Mixing Efficiency Analysis on Droplet Formation Process in Microchannels by Numerical Methods

Formation of magnetic double emulsions under steady and variable magnetic fields from a 3D-printed coaxial capillary device

BACKGROUND

Double emulsions (DEs) have attracted researchers' attention to be utilized as a promising platform in biomedical and chemical applications. Several actuation mechanisms have been proposed for the generation of DEs. The conventional DE formation approaches (e.g. two-stage emulsification) suffer from low monodispersity. The electric actuation (i.e. coaxial electrospray technology) has been demonstrated as a controllable method for the DE formation, while the capability of magnetic actuation has not been studied yet.

RESULT

In the present study, the generation of ferrofluid double emulsions (FDEs), made from water-based ferrofluid as a core and oil as a shell, under the magnetic actuation of a permanent magnet with a steady magnetic field and an electromagnet with DC and pulse width modulation (PWM) magnetic fields was investigated with a simple controllable setup fabricated using 3D printing. The effect of various parameters affecting the FDE formation, such as the fluid flow rates, the magnetic field type, the magnetic flux density, and the PWM frequency and duty cycle, on the FDE formation characteristics, including the inner and outer equivalent diameters, and the formation frequency was studied. Under the steady magnetic field, two regimes of the FDE formation were identified: inertia-dominated and magnet-dominated.

SIGNIFICANCE

Wireless power-free magnetic actuation provides better control over the FDE formation, enhancing this process by increasing the FDE formation frequency with high monodispersity. The PWM magnetic field offers excellent controllability over the FDE formation with low-volume or no, in some cases, satellite droplets by tuning the PWM frequency and the duty cycle.

Particle Image Velocimetry of 3D-Printed Anatomical Blood Vascular Models Affected by Atherosclerosis

Improvements in the diagnosis and treatment of cardiovascular diseases facilitate a better understanding of the ongoing process. The study of biomedical fluid dynamics using non-intrusive visualizing methods on a micro-scale has become possible using a proper 3D printing process. The computed tomography scan of a patient with atherosclerosis was processed, and a 3D-printed artery with an inlet diameter of 4.2 mm was developed and measured using three different constant flow rates. To mimic blood, a solution of glycerin and water was used. The procedure to obtain a proper 3D-printed model using low-force stereolithography technology with high-quality optical access usable for PIV was described and discussed. The paper presents the results of PIV as multi-stitched, color-coded vector maps from the axis cross section along the whole 3D-printed model. The obtained data allowed a resolution of 100 × 100 µm per single vector to be achieved. Furthermore, the results of the stitched 16 base images of the artery and the 3D-printed model prepared were included. The results of this study show that 3D prints allow for the creation of the desired geometry and can be used to investigate severe pathologies of the human circulatory system. The strengths and weaknesses of this methodology were discussed and compared to other techniques used to obtain transparent objects.

Secondary Flows, Mixing, and Chemical Reaction Analysis of Droplet-Based Flow inside Serpentine Microchannels with Different Cross Sections

Chemical bioreactions are an important aspect of many recent microfluidic devices, and their applications in biomedical science have been growing worldwide. Droplet-based microreactors are among the attractive types of unit operations, which utilize droplets for enhancement in both mixing and chemical reactions. In the present study, a finite-volume-method (FVM) numerical investigation is conducted based on the volume-of-fluid (VOF) applying for the droplet-based flows. This multiphase computational modeling is used for the study of the chemical reaction and mixing phenomenon inside a serpentine microchannel and explores the effects of the aspect ratio (i.e., AR = height/width) of rectangular cross-sectional geometries as well as three other cross-sectional geometries including trapezoidal, triangular, and circular, on consumption and production rates of chemical species. It is found that in these droplet bioreactors, the reaction begins from the forward section of the droplet. We investigate the secondary flows and chemical reactions inside the droplets in a serpentine microchannel with different cross-sectional geometries. Different transient Dean vortices and secondary flows in the presence and absence of the droplets are studied and explained based on the position of the droplets. It is found that as the droplets pass through the microchannel turns, the patterns and magnitude of the secondary flows change, depending on the cross-sectional geometry. Eventually, the results demonstrate that the AR = 2 rectangular cross-section is the most helpful geometry, whereas the trapezoidal cross-section takes into account the least efficient one between all geometries.

Influence of channel height on mixing efficiency and synthesis of iron oxide nanoparticles using droplet-based microfluidics

Experimental and CFD numerical analysis of mixing efficiency in droplet-based microfluidics for various channel heights and its impact on the preparation of iron oxide nanoparticles.

Lattice Boltzmann Simulation on Droplet Flow through 3D Metal Foam

The hydrodynamics of droplets passing through metal foam is investigated using the lattice Boltzmann method (LBM). The accurate 3D porous structure for the simulation is generated by X-ray micro-computed tomography. The simulated results are in good agreement with the experimental ones using high-speed video. The simulated results show that for droplets passing metal foam, there is a critical capillary number, Cac (around 0.061), above which the droplet continues to deform until it breaks up. The simulated results show that the capillary number, droplet size, pores diameter, and thickness of metal foam have the significant effect of droplets deforming and breaking up when the droplets pass through the metal foam. To avoid the calescence of two droplets at the inlet zone of the metal foam, the distance between droplets should be larger than three times the diameter of the droplet.

Effects of a Dynamic Injection Flow Rate on Slug Generation in a Cross-Junction Square Microchannel

The injection flow rates of two liquid phases play a decisive role in the slug generation of the liquid-liquid slug flow. However, most injection flow rates so far have been constant. In order to investigate the effects of dynamic injection flow rates on the slug generation, including the slug size, separation distance and slug generation cycle time, a transient numerical model of a cross-junction square microchannel is established. The Volume of Fluid method is adopted to simulate the interface between two phases, i.e., butanol and water. The model is validated by experiments at a constant injection flow rate. Three different types of dynamic injection flow rates are applied for butanol, which are triangle, rectangular and sine wave flow rates. The dynamic injection flow rate cycles, which are related to the constant slug generation cycle time t0, are investigated. Results show that when the cycle of the disperse phase flow rate is larger than t0, the slug generation changes periodically, and the period is influenced by the cycle of the disperse phase flow rate. Among the three kinds of dynamic disperse flow rate, the rectangular wave influences the slug size most significantly, while the triangle wave influences the separation distance and the slug generation time more prominently.

A Numerical Analysis of Pressure Pulsation Characteristics Induced by Unsteady Blood Flow in a Bileaflet Mechanical Heart Valve

The leaflet vibration phenomenon in bileaflet mechanical heart valves (BMHVs) can cause complications such as hemolysis, leaflet damage, and valve fracture. One of the main reasons for leaflet vibration is the unsteady blood flow pressure pulsation induced by turbulent flow instabilities. In this study, we performed numerical simulations of unsteady flow through a BMHV and observed pressure pulsation characteristics under different flow rates and leaflet fully opening angle conditions. The pressure pulsation coefficient and the low-Reynolds k-ω model in CFD (Computational Fluid Dynamics) software were employed to solve these problems. Results showed that the level of pressure pulsation was highly influenced by velocity distribution, and that the higher coefficient of pressure pulsation was associated with the lower flow velocity along the main flow direction. The influence of pressure pulsation near the trailing edges was much larger than the data obtained near the leading edges of the leaflets. In addition, considering the level of pressure pulsation and the flow uniformity, the recommended setting of leaflet fully opening angle was about 80°.

Cavitating Flow through a Micro-Orifice

Microfluidic systems have witnessed rapid development in recent years. As one of the most common structures, the micro-orifice is always included inside microfluidic systems. Hydrodynamic cavitation in the micro-orifice has been experimentally discovered and is harmful to microfluidic systems. This paper investigates cavitating flow through a micro-orifice. A rectangular micro-orifice with a l/d ratio varying from 0.25 to 4 was selected and the pressure difference between the inlet and outlet varied from 50 to 300 kPa. Results show that cavitation intensity increased with an increase in pressure difference. Decreasing exit pressure led to a decrease in cavitation number and cavitation could be prevented by increasing the exit pressure. In addition, the vapor cavity also increased with an increase in pressure difference and l/d ratio. Results also show the pressure ratio at cavitation inception was 1.8 when l/d was above 0.5 and the cavitation number almost remained constant when l/d was larger than 2. Moreover, there was an apparent difference in cavitation number depending on whether l/d was larger than 1.