arXiv subjects

Bong June Sung

Publications and source records attributed to Bong June Sung.

4 recordsLinked to original sources

arXiv2025

Non-Gaussian Rotational Diffusion and Swing Motion of Dumbbell Probes in Two Dimensional Colloids

Two dimensional (2D) colloids exhibit intriguing phase behaviors distinct from those in three dimensions, as well as dynamic heterogeneity reminiscent of glass-forming liquids. Here, using discontinuous molecular dynamics simulations, we investigate the reporting dynamics of dicolloidal dumbbell probes in 2D colloids across the liquid-hexatic phase transition, where hexagonal bond-orientational order (HBOO) extends to quasi-long-ranged one. The rotational dynamics of dumbbell probes faithfully capture the structural and dynamical features of the host: Brownian in the isotropic liquid, and non-Gaussian in the hexatic and solid phases, reflecting both HBOO and dynamic heterogeneity of the medium. In the 2D hexatic and solid phases, probe rotation reflects heterogeneity as the dumbbells sample multiple dynamical domains of the host system: in mobile domains, they undergo rotational jumps of $π/3$ in accordance with HBOO, whereas in immobile domains they librate within cages formed by surrounding discs. Such non-Gaussianity disappears upon reentrant melting of the host medium driven by size polydispersity, highlighting a close connection between HBOO and probe dynamics. Furthermore, probe dynamics reveal both coupling (at a single particle level) and decoupling (at an ensemble-averaged level) between translation and rotation: swing motion emerges as their primary diffusion mode, while the Debye-Stokes-Einstein relation breaks down regardless of how the rotational diffusion coefficient is defined.

cond-mat.soft↗

arXiv2025

Bridging Electrostatic Screening and Ion Transport in Lithium Salt-Doped Ionic Liquids

Alkali salt-doped ionic liquids are emerging as promising electrolyte systems for energy applications, owing to their excellent interfacial stability. To address their limited ionic conductivity, various strategies have been proposed, including modifying the ion solvation environment and enhancing the transport of selected ions (e.g., Li$^+$). Despite the pivotal role of electrostatic interactions in determining key physicochemical properties, their influence on ion transport in such systems has received relatively little attention. In this work, we investigate the connection between ion transport and electrostatic screening using atomistic molecular dynamics simulations of 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([pyr$_{14}$][TFSI]) doped with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at molar fractions x$_{LiTFSI}$ $\le$ 0.3. We find that the charge-charge and density-density correlation functions exhibit oscillatory exponential decay, indicating that LiTFSI doped [pyr$_{14}$][TFSI] is a charge- and mass-dense system. The electrostatic screening length decreases with increasing LiTFSI concentration, whereas the decay length of the density-density correlation functions remains nearly unchanged. Notably, we find that the x$_{LiTFSI}$-sensitive screening length serves as a central length scale for disentangling species-specific contributions of ion pairs to collective ion transport upon LiTFSI doping. This framework provides a unifying perspective on the interplay between structure and transport in ionic liquid systems.

cond-mat.soft↗

arXiv2025

Subcritical Pitchfork Bifurcation Transition of a Single Nanoparticle in Strong Confinement

Confinement influences fluid properties. We show, employing molecular dynamics simulations with explicit solvents, that slit confinement drives a first-order transition for a small nanoparticle between staying at the slit center and binding to the slit surfaces. The transition follows a subcritical pitchfork bifurcation, accompanying a similar transition of the nanoparticle's lateral diffusion, depending on interparticle interactions and confinement interfaces. Our findings underscore the necessity for advancing molecular hydrodynamics under strong confinement.

cond-mat.soft↗

arXiv2018

Universal Transport Dynamics of Complex Fluids

Thermal motion in complex fluids is a complicated stochastic process but ubiquitously exhibits initial ballistic, intermediate sub-diffusive, and long-time non-Gaussian diffusive motion, unless interrupted. Despite its relevance to numerous dynamical processes of interest in modern science, a unified, quantitative understanding of thermal motion in complex fluids remains a long-standing problem. Here, we present a new transport equation and its solutions, which yield a unified quantitative explanation of the mean square displacement (MSD) and the non-Gaussian parameter (NGP) of various fluid systems. We find the environment-coupled diffusion kernel and its time correlation function are two essential quantities determining transport dynamics of complex fluids. From our analysis, we construct a general, explicit model of the complex fluid transport dynamics. This model quantitatively explains not only the MSD and NGP, but also the time-dependent relaxation of the displacement distribution for various systems. We introduce the concepts of intrinsic disorder and extrinsic disorder that have distinct effects on transport dynamics and different dependencies on temperature and density. This work presents a new paradigm for quantitative understanding of transport and transport-coupled processes in complex disordered media.

cond-mat.dis-nn↗