5 Best Practices to Managing Bubbles in Microfluidics

It feels like no matter the project, if fluidics are involved, the topic of bubbles pops back up.  Lab-on-a-chip is a growing industry, and miniaturizing a fluidic system comes with a unique set of bubble challenges.  When down at the microfluidic scale, a bubble has very significant impacts on fluid flow. This impact can be leveraged, as in the case of separating fluid slugs, or it can pose a hindrance, such as when filling an empty cavity with fluid.  The focus of this discussion will be on filling cavities within microfluidic chambers.

All microfluidic systems begin their life empty of fluid, so all the channels and chambers will at some point need to fill with fluid.  But how do you coax the air out of the cavity in order to replace it with fluid?  There are two different directions this could go – initially filling the cavity without ever allowing a bubble to form, or filling the space around the cavity and then “debubbling”.

Methods for initially filling the cavity without air include the following:

  1. Create a vacuum. One approach is to not start with air in that cavity.  If the system allows, creating a vacuum inside the fluid chamber will remove the air, and therefore once fluid is introduced to the system it will have no choice but to fill the void.
  2. Modify the chip’s surface energy. A bubble of air does not collapse when surrounded by fluid because the surface tension of that fluid makes the fluid molecules want to continue sticking together.  As a fluid flows over a cavity, if that cavity is small enough, then the fluid will want to stick together as it flows over the top instead of falling into the space.  However if the surface energy of the chip is lowered, the fluid molecules will want to stick to it more than to each other, causing them to wick into the cavity.
  3. Modify the fluid’s surface tension. A similar approach to the previous point, but now fluids called surfactants are added to your desired fluid, making it less likely to stick to itself instead of the chip’s surface.

If it is inevitable to fill the chamber with a bubble remaining in the cavity, there are a few options which can be used to debubble it:

  1. Increase the size of the bubble. The bubble will form to be roughly the volume of the cavity when the fluid flows overtop.  By reducing the pressure or increasing the temperature, the volume of the bubble will increase (Boyle’s and Charles’ laws, respectively).  Make the bubble large enough and it will rise out of the well.
  2. Change the weight of the fluid. Weight is determined by both mass and gravity, so by increasing gravity in the chamber we are increasing the weight of the fluid above the bubble.  How do you increase gravity?  That is the principle behind a centrifuge, which uses rotary motion and acceleration to create a centrifugal force that separates materials based on their density.  If the chip can be placed in a centrifugal system, the increased weight of the fluid and resulting increase in buoyancy forces will force the bubble out of the cavity and to float to the top.  Then it can be similarly flowed out of the chamber.

The correct tool for avoiding bubbles in a microfluidic cavity is going to be dependent on the specifics of your lab-on-a-chip system, but don’t fear because there should be a way!

Danica Gordon
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