| biological physics | This started with my
PhD at Dept. de Física
Teórica de la Materia Condensada (UAM) under Pedro Tarazona
(ftmc, UAM), and Enrique Chacón
(ICMM-CSIC)
(PhD
title was Physics of amphiphile
aggregates). Many biological molecules are amphiphilic (or, amphipathic): one of its ends is hydrophobic, while the other is hydrophilic. The presence of these opposing tendencies result in the formation of many interesting supramolecular assemblies. The chief example in biology are phospholipids, the main ingredient in animal's cell walls. During my PhD, we studied simple models for this kind of molecules. The framework was taken from liquid state theory; basically, density functional theory. The aggregate most important in biology is the bilayer membrane, either flat or closed; this is what cell walls are (plus many other molecules, mainly proteins, whose purpose is functional rather than structural.) [1998] Other interesting aggregate is the micelle (see below). Later, I came across biological membranes again. This time, it was from polymer physics (see below), |
| polymer physics | Michael Schick, at
the Dep. of Physics,
University of Washington in Seattle, Washington, USA is well known
in the field of polymer physics (among others). Since then, I have been
working with the so-called SCFT (self consistent field theory), which
is just plain mean field applied to polymer physics. Diblock copolymers are polymers with two parts, made with two monomers whose mixture would segregate. (Notice analogy with amphiphilic molecules above.) SCFT successfully explains the structures that a melt of this copolymers forms. We first worked in explaining the beautiful structures that may form at the boundary between two ordered domains. In particular, twist grain boundaries, which have been compared to a minimal surface called Scherk's first surface. We also studied other boundaries called T junctions. [2000b, 2000b] Later, we applied the same theory to a model of... phospholipids again. We studied the insertion of a short protein across the membrane. Membrane proteins are recognized to be hugely important, but difficult to study experimentally. [2002a] I also returned to micelles (below). |
| micelles | Amphiphilic molecules
can form other kinds of aggregates. In some cases (intuitively, when
their hydrophilic "head" is larger than their hydrophobic "tails") they
may form micelles. These aggregates are interesting because they have a
well-defined size. Think about forming a ball with thumbtacks: too few
and you will see their points, too many and the interior will be
hollow. There are some simple models that are exactly solvable and show
this behavior. I studied some of them during my PhD thesis, and then a
similar one came about later. [1997,2001] Then, a mixture of an A-B copolymer and B homopolymer can form these structures too, if the A part is smaller. I have also studied this mixture. [2003] |
| interfaces |
This subject has a
long history. When two phases of a substance are in coexistence (think
water and its vapor in a jar), there should be some structure to the
boundary between the phases (called the interphase). Also, there is an
energy cost to the formation of the interphase, measured in energy per
area, called the interphacial tension (if one of the phases is a vapor,
this is the famous surface tension.) Like in the other cases, theory
and simulation have provided much information about this subject. When I arrived in Barcelona, to work Dr. Lourdes Vega (Molecular Simulation Group, ICMAB-CSIC), we performed simulation work on interphaces of short linear chains. The results were compared with the predictions from a theory called soft-SAFT (from the SAFT family). This work has been extended to binary mixtureswith Drs. Andrés Mejía and Hugo Segura, from Chile. The phase coexistence of SF6 (see below) has also been studied by Aurelio Olivet. [2004a, 2004b, 2005, 2006a] |
| materials science | Colloids are pieces
of material small enough that the usual techniques of statistical
mechanics can be applied to them. Its interaction is usually attractive
at large distances, which causes the particules to fuse (this has the
interesting name of flocculation).
Often, the opposite is needed, in order to obtain a suspension.
Traditionally, a repulsion can be induced by charging the
particles. Another means is to anchor molecules to the surface. We have
studied the resulting forces in these systems. [2006b] |
| dielectrics | This is a very recent line of work. The idea is to make progress in understanding and predicting what makes a substance a good gas insulater. The most well known gases are N2 , cheap and not very good, and SF6, artificial gas with exceptional performance. |