Acoustofluidics 22: multi-wavelength resonators, applications and considerations

Node formation mechanisms in acoustofluidic capillary bridges

Using acoustofluidic channels formed by capillary bridges two models are developed to describe nodes formed by leaky and by evanescent waves. The liquid channel held between a microscope slide (waveguide) and a strip of polystyrene film (fluid guide) avoids solid-sidewall interactions. With this simplification, our experimental and numerical study showed that waves emitted from a single plane surface, interfere and form the nodes without any resonance in the fluid. Both models pay particular attention to tensor elements normal to the solid-liquid interfaces they find that; initially nodes form in the solid and the node pattern is replicated by waves emitted into the fluid from antinodes in the stress. At fluids depths near half an acoustic wavelength, most nodes are formed by leaky waves. In the glass, water-loading reduces node-node separation and forms an overlay type waveguide which aligns the nodes predominantly along the channel. One new practical insight is that node separation can be controlled by water depth. At 0.2 mm water depths (which are smaller than a ¼ wavelength) nodes form from evanescent waves. Here a suspension of yeast cells formed a pattern of small dot-like clumps of cells on the surface of the polystyrene film. We found the same pattern in sound intensity normal, and close, to the water-polystyrene interface. The capillary bridge channel developed for this study is simple, low-cost, and could be developed for filtration, separation, or patterning of biological species in rapid immuno-sensing applications.

Selective particle separation on centimeter scale using a dual frequency dynamic acoustic field

This study investigated the application of dual-frequency type dynamic acoustic fields for size-selective particle separation on centimeter scale in a continuous flow. The 3D-printed X-shaped prototype has two inlets and two outlets. The dynamic acoustic field is generated by two transducers positioned under an angle of 60^° and operating at slightly different frequencies. The acoustic reflections are eliminated by placing sound-absorbing material inside the prototype and the non-resonant operation is confirmed by the electrical admittance measurements. Numerical calculations suggested that pressure generated by each transducer does not need to have equal amplitude. Computer simulations and lab experiments were carried out for different frequency differences and flow rates. The results demonstrated the ability of dual-frequency dynamic acoustic fields for size-selective particle filtration on centimeter scale, with a total flow rate up to.1Lh^-1.

A thermoelectrically stabilized aluminium acoustic trap combined with attenuated total reflection infrared spectroscopy for detection of Escherichia coli in water

Acoustic trapping is a non-contact particle manipulation method that holds great potential for performing automated assays. We demonstrate an aluminium acoustic trap in combination with attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) for detection of E. coli in water. The thermal conductivity of aluminium was exploited to thermo-electrically heat and hold the acoustic trap at the desired assay temperature of 37 °C. Systematic characterisation and optimisation of the acoustic trap allowed high flow rates while maintaining high acoustic trapping performance. The ATR element serves not only as a reflector for ultrasound standing wave generation but also as a sensing interface. The enzyme conversion induced by alkaline phosphatase-labelled bacteria was directly monitored in the acoustic trap using ATR-FTIR spectroscopy. Sequential injection analysis allowed automated liquid handling, including non-contact bacteria retention, washing and enzyme-substrate exchange within the acoustic trap. The presented method was able to detect E. coli concentrations as low as 1.95 × 106 bacteria per mL in 197 min. The demonstrated ultrasound assisted assay paves the way to fully automated bacteria detection devices based on acoustic trapping combined with ATR-FTIR spectroscopy.

Improved packing of preparative biochromatography columns by mechanical vibration

The bioprocessing industry relies on packed-bed column chromatography as its primary separation process to attain the required high product purities and fulfill the strict requirements from regulatory bodies. Conventional column packing methods rely on flow packing and/or mechanical compression. In this work, the application of ultrasound and mechanical vibration during packing was studied with respect to packing density and homogeneity. We investigated two widely used biochromatography media, incompressible ceramic hydroxyapatite, and compressible polymethacrylate-based particles, packed in a laboratory-scale column with an inner diameter of 50 mm. It was shown that ultrasonic irradiation led to reduced particle segregation during sedimentation of a homogenized slurry of polymethacrylate particles. However, the application of ultrasound did not lead to an improved microstructure of already packed columns due to the low volumetric energy input (~152 W/L) caused by high acoustic reflection losses. In contrast, the application of pneumatic mechanical vibration led to considerable improvements. Flow-decoupled axial linear vibration was most suitable at a volumetric force output of ~1,190 N/L. In the case of the ceramic hydroxyapatite particles, a 13% further decrease of the packing height was achieved and the reduced height equivalent to a theoretical plate (rHETP) was decreased by 44%. For the polymethacrylate particles, a 18% further packing consolidation was achieved and the rHETP was reduced by 25%. Hence, it was shown that applying mechanical vibration resulted in more efficiently packed columns. The application of vibration furthermore is potentially suitable for in situ elimination of flow channels near the column wall.

Selective Particle Filtering in a Large Acoustophoretic Serpentine Channel

The objective of this study is to investigate the performance of a serpentine channel for acoustically driven selective particle filtering. The channel consists of sharp corners and straight sections, and the acoustic field is affecting the particles throughout the channel. A prototype of the separator channel is manufactured using 3D printing. Acoustic waves are generated by a piezoelectric transducer operating near 2 MHz. Computer simulations are carried out to explore and visualize the flow field and acoustic field in the separator. Selective particle trapping is aimed to be achieved in the hairpin sections, which is confirmed by experiments. Spherical polyethylene particles of 34 µm, 70 µm and 100 µm diameter are used to demonstrate selective trapping by adjusting the flow rate in the channel or voltage input to the transducer. In addition, wheat beer containing yeast up to 20 µm size is selectively filtered by adjusting the flow rate to the channel. Experiments demonstrate that selective particle filtering is possible in the serpentine channel as both methods yield clear separation thresholds.

Phase separation technology based on ultrasonic standing waves: A review

The current understanding and developments of phase separation technology based on ultrasonic standing waves (USWs) are reviewed. Most previous reviews have focused on microscale applications of this technology in the fields of biological materials and food processing. This review covers different applications of ultrasonic separation technology, especially in petrochemical industry. The kinetic mechanism of ultrasonic, design of reactors, separation principles, and related applications are discussed in detail. We lay special stress on the motion characteristics of particles in USWs. According to the particle numbers, particle properties, and frequency characteristics, the separation principles are reasonably categorized as: (1) Bands effect; (2) Acoustophoretic coefficient; (3) Particle density; (4) Sweep frequency. Diverse separation principles improve the universality of ultrasonic separation technology. However, acoustic streaming and acoustic cavitation are two of the main challenges in the application of ultrasonic separation. Based on the current research, the future research can focus on the following aspects: (1) Explore the mechanism of ultrasonic demulsification; (2) Establish unified evaluation criteria for acoustic separation systems; (3) Develop the basis for determination of acoustic cavitation and non-cavitation.

Effect of temperature tuning on the aerosol acoustic aggregation process

Diesel exhaust aerosols (DEAs) can absorb and accumulate toxic metal particulates and bacteria suspended in the atmospheric environment, which impact human health and the environment. The use of acoustic standing waves (ASWs) to aggregate DEA is currently considered to be an efficient particle removal method; however, study of the effect of different temperatures on the acoustic aggregation process is scarce. To explore the method and technology to regulate and optimize the aerosol aggregation process through temperature tuning, an acoustic apparatus integrated with a temperature regulation function was constructed. Using this apparatus, the effect of different characteristic temperatures (CTs) on the aerosol aggregation process was investigated experimentally in the ASW environment. Under constant conditions of acoustic frequency 1.286kHz, voltage amplitude 17V and input electric power 16.7W, the study concentrated on temperature effects on the aggregation process in the CT range of 58-72°C. The DEA opacity was used. The results demonstrate that the aggregation process is quite sensitive to the CT, and that the optimal DEA aggregation can be achieved at 66°C. The aggregated particles of 68.17μm are composed of small nanoparticles of 13.34-62.15nm. At CTs higher and lower than 66°C, the apparatus in non-resonance mode reduces the DEA aggregation level. For other instruments, the method for obtaining the optimum temperature for acoustic agglomeration is universal. This preliminary demonstration shows that the use of acoustic technology to regulate the aerosol aggregation process through tuning the operating temperature is feasible and convenient.

Cavitation and non-cavitation regime for large-scale ultrasonic standing wave particle separation systems--In situ gentle cavitation threshold determination and free radical related oxidation

We here suggest a novel and straightforward approach for liter-scale ultrasound particle manipulation standing wave systems to guide system design in terms of frequency and acoustic power for operating in either cavitation or non-cavitation regimes for ultrasound standing wave systems, using the sonochemiluminescent chemical luminol. We show that this method offers a simple way of in situ determination of the cavitation threshold for selected separation vessel geometry. Since the pressure field is system specific the cavitation threshold is system specific (for the threshold parameter range). In this study we discuss cavitation effects and also measure one implication of cavitation for the application of milk fat separation, the degree of milk fat lipid oxidation by headspace volatile measurements. For the evaluated vessel, 2 MHz as opposed to 1 MHz operation enabled operation in non-cavitation or low cavitation conditions as measured by the luminol intensity threshold method. In all cases the lipid oxidation derived volatiles were below the human sensory detection level. Ultrasound treatment did not significantly influence the oxidative changes in milk for either 1 MHz (dose of 46 kJ/L and 464 kJ/L) or 2 MHz (dose of 37 kJ/L and 373 kJ/L) operation.

Scaling-up ultrasound standing wave enhanced sedimentation filters

Particle concentration and filtration is a key stage in a wide range of processing industries and also one that can be present challenges for high throughput, continuous operation. Here we demonstrate some features which increase the efficiency of ultrasound enhanced sedimentation and could enable the technology the potential to be scaled up. In this work, 20 mm piezoelectric plates were used to drive 100 mm high chambers formed from single structural elements. The coherent structural resonances were able to drive particles (yeast cells) in the water to nodes throughout the chamber. Ultrasound enhanced sedimentation was used to demonstrate the efficiency of the system (>99% particle clearance). Sub-wavelength pin protrusions were used for the contacts between the resonant chamber and other elements. The pins provided support and transferred power, replacing glue which is inefficient for power transfer. Filtration energies of ∼4 J/ml of suspension were measured. A calculation of thermal convection indicates that the circulation could disrupt cell alignment in ducts >35 mm high when a 1K temperature gradient is present; we predict higher efficiencies when this maximum height is observed. For the acoustic design, although modelling was minimal before construction, the very simple construction allowed us to form 3D models of the nodal patterns in the fluid and the duct structure. The models were compared with visual observations of particle movement, Chladni figures and scanning laser vibrometer mapping. This demonstrates that nodal planes in the fluid can be controlled by the position of clamping points and that the contacts could be positioned to increase the efficiency and reliability of particle manipulations in standing waves.

An ultrasonically enhanced inclined settler for microalgae harvesting

Microalgae have vast potential as a sustainable and scalable source of biofuels and bioproducts. However, algae dewatering is a critical challenge that must be addressed. Ultrasonic settling has already been exploited for concentrating various biological cells at relatively small batch volumes and/or low throughput. Typically, these designs are operated in batch or semicontinuous mode, wherein the flow is interrupted and the cells are subsequently harvested. These batch techniques are not well suited for scaleup to the throughput levels required for harvesting microalgae from the large-scale cultivation operations necessary for a viable algal biofuel industry. This article introduces a novel device for the acoustic harvesting of microalgae. The design is based on the coupling of the acoustophoretic force, acoustic transparent materials, and inclined settling. A filtration efficiency of 70 ± 5% and a concentration factor of 11.6 ± 2.2 were achieved at a flow rate of 25 mL·min(-1) and an energy consumption of 3.6 ± 0.9 kWh·m(-3) . The effects of the applied power, flow rate, inlet cell concentration, and inclination were explored. It was found that the filtration efficiency of the device is proportional to the power applied. However, the filtration efficiency experienced a plateau at 100 W L(-1) of power density applied. The filtration efficiency also increased with increasing inlet cell concentration and was inversely proportional to the flow rate. It was also found that the optimum settling angle for maximum concentration factor occurred at an angle of 50 ± 5°. At these optimum conditions, the device had higher filtration efficiency in comparison to other similar devices reported in the previous literature.

Separation of suspensions and emulsions via ultrasonic standing waves - a review

Ultrasonic standing waves (USW) separation is an established technology for micro scale applications due to the excellent control to manipulate particles acoustically achieved when combining high frequency ultrasound with laminar flow in microchannels, allowing the development of numerous applications. Larger scale systems (pilot to industrial) are emerging; however, scaling up such processes are technologically very challenging. This paper reviews the physical principles that govern acoustic particle/droplet separation and the mathematical modeling techniques developed to understand, predict, and design acoustic separation processes. A further focus in this review is on acoustic streaming, which represents one of the major challenges in scaling up USW separation processes. The manuscript concludes by providing a brief overview of the state of the art of the technology applied in large scale systems with potential applications in the dairy and oil industries.

Acoustic devices for particle and cell manipulation and sensing

An emerging demand for the precise manipulation of cells and particles for applications in cell biology and analytical chemistry has driven rapid development of ultrasonic manipulation technology. Compared to the other manipulation technologies, such as magnetic tweezing, dielectrophoresis and optical tweezing, ultrasonic manipulation has shown potential in a variety of applications, with its advantages of versatile, inexpensive and easy integration into microfluidic systems, maintenance of cell viability, and generation of sufficient forces to handle particles, cells and their agglomerates. This article briefly reviews current practice and reports our development of various ultrasonic standing wave manipulation devices, including simple devices integrated with high frequency (>20 MHz) ultrasonic transducers for the investigation of biological cells and complex ultrasonic transducer array systems to explore the feasibility of electronically controlled 2-D and 3-D manipulation. Piezoelectric and passive materials, fabrication techniques, characterization methods and possible applications are discussed. The behavior and performance of the devices have been investigated and predicted with computer simulations, and verified experimentally. Issues met during development are highlighted and discussed. To assist long term practical adoption, approaches to low-cost, wafer level batch-production and commercialization potential are also addressed.

Ultrasound assisted particle and cell manipulation on-chip

Ultrasonic fields are able to exert forces on cells and other micron-scale particles, including microbubbles. The technology is compatible with existing lab-on-chip techniques and is complementary to many alternative manipulation approaches due to its ability to handle many cells simultaneously over extended length scales. This paper provides an overview of the physical principles underlying ultrasonic manipulation, discusses the biological effects relevant to its use with cells, and describes emerging applications that are of interest in the field of drug development and delivery on-chip.