Phase separation of a nonionic surfactant aqueous solution in a standing surface acoustic wave for submicron particle manipulation

Engineering the acoustic field with a Mie scatterer for microparticle patterning

The utilization of acoustic fields offers a contactless approach for microparticle manipulation in a miniaturized system, and plays a significant role in medicine, biology, chemistry, and engineering. Due to the acoustic radiation force arising from the scattering of the acoustic waves, small particles in the Rayleigh scattering range can be trapped, whilst their impact on the acoustic field is negligible. Manipulating larger particles in the Mie scattering regime is challenging due to the diverse scattering modes, which impacts the local acoustic field. The rapid movement of free-moving Mie scatterers in an acoustic standing wave field makes it difficult to study the interaction between a sound field and a Mie scatterer in an engineering context. Here, a combined approach that integrates theoretical analysis and experimental investigation was developed to explore the influence of a Mie scatterer on the acoustic field by fabricating an acoustic trapping device featuring a fixed Mie scatterer at its center. We demonstrate that an insonified Mie scatterer can operate as an acoustic emitter in water, enabling dynamic and versatile modulation of the total acoustic field. Such a scatterer can interact with one or multiple incident propagating acoustic waves, leading to the generation of a localized standing wave field in the vicinity of the scatterer. This local field can be controlled by the relative location of the scatterer with respect to the incident field leading to control over the transformation from an incident 1D acoustic field into a 2D acoustic field. This control paves the way for localized and multi-scale micro-object manipulation.

Acoustofluidic manipulation for submicron to nanoparticles

Particles, ranging from submicron to nanometer scale, can be broadly categorized into biological and non-biological types. Submicron-to-nanoscale bioparticles include various bacteria, viruses, liposomes, and exosomes. Non-biological particles cover various inorganic, metallic, and carbon-based particles. The effective manipulation of these submicron to nanoparticles, including their separation, sorting, enrichment, assembly, trapping, and transport, is a fundamental requirement for different applications. Acoustofluidics, owing to their distinct advantages, have emerged as a potent tool for nanoparticle manipulation over the past decade. Although recent literature reviews have encapsulated the evolution of acoustofluidic technology, there is a paucity of reports specifically addressing the acoustical manipulation of submicron to nanoparticles. This article endeavors to provide a comprehensive study of this topic, delving into the principles, apparatus, and merits of acoustofluidic manipulation of submicron to nanoparticles, and discussing the state-of-the-art developments in this technology. The discourse commences with an introduction to the fundamental theory of acoustofluidic control and the forces involved in nanoparticle manipulation. Subsequently, the working mechanism of acoustofluidic manipulation of submicron to nanoparticles is dissected into two parts, dominated by the acoustic wave field and the acoustic streaming field. A critical analysis of the advantages and limitations of different acoustofluidic platforms in nanoparticles control is presented. The article concludes with a summary of the challenges acoustofluidics face in the realm of nanoparticle manipulation and analysis, and a forecast of future development prospects.

Formation of Alternating Surfactant-Enriched and Surfactant-Depleted Phases in the Taylor Cone of a Nanoelectrospray

The electrospray ionization of highly conductive solutions containing Triton X-100, a nonionic surfactant, is found to induce alternating periods of surfactant enrichment and depletion when the concentration of the surfactant is near the critical micelle concentration (CMC) and when the flow rate is on the order of 10 nL/min. Analyzing the surfactant-protein mixture shows that the protein is partially denatured during the surfactant enrichment. The measurement of the phospholipid and oligosaccharide mixture prepared in the surfactant solution shows that the ion signal of the lipid is in phase with, and the hydrophilic oligosaccharide is out of phase with the surfactant signal. The results suggest that this novel phenomenon can be exploited for in situ separation of compounds in ESI-MS. Besides the ion signal, the condition of the alternating phase is also reflected in the spray current and Taylor cone's apex angle. The phase separation is likely related to the formation of a micelle in the Taylor cone and can be selectively triggered by tuning the flow rate with emitter voltage for an on-demand application.

An enhanced tilted-angle acoustic tweezer for mechanical phenotyping of cancer cells

Acoustofluidic devices becomes one of the emerging and versatile tools for many biomedical applications. Most of the previous acoustofluidic devices are used for cells manipulation, and the few devices for cell phenotyping with a limitation in throughput. In this study, an enhanced tilted-angle (ETA) acoustofluidic device is developed and applied for mechanophenotyping of live cells. The ETA Device consists of an interdigital transducer which is positioned along a microfluidic channel. An inclination angle of 5° is introduced between the interdigital transducer and the liquid flow direction. The pressure nodes formed inside the acoustofluidic field in the channel deflect the biological cells from their original course in accordance with their mechanical properties, including volume, compressibility, and density. The threshold power for fully converging the cells to the pressure node is used to calculate the acoustic contrast factor. To demonstrate the ETA device in cell mechanophenotyping, and distinguishing between different cell types, further experimentation is carried out by using A549 (lung cancer cells), MDB-MA-231 (breast cancer cells), and leukocytes. The resulting acoustic contrast factors for the lung and breast cancer cells are different from that of the leukocytes by 27.9% and 21.5%, respectively. These results suggest this methodology can successfully distinguish and phenotype different cell types based on the acoustic contrast factor.

Manipulation of Particle/Cell Based on Compressibility in a Divergent Microchannel by Surface Acoustic Wave

The mechanical properties (compressibility or deformability) of cells are closely related to their death, migration, and differentiation. Accurate separation and manipulation of bioparticles based on these mechanical properties are still a challenging in the field of acoustofluidics. In this work, based on surface acoustic waves (SAW) and divergent microchannels, we developed a new method for separating and detecting particles or cells with different compressibility. The difference in acoustic radiation force (F_r) caused by compressibility are gradually amplified and accumulated by decreasing the flow velocity, and they are finally reflected in the particle migration distance. During the transverse migration process, the alternating dominance of the acoustic radiation force and the Stokes resistance force (F_s) drives the particles to create three typical migration patterns: intermittent migration, compound migration, and near-wall migration. In the present tilted SAW device, a 91% separation success rate of ∼10 μm polystyrene (PS) and polydimethylsiloxane (PDMS) particles can be achieved by optimizing the acoustic field input power and the fluid velocity. The application potential of the present divergent microchannel is validated by separating the myelogenous leukemia cell K562 and the natural killer cell NK92 that have similar densities and sizes (∼15 μm) but different compressibility. The results of this work are expected to provide valuable insights into the acoustofluidics separation and detection of the cells that are with different compressibility.

Bisymmetric coherent acoustic tweezers based on modulation of surface acoustic waves for dynamic and reconfigurable cluster manipulation of particles and cells

Acoustic tweezers based on surface acoustic waves (SAWs) have raised great interest in the fields of tissue engineering, targeted therapy, and drug delivery. Generally, the complex structure and array layout design of interdigital electrodes would restrict the applications of acoustic tweezers. Here, we present a novel approach by using bisymmetric coherent acoustic tweezers to modulate the shape of acoustic pressure fields with high flexibility and accuracy. Experimental tests were conducted to perform the precise, contactless, and biocompatible cluster manipulation of polystyrene microparticles and yeast cells. Stripe, dot, quadratic lattice, hexagonal lattice, interleaved stripe, oblique stripe, and many other complex arrays were achieved by real-time modulation of amplitudes and phase relations of coherent SAWs to demonstrate the capability of the device for the cluster manipulation of particles and cells. Furthermore, rapid switching among various arrays, shape regulation, geometric parameter modulation of array units, and directional translation of microparticles and cells were implemented. This study demonstrated a favorable technique for flexible and versatile manipulation and patterning of cells and biomolecules, and it has the advantages of high manipulation accuracy and adjustability, thus it is expected to be utilized in the fields of targeted cellular assembly, biological 3D printing, and targeted release of drugs.

Microfluidic acoustic sawtooth metasurfaces for patterning and separation using traveling surface acoustic waves

We demonstrate a sawtooth-based metasurface approach for flexibly orienting acoustic fields in a microfluidic device driven by surface acoustic waves (SAW), where sub-wavelength channel features can be used to arbitrarily steer acoustic fringes in a microchannel. Compared to other acoustofluidic methods, only a single travelling wave is used, the fluidic pressure field is decoupled from the fluid domain's shape, and steerable pressure fields are a function of a simply constructed polydimethylsiloxane (PDMS) metasurface shape. Our results are relevant to microfluidic applications including the patterning, concentration, focusing, and separation of microparticles and cells.

Versatile acoustic manipulation of micro-objects using mode-switchable oscillating bubbles: transportation, trapping, rotation, and revolution

Controllable on-chip multimodal manipulation of micro-objects in microfluidic devices is urgently required for enhancing the efficiency of potential biomedical applications. However, fixed design and driving models make it difficult to achieve switchable multifunction efficiently in a single device. In this study, a versatile bubble-based acoustofluidic device is proposed for multimodal manipulation of micro-objects in a biocompatible manner. Identical bubbles trapped over the bottom microcavities are made to flexibly switch between four different oscillatory motions by varying the applied frequency to generate corresponding modes of streaming patterns in the microchannel. Such regular modes enable stable transportation, trapping, 3D rotation, and circular revolution of the micro-objects, which were experimentally and numerically verified. The mode-switchable manipulations can be noninvasively applied to particles, cells, and organisms with different sizes, shapes, and quantities and can be controlled by key driving parameters. Moreover, 3D cell reconstruction is developed by applying the out-of-plane rotational mode and analyzed for illustration of cell surface morphology while quantifying reliably basic cell properties. Finally, a simple platform is established to integrate user-friendly function control and reconstruction analysis. The mode-switchable acoustofluidic device features a versatile, controllable, and contactless micro-object manipulation method, which provides an efficient solution for biomedical applications.