We study the phase diagram of a binary mixture of patchy particles which has been designed to form a reversible gel. For this we perform Monte Carlo and molecular dynamics simulations to investigate the thermodynamics of such a system and compare our numerical results with predictions based on the analytical parameter-free Wertheim theory. We explore a wide range of the temperature-density-composition space that defines the three-dimensional phase diagram of the system. As a result, we delimit the region of thermodynamic stability of the fluid. We find that for a large region of the phase diagram the Wertheim theory is able to give a quantitative description of the system. For higher densities, our simulations show that the system is crystallizing into a BCC structure. Finally, we study the relaxation dynamics of the system by means of the density and temperature dependences of the diffusion coefficient. We show that there exists a density range where the system passes reversibly from a gel to a fluid upon both heating and cooling, encountering neither demixing nor phase separation.

We exploit the concept of competing interactions to design a binary mixture of patchy particles that forms a reversible gel upon heating. Our molecular dynamics computer simulation of such a system shows that with increasing temperature the relaxation dynamics slows down by more than four orders of magnitude and then speeds up again. The system is thus a fluid both at high and at low temperatures and a solid-like disordered open network structure at intermediate temperature. We further discuss the feasibility of realizing a real material with this reversible behavior.

We have used first principles simulations in order to investigate the properties of a sodium borosilicate glass of composition 3Na2O-B2O3-6SiO2 (NBS). This composition is similar to that of the glass wool used in our daily life. The study was carried up using first principles molecular dynamics within the density functional theory framework as implemented in the VASP code [1]. In this talk, we will present the analysis of the local environments of the three building structural units of the glass network, namely silicon atoms in 4-fold coordination, and boron atoms with 3- or 4-fold coordination. We will also discuss the local distribution of the Na atoms around the basic structural units. Indeed we have identified their preferential neighborhoods and how the nature of network former and its coordination infer on the shape of these preferential regions of Na atoms. The vibrational properties have been equally studied, and the contributions of the various species have been identified. We have found that 3- and 4-fold coordinated boron atoms give rise to distinguished spectral features. Moreover, the partial vibrational density of the 3-fold coordinated B atoms has been found to be a weighted sum of 2 specific contributions so-called 3-fold symmetric coordinated B atoms and asymmetric coordinated B atoms. [1] Kresse & Hafner (1993) PRB 47, 558; Kresse & Furthmüller (1996) Comp.Mater. Sci. 6, 15; Kresse & Furthmüller (1996) PRB 54, 1116