Feiyan Cai

Acoustic Transmission Enhancement through a Periodically Structured Stiff Plate without Any Opening

We report both experimentally and theoretically that enhanced acoustic transmission can occur in the subwavelength region through a thin but stiff structured plate without any opening. This exotic acoustic phenomenon is essentially distinct from the previous related studies originated from, either collectively or individually, the interaction of the incident wave with openings in previous structures. It is attributed to the structure-induced resonant excitation of the nonleaky Lamb modes that exist intrinsically in the uniform elastic plate. Our finding should have an impact on ultrasonic applications.We report an observation of the extraordinary high reflection of acoustic waves in water by thin epoxy plates partitioned by subwavelength cuts, whereas such plates without structure are acoustically-transparent as the acoustic properties of epoxy are close to water. It is demonstrated that this exotic phenomenon results from the resonant excitation of the local modes within the individual pieces derived by the cuts. The experiment agrees well with the theory.

A Force to Be Reckoned With: A Review of Synthetic Microswimmers Powered by Ultrasound

Synthetic microswimmers are a class of artificial nano- or microscale particle capable of converting external energy into motion. They are similar to natural microswimmers such as bacteria in behavior and are, therefore, of great interest to the study of active matter. Additionally, microswimmers show promise in applications ranging from bioanalytics and environmental monitoring to particle separation and drug delivery. However, since their sizes are on the nano-/microscale and their speeds are in the μm s(-1) range, they fall into a low Reynolds number regime where viscosity dominates. Therefore, new propulsion schemes are needed for these microswimmers to be able to efficiently move. Furthermore, many of the hotly pursued applications call for innovations in the next phase of development of biocompatible microswimmers. In this review, the latest developments of microswimmers powered by ultrasound are presented. Ultrasound, especially at MHz frequencies, does little harm to biological samples and provides an advantageous and well-controlled means to efficiently power microswimmers. By critically reviewing the recent progress in this research field, an introduction of how ultrasound propels colloidal particles into autonomous motion is presented, as well as how this propulsion can be used to achieve preliminary but promising applications.

Transportation of single cell and microbubbles by phase-shift introduced to standing leaky surface acoustic waves

A microfluidic device was developed to precisely transport a single cell or multiple microbubbles by introducing phase-shifts to a standing leaky surface acoustic wave (SLSAW). The device consists of a polydimethyl-siloxane (PDMS) microchannel and two phase-tunable interdigital transducers (IDTs) for the generation of the relative phase for the pair of surface acoustic waves (SAW) propagating along the opposite directions forming a standing wave. When the SAW contacts the fluid medium inside the microchannel, some of SAW energy is coupled to the fluid and the SAW becomes the leaky surface wave. By modulating the relative phase between two IDTs, the positions of pressure nodes of the SLSAW in the microchannel change linearly resulting in the transportation of a single cell or microbubbles. The results also reveal that there is a good linear relationship between the relative phase and the displacement of a single cell or microbubbles. Furthermore, the single cell and the microbubbles can be transported over a predetermined distance continuously until they reach the targeted locations. This technique has its distinct advantages, such as precise position-manipulation, simple to implement, miniature size, and noninvasive character, which may provide an effective method for the position-manipulation of a single cell and microbubbles in many biological and biomedical applications.

Subwavelength imaging of acoustic waves by a canalization mechanism in a two-dimensional phononic crystal

In this letter, the subwavelength imaging of acoustic waves is reported based on a mechanism that the evanescent modes of a source are canalized by the Bloch modes of a two-dimensional phononic crystal that served as the lens. The phononic crystal was designed to have a thickness that meets the condition of Fabry–Perot resonance in order to enhance wave transmission and hence to improve imaging performance. Numerical simulations demonstrated that for a point acoustic source an image as small as 0.16λ can be formed.

Precise and programmable manipulation of microbubbles by two-dimensional standing surface acoustic waves

A microfluidic device is developed to transport microbubbles (MBs) along a desired trajectory in fluid by introducing the phase-shift to a planar standing surface acoustic wave (SSAW). The radiation force of SSAW due to the acoustic pressure gradient modulated by a phase-shift can move MBs to anticipated potential wells in a programmable manner. The resolution of the transportation is approximately 2.2 µm and the estimated radiation force on the MBs is on the order of 10−9 N. This device can be used for manipulation of bioparticles, cell sorting, tissue engineering, and other biomedical applications.

Tunable transmission spectra of acoustic waves through double phononic crystal slabs

Acoustic resonant modes in a slab with a periodic array of holes are analyzed by using three-dimensional finite-difference time-domain method. We show that there exist two different types of resonant modes in this slab: the coupled Stoneley wave resonant modes and the waveguide resonant modes. Both resonant modes can lead to complex resonant line shapes in the transmission spectrum. By using the distinct property of these resonant modes, a tunable phononic crystal structure consisting of coupled phononic crystal slabs is introduced for achieving acoustic filters and sensors.

High refractive-index sonic material based on periodic subwavelength structure

We show that a steel plate with periodic array of subwavelength slits or holes can serve as a material of tunable refractive index for acoustic waves. The effective refractive index is inversely proportional to the filling factor of the slits or holes, which enables the realization of high refractive-index material for acoustic waves. An acoustic wave focusing lens based on this material is exemplified.

Computation of the acoustic radiation force using the finite-difference time-domain method

The computational details related to calculating the acoustic radiation force on an object using a 2-D grid finite-difference time-domain method (FDTD) are presented. The method is based on propagating the stress and velocity fields through the grid and determining the energy flow with and without the object. The axial and radial acoustic radiation forces predicted by FDTD method are in excellent agreement with the results obtained by analytical evaluation of the scattering method. In particular, the results indicate that it is possible to trap the steel cylinder in the radial direction by optimizing the width of Gaussian source and the operation frequency. As the sizes of the relating objects are smaller than or comparable to wavelength, the algorithm presented here can be easily extended to 3-D and include torque computation algorithms, thus providing a highly flexible and universally usable computation engine.

A Disposable Microfluidic Device for Controlled Drug Release from Thermal-Sensitive Liposomes by High Intensity Focused Ultrasound.

The drug release triggered thermally by high intensity focused ultrasound (HIFU) has been considered a promising drug delivery strategy due to its localized energy and non-invasive characters. However, the mechanism underlying the HIFU-mediated drug delivery remains unclear due to its complexity at the cellular level. In this paper, micro-HIFU (MHIFU) generated by a microfluidic device is introduced which is able to control the drug release from temperature-sensitive liposomes (TSL) and evaluate the thermal and mechanical effects of ultrasound on the cellular drug uptake and apoptosis. By simply adjusting the input electrical signal to the device, the temperature of sample can be maintained at 37 °C, 42 °C and 50 °C with the deviation of ± 0.3 °C as desired. The flow cytometry results show that the drug delivery under MHIFU sonication leads to a significant increase in apoptosis compared to the drug release by incubation alone at elevated temperature of 42 °C. Furthermore, increased squamous and protruding structures on the surface membrane of cells were detected by atomic force microscopy (AFM) after MHIFU irradiation of TSL. We demonstrate that compared to the routine HIFU treatment, MHIFU enables monitoring of in situ interactions between the ultrasound and cell in real time. Furthermore, it can quantitatively analyze and characterize the alterations of the cell membrane as a function of the treatment time.

Multi-scale patterning of microparticles using a combination of surface acoustic waves and ultrasonic bulk waves

Standing surface acoustic waves (SAWs) and standing bulk waves (BWs) are combined to pattern two populations of particles with differing sizes. Patterns with large differences in wavelength in each direction and simultaneous generation of different patterns for each population are demonstrated. Particles are trapped at nodal positions of orthogonal standing wave fields in patterns determined by device voltage amplitudes and frequencies. 10-μm beads are trapped at points at the intersection of the pressure nodes of the SAW and BW fields, and 1-μm beads are trapped in lines at the pressure nodes of the SAW field, producing a multi-scale pattern.