arXiv subjects

Tzortzis Koulaxizis

Publications and source records attributed to Tzortzis Koulaxizis.

3 recordsLinked to original sources

arXiv2026

Axial dispersion in dilute solutions of linear and branched polymers in parallel-plate and expansion-contraction microchannels

The axial dispersion of polymers in microchannels depends on an interplay between microchannel geometry, polymer architecture, and hydrodynamics. Here, we investigate the axial dispersion of linear, comb, and star polymers in parallel-plate and sinusoidal expansion-contraction microchannels at dilute concentrations using multiparticle collision dynamics simulations. The polymers all contain the same number of monomers but differ in their architecture, and their concentration is fixed at either one value that is dilute for all polymers or the same value relative to the overlap concentration for each polymer. The dispersion coefficients measured at a nominal solvent volumetric flow rate are found to depend on both architecture and concentration. We show that the dispersion coefficients collapse as a function of the Péclet number after accounting for confinement effects on the polymer diffusion coefficient and polymer contributions to the flow field, and the dispersion coefficients in the parallel-plate microchannel can be reasonably predicted using a theory that accounts for inhomogeneous distribution of the polymers in the microchannel.

cond-mat.soft↗

arXiv2026

Effect of Pre-Shear and Dispersity on Crystallization of a Model Polymer with Soft Pair Interactions using Molecular Dynamics Simulations

Polymer crystallization is a process of great interest in both fundamental theory and industrial settings, particularly in polymer processing and applications involving semi-crystalline materials. The effect of processing on the initial stages of crystallization is not fully understood. Our study investigates the influence of pre-shear on monodisperse melts and bidisperse blends of a generic, segmentally coarse-grained polymer model. Through molecular dynamics simulations, we explore how polydispersity affects crystallization, where we found that the addition of short chains to a melt of longer chains increased the final crystallinity by about 10%, and increased the initial growth rate by roughly a factor of two. In contrast, however, pre-shearing the hot melt before quenching only showed a minor increase in both growth rates and final crystallinty, except in monodisperse melts of short chains. Crystal grain shapes were most influenced by pre-shearing monodisperse melts, where both asphericity and prolateness decreased. Additionally, we determined topological connectivity of crystal grains through tie- and loop-chain analysis. Again, only monodisperse melts showed a significant increase of tie chain fractions with pre-shear, while all other systems showed only modest increases. Our findings provide insight into the changes of crystallinity and cluster morphologies that emerge when pre-sheared, offering a deeper understanding of the initial crystallization processes in polymer melts when subjected to pre-shear.

cond-mat.soft↗

arXiv2025

Mesoscale particle-based simulations of flow in expansion-contraction microchannels at low Reynolds number

We computationally study the flow of Newtonian fluids through sinusoidal expansion-contraction microchannels at low Reynolds number. We first use a perturbation method to analytically derive series solutions for the stream function and volumetric flow rate that extend prior work [P.K. Kitanidis and B.B. Dykaar, Transport in Porous Media 26, 89-98 (1997)] up to tenth order. We then employ two particle-based mesoscale methods, dissipative particle dynamics (DPD) and multiparticle collision dynamics (MPCD), to simulate the same flows. We find that the fluid velocity at the expansion and contraction points as well as the volumetric flow rate are in good agreement between DPD, MPCD, and the fourth-order series solution for a wide range of microchannel geometries. The mesoscale fluid models exhibit some slip at the walls, leading to a small but consistent overprediction of the velocity and volumetric flow rate. The series solution fails for short microchannel lengths and large amplitudes; we identify lengths and amplitudes for which it converges to a given order. Overall, we find that DPD and MPCD are convenient and reasonably accurate methods, particularly for microchannel geometries where the series solution fails or is cumbersome to implement.

physics.flu-dyn↗