Avoiding Air Bubbles in Microfluidic Setups
Air bubbles are annoying in microfluidic setups, since they can disturb or even hinder an experiment. Even after filling of the whole system with liquids, air bubbles can occur because of water nucleation, e.g. when using unfavorable combination of inner diameters between the single modules of the system.
Small air bubbles can temporally increase the flow resistance, thus, the flow rate will drop. In such small length scales, the surface tension of water-air-interfaces is a relevant factor. Air bubbles can occur in various ways. Assuming that all connectors are airtight, the most common cause is that air and/or air bubbles where already in the system at the beginning. To get rid of dead volume air bubbles, the whole system should be flushed with (degassed) water or medium first in all channels. This typically removes >90% of air bubble-related issues in most microfluidic systems. The two other causes of air bubbles are the small air diffusivity through tubing (practically relevant for many very long tubings) and the pressure-depending air solubility of water (relevant when applying negative pressures or strong pressure gradients in the system). For those two cases, bubble traps are perfect.
Thus, to avoid air bubbles you can:
– flush the whole setup beforehand
– degas fluids
– consider the changes of the inner diameters between single modules to avoid a pressure drop within the system
– double-check the system on tightness
– use bubble traps (if absolutely necessary)