Phase Behaviors of Soft-core Particle Systems

Core-softened colloid under extreme geometrical confinement

Geometrical constraints offer a promising strategy for assembling colloidal crystal structures that are not typically observed in bulk or under 2D conditions. Core-softened colloids, in particular, have emerged as versatile chemical building blocks with applications across various scientific and technological areas. In this study, we investigate the behavior of a core-softened model confined between two parallel walls. Employing molecular dynamics simulations, we analyze the system's response under extreme confinement, where only one or two layers of colloids are permitted. The system comprises particles modeled by a ramp-like potential confined within slit nanoslits created by two flat, purely repulsive walls with a lateral side L separated by a distance Lz. Through a systematic analysis of the phase behavior as Lz increases, or as the system undergoes decompression, for different values of L, we identified a mono-to-bilayer transition associated with changes in the colloidal structure. In the monolayer regime, we observed solid phases at lower densities than those observed in the 2D case. Importantly, we demonstrated that confinement at specific Lz values, allowing particle arrangement into two layers, can lead to the emergence of the square phase, which was not observed under monolayer or 2D conditions. By correlating thermodynamic, translational, and orientational ordering, as well as the dynamics of this confined colloidal system, our findings offer valuable insights into the utilization of geometrical constraints to induce and manipulate structural changes.

Understanding the glassy dynamics from melting temperatures in binary glass-forming liquids

It is natural to expect that small particles in binary mixtures move faster than large ones. However, in binary glass-forming liquids with soft-core particle interactions, we observe the counterintuitive dynamic reversal between large and small particles along with the increase of pressure by performing molecular dynamics simulations. The structural relaxation (dynamic heterogeneity) of small particles is faster (weaker) than large ones at low pressures, but becomes slower (stronger) above a crossover pressure. In contrast, this dynamic reversal never happens in glass-forming liquids with hard-core interactions. We find that the difference of the effective melting temperatures felt by large and small particles can be used to understand the dynamic reversal. In binary mixtures, we derive effective melting temperatures of large and small particles simply from the conversion of units and find that particles with a higher effective melting temperature usually undergo a slower and more heterogeneous relaxation. The presence (absence) of the dynamic reversal in soft-core (hard-core) systems is simply due to the non-monotonic (monotonic) behavior of the melting temperature as a function of pressure. Interestingly, by manipulating the relative softness between large and small particles, we obtain a special case of soft-core systems, in which large particles always have higher effective melting temperatures than small ones. As a result, the dynamic reversal is totally eliminated. Our work provides another piece of evidence of the underlying connections between the properties of non-equilibrium glass-formers and equilibrium crystal-formers.

A DPD model of soft spheres with waterlike anomalies and poly(a)morphism

Core-softened approaches have been employed to understand the behavior of a large variety of systems in soft condensed matter, from biological molecules to colloidal crystals, glassy phases, and water-like anomalies. At the same time, dissipative particle dynamics (DPD) is a powerful tool suitable for studying larger length and time scales. In this sense, we propose a simple model of soft molecules that exhibits a wide range of interesting phenomena: polyamorphism, with three amorphous phases, polymorphysm, including a recently found gyroid phase and a cubic diamond structure, reentrant liquid phase, and density, diffusion, and structural water-like anomalies. Each molecule is constituted by two collapsing beads, representing a harder central core and a softer corona. This induces a competition between distinct conformations that leads to their unique behavior. This provides a basis for the development of more accurate water-like DPD models that can then be parameterized for specific systems and even used to model and understand the self-assembly of colloidal crystals.

Revealing the characteristic length of random close packing via critical-like random pinning

By randomly pinning particles in fluidized states and finding the local energy minima, we form static packings of mono-disperse disks that resemble random close packing, when only n_c = 2.6% of the particles are pinned. The packings are isostatic and exhibit typical critical scalings of the jamming transition. The non-triviality of n_c is manifested mainly in two aspects. First, n_c acts as a critical point, leading to bifurcated critical scalings in its vicinity. The criticality of n_c is also demonstrated in the packings of weakly polydisperse disks. Second, n_c sets a length scale in agreement with the characteristic length of random close packing. With robust evidence, we show that this agreement is generally true for both mono- and poly-disperse particles and in both two and three dimensions. The randomness inherited from fluidized states by random pinning thus interprets the randomness of random close packing from a unique perspective.

Signatures of continuous hexatic-liquid transition in two-dimensional melting

Recent studies have shown that the melting of two-dimensional crystals can be either continuous or discontinuous, relying on multiple parameters such as particle stiffness, density, and particle size dispersity. However, what determines the continuity or discontinuity of the two-dimensional melting remains elusive. Here we study the two-dimensional melting of binary mixtures of soft-core particles. The two particle species are different in either particle size or particle stiffness. Starting with the mono-component systems which exhibit discontinuous hexatic-liquid transition, we gradually increase the particle size or stiffness dispersity and find that the hexatic-liquid coexistent region shrinks and eventually vanishes above a critical dispersity. Therefore, the growth of disorder caused by the particle size or stiffness dispersity leads to the discontinuous-continuous transition of the two-dimensional melting. We further find that as long as the melting is continuous the defect concentrations on the boundary between hexatic and liquid phases remain almost constant, accompanied by an almost constant correlation length. These characteristic defect concentrations and correlation length are universal and independent of particle interactions, temperature, and type of particle dispersity, which act as signatures of the continuous two-dimensional melting.

Two-Scale Scenario of Rigidity Percolation of Sticky Particles

In the presence of attraction, the jamming transition of packings of frictionless particles corresponds to the rigidity percolation. When the range of attraction is long, the distribution of the size of rigid clusters, P(s), is continuous and shows a power-law decay. For systems with short-range attractions, however, P(s) appears discontinuous. There is a power-law decay for small cluster sizes, followed by a low probability gap and a peak near the system size. We find that this appearing "discontinuity" does not mean that the transition is discontinuous. In fact, it signifies the coexistence of two distinct length scales, associated with the largest cluster and smaller ones, respectively. The comparison between the largest and second largest clusters indicates that their growth rates with system size are rather different. However, both cluster sizes tend to diverge in the large system size limit, suggesting that the jamming transition of systems with short-range attractions is still continuous. In the framework of the two-scale scenario, we also derive a generalized hyperscaling relation. With robust evidence, our work challenges the former single-scale view of the rigidity percolation.

Connecting glass-forming ability of binary mixtures of soft particles to equilibrium melting temperatures

The glass-forming ability is an important material property for manufacturing glasses and understanding the long-standing glass transition problem. Because of the nonequilibrium nature, it is difficult to develop the theory for it. Here we report that the glass-forming ability of binary mixtures of soft particles is related to the equilibrium melting temperatures. Due to the distinction in particle size or stiffness, the two components in a mixture effectively feel different melting temperatures, leading to a melting temperature gap. By varying the particle size, stiffness, and composition over a wide range of pressures, we establish a comprehensive picture for the glass-forming ability, based on our finding of the direct link between the glass-forming ability and the melting temperature gap. Our study reveals and explains the pressure and interaction dependence of the glass-forming ability of model glass-formers, and suggests strategies to optimize the glass-forming ability via the manipulation of particle interactions.

Glass-forming ability is an important parameter for manufacturing glassy materials, but it remains challenging to be characterized due to its nonequilibrium nature. Nie et al. provide a solution by linking it to the pressure dependence of melting temperature of constituent components in binary mixtures.