Frontiers in Correlated Matter:
Physicists meet to discuss the intellectual challenges of complex matter.
Piers Coleman and Leo Kadanoff
Soft and biological matter
At the meeting, there was a huge surge of interest in soft matter and biological systems. Condensed matter physics is generally divided in "hard" and "soft" condensed matter physics. "Hard" matter refers
to matter governed by atomic forces and quantum mechanics . Hard matter is generally not biological, and involves crystals, glasses, metals, semiconductors and oxides. "Soft" matter refers to the myriad varieties of matter that form on the macroscopic scale, often from biological, or organic molecules. Here the physics, though collective,
does not involve quantum mechanics, and the materials of interest are literally "softer".
Crumpled and Granular Matter
Towards a science of Matter Singularities : Thomas WittenStructure and function of Biological matter
The Dynamics of Granular Flows : Jerry Gollub
Packings, Jammings, Correlations and Entropies in Condensed Matter and Candy : Paul Chaikin
The statics and dynamics of biological molecules
Electrodynamics in soft matter and biophysics: Phil PincusMatter with Memory: Glasses and Gloopy Matter.
Mechanics and Statistical Mechanics of Living Cells :David Weitz
Disciplined Disorder and Disordered Disciplines :Peter Wolynes
Why don't glasses flow? Philip Bouchaud
One broad class of resarch work looked at how materials could fall into jammed configurations in which they would move very slowly, remember their past, and maintain interesting structures over long periods of time.
Much of the work was related to understanding the functioning of a groups of atoms or molecules by taking into account the shapes and forms they took. People looked at new kinds of materials which have a much richer family of shapes and stable behaviors than our usual materials. A bucket of water has shape given to it by the bucket. That is a familiar behavior. However, a bucket of sand may have your footprint or mine or anyone's. These materials with memory may have very interesting and novel properties.
Another class of work focused upon the forms and groupings of molecules in biological systems, and how those structures affect dynamical properties and biological functioning. Several of the participants discussed how ideas of condensed matter are starting to shape problems of biological complexity.
From Molecules to
Complexity: George Whitesides
George Whitesides of Harvard University observed life and cells are just one of many complex systems that populate the world, including weather and economic systems; large networks such as the nervous system, computers, information systems and energy distribution grids. Our ability to understand and control the complex behavior of these systems he argued, is a central problem in science and engineering. Whitesides expressed optimism that biology is slowly developing the kinds of reproducible experiments that bring it into the realm of physics. In Physics and Chemistry we are accustomed to sets of rules and laws that govern our understanding - Newton laws, Arrhenius' laws, the application of statistical mechanics - but what, he asked, are the rules for biology? The answer, Whiteside conjectures, will lie in in discovering the principles that regulate and describe complex networks.
from UC Santa Barbara asked what is the the physical
mechanism connected with the functioning of the long-chain biological
actin and DNA? Both molecules feel important forces from their
electrical charges, and from the chargers drawn to them in
pointed out that we have recently achieved a pretty good understanding
of the static properties of the complex structures which form around
the molecules as
they sit in solution. The next decades show promise of giving us
understanding of the crucial dynamical properties of these surrounding
Some of this dynamics will come from the universal properties of the
charges in these structures. Other pieces, involving molecular
will come from more detailed "chemical" properties produced by the
details of the molecules and their environment. The use of
physics and chemistry
together to explain biological phenomena is likely to be a hallmark of
near future investigations in this area.
|Dave Weitz of
Harvard University, extended the discussion of biology
from liquid properties to those of elastic networks. Part of this
discussion was an illustration of the use of in vitro simulations of
biological systems. Cells use networks of actin--a molecule in the form
of a long
chain-- to provide themselves rigidity and strength. These qualities
the actions are connected and welding into a kind of network. To
construct an artificial structure like that in the cell, he placed
proteins, actin, a second protein which cross-linked the actin, and a
helps cut the actin into the right lengths. He then studied the
stretching properties of this artificial system and compared it to the
of cells. He found a detailed correspondence between the two,
enabling him to argue that it was just this network which produced the
behavior of cells. He then supplemented this work with a study of
a real cell. He added a dye which could fluoresce to the actin of
cell, which is arranged into a network of tubes. Using this dye
he can observe
the actual motion as it occurs in a living cell. This pair of
the beginning of a set of work which we might hope will teach us about
cells by constructing their parts and comparing the behavior of aptly
constructed parts with that of the real thing.
Memory effects were taken up by Peter Wolynes of UC San Diego and J-P Bouchard from Saclay, France, in their surveys of recent advances in our understanding of glasses. Glasses have the capacity of memory as as concomitant of their being able to be responsive or "adaptive". Glasses have this property. The detailed local arrangement of atoms provides a sort of trap in which the material gets stuck or "jammed" (that is the technical word used in this subject). As a result a glassy material, like common window glass, is a kind of fluid, which however shows great resistance to flow. Glasses "remember" when they were formed. Their configuration slowly changes or "ages".
|Both speakers argues that the behavior of our familiar glasses was produced by partially realized phase transitions. There are two such transitions. The higher temperature transition dropped the fluid into a glassy state in which the system permitted jumping back and forth between some sorts of configurations but greatly inhibited jumps to other configurations. As the system aged, it slowly found more favorable configurations. However, it could not directly return to the previous, less favorable, ones. If the temperature were lowered still further, the system would undergo another phase transition and fall into a disordered low temperature state. However this lower temperature transition is almost always preempted by a drop into a solid configuration. This conceptual framework was used to describe how structures form within the glasses, how these structures get jammed, how glasses age, and how the material retains memory of its past.|
Michael Cates, from the University of Edinburgh, talked about how memory and landscapes become very important at the interface between biology and soft matter, playing a vital role in our understanding of how matter self-assembles.
In glassy systems, he said, this is question of learning how to steer soft matter through an energy landscape. A huge class of soft matter is "gloopy" - matter which flows like a liquid, but which remember's its shape on short time-scales, so that it can be cut with a knife, and it flows in sudden bursts, as in the classic example of ketchup "glooping" out of the bottle.
does gloop gloop?" actually encodes a huge number of unsolved questions
physics, such as what is different about the interactions of molecules
matter? How do these interactions control when flow starts and
Examples of gloopy matter include polymers - molecular spaghetti -
colloids- fluids containing molecular billiard balls - and even
materials might be thought of as a generalized class of
Cates described his group's work on colloids colloids of lecithin inside a bile salt, in which a suddent change in the concentration of bile salt causes the disk-like colloids of lecithin to become unstable, starting to evolve into spherical colloids. Gloopy matter, forms because this evolution becomes arrested in a state containing a network of curved bilayers - or vesicules. Cates emphasized that by looking at such mixtures in parameter regimes that are far outside those seen in real biological matter, they have been able to learn far more about the mechanism by which the gloopy matter develops. In particular, Cate's group has been able to show that like a glass, the gloopy matter is actually metastable, and they've been able to simulate the formation of these arrested states.
Cates ended by remarking that the problem of understanding how non-equilibrium matter moves through a complex energy landscape is a central issue that needs to be addressed in the context of both nano and biological physics. There's lots of new physics out there, he says, but we can't apply it until we understand it first.