Glass-forming liquids subjected to sufficiently strong shear universally exhibit striking nonlinear behavior; for example, a power-law decrease of the viscosity with increasing shear rate. This ...phenomenon has attracted considerable attention over the years from both fundamental and applicational viewpoints. However, the out-of-equilibrium and nonlinear nature of sheared fluids have made theoretical understanding of this phenomenon very challenging and thus slower to progress. We find here that the structural relaxation time as a function of the two-body excess entropy, calculated for the extensional axis of the shear flow, collapses onto the corresponding equilibrium curve for a wide range of pair potentials ranging from harsh repulsive to soft and finite. This two-body excess entropy collapse provides a powerful approach to predicting the dynamics of nonequilibrium liquids from their equilibrium counterparts. Furthermore, the two-body excess entropy scaling suggests that sheared dynamics is controlled purely by the liquid structure captured in the form of the two-body excess entropy along the extensional direction, shedding light on the perplexing mechanism behind shear thinning.
Hydrodynamic interactions (HIs) play a critical role in the self-organization of colloidal suspensions and biological solutions. However, their roles have remained elusive particularly for charged ...soft matter systems. Here we consider the role of HIs in the self-assembly of oppositely charged colloidal particles, which is a promising candidate for electrical tunable soft materials. We employ the fluid particle dynamics method to consider many-body HIs and the coupling between the colloid, ion, and fluid motions. We find that, under a constant electric field, oppositely charged colloidal particles form clusters and percolate into a gel network, unlike bundlelike aggregates aligned in the field direction observed by Brownian dynamics simulations neglecting HIs. We reveal that the cluster-forming tendency originates from the incompressibility-induced "inverse squeezing flow" effect that dramatically slows down the disaggregation of attached colloids. Our findings indicate that the HI selects a unique kinetic pathway to the nonequilibrium colloidal self-assembly.
One of the biggest unresolved problems in crystallization phenomena is the significant discrepancy in the nucleation rate between experiments and simulations even for the simplest liquid, i.e., the ...hard-sphere system. A popular explanation for this discrepancy is the neglect of hydrodynamic interactions (HI) in simulation studies. By comparing simulations with and without HI, we show that the long-time diffusive dynamics of the colloids is slowed down more rapidly by hydrodynamic lubrication effects with increasing volume fraction. We find that the kinetics of both nucleation and growth are controlled by this long-time diffusion and that it is possible to account for most of the effects of HI by rescaling with this timescale. Therefore, we conclude that HI is not the primary cause of the accelerated nucleation rates observed in experiments.
The principle of polymorph selection upon crystal nucleation is one of the fundamental problems in crystallization. Recently we found that for hard spheres the crystal polymorph is already selected ...by locally favoured packing symmetry in a metastable supercooled state. Here we study whether this scenario is also valid for soft spheres. To do so, we investigate the homogeneous nucleation process of the Gaussian core model (GCM) in supercooled states by means of Monte Carlo computer simulations. We use bond orientational order parameters, which characterize local packing symmetries, to follow the formation of solid nuclei and to distinguish between different polymorphs. We concentrate on two state points, at low and high pressure respectively, for which macroscopic thermodynamics dictates the formation of the different polymorphs (fcc and bcc crystals respectively). We show that the nucleation of the different crystalline structures does not follow Ostwald's step rule of crystallization, and that, despite the underlying phase diagram, the bcc phase is always favoured. In analogy to hard sphere systems, we find a new criterion for polymorph selection: crystallization occurs in precursor regions of high bond orientational order, and the crystal which first nucleates is the one that has the closest symmetry to these ordered regions in the supercooled state.
We propose a novel mechanism of polymorph selection, and test it with simulations of the Gaussian core model.
Some liquids do not crystallize below the melting point, but instead enter into a supercooled state and on cooling eventually become a glass at the glass-transition temperature. During this process, ...the liquid dynamics not only drastically slow down, but also become progressively more heterogeneous. The relationship between the kinetic slowing down and growing dynamic heterogeneity is a key problem of the liquid-glass transition. Here, we study this problem by using a liquid model, with a crystalline ground state, for which we can systematically control frustration against crystallization. We found that slow regions having a high degree of crystalline order emerge below the melting point, and their characteristic size and lifetime increase steeply on cooling. These crystalline regions lead to dynamic heterogeneity, suggesting a connection to the complex free-energy landscape and the resulting slow dynamics. These findings point towards an intrinsic link between the glass transition and crystallization.
Distance-Regular Graphs Van Dam, Edwin R.; Koolen, Jack H.; Tanaka, Hajime
The Electronic journal of combinatorics,
04/2016, Letnik:
1000
Journal Article
Recenzirano
Odprti dostop
This is a survey of distance-regular graphs. We present an introduction to distance-regular graphs for the reader who is unfamiliar with the subject, and then give an overview of some developments in ...the area of distance-regular graphs since the monograph 'BCN' Brouwer, A.E., Cohen, A.M., Neumaier, A., Distance-Regular Graphs, Springer-Verlag, Berlin, 1989 was written.
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