When embedding tissues in BEEM capsules with Epon resin, the appearance of four equidistant bubbles at the capsule's bottom is mainly due to:

• Capsule Geometry: Most BEEM capsules have subtle, symmetrical markings or mold imperfections at the bottom from the manufacturing process. These act as nucleation sites where air becomes trapped when the viscous resin is dispensed.
• Air Entrapment: Pouring the resin can trap air, and the bubbles naturally migrate to these energetically favorable, symmetric points.
• Surface Tension: Resin’s wetting properties and surface tension further stabilize bubbles at those sites. The 4-bubble pattern is caused by the interaction of plastic mold geometry, trapped air, and surface tension during the embedding process. It is a well-known, largely physical phenomenon that can be minimized by slower resin dispensing, proper degassing, and ensuring capsule quality.

Thank you Shweta.