Introduction
Bubble rafts have long been used to model the bonding and packing of atoms. This tool has also been useful in investigating the behavior of dislocations. Since the bonding of the bubbles is analogous to the non-directional metallic bonding, the bubbles in the bubble raft behave like metal atoms. The non-directional bonding leads to highest coordination packing (each atom is surrounded by a maximum number of neighbors). Since the attractive and repulsive forces are similar to those of systems with metallic bonding, the bubble raft model is a convenient macroscopic analog for atomic behavior.
The ideal state (lowest energy) of the system is a large single crystal, but not all the bubbles pack perfectly. The packing imperfections in the bubble raft represent defects within real materials. Several different kinds of defects can exist within materials. Zero-dimensional defects form when individual bubbles are improperly placed in the crystal structure. Wrong sized bubbles (solute atoms) can cause irregularities in the bubble packing. Vacancies can form when bubbles coalesce around an open area. More commonly a vacancy forms when a bubble pops.
One-dimensional defects are also observed in the bubble raft model. Dislocations fall into this category. While direct identification of a dislocation takes some time, the region in which dislocation exists may be identified by the lower density packing of the bubbles. This is shown on the projection as a slightly brighter area.
Two-dimensional defects present in real materials are not fully represented in the model since the bubble raft is confined to two dimensions. Instead, two-dimensional defects (e.g. grain boundaries) are compressed into one dimension within the bubble raft. These one-dimensional representations are readily recognizable due to the lower packing density at their core. These boundaries represent the boundary between two crystals with different crystallographic orientations.
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