LIQUIDS, PROTEINS, POLYMER AND COLLOIDAL SOLUTIONS,
  LIQUID INTERFACES, PHASE TRANSITIONS AND CRITICAL PHENOMENA

  

                                                                                                                             

Research area: Statistical Physics of liquids - Dr. D. Pini (University of Milano)

The latest years have witnessed a surge in the study of soft matter. This highly cross-disciplinary field encompasses Physics, Chemistry, and Biology, and includes liquids, mixtures, colloidal solutions, membranes, and liquid crystals. The description of the thermodynamic properties of these systems is generally based upon effective potentials, which take approximately into account both the complexity of the constituents and the influence of other species which may be present in the solution, such as electrolytes and polymers. The overall interaction results from the superposition of repulsive and attractive forces, whose strengths and ranges depend on the thermodynamic state as well as on other parameters specific to the dispersion, such as the concentration of the various species, their charge, and their size. As a consequence, these potentials are generally more structured than those appropriate for simple liquids, and their profile can be modified by acting on the dispersion parameters. It is then important to understand which changes in the thermodynamics and phase diagram of soft-matter systems are brought about by a change in the shape of the interaction. Our group has a wide experience in the development and application of accurate theories of liquid state. One of these microscopic approaches, namely the hierarchical reference theory (HRT) [1], has been developed entirely by us in order to implement the renormalization group ideas in the context of liquid-state theory. Recently, we have proposed a new formulation of this method, the smooth cut-off HRT [2], which improves the description of the fluid-fluid transition, and is also capable of enforcing exactly the constraint on the correlations stemming from the short-range repulsion in a certain class of potentials. Whenever necessary, numerical simulation is also employed. The topics under investigation include: colloidal dispersions with depletion interactions [3]; soft-core fluids such star polymer solutions [4]; microphase formation in colloidal systems with competing interactions [5]. The latter is an interesting instance of self-organization, whereby complex structures are spontaneously created, and has recently been given considerable attention by the liquid-state community. The possibility of modifying these structures by external fields, e.g. via a substrate playing the role of a spatially modulated potential, is also relevant for technological applications. Even when the competition is not so strong as to cause the occurrence of microphases, it still triggers large density fluctuations, which in turn strongly affect the thermodynamics and the correlations of the fluid [6]. Our approach also allows to study these systems in the critical region [7], where other theories either behave unrealistically, or simply fail to give results at all. Some directions for future development include:

• The theoretical investigation of pattern formation due to competing interactions in three-dimensional fluids

• The extension of the smooth cut-off HRT either by:

  i) Implementing it for a wider class of potentials than those considered so far

  ii) Improving the functional form of the correlations

                                                       

References:

[1] A. Parola and L. Reatto, Adv. in Physics 44, 211 (1995).
[2] C. D. Ionescu, A.~Parola, D.~Pini, and L.~Reatto, Phys. Rev. E 76, 031113 (2007);
     A. Parola, D. Pini, and L. Reatto, Phys. Rev. Lett. 100, 165704 (2008).
[3] F. Lo Verso, D. Pini, and L. Reatto, J. Phys.: Condens. Matt. 17, 771 (2005);
     F. Lo Verso, R. L. C. Vink, D. Pini, and L. Reatto, Phys. Rev. E 73, 061407 (2006).
[4] F. Lo Verso, M. Tau, and L. Reatto, J. Phys.: Condens. Matt. 15, 1505 (2003);
     G. Foffi, F. Sciortino, P. Tartaglia, E. Zaccarelli, F. Lo Verso, L. Reatto, K. A. Dawson, and
     C. N. Likos, Phys. Rev. Lett. 90, 238301 (2003).
[5] A. Imperio and L. Reatto, Phys. Rev. E 76, 040402 (2007);
     A. Imperio, L. Reatto, and S. Zapperi, Phys. Rev. E 78, 021402 (2008);
     A. J. Archer, C. D. Ionescu, D. Pini, and L. Reatto, J. Phys. Condens. Matter 20, 415106 (2008).
[6] D. Pini, A. Parola, and L. Reatto, J. Phys.: Condens. Matt. 18, S2305 (2006);
     A. J. Archer, D. Pini, R. Evans, and L. Reatto, J. Chem. Phys. 126, 014104 (2007).
[7] D. Pini, F. Lo Verso, M. Tau, A. Parola, and L. Reatto, Phys. Rev. Lett. 100, 055703 (2008);
     D. Pini, A. Parola, L. Reatto, F. Lo Verso, and M. Tau, J. Phys.: Condens. Matter 20, 494246 (2008).