My present research activities concern principally in understanding the link between specific forms of interactions and the resulting macroscopic system. My favorite system to study this problem is aqueous mixtures. Indeed, the strongly orientational nature of the hydrogen bonding "interaction" make water a very special solvent. A characteristic of aqueous mixtures, which is not observed in mixtures of simpler liquids, is the permanent heterogeneity which is seen in snapshots of simulations, independently of the choices of models for water or the solutes. One sees water and the solute segregated in pockets at the size of the nanometer. What is very intriguing, is that such systems look very much like mixtures of simple liquids in the verge of a liquid-liquid phase separation, and yet these aqueous mixtures are fully stable. Studying such systems raises many questions. What is the microscopic definition of stability and metastability? How finite size computer simulations can deal with this problem? What is the role of the underlying Coulomb interactions, specifically the long range nature of this interaction, through its representation as the hydrogen-bond? What liquid state theories can say about such systems? Answers to such questions meet rapidly the limits of what is currently known and understood in the framework of the statistical mechanics of liquids. As an example, finding what is integral of any of the pair correlation functions (called Kirkwood-Buff integrals in our jargon) in such systems, as determined by experiments (thermodynamics and scattering), simulations and theory, turned out to be a real problem that took a decade to solve. In order to solve this problem, it was necessary to seek a unifying description of mixtures ranging from simple aqueous mixtures (called solutions) all the way to micro-emulsions. The heart of the problem was to understand the difference between concentration fluctuations and the omni-present micro-heterogeneity. It is quite fascinating that aqueous mixtures which are thought to be well understood, could be the siege of so many borderline questions and problems. What started as simple problem -computing the correlation functions in computer simulations- has taken me into unexpected directions: micro-heterogeneity can be described as an "emergent particle" while fluctuations of the solvent provide a way these "particles" interact or not -depending on the nature of the solvent interactions. This way of looking at these systems bring us very close to other areas of physics -condensed matter, high energy physics. This is not unexpected, since the link between emulsions and exotic domains of Physics have been explored decades ago, mainly by de Gennes. The originality of my approach to the problem is that I strive to maintain is a permanent contact with the statistical description of the liquid state, close to the tradition of the former Laboratoire de Physique Théorique des Liquides, as founded by J.P. Hansen and S. Bratos.
My research is intimately connected to the Split group in Dalmatia (Croatia): Franjo Sokolić, Larisa Zoranić, Bernarda Kežić, Martina Požar and Marijana Mijaković
Aqueous tert-butanol mixtures: a molecular-emulsion, B. Kežić and A. Perera, J. Chem. Phys. 137, 014501 (2012)
A model for molecular emulsions: water and weak-water mixtures, B. Kežić, R. Mazighi, and A. Perera, Physica A 392, 567 (2013)
Fluctuation and micro-heterogeneity in mixtures of complex liquids, A. Perera and B. Kežić, Faraday Discussions (2013) DOI: 10.1039/C3FD00072A
Microheterogeneity in molecular liquids, A. Perera, B. Kežić, F. Sokolić and L. Zoranić in “Molecular Dynamics” (Vol 2), Ed. L. Wang (InTech, Rijeka) 2012.ISBN 978-1-4398-9922-9
Concentration fluctuations and micro-heterogeneity in aqueous mixtures: new developments in analogy with micro-emulsions, A. Perera in “Fluctuation Theory of Solutions: Applications in Chemistry, Chemical Engineering, and Biophysics”, Ed. P. E. Smith, J. P. O'Connell and E. Matteoli, CRC Press Taylor and Francis (2012) , ISBN 978-953-51-0444-5