2
Introduction
Marchetti et al (2013), in an all-round review co-authored by seven physicists, define
active matter as “composed of self-driven units, active particles, each capable of
converting stored or ambient free energy into systematic movement”, referring to
Schweitzer (2003), who did not use this term, however.
Both points of view have their merit. Certainly, neither stars nor geologically
active planets are driven externally. A star is also a structured “machine” driven by
its own nuclear fusion energy, and governing its coterie of planets, some of which
themselves are structured machines. Our Earth possesses, besides Sun’s radiation,
her own radioactive energy source that keeps her interior fluid, driving the magnetic
dynamo and the continental plate tectonics. Her oceans are convective machines, with
their network of mighty currents governing the climate, and occasional outbursts of
hurricanes. And perhaps we should recall at this point Lovelock’s (1979) Gaia, the
living planet, of which we, as well as other living “machines”, are constituent parts.
Leibniz could not know anything like this, but if he were resurrected in our day, he
couldn’t fail to acquire a laptop and a smartphone and, with his natural curiosity,
would agree that the parts of our artificial machines are machines. However, all this
would bring us too far. As once remarked a fictitious Russian writer, one cannot
embrace the unembraceable. The working definition of “active matter” may be
confined to the problems investigated by scientists who are actually working in the
field.
What does it include? First of all, indeed, self-driven “active” particles. What
exactly constitutes a “particle” is interpreted very widely. It might just be a particle
moving in a fluid under the action of a gradient of some field, e.g., the concentration
of some species, or electric potential, or magnetic force. It might be a particle,
not active in its own right, but entrained in a vibrating granular layer; part of a
“dissipative structure” not unlike convective structures in the oceans, atmosphere,
or laboratory studies. A granular layer was indeed the first medium in which a
dissipative non-equilibrium structure was discovered, by Faraday (1831). It might
be a microbe wandering in search of nutrients. It might be a bird or a fish, a part of a
flock or a shoal. It might be a person in a crowd. Note that this list includes a rather
loose notion of self -driving, but in any case, the “self” of the “particle” is always
subjected to at least some degree to external forces and/or collective interactions.
There is a rather artificial distinction between “dry” and “wet” active matter,
which originated more from the kind of modeling than from the nature of the
“particles” themselves. Interactions in “wet” matter are mediated by the medium
they are immersed in, while in the case of “dry” matter, the medium is ignored,
sometimes rightly, when its influence is a minor factor, and sometimes just to make
the problem tractable, e.g., ignoring fluid mechanics when modeling flocks.
The “particles” are often assigned certain intrinsic characteristics, most commonly, their orientation. The orientation may determine their preferred direction
of motion, e.g., with respect to gradients of an external field, or the character of
their interactions, e.g., the tendency to align with their neighbors. The basic types
of orientation are vector, denoting the direction, or nematic, implying an alignment
without a definite direction, as in a vector without an arrow. Both kinds of orientation
may or may not be qualified by their strength.
Introduction
Marchetti et al (2013), in an all-round review co-authored by seven physicists, define
active matter as “composed of self-driven units, active particles, each capable of
converting stored or ambient free energy into systematic movement”, referring to
Schweitzer (2003), who did not use this term, however.
Both points of view have their merit. Certainly, neither stars nor geologically
active planets are driven externally. A star is also a structured “machine” driven by
its own nuclear fusion energy, and governing its coterie of planets, some of which
themselves are structured machines. Our Earth possesses, besides Sun’s radiation,
her own radioactive energy source that keeps her interior fluid, driving the magnetic
dynamo and the continental plate tectonics. Her oceans are convective machines, with
their network of mighty currents governing the climate, and occasional outbursts of
hurricanes. And perhaps we should recall at this point Lovelock’s (1979) Gaia, the
living planet, of which we, as well as other living “machines”, are constituent parts.
Leibniz could not know anything like this, but if he were resurrected in our day, he
couldn’t fail to acquire a laptop and a smartphone and, with his natural curiosity,
would agree that the parts of our artificial machines are machines. However, all this
would bring us too far. As once remarked a fictitious Russian writer, one cannot
embrace the unembraceable. The working definition of “active matter” may be
confined to the problems investigated by scientists who are actually working in the
field.
What does it include? First of all, indeed, self-driven “active” particles. What
exactly constitutes a “particle” is interpreted very widely. It might just be a particle
moving in a fluid under the action of a gradient of some field, e.g., the concentration
of some species, or electric potential, or magnetic force. It might be a particle,
not active in its own right, but entrained in a vibrating granular layer; part of a
“dissipative structure” not unlike convective structures in the oceans, atmosphere,
or laboratory studies. A granular layer was indeed the first medium in which a
dissipative non-equilibrium structure was discovered, by Faraday (1831). It might
be a microbe wandering in search of nutrients. It might be a bird or a fish, a part of a
flock or a shoal. It might be a person in a crowd. Note that this list includes a rather
loose notion of self -driving, but in any case, the “self” of the “particle” is always
subjected to at least some degree to external forces and/or collective interactions.
There is a rather artificial distinction between “dry” and “wet” active matter,
which originated more from the kind of modeling than from the nature of the
“particles” themselves. Interactions in “wet” matter are mediated by the medium
they are immersed in, while in the case of “dry” matter, the medium is ignored,
sometimes rightly, when its influence is a minor factor, and sometimes just to make
the problem tractable, e.g., ignoring fluid mechanics when modeling flocks.
The “particles” are often assigned certain intrinsic characteristics, most commonly, their orientation. The orientation may determine their preferred direction
of motion, e.g., with respect to gradients of an external field, or the character of
their interactions, e.g., the tendency to align with their neighbors. The basic types
of orientation are vector, denoting the direction, or nematic, implying an alignment
without a definite direction, as in a vector without an arrow. Both kinds of orientation
may or may not be qualified by their strength.