Reduction of Microfluidic Logical Pneumatic Mono Stable-Oscillators’ Reversible Outlet Flow

1. Introduction

Microfluidic technologies have experienced significant growth and development in recent years, emerging as one of the most rapidly advancing fields in scientific research. Among these technologies, microactuators have garnered considerable attention due to their critical role in regulating and manipulating various processes. The development of microactuators dates back to the late 1970s, and since then, there have been substantial advancements in the field of Micro-Electro-Mechanical Systems (MEMS) actuators, particularly in terms of efficiency, power, and force output. These advancements have made microactuators indispensable in emerging industrial and medical applications, where compact, cost-effective, and high-performance devices are increasingly sought after [1].

Fluidic microactuators, which utilize pressurized fluids—either gases (pneumatic) or liquids (hydraulic)—to induce motion through the deformation of inflatable chambers, have demonstrated exceptional power and force densities at the microscale [2]. Their notable durability and resistance to environmental factors further enhance their appeal. Despite these advantages, fluidic microactuators often receive less attention compared to other MEMS technologies, leaving significant potential for further exploration and innovation.

This study focuses on fluidic actuators, with particular emphasis on their operational principles, classifications, and characteristics. Fluidic actuators enable fluid motion through two primary mechanisms: (1) direct peristaltic motion induced by actuated solid membranes, or (2) indirect motion driven by hydrodynamic or osmotic effects. These mechanisms facilitate rotational, translational, and deflection motions, making fluidic actuators versatile tools for a wide range of applications.

One notable category of fluidic actuators is the amplifier fluidic actuator, which operates based on the Coanda effect. This effect ensures the attachment of the main jet from the supply flow to the wall, producing an output flow at the device's end. By altering the control flow or pressure, the main jet can be redirected to another output, achieving stable functionality. However, the process of jet attachment and switching is complex and has been the subject of extensive research [4,5]. These fluidic elements perform functions analogous to electronic systems, offering unique advantages in microfluidic applications.

Despite their potential, fluidic actuators face challenges, particularly in controlling reversible outlet flow in mono-stable oscillators. This study addresses this issue by proposing a novel approach to reduce reversible outlet flow, thereby enhancing the performance and reliability of microfluidic systems. The general design for a fluidic element from this category is presented in Figure 1

This overview primarily examines fluidic actuators, with a particular emphasis on their operational principles, classifications (see Table 1), and distinguishing features.

2. Simulation of Fluidic Actuation with Mobile Object

The increasing demand for microsystems in medical and military applications has driven the development of advanced actuation systems. In this study, we simulate mono-stable fluidic systems incorporating mobile components to manipulate the shape and output chamber of the moving object, thereby optimizing actuator performance.

Description of the Geometrical Model (Combination of Oscillator and Actuator)

A fluidic actuator measuring 5 × 15 mm² was designed for numerical simulation using FLUENT. The actuator features a moving part with two inlets and one outlet. To evaluate the system's behavior and optimize performance, the moving part was modeled in three distinct shapes: sphere, square, and H-shaped, each with dimensions of 4.8 mm.

The simulation process involved the following steps:

1-Geometrical Modeling: The actuator and oscillator were combined into a single model.

2-Mesh Generation: A triangular mesh was created, consisting of 166,782 nodes, to ensure accurate simulation results.

3-Dynamic Mesh Model: A dynamic mesh model was employed to track the movement of the mobile object under fluid forces.

4-The geometry of the actuator was modified to ensure convergence at the two inlets, aligning with the oscillator's outlet dimensions. This configuration is illustrated in Figure 2

The second part of this study constitutes its core contribution. It involves the integration of the ball actuator with the fluidic oscillator to enhance the oscillator's performance by minimizing reverse fluid flow through the right and left outlet holes, as well as the oscillator itself. To achieve this, the actuator's geometry was modified to ensure convergence at the two inlets, aligning it with the oscillator's outlet dimensions. This modified geometry is illustrated in Figure 3

To accommodate the new geometry, several modifications were made to the mesh. The primary objective was to achieve a mesh with a low maximum aspect ratio and minimal asymmetry. The final mesh used in the FLUENT simulations, presented in Figure 4, consists of a triangular structure comprising 166,782 nodes. To ensure optimal balance between result accuracy and computational efficiency, additional meshes with varying densities (41,000 nodes and 442,986 nodes) were also generated and evaluated.

As the moving part (the sphere) shifts in response to fluid forces acting on its boundaries, the mesh boundaries must adapt accordingly. To address this, a dynamic mesh model was employed. The dynamic mesh capabilities of FLUENT, combined with a user-defined function, were utilized to accurately track the movement of the sphere throughout the simulation.

3. Simulation Results

The mass flow rate at the exit of the actuator, corresponding to the geometry of case (b), is depicted in Figure 5. This figure illustrates the mass flow rate under a pressure variation of P = 3 bar during both the forward and return motion of the spherical mobile object Figure 6..

Mobile Forward phase

Mobile return phase

3.1. Evolution of Mass Flow at the Left and Right Outputs of the Oscillator-Actuator Combination

Figure 7 depicts the mass flow rate as a function of time at the actuator's output under applied pressures of 2 bar and 2.5 bar, respectively. The results indicate that the return flow at both the left and right outputs of the oscillator is nearly negligible.

3.2. Evolution of Mobile Velocity

Figure 8 illustrates the axial velocity profiles of the sphere's motion for supply pressures of P=1.5 bar and P=2 bar at the oscillator inlet. The profiles exhibit a distinct sawtooth pattern, with the velocity reaching a peak of 10 m/s at a pressure of 2 bar.

3.3. Trend Curve

The fluid oscillator frequency at the millimeter scale was simulated using CFD. The initial numerical results indicate that the fluid oscillator frequency increases with an increasing ΔP/P ratio, within a pressure range of 1.5 to 3 bars Figure 9.

5. Conclusions

The performance and compactness of microfluidic actuators have been significantly enhanced through the implementation of various mechanisms and innovative combinations. High-performance actuators, characterized by their elevated force and power density alongside minimal power consumption, are in great demand for a wide range of applications, including minimally invasive surgical procedures and micro-robotic systems.

Numerical simulations have been conducted to assess the efficiency of these microactuators, with particular attention given to the internal channel length scale and the diameter of the digital flow. However, accurately modeling the complexity of the diode geometry remains challenging due to the involvement of high-pressure conditions.

This study has proven to be instrumental in addressing several challenges faced by our team. Notably, it has provided effective solutions to the backflow issues encountered with mini-injectors, as reported in previous works in literature.