The Microfluidics Module
The Microfluidics Module is used by engineers and scientists to understand, predict, and design microfluidic systems. The use of simulation tools in the design cycle is becoming more widespread as they can improve understanding, reduce prototyping costs, and speed up development. The Microfluidics Module allows users to quickly and accurately model single-phase flows, multiphase flows, flow through porous media, electrokinetic flows, and slightly rarefied gas flows.
The Microfluidics Module can solve stationary and time-dependent flows in two-dimensional and three-dimensional spaces. Formulations suitable for different types of flow are set up as predefined physics interfaces, referred to as Microfluidics physics interfaces. The Fluid Flow interfaces use physical quantities, such as pressure and flow rate, and physical properties, such as viscosity and density, to define a fluid-flow problem. Different physics interfaces are available to cover a range of microfluidic flows. Examples include: laminar flow, creeping flow, two-phase flow (phase field, level set, and moving mesh), three-phase flow (phase field), porous media flow (Darcy’s Law, the Brinkman equations, Free and Porous Media Flow, Brinkman — which combines the Brinkman equations with laminar flow, or Free and Porous Media Flow, Darcy — which combines Darcy’s Law with laminar flow) and slip flow. The transport of multiple species can also be treated with the Transport of Diluted Species interface. These physics interfaces can easily be coupled with, for example, the Electrostatics or Electric Currents interfaces in COMSOL Multiphysics to solve multiphysics problems such as electrokinetic flows.
For each of the Microfluidics physics interfaces, the underlying physical principles are expressed in the form of partial differential equations, together with corresponding initial and boundary conditions. COMSOL’s design emphasizes the physics by providing users with the equations solved by each feature and offering the user full access to the underlying equation system. There is also tremendous flexibility to add user-defined equations and expressions to the system. For example, to model the transport of a species that significantly affects the viscosity of the fluid, simply type in a concentration-dependent viscosity — no scripting or coding is required (this is demonstrated in the Tutorial Model: A Controlled Diffusion Separator). When COMSOL Multiphysics compiles the equations, the complex couplings generated by these user-defined expressions are automatically included in the equation system. The equations are then solved using the finite element method and a range of industrial-strength solvers. Once a solution is obtained a vast range of postprocessing tools are available to interrogate the data, and predefined plots are automatically generated to show the device response. COMSOL Multiphysics offers the flexibility to evaluate a wide range of physical quantities including predefined quantities such as the pressure, velocity, shear rate, or the vorticity (available through easy-to-use menus), as well as arbitrary user-defined expressions.
To model a microfluidic device, the geometry is first defined in the software. Then appropriate materials are selected and a suitable Microfluidics physics interface is added. Initial conditions and boundary conditions are set up within the physics interface. Next, the mesh is defined; in many cases the default mesh, which is produced from physics-dependent defaults, will be appropriate for the problem. A solver is selected, again with defaults appropriate for the relevant physics interface, and the problem is solved. Finally the results are visualized. All these steps are accessed from the COMSOL Desktop.