacuum degree is too high (<;10 ^-3 Pa).

Controllable enrichment of micro/nanoscale objects plays a significant role in many biomedical and biochemical applications, such as increasing the detection sensitivity of assays, or improving the structures of bio-engineered tissues.

In this study, a theoretical model of the steady-state temperature field in a bar-shaped piezoelectric transformer operating in longitudinal vibration mode is developed, and the characteristics of the temperature field are analyzed numerically. Being different from previous work, the effects of the stress distribution and the relative positions of input and output parts on the internal loss distribution are taken into account in this model. Using this model, the temperature rise and its distribution are calculated, and the effects of ambient temperature, internal loss distribution, size, heat dissipation coefficient, and heat conductivity of the transformer on the temperature rise are estimated. The calculated temperature rise and its distribution agree quite well with the experimental data. It is found that the allowable output power of the transformer should be reduced markedly when the ambient temperature increases to maintain a reasonable working temperature for the piezoelectric material of the transformer. It also is found that decreasing wavelength, making internal loss distribution uniform, and increasing the ratio of perimeter to area of the cross section (for example, decreasing the cross-sectional area and using a thin plate structure), decrease the temperature rise for a given internal loss per unit volume. Furthermore, it is revealed that the heat dissipation coefficient has a remarkable effect on the temperature rise at a large internal loss, and the variation in the heat conductivity of the piezoelectric material has little effect on the temperature rise.