How To Avoid Bubbles in Microfluidics

The saturated solubility is the amount of gas dissolved in a solution when it is at equilibrium, and the gas entering and leaving the solution are balanced. Saturated solubility is dependent on many factors including, solution type, gas type, temperature and pressure. Bubbles form in a solution when the amount of dissolved gas exceeds its saturated solubility.

Generally speaking, bubble formation becomes problematic with increasing temperature, decreasing pressure, and mixing solvents. The presence of bubbles in a liquid can compromise experimental outcomes, and microfluidics, which uses small dimensioned channels and tubing, is particularly susceptible to bubbles. Therefore with microfluidics, it is beneficial to consider the issues bubbles can create, along with their cause and how to minimize their presence.

Issues Created by Bubbles

Bubbles contribute to flow instability as they are dynamic and expand/contract in the fluidic set-up. A bubble in the flow pathway contributes a varying amount of fluidic resistance by changing the effective channel diameter as the bubble expands/contracts. In addition to flow instability brought about by varying fluidic resistance, bubbles can also negatively affect the system’s response time. With pressure changes, a bubble can absorb some of the pressure change by expansion/contraction, which will cause the system to reach pressure equilibrium more slowly and thus have a slower system response time. Furthermore bubbles can create detrimental interactions within certain experiments. For example, they can cause aggregation problems with particles or proteins, they can damage chemical functionalization of surfaces, and in cell culture, bubbles can lead to cellular death.

Use an In-Line Degasser to Achieve Optimal Performance of Your Analytical Instrumentation

Dissolved gases in a fluidic system can cause trouble when the temperature or pressure vary, as dissolved gas can come out of solution, forming bubbles. These bubbles can negatively affect the accuracy, precision and performance of your equipment. In-line degassing with a CorSolutions’ degasser is an efficient means for removing dissolved gas from liquid, preventing bubble formation.

What Causes Bubbles and How to Avoid Them

A table of common causes of bubbles, along with methods for abating their occurrence, is provided below.

What Causes Bubbles?		How to Minimize Bubbles
At on-set of an experiment, liquid must replace all the air in the flow path, and during this process bubbles can trap or collect in certain regions, making their elimination difficult.		· Design microfluidic devices without corners, sharp angles or other geometries which are conducive to trapping bubbles. Design to minimize system dead volume and use low dead volume CorSolutions’ connectors. · Flush system with isopropyl alcohol or a surfactant, such as SBS, to remove trapped bubbles · Using solvent degassed with a CorSolutions’ degasser will gradually dissolve trapped bubbles
When changing liquids air can be introduced into the system.		· Use an injection valve to introduce a new liquid which will keep the system closed
If there is a system leak, bubbles can be introduced		· Minimize the use of fittings and ensure the ones that are being used are not leaking.
Liquids can contain dissolved gas, which can outgas during experimentation		· Add CorSolutions’ in-line vacuum degassing unit

Benefits

Ultra-high degassing efficiency
Integrated vacuum pump
Low volume
Easy to prime
Short evacuation times
Inert flow path

Vacuum Degassing Using a Gas-Liquid Separation Membrane

A degassing unit, when installed in-line between the solvent bottle and the fluid delivery pump, offers a facile means to minimize bubbles. The degasser has a short length of polymer tubing through which the solvent flows and through which gas molecules are selectively permeable. This tubing is located in a vacuum chamber maintained by a vacuum pump which is constantly running at a low speed. The unit maintains a vacuum pressure on the outside of the tubing. Dissolved gases migrate across the tubing wall under a concentration gradient produced by the vacuum as the solvent flows within the tubing. By applying vacuum on the outside of the highly permeable membrane, the dissolved gas is removed through the membrane while the liquid remains on the interior. The gases are then expelled from the system. Typically, approximately 50% of the dissolved gas in a liquid must be removed to prevent problems with bubble formation in flow lines. It is not necessary to eliminate all dissolved gas, as it only needs to be reduced to a concentration that is below the saturation point of the mixture.