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. [2000, 2002]

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. [2002]

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. [1998]

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 interface). Also, there is an energy cost to the formation of the interface, measured in energy per area, called the interfacial 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 interfaces 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 mixtures with Drs. Andrés Mejía and Hugo Segura, from Chile. The phase coexistence of SF₆ (see below) has also been studied by Aurelio Olivet. [2004a, 2004b, 2005a, 2005b].

The most recent work in this area is related to the dynamics of the gas/liquid interface [2008]. Some proposals concerning its determination and structure have also been put forward (see this introductory page to the subject, and some info about our recent proposal).

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 particles 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. [2006, 2007]

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 insulator. The most well known gases are N₂ , cheap and not very good, and SF₆, artificial gas with exceptional performance. [2007]