3.2 Crystals on the Mesoscale
27
thus, bronze is stronger than pure copper, as was understood when new kinds of
tools and weapons initiated the Bronze Age. Even single crystals are weakened by
dislocations (more on them in Sect. 3.4). Ways to attain better order may be counterintuitive. Reheating a crystal may remove imperfections, as the system is teased
out of a metastable equilibrium. In this way, steel is tempered and particle arrays are
stirred up to better compact.
3.2 Crystals on the Mesoscale
The structure of crystals is determined by interactions between atoms, which can
be both attractive and repulsive, as seen most clearly in ionic crystals. Atoms and
molecules (or even people) preferring neighbors of their own kind tend to separate
(Sect. 2.5), but aggregation of separated phases on larger and larger scales is prevented if they are tied together. This happens in block copolymers which include
blocks of mutually repulsive units, like AAAABBBBAAAABBBB. Failing to separate completely, such polymers form patterned structures, not so neat as crystals
but approaching an ordered structure on a mesoscopic scale that depends on the size
of the blocks (Fig. 3.5).
The characteristic inner scale of structures of this kind is very different from that
of atomic or molecular crystals. It is called the mesoscale – in-between the atomic
and macroscopic scales. It could literally be called microscopic, as it lies just below
the micron in most applications, within the range of an optical microscope. However, the prefix micro has always been reserved for the tiniest things – a scale that
kept decreasing over time as science penetrated deeper into the makeup of matter.
Other labels are colloidal or nanoscale, referring to colloids, suspensions of particles with sizes from just a few to a few hundred nanometers.
Fig. 3.5 Left: A scheme of separation of block copolymer units. Right: The resulting pattern
27
thus, bronze is stronger than pure copper, as was understood when new kinds of
tools and weapons initiated the Bronze Age. Even single crystals are weakened by
dislocations (more on them in Sect. 3.4). Ways to attain better order may be counterintuitive. Reheating a crystal may remove imperfections, as the system is teased
out of a metastable equilibrium. In this way, steel is tempered and particle arrays are
stirred up to better compact.
3.2 Crystals on the Mesoscale
The structure of crystals is determined by interactions between atoms, which can
be both attractive and repulsive, as seen most clearly in ionic crystals. Atoms and
molecules (or even people) preferring neighbors of their own kind tend to separate
(Sect. 2.5), but aggregation of separated phases on larger and larger scales is prevented if they are tied together. This happens in block copolymers which include
blocks of mutually repulsive units, like AAAABBBBAAAABBBB. Failing to separate completely, such polymers form patterned structures, not so neat as crystals
but approaching an ordered structure on a mesoscopic scale that depends on the size
of the blocks (Fig. 3.5).
The characteristic inner scale of structures of this kind is very different from that
of atomic or molecular crystals. It is called the mesoscale – in-between the atomic
and macroscopic scales. It could literally be called microscopic, as it lies just below
the micron in most applications, within the range of an optical microscope. However, the prefix micro has always been reserved for the tiniest things – a scale that
kept decreasing over time as science penetrated deeper into the makeup of matter.
Other labels are colloidal or nanoscale, referring to colloids, suspensions of particles with sizes from just a few to a few hundred nanometers.
Fig. 3.5 Left: A scheme of separation of block copolymer units. Right: The resulting pattern