Concentration of Microparticles Using Flexural Acoustic Wave in Sessile Droplets

Cavity-agnostic acoustofluidic manipulations enabled by guided flexural waves on a membrane acoustic waveguide actuator

This article presents an in-depth exploration of the acoustofluidic capabilities of guided flexural waves (GFWs) generated by a membrane acoustic waveguide actuator (MAWA). By harnessing the potential of GFWs, cavity-agnostic advanced particle manipulation functions are achieved, unlocking new avenues for microfluidic systems and lab-on-a-chip development. The localized acoustofluidic effects of GFWs arising from the evanescent nature of the acoustic fields they induce inside a liquid medium are numerically investigated to highlight their unique and promising characteristics. Unlike traditional acoustofluidic technologies, the GFWs propagating on the MAWA's membrane waveguide allow for cavity-agnostic particle manipulation, irrespective of the resonant properties of the fluidic chamber. Moreover, the acoustofluidic functions enabled by the device depend on the flexural mode populating the active region of the membrane waveguide. Experimental demonstrations using two types of particles include in-sessile-droplet particle transport, mixing, and spatial separation based on particle diameter, along with streaming-induced counter-flow virtual channel generation in microfluidic PDMS channels. These experiments emphasize the versatility and potential applications of the MAWA as a microfluidic platform targeted at lab-on-a-chip development and showcase the MAWA's compatibility with existing microfluidic systems.

Analysis and numerical investigation of bile flow dynamics within the strictured biliary duct

The mechanics of bile flow in the biliary system plays an important role in studying bile stasis and gallstone formation. Bile duct stricture is an abnormal phenomenon that refers to the bile duct getting smaller or narrower. The main objective of this study is to study the influence of stricture on bile flow dynamics using numerical methods. We employed a numerical Computational Fluid Dynamics model of the bile flow within a strictured hepatic duct. We studied and compared the influence of stricture severity, stricture length, eccentricity, and bile flow property on the bile flow dynamics. The bile flow velocity, pressure distribution, pressure drop, and wall shear stress are provided in detail. The stricture alters the normal bile flow pattern and increases flow resistance. At the location upstream and downstream of the stricture, bile flow slows down. In the area of the stricture throat, bile flow is accelerated, and recirculation forms behind the stricture. The maximum pressure drop of the biliary system increases with the stricture length. The eccentricity makes the flow deflect away from the duct's centerline. The behavior of the deflected flow is significantly altered downstream of the stricture. Such bile flow behavior as deceleration and recirculation may lead to cholestasis. Stricture alters bile flow in the biliary tract, causing changes in biliary hydrodynamic indexes, which could potentially serve as an omen for gallstone formation and other related diseases. The consideration of the bile duct stricture could lead to better patient stratification.

Skipping the Boundary Layer: High-Speed Droplet-Based Immunoassay Using Rayleigh Acoustic Streaming

Acoustic mixing of droplets is a promising way to implement biosensors that combine high speed and minimal reagent consumption. To date, this type of droplet mixing is driven by a volume force resulting from the absorption of high-frequency acoustic waves in the bulk of the fluid. Here, we show that the speed of these sensors is limited by the slow advection of analyte to the sensor surface due to the formation of a hydrodynamic boundary layer. We eliminate this hydrodynamic boundary layer by using much lower ultrasonic frequencies to excite the droplet, which drives a Rayleigh streaming that behaves essentially like a slip velocity. At equal average flow velocity in the droplet, both experiment and three-dimensional simulations show that this provides a three-fold speedup compared to Eckart streaming. Experimentally, we further shorten a SARS-CoV-2 antibody immunoassay from 20 min to 40 s taking advantage of Rayleigh acoustic streaming.

Particle Manipulation in 2D Space Using a Capacitive Micromachined Ultrasonic Transducer

Ultrasonic particle manipulation is a noncontact method for controlling microscale objects, such as cells or microparticles, using an acoustic field. In this study, a 2D array of capacitive micromachined ultrasonic transducers (CMUTs), placed horizontally in immersion, generated ultrasonic waves in the vertical direction, and the oil’s surface increased due to the radiation force of the ultrasonic waves. In addition, the radiation force directly exerted a force on a floating particle. By measuring the movement of the reflected laser light by the moving oil surface, the height of the oil’s surface deformed by the acoustic radiation force (ARF) was measured. The ARF made a floating particle, as well as the oil’s surface, move. The particle moved radially away from the surface position above the transducer, and its velocity was determined by its position on the fluid’s surface. When a single channel was operated, it moved 0.4 mm at an average speed of 90 μm/s, and when two adjacent channels were operated, it moved 1.2 mm at a speed of 272 μm/s. The particles moved in any direction on the surface of the oil by controlling the actuation channel using an electrical switch.