A Numerical Investigation of the Mixing Performance in a Y-Junction Microchannel Induced by Acoustic Streaming.

In this study, the mixing performance in a Y-junction microchannel with acoustic streaming was investigated through numerical simulation. The acoustic streaming is created by inducing triangular structures at the junction and sidewalls regions. The numerical model utilizes Navier-Stokes equations in conjunction with the convection-diffusion equations. The parameters investigated were inlet velocities ranging from 4.46 to 55.6 µm/s, triangular structure's vertex angles ranging from 22° to 90° oscillation amplitude ranging from 3 to 6 µm, and an oscillation frequency set to 13 kHz. The results show that at the junction region, a pair of counter-rotating streaming vortices were formed, and unsymmetrical or one-sided vortices were formed when additional triangles were added along the sidewalls. These streaming flows significantly increase the vorticity compared with the case without the acoustic stream. Mixing performances were found to have improved with the generation of the acoustic stream. The mixing performance was evaluated at various inlet velocities, the vertex angles of the triangular structure, and oscillation amplitudes. The numerical results show that adding the triangular structure at the junction region considerably improved the mixing efficiency due to the generation of acoustic streaming, and further improvements can be achieved at lower inlet velocity, sharper vertex angle, and higher oscillation amplitude. Integrating with more triangular structures at the sidewall regions also improves the mixing performance within the laminar flow regime in the Y-microchannel. At Y = 2.30 mm, oscillation amplitude of 6 µm, and flow inlet velocity of 55.6 µm/s, with all three triangles integrated and the triangles' vertex angles fixed to 30°, the mixing index can achieve the best results of 0.9981, which is better than 0.8355 in the case of using only the triangle at the junction, and 0.6642 in the case without acoustic streaming. This is equal to an improvement of 50.27% in the case of using both the junction and the two sidewall triangles, and 25.79% in the case of simply using a junction triangle.

Mixing enhancement in T-junction microchannel with acoustic streaming induced by triangular structure.

The T-shaped microchannel system is used to mix similar or different fluids, and the laminar flow nature makes the mixing at the entrance junction region a challenging task. Acoustic streaming is a steady vortical flow phenomenon that can be produced in the microchannel by oscillating acoustic transducer around the sharp edge tip structure. In this study, the acoustic streaming is produced using a triangular structure with tip angles of 22.62°, 33.4°, and 61.91°, which is placed at the entrance junction region and mixes the inlets flow from two directions. The acoustic streaming flow patterns were investigated using micro-particle image velocimetry (µPIV) in various tip edge angles, flow rate, oscillation frequency, and amplitude. The velocity and vorticity profiles show that a pair of counter-rotating streaming vortices were created around the sharp triangle structure and raised the Z vorticity up to 10 times more than the case without acoustic streaming. The mixing experiments were performed by using fluorescent green dye solution and de-ionized water and evaluated its performance with the degree of mixing (M) at different amplitudes, flow rates, frequencies, and tip edge angles using the grayscale value of pixel intensity. The degree of mixing characterized was found significantly improved to 0.769 with acoustic streaming from 0.4017 without acoustic streaming, in the case of 0.008 µl/min flow rate and 38 V oscillation amplitude at y = 2.15 mm. The results suggested that the creation of acoustic streaming around the entrance junction region promotes the mixing of two fluids inside the microchannel, which is restricted by the laminar flow conditions.

Comparison of Acoustic Streaming Flow Patterns Induced by Solid, Liquid and Gas Obstructions.

In this study, acoustic streaming flows inside micro-channels induced by three different types of obstruction-gaseous bubble, liquid droplet and solid bulge-are compared and investigated experimentally by particle tracking velocimetry (PTV) and numerically using the finite element method (FEM). The micro-channels are made by poly(dimethylsiloxane) (PDMS) using soft lithography with low-cost micro-machined mold. The characteristic dimensions of the media are 0.2 mm in diameter, and the oscillation generated by piezoelectric actuators has frequency of 12 kHz and input voltages of 40 V. The experimental results show that in all three obstruction types, a pair of counter-rotating vortical patterns were observed around the semi-circular obstructions. The gaseous bubble creates the strongest vortical streaming flow, which can reach a maximum of 21 mm/s, and the largest u component happens at Y/D = 0. The solid case is the weakest of the three, which can only reach 2 mm/s. The liquid droplet has the largest v components and speed at Y/D = 0.5 and Y/D = 0.6. Because of the higher density and incompressibility of liquid droplet compared to the gaseous bubble, the liquid droplet obstruction transfers the oscillation of the piezo plate most efficiently, and the induced streaming flow region and average speed are both the largest of the three. An investigation using numerical simulation shows that the differing interfacial conditions between the varying types of obstruction boundaries to the fluid may be the key factor to these differences. These results suggest that it might be more energy-efficient to design an acoustofluidic device using a liquid droplet obstruction to induce the stronger streaming flow.

Role of vessel-to-prosthesis size mismatch in venous valve performance.

BACKGROUND: Efforts to treat chronic venous insufficiency have focused on the development of prosthetic venous valves. The role of prosthetic valve-to-vessel size matching has not been determined. The purpose of this investigation was to assess the effect of size mismatching on venous valve function and to establish a mismatch limit that affects valve hemodynamic performance and venous wall stress to improve future valve designs and implants. METHODS: Flow dynamics of prosthetic venous valves were studied in vitro using a pulse duplicator flow loop. Valve performance based on flow rate and pressure measurements was determined at oversizing ratios ranging from 4.2% to 25%. Valve open area ratios at different size mismatching ratios were investigated by image analysis. Finally, a wall stress analysis was used to determine the magnitude of circumferential (hoop) stress in the venous wall at various degrees of oversizing. RESULTS: Our findings indicate that valve regurgitate volume, closing time, and pressure difference across the valve are significantly elevated at mismatch ratios greater than ∼15%. This is supported by increases in regurgitate velocity and open area relative to valves tested at near-nominal diameters. At this degree of size mismatch, the wall stress is increased by a factor of two to three times relative to physiologic pressures. CONCLUSIONS: These findings establish a relationship between valve size matching and valve hemodynamic performance, including vessel wall stress, which should be considered in future valve implants. The size of the prosthetic valve should be within 15% of maximum vein size to optimize venous valve hemodynamic performance and to minimize the hoop wall stress.

Hemodynamic coupling of a pair of venous valves.

BACKGROUND: In vivo studies have shown that valves in veins are paired in an orthogonal configuration. The aim of this study is to characterize the flow interaction of paired valves under controlled in vitro bench conditions. METHODS: A bench top in vitro experiment was set up at physiological flow conditions to simulate the flow inside a venous valve. Two bicuspid bioprosthetic valves paired in 0° and 90° orientations were tested in a 12-mm-diameter tube, and the two-dimensional velocity fields around the valve were measured by particle image velocimetry. The distance between the two valves was varied from 3 to 5 cm, and the corresponding velocities and vorticities were determined. RESULTS: Velocity field shows the flow exit from the orthogonal valve-pairing configurations forced the main jet stream to turn to the outer region of the tube. Flow patterns between the valves show significantly less stagnation region from the 90° valve pairing over a 0° valve pairing case. The variation in valves distance shows that the coupling effect of the two valves extends to a range beyond four times of the tube diameter, albeit the ability to alter the flow decreases at larger distances. CONCLUSIONS: The findings suggest that the 90° valve pairing configuration regulates the flow between the valves, and the separation distance affects the hemodynamic efficiency of the two valves by reducing the total reverse flow volume.