For decades, scientists have pursued the goal of performing automated reactions in a compact fluid processor with minimal human intervention. Most advanced fluidic handling technologies (e.g., microfluidic chips and micro-well plates) lack fluid rewritability, and the associated benefits of multi-path routing and re-programmability, due to surface-adsorption-induced contamination on contacting structures. This limits their processing speed and the complexity of reaction test matrices. We present a contactless droplet transport and processing technique called digital acoustofluidics which dynamically manipulates droplets with volumes from 1 nL to 100 µL along any planar axis via acoustic-streaming-induced hydrodynamic traps, all in a contamination-free (lower than 10−10% diffusion into the fluorinated carrier oil layer) and biocompatible (99.2% cell viability) manner. Hence, digital acoustofluidics can execute reactions on overlapping, non-contaminated, fluidic paths and can scale to perform massive interaction matrices within a single device.

Acoustic tweezers have recently raised great interest across many fields including biology, chemistry, engineering, and medicine, as they can perform contactless, label-free, biocompatible, and precise manipulation of particles and cells. Here, we present wave number–spiral acoustic tweezers, which are capable of dynamically reshaping surface acoustic wave (SAW) wavefields to various pressure distributions to facilitate dynamic and programmable particle/cell manipulation. SAWs propagating in multiple directions can be simultaneously and independently controlled by simply modulating the multitone excitation signals. This allows for dynamic reshaping of SAW wavefields to desired distributions, thus achieving programmable particle/cell manipulation. We experimentally demonstrated the multiple functions of wave number–spiral acoustic tweezers, among which are multiconfiguration patterning; parallel merging; pattern translation, transformation, and rotation; and dynamic translation of single microparticles along complex paths. This wave number–spiral design has the potential to revolutionize future acoustic tweezers development and advance many applications, including microscale assembly, bioprinting, and cell-cell interaction research.

Metasurfaces open up unprecedented potential for wave engineering using subwavelength sheets. However, a severe limitation of current acoustic metasurfaces is their poor reconfigurability to achieve distinct functions on demand. Here a programmable acoustic metasurface that contains an array of tunable subwavelength unit cells to break the limitation and realize versatile two-dimensional wave manipulation functions is reported. Each unit cell of the metasurface is composed of a straight channel and five shunted Helmholtz resonators, whose effective mass can be tuned by a robust fluidic system. The phase and amplitude of acoustic waves transmitting through each unit cell can be modulated dynamically and continuously. Based on such mechanism, the metasurface is able to achieve versatile wave manipulation functions, by engineering the phase and amplitude of transmission waves in the subwavelength scale. Through acoustic field scanning experiments, multiple wave manipulation functions, including steering acoustic waves, engineering acoustic beams, and switching on/off acoustic energy flow by using one design of metasurface are visually demonstrated. This work extends the metasurface research and holds great potential for a wide range of applications including acoustic imaging, communication, levitation, and tweezers.

Advances in gene editing are leading to new medical interventions where patients’ own cells are used for stem cell therapies and immunotherapies. One of the key limitations to translating these treatments to the clinic is the need for scalable technologies for engineering cells efficiently and safely. Toward this goal, microfluidic strategies to induce membrane pores and permeability have emerged as promising techniques to deliver biomolecular cargo into cells. As these technologies continue to mature, there is a need to achieve efficient, safe, nontoxic, fast, and economical processing of clinically relevant cell types. We demonstrate an acoustofluidic sonoporation method to deliver plasmids to immortalized and primary human cell types, based on pore formation and permeabilization of cell membranes with acoustic waves. This acoustofluidic-mediated approach achieves fast and efficient intracellular delivery of an enhanced green fluorescent protein-expressing plasmid to cells at a scalable throughput of 200,000 cells/min in a single channel. Analyses of intracellular delivery and nuclear membrane rupture revealed mechanisms underlying acoustofluidic delivery and successful gene expression. Our studies show that acoustofluidic technologies are promising platforms for gene delivery and a useful tool for investigating membrane repair.

The valley degree of freedom in crystals offers great potential for manipulating classical waves, however, few studies have investigated valley states with complex wavenumbers, valley states in graded systems, or dispersion tuning for valley states. Here, we present tunable valley phononic crystals (PCs) composed of hybrid channel-cavity cells with three tunable parameters. Our PCs support valley states and Dirac cones with complex wavenumbers. They can be configured to form chirped valley PCs in which edge modes are slowed to zero group velocity states, where the energy at different frequencies accumulates at different designated locations. They enable multiple functionalities, including tuning of dispersion relations for valley states, robust routing of surface acoustic waves, and spatial modulation of group velocities. This work may spark future investigations of topological states with complex wavenumbers in other classical systems, further study of topological states in graded materials, and the development of acoustic devices.

Lamb waves have shown great potentials in damage detection of thin-walled structures due to their long propagation capability and sensitivity to a variety of damage types. However, their practical adoption has been hindered due to the complexity caused by their multimodal nature. Various wave modes that propagate at various velocities in the structure make the interpretation of Lamb wave signals very difficult. It is desired that the modes can be separated for independent analysis or further employment. In this article, we present our studies on the multimodal Lamb wave propagation and wave mode decomposition using frequency–wavenumber analysis. Wave representation in the frequency–wavenumber domain is obtained using multidimensional Fourier transform, where various Lamb wave modes can be easily discerned. This allows for separating them or extracting a desired wave mode through a filtering process, thus making it possible to use a single-mode Lamb wave for the detection of a certain type of damage in structural health monitoring applications. To retain the temporal and spatial information that is lost during Fourier transformation, a novel wavenumber analysis is also presented. These concepts are illustrated through experimental testing where high spatial resolution wavefields are measured by a scanning laser Doppler vibrometer.

Laminated composites are susceptible to delamination due to their weak transverse tensile and interlaminar shear strengths as compared to their in-plane properties. Delamination damage can occur internally, where it is not visible to the naked eye. Development of reliable, quantitative techniques for detecting delamination damage in laminated composite components will be imperative for safe and functional optimally designed next-generation composite structures. In this article, we study the potential of using Lamb waves for delamination detection and quantification, using model-assisted data acquisition. Novel wavenumber analysis approaches are developed and discussed to show how they can be used to investigate Lamb wave interactions with delaminated plies. Ultrasonic wave simulations are implemented to provide both in-plane and out-of-plane wave motion for the wavenumber studies. The out-of-plane results are verified against data obtained from experimental tests. It is found that the wavenumber methods can not only determine the delaminated region of the plate and its length, but can also identify the plies between which the delamination occurs. We envision that the wavenumber approaches can lead to a complete delamination quantification in the future.

Lamb waves are dispersive and multi-modal, which makes the interpretation of Lamb wave signals very difficult in either the time or frequency domain. In the this article, we present our studies on Lamb wave propagation characterization and crack detection using a hybrid lead zirconate titanate (PZT)-laser vibrometer system and frequency-wave number analysis. A scanning laser Doppler vibrometer is used to acquiring high-resolution time-space Lamb wavefield excited by a PZT actuator. The recorded wavefield is then transformed to frequency-wave number domain by two-dimensional Fourier transform. Wave spectrum in the frequency-wave number domain shows clear distinction among Lamb wave modes being present. These concepts are illustrated through several experimental tests. However, the space information is lost during this transformation. A short-space two-dimensional Fourier transform is then adopted to obtain the frequency-wave number spectra at various spatial locations, resulting in the space-frequency-wave number representation, which can show how the frequency-wave number component varies with respect to space dimension. It provides a means to study the wave propagation from the perspective of wave number domain. The space-frequency-wave number analysis has successfully been used for the study of wave interaction with structural discontinuity and crack detection on an aluminum plate.