Droplet Manipulation Using Acoustic Streaming Induced by a Vibrating Membrane

Surface acoustic wave-driven pumpless flow for sperm rheotaxis analysis

Sperm rheotaxis, the phenomenon where sperm cells swim against the direction of fluid flow, is one of the major guiding mechanisms for long-distance sperm migration within the female reproductive tract. However, current approaches to study this pose challenges in dealing with rare samples by continuously introducing extra buffer. Here, we developed a device utilising acoustic streaming, the steady flow driven by an acoustic perturbation, to drive a tuneable, well-regulated continuous flow with velocities ranging from 40 μm s^-1 to 128 μm s^-1 (corresponding to maximum shear rates of 5.6 s^-1 to 24.1 s^-1) in channels of interest - a range suitable for probing sperm rheotaxis behaviour. Using this device, we studied sperm rheotaxis in microchannels of distinct geometries representing the geometrical characteristics of the inner-surfaces of fallopian tubes, identified sperm dynamics with the presence of flow in channels of various widths. We found a 28% higher lateral head displacement (ALH) in sufficiently motile rheotactic sperm in a 50 μm channel in the presence of acoustically-generated flow as well as a change in migration direction and a 52% increase in curvilinear velocity (VCL) of sufficiently motile sperm in a 225 μm channel by increasing the average flow velocity from 40 μm s^-1 to 130 μm s^-1. These results provided insights for understanding sperm navigation strategy in the female reproductive tract, where rheotactic sperm swim near the boundaries to overcome the flow in the female reproductive tract and reach the fertilization site. This surface acoustic wave device presents a simple, pumpless alternative for studying microswimmers within in vitro models, enabling the discovery of new insights into microswimmers' migration strategies, while potentially offering opportunities for rheotaxis-based sperm selection and other flow-essential applications.

Surface behaviors of droplet manipulation in microfluidics devices

In recent years, the rapid development of microfluidic technology has caused a revolutionary impact in the fields of chemistry, medicine, and life sciences. Also, droplet control is one of the most important technologies in the field of microfluidics. In order to achieve different degrees of droplet transport, the dynamic balance of the competing processes of droplet driving force and fluid resistance should be controlled to achieve good selectivity of droplet transport. Here, we focus on the principles of droplet transport in microfluidic devices, including the driving forces for droplet transport in fluids and the effects of transport properties on droplet transport. After that, the effects of external fields on the directional transport of droplets and the advantages and disadvantages of each external field in droplet transport are discussed in detail. Finally, the applications and challenges of droplet microfluidics in chemical, biomedical, and mechanical systems are comprehensively introduced.

Hierarchically Structured, All-Aqueous-Coated Hydrophobic Surfaces with pH-Selective Droplet Transfer Capability

Often inspired by nature, techniques for precise droplet manipulation have found applications in microfluidics, microreactors, and water harvesting. However, a widely applicable strategy for surface modification combining simultaneous hydrophobicity and pH-sensitivity has not yet been achieved by employing environmentally friendly assembly conditions. The introduction of pH-responsive groups to an otherwise fluorinated polyphosphazene (PPZ) unlocks pH-selective droplet capture and transfer. Here, an all-aqueous layer-by-layer (LbL) deposition of polyelectrolytes is used to create unique hydrophobic coatings, endowing surfaces with the ability to sense environmental pH. The high hydrophobicity of these coatings (ultimately reaching a contact angle >120° on flat surfaces) is enabled by the formation of hydrophobic nanoscale domains and controllable by the degree of fluorination of PPZs, polyamine-binding partners, deposition pH, and coating thickness. Inspired by the hierarchical structure of rose petals, these versatile coatings reach a contact angle >150° when deposited on structured surfaces while introducing a tunable adhesivity that enables precise droplet manipulation. The films exhibited a strongly pronounced parahydrophobic rose petal behavior characterized through the contact angle hysteresis. Depositing as few as five bilayers (∼25 nm) on microstructured rather than smooth substrates resulted in superhydrophobicity with water contact angles >150° and the attenuation of the contact angle hysteresis, enabling highly controlled transfer of aqueous droplets. The pH-selective droplet transfer was achieved between surfaces with either the same microstructure and LbL film building blocks, which were assembled at different pH, or between surfaces with different microstructures coated with identical films. The demonstrated capability of these hydrophobic LbL films to endow surfaces with controlled hydrophobicity through adsorption from aqueous solutions and control the adhesion and transfer of water droplets between surfaces can be used in droplet-based microfluidics applications and water collection/harvesting.

Manipulation of single cells via a Stereo Acoustic Streaming Tunnel (SteAST)

At the single-cell level, cellular parameters, gene expression and cellular function are assayed on an individual but not population-average basis. Essential to observing and analyzing the heterogeneity and behavior of these cells/clusters is the ability to prepare and manipulate individuals. Here, we demonstrate a versatile microsystem, a stereo acoustic streaming tunnel, which is triggered by ultrahigh-frequency bulk acoustic waves and highly confined by a microchannel. We thoroughly analyze the generation and features of stereo acoustic streaming to develop a virtual tunnel for observation, pretreatment and analysis of cells for different single-cell applications. 3D reconstruction, dissociation of clusters, selective trapping/release, in situ analysis and pairing of single cells with barcode gel beads were demonstrated. To further verify the reliability and robustness of this technology in complex biosamples, the separation of circulating tumor cells from undiluted blood based on properties of both physics and immunity was achieved. With the rich selection of handling modes, the platform has the potential to be a full-process microsystem, from pretreatment to analysis, and used in numerous fields, such as in vitro diagnosis, high-throughput single-cell sequencing and drug development.

Recent developments, challenges, and prospects of ultrasound-assisted oil technologies

There has been consistent drive towards research and innovation in oil production technologies in order to achieve improved effectiveness and efficiency in their operation. This drive has resulted in breakthrough in technologies such as the application of ultrasound (US) in demulsification and enhanced oil recovery (EOR), and usage of high-volume hydraulic fracturing and special horizontal well for shale oil and gas extraction. These can be observed in the increment in the number of commercial oil technologies such as EOR projects that rose from 237 in 1996 to 375 in 2017. This sustained expansion in EOR resulted in their total oil production rising from 1.5 million barrels per day in 2005 to 2.3 million barrels per day in 2020. And this is predicted to increase to about 4.7 million barrels per day in 2040, which represent about 4% of total production. Consequently, in this review, the developments in the utilization of US either as standalone or integrated with other technologies in EOR and dehydration of water in oil emulsions were analyzed. The studies include the optimization of fluid and US properties in EOR and demulsification. Reports on the treatment of formation damage resulting from inorganic salts, organic scales, drilling fluid plugs, condensate, paraffin wax and colloidal particle with US-assisted EOR were also highlighted. Moreover, the mechanisms were examined in order to gain insightful understanding and to aid research investigations in these areas. Technologies such as US assisted green demulsification, high intensity focused ultrasound, and potential pathways in field studies were assessed for their feasibilities. It is essential to evaluate these technologies due to the significant accrued benefits in them. The usage of green demulsifiers such as deep eutectic solvents, ionic liquids and bio-demulsifiers has promising future outlook and US could enhance their technical advancement. HiFU has been applied successfully in clinical research and developments in this area can potentiality improve demulsification and interfacial studies (fluid-fluid and solid-fluid interactions). As regards field studies, there is need to increase actual well investigations because present reports have few on-site measurements with most studies being in laboratory scale. Furthermore, there is need for more detailed modeling of these technologies as it would assist in conserving resources, saving research time and fast-tracking oil production. Additional evaluative studies of conditions such as the usage of Raschig rings, crude oil salinity and high temperature which have improved demulsification of crude oil emulsions should be pursued.

Poly-Phenylene-Sulfide Wedge Transducer for Exciting Surface Acoustic Waves for Removing Droplets on a Glass Plate

A wedge transducer is developed using poly-phenylene-sulfide (PPS) as an exciter of high-intensity surface acoustic wave (SAW) to remove droplets adhering to automobiles' front shield glass. Since the front shield glass has a glass/rubber/glass triple-layered configuration, SAW is needed to be employed to confine the vibration energy within the first glass layer and avoid the energy absorption in the rubber layer. PPS is chosen among popular engineering polymers for the vibrating body because its attenuation at ultrasonic frequency is lower than other polymer materials. To assess the feasibility of our proposal, we designed and fabricated a PPS wedge transducer, and investigated the vibration characteristics and the capability to remove droplets. The transducer, which incorporated a lead-zirconate-titanate (PZT) plate and a PPS wedge, had the total volume of 24×20×8.5 mm³. At the voltage of 24.8 V, a SAW having the wavelength of 2 mm and the vibration velocity of 14.5 mm/s was excited on the glass plate, and the droplets were removed with the SAW.

A practical microfluidic pump enabled by acoustofluidics and 3D printing

Simple and low-cost solutions are becoming extremely important for the evolving necessities of biomedical applications. Even though, on-chip sample processing and analysis has been rapidly developing for a wide range of screening and diagnostic protocols, efficient and reliable fluid manipulation in microfluidic platforms still require further developments to be considered portable and accessible for low-resource settings. In this work, we present an extremely simple microfluidic pumping device based on three-dimensional (3D) printing and acoustofluidics. The fabrication of the device only requires 3D-printed adaptors, rectangular glass capillaries, epoxy and a piezoelectric transducer. The pumping mechanism relies on the flexibility and complexity of the acoustic streaming patterns generated inside the capillary. Characterization of the device yields controllable and continuous flow rates suitable for on-chip sample processing and analysis. Overall, a maximum flow rate of ~ 12 μL/min and the control of pumping direction by frequency tuning is achieved. With its versatility and simplicity, this microfluidic pumping device offers a promising solution for portable, affordable and reliable fluid manipulation for on-chip applications.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10404-020-02411-w.

On-Chip Generation of Vortical Flows for Microfluidic Centrifugation

Microcentrifugation constitutes an important part of the microfluidic toolkit in a similar way that centrifugation is crucial to many macroscopic procedures, given that micromixing, sample preconcentration, particle separation, component fractionation, and cell agglomeration are essential operations in small scale processes. Yet, the dominance of capillary and viscous effects, which typically tend to retard flow, over inertial and gravitational forces, which are often useful for actuating flows and hence centrifugation, at microscopic scales makes it difficult to generate rotational flows at these dimensions, let alone with sufficient vorticity to support efficient mixing, separation, concentration, or aggregation. Herein, the various technologies-both passive and active-that have been developed to date for vortex generation in microfluidic devices are reviewed. Various advantages or limitations associated with each are outlined, in addition to highlighting the challenges that need to be overcome for their incorporation into integrated microfluidic devices.

Focusing surface acoustic waves assisted electrochemical detector in microfluidics

This article demonstrates a novel electrochemical detection device. The device is composed by two focusing interdigital transducers for exciting focused surface acoustic waves by applying an AC signal, a three-electrode system for electrochemical measurement, and a liquid pool for holding liquid on a LiNbO₃ wafer. The amperometry current of ferrocenecarboxylic acid and potassium phosphate buffer solution is used to characterize the detection sensitivity. Two experiments are carried out to optimize the device design. The result shows that the two focusing interdigital transducers with arc degree 30° and distance 5 mm can remarkably enhance the liquid mixing rate. Under this condition, the oxidation current is about 27 times larger than that without surface acoustic wave stirring.

Versatile platform for performing protocols on a chip utilizing surface acoustic wave (SAW) driven mixing

We present and demonstrate a dextrous microfluidic device which features a reaction chamber with volume flexibility. This feature is critical for developing protocols directly on chip when the exact reaction is not yet defined, enabling bio/chemical reactions on chip to be performed without volumetric restrictions. This is achieved by the integration of single layer valves (for reagent dispensing) and surface acoustic wave excitation (for rapid reagent mixing). We show that a single layer valve can control the delivery of fluid into, an initially air-filled, mixing chamber. This chamber arrangement offers flexibility in the relative volume of reagents used, and so offers the capability to not only conduct, but also develop protocols on a chip. To enable this potential, we have integrated a SAW based mixer into the system, and characterised its mixing time based on frequency and power of excitation. Numerical simulations on the streaming pattern inside the chamber were conducted to probe the underlying physics of the experimental system. To demonstrate the on-chip protocol capability, the system was utilised to perform protein crystallization. Furthermore, the effect of rapid mixing, results in a significant increase in crystal size uniformity.

A simple capillary-based open microfluidic device for size on-demand high-throughput droplet/bubble/microcapsule generation

We report an easily-established capillary-based open microfluidic device (COMD) as a simple and robust method for size on-demand generation of monodisperse droplets of various fluidic materials with controllable volume. A device is set up in which a capillary is positioned with its tip close to a flat surface with a precise gap distance in a container. The continuous phase remains static in the container, and the dispersed phase is pumped through the capillary and forms droplets at the exit of the gap. Monodisperse droplets, bubbles and microcapsules of various fluids with diameters of 10-300 μm (picoliter to nanoliter) and generation frequency of 1-1000 Hz are obtained by controlling the gap distance in the range of 5-500 μm. The droplet formation is caused by capillarity-induced narrowing of the dispersed phase at the capillary exit, with droplet size being determined by the gap volume and fluid flow. We find that, at low flow rate, using the same COMD, the generated droplet size is constant, being determined by the gap size; however, at higher flow rate, droplet size increases with the flow rate. Droplet types can be managed by fluids and surface modification of the capillary and bottom surfaces. High throughput droplet generation is achieved by in-parallel integration of multiple capillaries in one device. Such a COMD is simple and easy-to-build without complex microfabrication requirements; however, it is highly robust, flexible and easy-to-operate for size on-demand droplet generation. It offers an opportunity for common laboratories to perform droplet-based assays, and has high potential for high throughput industrial emulsification applications as well.

Development of a Micro-Manipulator for Bio-Targets Using Micro Flows

In the field of bio-engineering, there is a need for a specialized manipulator that protect targets from hard physical contact. The solution proposed in this paper is a system that controls micro flows to manipulate a micro-sized bio-target. The manipulation principle and system structure are presented. In addition, a simple model that uses physically observable parameters is constructed to capture the flow-target interactions following a chemically assisted flow visualization. The experimental results of a prototype are presented, and the feasibility is discussed. The purpose of this paper is to prove the feasibility of this manipulation method. Further developments are introduced, highlighting the full potential of the prototype.

Microfluidic flow switching via localized acoustic streaming controlled by surface acoustic waves

Acoustic streaming flow induced by high-frequency surface acoustic waves has been used to switch streams of two immiscible fluids flowing in parallel through a bifurcating microchannel with an H-shaped junction at the centre.

Controlled removal of micro/nanoscale particles in submillimeter-diameter area on a substrate

In this paper, a removal method of micro/nanoscale particles in a submillimeter-diameter area at the interface between an aqueous suspension droplet and silicon substrate surface around a selectable point is proposed and demonstrated. It employs the acoustic streaming generated by an ultrasonically vibrating micro manipulating probe (MMP). The operating frequency of the device is 124.5 kHz, at which the micro manipulating probe oscillates approximately linearly. The experiments show that microscale particles with a diameter of 3-5 μm and nanoscale particles with a diameter of 300-500 nm in submillimeter-diameter areas can be removed in about 1.5 min. The principle of the cleaning method is analyzed by measuring the device's vibration mode and computing the 3D acoustic streaming field around the MMP. The diameter of cleaned area versus sonication time is clarified by experiments as well as the stable diameter of cleaned area versus vibration velocity. The dependency of the acoustic streaming field on the working parameters, which include the distance between the MMP's tip and substrate, the angle between the MMP and substrate, and the ratio of the normal vibration components of the MMP, is also investigated by the FEM (finite element method) computation. To the best of our knowledge, this is the first time to report a method for the removal of micro/nanoscale particles in a submillimeter range around a selectable point.

Droplet control technologies for microfluidic high throughput screening (μHTS)

The transition from micro well plate and robotics based high throughput screening (HTS) to chip based screening has already started. This transition promises reduced droplet volumes thereby decreasing the amount of fluids used in these studies. Moreover, it significantly boosts throughput allowing screening to keep pace with the overwhelming number of molecular targets being discovered. In this review, we analyse state-of-the-art droplet control technologies that exhibit potential to be used in this new generation of screening devices. Since these systems are enclosed and usually planar, even some of the straightforward methods used in traditional HTS such as pipetting and reading can prove challenging to replicate in microfluidic high throughput screening (μHTS). We critically review the technologies developed for this purpose in depth, describing the underlying physics and discussing the future outlooks.

Surface acoustic wave enabled pipette on a chip

Mono-disperse droplet formation in microfluidic devices allows the rapid production of thousands of identical droplets and has enabled a wide range of chemical and biological studies through repeat tests performed at pico-to-nanoliter volume samples. However, it is exactly this efficiency of production which has hindered the ability to carefully control the location and quantity of the distribution of various samples on a chip - the key requirement for replicating micro well plate based high throughput screening in vastly reduced volumetric scales. To address this need, here, we present a programmable microfluidic chip capable of pipetting samples from mobile droplets with high accuracy using a non-contact approach. Pipette on a chip (PoaCH) system selectively ejects (pipettes) part of a droplet into a customizable reaction chamber using surface acoustic waves (SAWs). Droplet pipetting is shown to range from as low as 150 pL up to 850 pL with precision down to tens of picoliters. PoaCH offers ease of integration with existing lab on a chip systems as well as a robust and contamination-free droplet manipulation technique in closed microchannels enabling potential implementation in screening and other studies.

The importance of travelling wave components in standing surface acoustic wave (SSAW) systems

The use of ultrasonic fields to manipulate particles, cells and droplets has become widespread in lab on a chip (LOC) systems. There are two dominant actuation methods, the use of bulk acoustic waves (BAW) or surface acoustic waves (SAW). The development of BAW actuated systems have been underpinned by a robust understanding of the link between the ultrasonic field and forces which can be generated. In this work, we examine this link for standing surface acoustic waves (SSAW) comparing the relative strengths of streaming induced drag and acoustic radiation forces on suspended particles. To achieve this we have employed boundary conditions which accurately capture the travelling wave components of the pseudo-standing wave field, describe the key features of the acoustic radiation force fields and the acoustic streaming fields which can be generated, and finally we show that the relative importance of these two mechanisms is spatially dependant within a fluid chamber. The boundary condition used models the SSAW as two counter-propagating travelling waves, rather than assuming a standing wave directly. This allows the accurate inclusion of energy decay as the SAW couples into the fluid chamber and the resulting travelling wave component. This study shows that this previously neglected complexity of the SAW field is a critical factor in the nature of the resultant streaming field, as it gives rise to strong streaming rolls at the channel walls, which we validate experimentally. These rolls result in spatial variations of the dominant forces which in turn varies particle migration patterns spatially across the fluid domain.