Which specimens are susceptible to air bubbles?

Air bubbles, unless they are too large, are not generally a problem in wet mounts. If larger parts of the specimen are submerged in water, then these parts can be viewed without problems. Some air bubbles are certainly tolerable and unless one wants to produce high-quality pictures it is often not worth the effort to make a completely bubble-free specimen. It is easily possible to simply move the slide and observe a different part of the specimen.

Air bubbles that are too large may not be recognized as air bubbles. Air bubbles are often very prominent and can easily be seen. Naturally, the bubbles should not be confused with the actual specimen, something that beginners sometimes do because the bubbles are so prominent and can be seen even if the specimen itself is not in focus.

The bubbles cause optical artifacts at the place where the air meets the water. The air bubble appears to be surrounded by a dark ring. This dark ring covers some parts of the specimen and makes observation more difficult.

The microscope optics are designed to give optimum resolution for a specimen which is surrounded by water. If the bubble is large and the specimen completely surrounded by air, then the resolution is lower.

Under some rare circumstances, air bubbles can even be beneficial. The bubbles can serve as a source of oxygen for some organisms, such as paramecia and other ciliates. It is possible to see them collect around the bubbles. Air bubbles can also limit the movement of ciliates, which slows them down and makes it possible to see them better. certain organisms such as algae produce oxygen by photosynthesis. The oxygen can accumulate and contribute to the formation of bubbles. In that sense the bubbles are a sign of metabolic activity of the cells.

Bubbles can be more problematic if the specimen is thin and flat (such as larger microtome sections or onion skin). In this case it is possible that bubbles are caught below the specimen and it can be more difficult to remove these bubbles. Hydrophobic, furry specimens (insect hair etc.) also catch a lot of air. The water may therefore not be able to reach all parts of the specimen. In this case it may be easier to use a hydrophobic mounting medium (Euparal etc.) or to add a small amount of detergent to the water to break the surface tension. The detergent may also remove some oil from the surface of the insect hair, which makes it easier for the water to reach the hair.

Not all specimens are the same. Some specimens can be the cause for more air bubbles than others. This depends on a variety of factors. The following characteristics may result in more bubbles:

  • Large sheet-like specimens (e.g. onion skin): These specimens may catch air bubbles underneath them and prevent them from escaping. Push out the air bubbles before adding a cover slip.
  • Specimens with many fine hair: The hair catch much air and prevent the water from reaching all the parts of the specimen. The surface tension of the water is too high, and the water therefore does not “flow” into all parts of the specimen. This is comparable to the “Lotus Effect”, where the water does not wet the surface of the lotus leaf.
  • Fatty and hydrophobic specimens: These too do not accept water well, especially if the surface area of the specimen is large (many fine hair, etc). It may help to treat the specimen in alcohol or an alcohol-water mixture to remove the fatty surface.
  • Porous specimens: The pores of the specimen may be filled with air, which can be difficult to remove. The cells of plant stems, the vascular tissue, for example, are able to hold air. It is possible to remove the air by placing the specimen into a vacuum while it is submerged in the fixing solution. Aspirators (eductor-jet pumps) can be mounted to a water tap to produce a vacuum.