Acoustic Actuation Research Articles

Soft robotic patterning of liquids

Patterning of two or more liquids, either homogeneous in each phase or mixed with particles (including biological matter, such as cells and proteins), by controlling their flow dynamics, is relevant to several applications. Examples include dynamic spatial confinement of liquids in microfluidic systems (such as lab-on-a-chip and organ-on-a-chip devices) or structuring of polymers to modulate various properties (such as strength, conductivity, transparency and surface finishing). State-of-the-art strategies use various technologies, including positioners, shakers and acoustic actuators, which often combine limited versatility of mixing with significant inefficiency, energy consumption, and noise, as well as tendency to increase the temperature of the liquids. Here, we describe a new kind of robotic mixers of liquids, based on electro-responsive smart materials (dielectric elastomer actuators). We show for the first time how an efficient soft robotic device can be used to produce, via combinations of rotations and translations, various spatial patterns in liquids and maintain them stable for a few minutes. Moreover, we show that, as compared to a conventional orbital shaker, the new type of robotic device can mix liquids with a higher efficacy (~ 94% relative to ~ 80%, after 8 min of mixing) and with a significantly lower increase of the liquids’ temperature (+ 1 °C relative to + 5 °C, after 6 h of mixing). This is especially beneficial when mixing should occur according to controllable spatial features and should involve temperature-sensitive matter (such as biological cells, proteins, pre-polymers and other thermolabile molecules).

A mechanistic model for acoustically enhanced condensation heat transfer and pressure drop

Enhancing low-quality condensation where thermal resistance is the highest is critical for size reduction and efficiency improvements for many power generation, HVAC&R, and chemical processing applications. Recently, acoustic actuation has been shown to enhance low-quality condensation by ∼30% by introducing a sound wave that oscillates between the condenser headers and resonates within the tube, agitating the liquid-vapor interface and significantly altering the distribution of the liquid phase towards the end of condensation, thereby increasing the heat transfer rate. This paper presents a multiple flow-regime model to predict the pressure drop and heat transfer during acoustically actuated condensation based on hydrodynamic changes induced by forced oscillation. Experimental results for R134a flowing through a horizontal tube across the entire quality range for mass fluxes ranging from 120 to 235 kg m−2 s−1, actuation frequencies from 2 to 20 Hz, header volumes from 150 to 1000 mL, and wall-to-refrigerant temperature differences ranging from 6 to 12 K are used for the development of the models. Enhancement mechanisms are discussed and heat-and-momentum analogy-based correlations are developed to accurately predict actuated and unactuated condensation pressure drop and heat transfer data simultaneously within 23% and 7% on average, respectively.

Condensation Heat Transfer and Pressure Drop of Acoustically Actuated Horizontal Two-Phase Flow

An acoustic actuation technique to enhance condensation inside horizontal tubes is developed and experimentally demonstrated. Heat exchanger headers are used as Helmholtz resonators, allowing low-frequency (∼10 Hz) acoustic waves to propagate and be reflected within the condenser, improving mixing saturated vapor and condensed liquid, thereby enhancing heat transfer. Heat transfer coefficients and pressure drops are measured to evaluate the heat transfer enhancement and pressure drop penalty as a function of vapor quality, mass flux, temperature difference, and acoustic wave amplitude and frequency. Experiments are conducted using a tube-in-tube heat exchanger with an inner diameter of 14.45 mm that partially condenses refrigerant R134a at 950 kPa across vapor qualities ranging from 5% to 80% as mass fluxes of 120 and 180 kg m−2 s−1 and temperature differences of 6 and 12 K. The actuation amplitude and frequency are varied from 0.5 to 6 mm and 2 – 20 Hz, respectively to characterize the acoustic response. Heat transfer enhancement mechanisms are hypothesized and evaluated based on the experimental results, concluding that changes in two-phase flow patterns and increased convective effects lead to significant enhancement at low qualities but negligible enhancement at high qualities when the flow is annular. Existing correlations and modifications to existing models are proposed to explain the observed enhancement, and a simple correlation based on a heat-and-momentum analogy is proposed, which predicts both baseline and enhanced heat transfer data within 2.7%.

Circumferential SH wave in piezo-reinforced composite structure with imperfect interface bonding

The present study fundamentally focuses on analyzing the limitations and transference of circumferential shear horizontal (CSH) waves in a four-layered compounded cylinder. The geometrical structure consists of cylinders of infinite length composed of self-reinforced (SR), fibre-reinforced (FR), piezo-magnetic (PM), and piezo-electric(PE) materials. In order to make the structure sensitive to the application pertaining to sensors and actuators, the PM and PE cylinders have been categorically placed in the outer part of the geometry. Whereas in order to provide stiffness and stability to the structure, the inner part consists of SR and FR materials. The common boundary between each of the cylinders has been essentially considered as imperfectly bounded. At the interface of PE and PM media, mechanical, electrical, magnetic, and inter-coupled types of imperfections have been exhibited. The closed-form of dispersion relation has been deduced for two contrast cases i.e. electrically open magnetically short (EOMS) and electrically short and magnetically open (ESMO) circuit conditions. Dispersion curves have been plotted to illustrate the salient features of parameters like normalized imperfect interface parameters, initial stresses, and radii of the concentric cylinders. The comparative effect of each one of these parameters on the phase velocity of the wave has been enlisted and marked individually. Every graph has been presented with two successive modes for a comprehensive understanding. This theoretical study may be implemented to improvise the performance of surface acoustic wave (SAW) sensors, actuators, and Piezo-cables in the form of Piezo-polymer sensors consisting of piezo-electric quartz, piezo-composite concentric cylinders, and coaxial piezo-cables.

Modelling the response of a turbulent jet flame to acoustic forcing in a linearized framework using an active flame approach

This study performs a linear mean field analysis of a turbulent reacting methane-air jet flame, with the goal of predicting the response of the reacting flow to upstream acoustic actuation. Unlike previous studies, this work develops and applies an active flame approach by taking the heat release oscillations of the flame resulting from the acoustic fluctuations into account. For an active flame approach in the linear mean field analysis, a linearized combustion model is necessary. Linearizing Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) combustion models leads to closure problems, making their application in this context troublesome, whereas Reynolds-averaged Navier Stokes (RANS) combustion models prove to circumvent this problem making them suitable candidates for this purpose. The RANS combustion models are linearized around the temporal mean state variables of the turbulent jet flame, which is obtained by LES. An a priori analysis shows that a linearized RANS–Eddy Break Up (EBU) model is the best suited among all investigated combustion models for the investigated set-up and reproduces with high accuracy the fluctuations in reaction rate obtained in the LES. Furthermore, the linearized governing equations of the flow including the linearized EBU model for the reaction rate are solved for incoming acoustic perturbations. The response modes show that the reaction rate oscillations are caused by Kelvin–Helmholtz vortex rings, which perturb the jet flame. The results are in good agreement with the LES simulations in terms of the mode shapes of both reaction rate and velocity fluctuations. This study represents a basis for linear mean field analysis of turbulent flames and monolithic modelling approaches for thermoacoustic instabilities in gas turbine combustors.

Magnetic-Acoustic Sequentially Actuated CAR T Cell Microrobots for Precision Navigation and In Situ Antitumor Immunoactivation.

Despite its clinical success, chimeric antigen receptor T (CAR T)-cell immunotherapy remains limited in solid tumors, owing to the harsh physical barriers and immunosuppressive microenvironment. Here a CAR-T-cell-based live microrobot (M-CAR T) is created by decorating CAR T with immunomagnetic beads using click conjugation. M-CAR Ts are capable of magnetic-acoustic actuation for precision targeting and in situ activation of antitumor immune responses. Sequential actuation endows M-CAR Ts with magnetically actuated anti-flow and obstacle avoidance as well as tissue penetration driven by acoustic propulsion, enabling efficient migration and accumulation in artificial tumor models. In vivo, sequentially actuated M-CAR Ts achieves long-distance targeting and accumulate at the peritumoural area under programmable magnetic guidance, and subsequently acoustic tweezers actuate M-CAR Ts to migrateinto deep tumor tissues, resulting in a 6.6-fold increase in accumulated exogenous CD8+ CAR T cells compared with that without actuation. Anti-CD3/CD28 immunomagnetic beads stimulate infiltrated CAR T proliferation and activation in situ, significantly enhancing their antitumor efficacy. Thus, thissequential-actuation-guided cell microrobot combines the merits of autonomous targeting and penetration of intelligent robots with in situ T-cell immunoactivation, and holds considerable promise for precision navigation and cancer immunotherapies.

A bar-joint model based on the corrected resistive force theory for artificial flagellated micro-swimmers propelled by acoustic waves

In this work, we proposed a bar-joint model based on the corrected resistive force theory (CRFT) for studying artificial flagellated micro-swimmers (AFMSs) propelled by acoustic waves in a two-dimensional (2D) flow field or with a rectangular cross-section. Note that the classical resistive-force theory for 3D cylindrical flagellum leads to over 90% deviation in terminal velocity from those of 2D fluid-structure interaction (FSI) simulations, while the proposed CRFT bar-joint model can reduce the deviation to below 5%; hence, it enables a reliable prediction of the 2D locomotion of an acoustically actuated AFMS with a rectangular cross-section, which is the case in some experiments. Introduced in the CRFT is a single correction factor K determined by comparing the linear terminal velocities under acoustic actuation obtained from the CRFT with those from simulations. After the determination of K, detailed comparisons of trajectories between the CRFT-based bar-joint AFMS model and the FSI simulation were presented, exhibiting an excellent consistency. Finally, a numerical demonstration of the purely acoustic or magneto-acoustic steering of an AFMS based on the CRFT was presented, which can be one of the choices for future AFMS-based precision therapy.

Pneumatic and acoustic suit: multimodal haptic suit for enhanced virtual reality simulation

A haptic device provides users with physical feedback to enhance their experience in virtual reality (VR). We developed a multimodal haptic suit, called as Pneumatic and Acoustic (PA) suit, which exhibits high-resolution haptic feedback, and applies high pressure and realistic vibration to a user. The PA suit artificially simulates the sensation of brief and strong collisions such as the impact of an explosion, ball, or fist. The pneumatic actuators, consisting of 40 air bladders, are designed as bellows-type pneumatic devices for vertical inflation. The air bladders are placed on the chest at an equal interval distance of 30 mm for high-resolution feedback. The acoustic actuators use an effective sound signal of a collision similar to realistic vibrations. This study aims to examine the effectiveness of our multimodal haptic suit in improving VR experience of users. The recognition tests clearly show that participants distinguish between the haptic patterns and position of collided virtual objects with the suit. The user study involving a collision of a ball shows that the PA suit transmits the approximate pressure of a real ball collision with artificial haptic feedback. Our receiving ball and explosion VR simulations confirm that the PA suit improves a VR experience depending on the types of actuators and VR contents. The results prove that the PA suit creates distinguishable haptic patterns for guiding a task and improves the VR experience of users with powerful and multimodal haptic feedback hence providing high-quality VR simulation.

Acoustophoretic trapping of particles by bubbles in microfluidics

We present in this paper a simple method to produce strategic acoustic particle capture sites in microfluidic channels in a controlled way. Air bubbles are intermittently injected into a micro-capillary with rectangular cross section during a flow motion of liquid suspensions containing micron-sized particles or particles to create bubble-defined “micro-gaps” with size close to 200 µm and spheroidal geometry. The establishment of a 3D standing acoustic wave inside the capillary at a frequency close to 1 MHz produces different radiation forces on solid particles and bubbles, and acoustic streaming around the bubble. While the sample flows, part of the particles collect along the acoustic pressure node established near the central axis and continue circulating aligned through the capillary until reaching the end, where are released enriched. Meanwhile, the bubble travels very fast toward positions of maximum pressure amplitude beside the channel wall, driven by Bjerkness forces, and attach to it, remaining immovable during the acoustic actuation. Some particles adhere to its membrane trapped by the acoustic streaming generated around the oscillating bubble. Changes of frequency were applied to analyze the influence of this parameter on the bubble dynamics, which shows a complete stability once attached to the channel wall. Only increasing the flow motion induces the bubble displacements. Once reached the open air at the end of the capillary, the bubble disappears releasing the trapped particles separated from their initial host suspension with a purity degree. The device presents a very simple geometry and a low-cost fabrication.

Dynamics of vapor bubble condensation under directional ultrasonic actuation

Direct-contact condensation of vapor bubbles injected into a subcooled liquid is enhanced using ultrasonic O(1 MHz) acoustic actuation. In the absence of actuation, the surface tension-driven pinch-off process of the vapor bubble from the injection orifice induces a liquid spear that travels upward through the bubble and ruptures the top interface to form a toroidal bubble. Similarly, the acoustic actuator produces a narrow high-intensity acoustic beam that deforms the top interface of the vapor bubble via radiation pressure to form a liquid spear that travels downward though the bubble and ruptures the bottom interface to form a toroidal bubble. Comparisons between the growth and collapse of vapor bubbles in these two cases were performed using high-speed video imaging and particle image velocimetry. The results show that the actuated bubble collapsed about 35% faster than the unactuated bubble. The flow fields around the bubbles induced by the motion of the liquid spears are similar in both cases. By comparing vapor bubbles under different subcooling conditions with an unactuated, noncondensing air bubble, it was shown that condensation at the liquid–vapor interface strongly influences bubble collapse times and the velocity field surrounding each of the bubbles and that these effects increase as the level of subcooling increases.

Microfabricated acoustofluidic membrane acoustic waveguide actuator for highly localized in-droplet dynamic particle manipulation.

Precision manipulation techniques in microfluidics often rely on ultrasonic actuators to generate displacement and pressure fields in a liquid. However, strategies to enhance and confine the acoustofluidic forces often work against miniaturization and reproducibility in fabrication. This study presents microfabricated piezoelectric thin film membranes made via silicon diffusion for guided flexural wave generation as promising acoustofluidic actuators with low frequency, voltage, and power requirements. The guided wave propagation can be dynamically controlled to tune and confine the induced acoustofluidic radiation force and streaming. This provides for highly localized dynamic particle manipulation functionalities such as multidirectional transport, patterning, and trapping. The device combines the advantages of microfabrication and advanced acoustofluidic capabilities into a miniature "drop-and-actuate" chip that is mechanically robust and features a high degree of reproducibility for large-scale production. The membrane acoustic waveguide actuators offer a promising pathway for acoustofluidic applications such as biosensing, organoid production, and in situ analyte transport.

Fabrication, Acoustic Characterization and Phase Reference-Based Calibration Method for a Single-Sided Multi-Channel Ultrasonic Actuator.

The ultrasonic actuator can be used in medical applications because it is label-free, biocompatible, and has a demonstrated history of safe operation. Therefore, there is an increasing interest in using an ultrasonic actuator in the non-contact manipulation of micromachines in various materials and sizes for therapeutic applications. This research aims to design, fabricate, and characterize a single-sided transducer array with 56 channels operating at 500 kHz, which provide benefits in the penetration of tissue. The fabricated transducer is calibrated using a phase reference calibration method to reduce position misalignment and phase discrepancies caused by acoustic interaction. The acoustic fields generated by the transducer array are measured in a 300 mm × 300 mm × 300 mm container filled with de-ionized water. A hydrophone is used to measure the far field in each transducer array element, and the 3D holographic pattern is analyzed based on the scanned acoustic pressure fields. Next, the phase reference calibration is applied to each transducer in the ultrasonic actuator. As a result, the homogeneity of the acoustic pressure fields surrounding the foci area is improved, and the maximum pressure is also increased in the twin trap. Finally, we demonstrate the capability to trap and manipulate micromachines with acoustic power by generating a twin trap using both optical camera and ultrasound imaging systems in a water medium. This work not only provides a comprehensive study on acoustic actuators but also inspires the next generation to use acoustics in medical applications.

Design of an Acoustic Synthetic Jet Actuator for Flow Control

Synthetic jet technology is widely adopted in active flow control. An actuator with an oscillating diaphragm is a commonly used excitation device for synthetic jet generation. However, it has a disadvantage wherein the volume at the cross-section of the cavity varies unevenly when the diaphragm vibrates, which makes it difficult to use multiple jets corresponding to one diaphragm. In this paper, an acoustic synthetic jet actuator that can generate multiple jets with one diaphragm was designed. The diaphragm vibrated in a cylindrical cavity, transferring air to another constant-volume square cavity through pipes. The square cavity was covered with a multiple-orifice plate for the expulsion and suction of the ambient air. Through this means, the implementation of multiple jets corresponding to one diaphragm was achieved. The multiple jets are called distributed synthetic jets in this paper. Governing parameters that determined the performance of the distributed synthetic jets were given by theoretical derivation. It was found that, under specific geometry conditions, the governing parameters were mainly the frequency and voltage of the input signal to the actuator. Then, the velocity characteristics of the distributed synthetic jets were measured by using a constant-temperature anemometer and the parameter space was determined. The results showed that it was practicable to apply the acoustic actuator to turbulent boundary layer flow control.

Acoustically Driven Cell-Based Microrobots for Targeted Tumor Therapy.

Targeted drug delivery using microrobots manipulated by an external actuator has significant potential to be a practical approach for wireless delivery of therapeutic agents to the targeted tumor. This work aimed to develop a novel acoustic manipulation system and macrophage-based microrobots (Macbots) for a study in targeted tumor therapy. The Macbots containing superparamagnetic iron oxide nanoparticles (SPIONs) can serve as drug carriers. Under an acoustic field, a microrobot cluster of the Macbots is manipulated by following a predefined trajectory and can reach the target with a different contact angle. As a fundamental validation, we investigated an in vitro experiment for targeted tumor therapy. The microrobot cluster could be manipulated to any point in the 4 × 4 × 4 mm region of interest with a position error of less than 300 μm. Furthermore, the microrobot could rotate in the O-XY plane with an angle step of 45 degrees without limitation of total angle. Finally, we verified that the Macbots could penetrate a 3D tumor spheroid that mimics an in vivo solid tumor. The outcome of this study suggests that the Macbots manipulated by acoustic actuators have potential applications for targeted tumor therapy.

3D Printed Device for Separation of Cells and Particles by Tilted Bulk Acoustic Wave Actuation

Three-dimensional (3D) printing has been proven to be a reliable manufacturing method for a diverse set of applications in engineering. Simple benchtop tools such as mini centrifuges, automated syringe pumps, and basic-robotic platforms have been successfully printed by basic 3D printers. The field of lab-on-a-chip offers promising functions and convenience for point-of-care diagnostics and rapid disease screening for limited resource settings. In this work, stereolithography (SLA) 3D resin printing is implemented to fabricate a microfluidic device to be used for separation of HeLa cells from smaller polystyrene particles through titled angle standing bulk acoustic wave actuation. The demonstrated device achieved continuous and efficient separation of target cells with over 92% HeLa cell purity and 88% cell recovery rates. Overall, 3D printing is shown to be a viable method for fabrication of microfluidic devices for lab-on-a-chip applications.

A discrete-adjoint framework for optimizing high-fidelity simulations of turbulent reacting flows

An adjoint-based framework is presented that measures exact sensitivity from high-fidelity simulations of turbulent reacting flows. The framework leverages and extends state-of-the-art numerical methods in a manner that is compatible with a discrete adjoint solver. To ensure energy stability and accuracy, high-order narrow-stencil finite-difference operators satisfying the summation-by-parts (SBP) property are combined with simultaneous-approximation-term boundary treatment. An adaptive SBP dissipation operator and its corresponding adjoint are formulated in a way to dampen unresolved modes and preserve scalar boundedness. A flamelet/progress variable approach is employed to handle chemical reactions using tabulated chemistry. The utility of using pre-computed lookup tables is that discrete adjoint sensitivity can be computed efficiently for arbitrary chemical mechanisms. Gradient-based optimization utilizing the sensitivity obtained from the adjoint solution is demonstrated on two configurations. First, momentum actuation is applied upstream in a spatial mixing layer to control the evolution of a passive scalar in a target region downstream. Then, the framework is used to optimize an acoustic actuator in a three-dimensional turbulent jet with the aim of anchoring an H2 lifted flame at a target location. Both cases involve manipulating discrete space-time fields with O(108)−O(109) degrees of freedom, which would not be possible via a brute-force trial-and-error approach.

Uncertain Quasi-static and Nonlinear Dynamic Analysis of Viscoelastic Dielectric Elastomer with Interval Parameters

Dielectric elastomers as a soft active material have been widely used in the field of artificial muscle actuator, acoustic actuator, loudspeaker, active control of vibration, soft robots and membrane resonators. Compared with traditional materials, there are many unknown uncertainties in the properties of the DE actuators. In this work, a viscoelastic dynamic model of dielectric elastomer is proposed with considering the uncertainties in material parameters, external mechanical load and voltage. By introducing the interval perturbation method and first-order Taylor series expansion method, the creep analysis, relaxation analysis and dynamic analysis of the dielectric elastomer with interval uncertain parameters are implemented. The effectivity of the proposed interval method is verified by the Monte Carlo simulation. This uncertain prediction method could be used in the design of active control systems with dielectric elastomers as actuators or sensors in the future.

Acoustics-Actuated Microrobots.

Microrobots can operate in tiny areas that traditional bulk robots cannot reach. The combination of acoustic actuation with microrobots extensively expands the application areas of microrobots due to their desirable miniaturization, flexibility, and biocompatibility features. Herein, an overview of the research and development of acoustics-actuated microrobots is provided. We first introduce the currently established manufacturing methods (3D printing and photolithography). Then, according to their different working principles, we divide acoustics-actuated microrobots into three categories including bubble propulsion, sharp-edge propulsion, and in-situ microrotor. Next, we summarize their established applications from targeted drug delivery to microfluidics operation to microsurgery. Finally, we illustrate current challenges and future perspectives to guide research in this field. This work not only gives a comprehensive overview of the latest technology of acoustics-actuated microrobots, but also provides an in-depth understanding of acoustic actuation for inspiring the next generation of advanced robotic devices.

Miniature sono-electrochemical platform enabling effective and gentle electrode biofouling removal for continuous sweat measurements

• Sono-electrochemical lab-on-a-chip device with gigahertz bulk acoustic resonator. • Acoustic actuation in-situ releasing fouled electrode′s electrochemical activity. • Non-invasiveness to intrinsic film electrode and the decorated nanoparticles. • Continuous and repeatedly electrochemical measurements to human sweat. Developing miniature smart electrochemical sweat sensors to perform continuous and noninvasive health diagnosis at molecule level is in the spotlight. Sensor's biofoulings hinder the targets' approaching, decreasing the detection sensitivity over time and further limiting the accurate analysis. Effective cleaning to remove biofoulings on electrochemical sensors is a main challenge and needs to overcome urgently for such purpose. We propose a miniature sono-electrochemical platform utilizing submillimeter gigahertz acoustic resonator to effectively clean away the biofoulings and release sensors inherent electrochemical activities. Resonator induced super strong fluid streaming can completely and repeatedly remove sweat biofoulings from gold electrode surface, leading to 100% recovery of sensor’s activities for continuous reliable measurements. Compared with common sensor renewal strategies, such as acidic or basic solution cleaning, mechanical polishing, traditional low frequency ultrasound treatment, this electrode surface renewal technique is stable, mathematically predictable, effective and gentle so that it does not cause any destructions to inherent electrode surface, and therefore the sensed electrochemical signals on raw and cleaned electrodes are comparable and improvable with no unexpectable deviation. Together with submillimeter size of the resonator, it shows potential to be integrated within wearable electroanalytical microfluidics for developing smart device to carry out continuous health diagnosis.

Spectral mass gauging in terrestrial gravity and microgravity conditions

Mass gauging of fluids in space systems (such as propellant tanks in upper stages and satellites, or life support systems) is a challenging task due to the absence of buoyancy. The vast number of possible liquid–gas configurations makes some current mass gauging ground techniques unreliable. The Spectral Mass Gauging technique has been recently presented as a promising approach for mass gauging in microgravity conditions given its independence on the liquid configuration. In this technique, the volume of liquid in a tank is determined from the spectral analysis of the frequency response of the tank walls to an acoustic excitation. We present an experimental study of the technique in terrestrial gravity and microgravity. Experiments in weightlessness were carried out in 15 drops at the drop tower of the Center of Applied Space Technology and Microgravity (ZARM) in Germany. We provide a criterion for the spectral analysis based on some characteristics of the Fast Fourier Transform of the acquired data, namely the Root Mean Square and the prominence of the spectral peaks. This criterion is used to study the effects of the acoustic actuation characteristics and the location of the data acquisition devices on the application of the acoustic technique in the considered scenarios. Our analysis also shows an independence of the technique on the gravity level, which confirms the feasibility of its use in space. The influence of the tank size on the applicability of this technique is discussed too.