Image made using the COMSOL Multiphysics® software and is provided courtesy of COMSOL.
An ink droplet is ejected through a nozzle and travels through air until it hits the target. The model can be used to understand the effect of the ink properties and the pressure profile at the nozzle on the drop velocity, drop volume, and the presence of satellite drops.
The Microfluidics Module brings you easily-operated tools for studying microfluidic devices. Important applications include simulations of lab-on-a-chip devices, digital microfluidics, electrokinetic and magnetokinetic devices, and inkjets. The Microfluidics Module includes ready-to-use user interfaces and simulation tools, so called physics interfaces, for single-phase flow, porous media flow, two-phase flow, and transport phenomena.
Microfluidic flows occur on length scales that are orders of magnitude smaller than macroscopic flows. Manipulation of fluids at the microscale has a number of advantages – typically microfluidic systems are smaller, operate faster, and require less fluid than their macroscopic equivalents.
Energy inputs and outputs are also easier to control (for example, heat generated in a chemical reaction) because the surface-to-area volume ratio of the system is much greater than that of a macroscopic system. In general, as the length scale of the fluid flow is reduced, properties that scale with the surface area of the system become comparatively more important than those that scale with the volume of the flow.
This is apparent in the fluid flow itself as the viscous forces, which are generated by shear over the isovelocity surfaces, dominate over the inertial forces. The Reynolds number (Re) that characterizes the ratio of these two forces is typically low, so the flow is usually laminar. In many cases, the creeping (Stokes) flow regime applies (Re«1). Laminar and creeping flows make mixing particularly difficult, so mass transport is often diffusion limited, but even in microfluidic systems diffusion is often a slow process. This has implications for chemical transport within microfluidic systems. The Microfluidics Module is designed specifically for handling momentum, heat, and mass transport with special considerations for fluid flow at the microscale.
1 Together with the Structural Mechanics Module or MEMS Module
2 Together with the Particle Tracing Module
This example studies a narrow vertical cylinder placed on top of a reservoir filled with water. Because of wall adhesion and surface tension at the air/water interface, water rises through the channel.
Surface tension and wall adhesive forces are often used to transport fluid through microchannels in MEMS devices or to measure, transport and position small amounts of fluid using micropipettes. Multiphase flow through a porous medium and droplets on solid walls are other examples where wall adhesion and surface tension strongly influence the dynamics of the flow.
To model the adhesive forces at the walls correctly, the treatment of the boundary conditions is important. If you fix the velocity to zero on the walls, the interface cannot move along the walls. Instead, you need to allow a non-zero slip velocity and to add a frictional force at the wall. With such a boundary condition, it is possible to explicitly set the contact angle, that is, the angle between the fluid interface and the wall.
The model calculates the pressure field, the velocity field, and the water surface’s shape and position. It uses a level set method, or a phase field method, to track the air/water interface and shows how to add friction and specify the contact angle at the channel walls. The capillary forces dominate over gravity throughout the simulation so that the interface moves upwards during the entire simulation.
This application presents an example of pressure driven flow and electrophoresis in a 3D micro channel system. Researchers often use a device similar to the one in this model as an electrokinetic sample injector in biochips to obtain well-defined sample volumes of dissociated acids and salts and to transport these volumes.
Focusing is obtained through pressure driven flow of the sample and buffer solution, which confines the sample in the focusing channel. When steady state has been obtained, the pressure driven flow is turned off and an electric field is applied along the channels.
The electric field drives the dissociated sample ions that are found in the focusing zone at right angles to the focusing channel, through the injection channel. This is solved in the time-domain.
This model requires the Chemical Reaction Engineering Module and either the CFD Module or the Microfluidics Module.
At the macroscopic level, systems usually mix fluids using mechanical actuators or turbulent 3D flow. At the microscale level, however, neither of these approaches is practical or even possible. This model demonstrates the mixing of fluids using laminar-layered flow in a MEMS mixer. This model analyzes the steady-state condition of the fluid flow as well as the convection and diffusion of a dissolved substance in a lamella mixer.
This model simulates an H-shaped micro-cell designed for controlled diffusive mixing. The cell puts two different laminar streams in contact for a controlled period of time. The contact surface is well-defined and, by controlling the flow rate, it is possible to control the amount of species that are transported from one stream to the other through diffusion. This example was originally formulated by Albert Witarsa under Professor Bruce Finlayson’s supervision at the University of Washington in Seattle. The work was done as part of a graduate course assignment to evaluate the potential of patents in microfluidics through mathematical modeling.
Microlaboratories for biochemical applications often require rapid mixing of different fluid streams. At the microscale, flow is usually highly ordered laminar flow, and the lack of turbulence makes diffusion the primary mechanism for mixing.
While diffusional mixing of small molecules (and therefore of rapidly diffusing species) can occur in a matter of seconds over distances of tens of micrometers, mixing of larger molecules such as peptides, proteins, and high molecular-weight nucleic acids can require equilibration times from minutes to hours over comparable distances. Such delays are impractically long for many chemical analyses. These problems have led to an intense search for more efficient mixers for microfluidic systems.
This model takes advantage of electroosmosis to mix fluids. The system applies a time-dependent electric field, and the resulting electroosmosis perturbs the parallel streamlines in the otherwise highly ordered laminar flow.
Although initially invented to be used in printers, inkjets have been adopted for other application areas, such as within the life sciences and microelectronics. Simulations can be useful to improve the understanding of the fluid flow and to predict the optimal design of an inkjet for a specific application.
The purpose of this application is to adapt the shape and operation of an inkjet nozzle for a desired droplet size, which depends on the contact angle, surface tension, viscosity, and density of the injected liquid. The results also reveal whether the injected volume breaks up into several droplets before merging into a final droplet at the substrate.
The fluid flow is modeled by the incompressible Navier-Stokes equations together with surface tension, using the level set method to track the fluid interface.
Dielectrophoresis (DEP) occurs when a force is exerted on a dielectric particle as it is subjected to a nonuniform electric field. DEP has many applications in the field of biomedical devices used for biosensors, diagnostics, particle manipulation and filtration (sorting), particle assembly, and more.
The DEP force is sensitive to the size, shape, and dielectric properties of the particles. This allows DEP to be used to separate different kinds of particles, such as various kinds of cells from a mixture. The Red Blood Cell Separation application shows how red blood cells can be selectively filtered from a blood sample in order to isolate red blood cells from platelets.
In the DEP filter device, the red blood cells, which are larger than the platelets, feel a larger force and are thereby deflected more. The two outlets are arranged so that the top outlet catches undeflected particles and only particles that have been deflected can exit from the lower outlet.
The app allows you to vary characteristics of the red blood cells and platelets, as well as the electric field.
Emulsions consist of small liquid droplets immersed in an immiscible liquid and widely occur in the production of food, cosmetics, fine chemicals, and pharmaceutical products. The quality of the product is typically dependent on the size of the droplets. Simulating these processes can help in optimizing these droplets as well as other process variables.
This model studies the volume mass fraction of the immersed fluid in an emulsion. From the results, the creation of the droplet is clearly seen. Factors such as fluid flow and additive chemicals can also be studied to see how they affect droplet size and formation.
This example describes the operation of a drug delivery system that supplies a variable concentration of a water soluble drug. A droplet with a fixed volume of water travels down a capillary tube at a constant velocity. Part of the capillary wall consists of a permeable membrane separating the interior of the capillary from a concentrated solution of the drug. As the drop passes by the membrane, its contact angle changes and the drug dissolves into the water. To model this process a constant flux of the drug is assumed on the capillary wall for the duration of its contact with the membrane. By altering the droplet velocity, the final concentration of the drug in the drop can be adjusted.
The contact angle of a two-fluid interface with a solid surface is determined by the balance of the forces at the contact point. In electrowetting the balance of forces at the contact point is modified by the application of a voltage between a conducting fluid and the solid surface.
In many applications the solid surface consists of a thin dielectric deposited onto a conducting layer; this is often referred to as Electrowetting on Dielectric (EWOD). Electrowetting can be used to modify the contact angle dynamically by changing the voltage applied to the conducting liquid. In this example, the meniscus between two immiscible liquids is used as an optical lens. A change in curvature of the meniscus caused by the electrowetting effect is used to change the focal length of the lens over a large range. This model is based on the work of the Philips FluidFocus team.
This model uses the Laminar Two-Phase Flow, Moving Mesh interface and a time dependent study.
