Analysis of fluid-structure interaction in micromixers with vibrating walls

Rapid mixing in micro-channels plays a significant role in the chemical, biological and medical fields. The majority of microfluidic applications is encountered in very low Reynolds number regime. Thus, two reactive fluids are predominantly parallel when they flow along the length of the microchannel. Generally, obstacles or surface modifications are made in the flow path to increase mixing of the fluids. With the development of deformable and flexible micro-channels, the focus has now shifted to the development of micromixers with moving or vibrating walls. In the present work an attempt has been made to numerically model the working of a micromixer with vibrating walls. This has been accomplished by computationally solving the coupled set of equations governing the elasticity of the walls and fluid motion (Fluid structure Interaction). Mixing index, which is a measure of effectiveness of mixing is calculated based on the species concentration at a particular region of interest. External sinusoidal vibrations are applied on the walls of the microchannel of size 225 μm. The effect of vibration on mixing index is studied. Optimization study is taken up to obtain the optimal value of vibration amplitude and frequency to achieve enhanced mixing with reduced pressure drop. The optimal parameters so obtained are observed to function well in the Reynolds number range 0.5-50 in which the entire numerical analysis has been carried out INTRODUCTION Microreaction technology [1] has enormous applications in the field of chemical and biological sciences. Increased surface to volume ratio in microchannel leads to high heat and mass transfer rates, which makes it more suitable for chemical reactions than conventional channels. A few applications in which microreaction technology has proved its impact include biological screening, polymerization, protein folding, drug delivery and organic compound synthesis. Several advantages of the micro total analysis system (μ-TAS) include an abated amount of sample and reagent volume consumption, low cost, reasonable portability and augmented reaction time [2]. However, the flow behaviour in microchannels limits the development in the field of microfabrication technology. The flow through a microchannel occurs at very low Reynolds numbers. Most flows in microchannels are observed in low laminar or creeping regimes. Thus the flow of two fluids during reaction is predominantly parallel or separated [3]. The separated flow of the two components reduces the pervasion of one flow stream component into the other which slows down the chemical reaction rate. This necessitates development of a device which induces chaos or diffusion in the fluid regime that enhances mixing. Micromixer [4] is an important component in chemical reaction technology that performs the task of inducing chaos or diffusion to achieve rapid mixing between the fluids. Effective mixing is established in micromixers of two kinds, active and passive [5]. Active micromixers use external force which may be electric [6], magnetic [7] or acoustic in nature [8] to induce chaos in the flow regime. In passive mixers, mixing occurs by virtue of modification in geometry and spatial orientation that either induces re-circulations or increases the effect of diffusion [9]. The primary objective of a passive micromixer is to increase the contact surface between the fluids by modifying the flow surfaces. The most primitive design of a passive micromixer essentially implements parallel laminations [10]. In this type of micromixer, a single inlet fluid stream is divided into multiple streams which are combined together later. This induces increased diffusion at the downstream of the flow channel. Different modifications were made to parallel micromixers and were presented in detail in [11-12]. In one such modification called injection micromixers, solute flow alone is divided into sub-streams which are later injected into the solvent [13]. The injection of solutes into solvent is carried out using nozzles. The above concepts were based on diffusion of one component into another. Yet another way of classifying micromixers could be based on the nature and magnitude of chaos induced in the flow field. A few examples of these micromixers include mixers with cylindrical obstacles [14], zig-zag micromixers [15] and micromixers with tesla structures. The latter class of micromixers that induce chaotic advection are quite desirable in very low flow rate applications where enhancement of diffusion is difficult. However, these micromixers face a major drawback because they have a large pressure drop associated with them. In the present work, an attempt is made to strengthen the concept of vibrating micromixers. Developments in material science in micro-environment have led to the use of deformable or flexible micromixers [16-17]. In the present work, an attempt is made to numerically model the working of a micromixer with vibrating walls and to optimize the physical variables such as vibration frequency and amplitude. Finite element modelling of fluid structure interaction is done to study the enhancement of mixing in micromixers with vibrating walls. Working physics of the vibrating wall micromixers is elucidated and the parametric 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics