arXiv subjects

Kyu Hwan Choi

Publications and source records attributed to Kyu Hwan Choi.

5 recordsLinked to original sources

arXiv2026

Mechanics of heterogeneous fiber networks

Internally generated active stresses drive soft materials into architectures inaccessible to thermal self-assembly. We use a microtubule-based active fluid to assemble and irreversibly restructure actin-fascin networks. Subsequently, we probe the mesoscale mechanics of such networks by combining active microrheology with fluorescence imaging of the strain field around the probe. Increasing motor concentration broadens the pore-size distribution and thickens load-bearing bundles, raising the mean local elastic modulus and its spatial variability. Displacement fields of actively-processed networks propagate over longer range when compared to unprocessed networks. At large strains, both networks strain soften and plastically restructure. The combined microrheology and strain-imaging approach show that tunable active stresses reprogram the structure and viscoelastic response of fiber networks at the scale of their structural heterogeneity.

cond-mat.soft↗

arXiv2026

Anisotropic Diffusion in Lyotropic Chromonic Liquid Crystal using Fluorescence Recovery After Photobleaching

Anisotropic diffusion governs transport in a wide range of soft and biological materials, where microstructure and molecular interactions jointly shape how matter moves. Here, we quantitatively investigate anisotropic molecular transport in lyotropic chromonic liquid crystals (LCLCs) using fluorescence recovery after photobleaching (FRAP). Disodium cromoglycate (DSCG) serves as a model LCLC system, and diffusion is measured across isotropic, nematic, and columnar phases as concentration and temperature are varied. To disentangle the roles of microstructure and molecular interactions, we employ two fluorescent tracers with distinct affinities for the LCLC aggregates: Acridine Orange (AO), which intercalates into DSCG aggregates, and Bodipy, which interacts weakly and remains largely in the aqueous phase. Fourier-space FRAP analysis independently resolves the parallel and perpendicular diffusion coefficients for both dyes relative to the liquid-crystal alignment. In the nematic phase, diffusion becomes anisotropic, with faster transport along the liquid-crystal director. As the DSCG concentration increases, AO dye molecules that are strongly coupled to the aggregates exhibit a slowdown in all directions, reflecting enhanced packing and steric confinement of the LC microstructure. In contrast, weakly interacting Bodipy dye molecules display enhanced transport along the alignment direction as the DSCG concentration increases in the nematic regime, suggesting the emergence of microscopic channels that guide motion, analogous to transport in oriented porous media. These results reveal how the evolving microstructure of LCLCs controls effective diffusion and provide a quantitative framework for understanding and designing anisotropic transport in aligned soft materials.

cond-mat.soft↗

arXiv2025

Direct experimental measurement of many-body hydrodynamic interactions with optical tweezers

Many-body hydrodynamic interactions (HIs) play an important role in the dynamics of fluid suspensions. While many-body HIs have been studied extensively using particle simulations, there is a dearth of experimental frameworks with which to quantify fluid-mediated multi-body interactions. To address this, we design an experimental method that utilizes optical laser tweezers for quantifying fluid-mediated colloidal interactions with exquisite precision and control. By inducing translation-rotation hydrodynamic coupling between trapped fluorescently-labeled colloids, we obtain a direct reporter of few- to many-body HIs experimentally. We leverage the torque-free nature of laser tweezers to enable sensitive measurements of signals between trapped colloids. First, we measure the pair HI between a stationary tracer probe and a translating particle as a function of their separation distance. We discover that our technique can precisely quantify distant fluid disturbances that are generated by ~2 pN of hydrodynamic force at 12 particle radii of separation. To study the effect of many-body HIs, we measure the rotational mobility of a probe in a three-particle setup and in a model material, a two-dimensional hexagonally-close-packed lattice, that undergoes oscillatory strain. Respectively, we discover that the probe's rotation can reverse in certain three-body configurations, and we find that rotational mobility in the crystalline array is strongly attenuated by particle rigidity. Experimental measurements are corroborated by microhydrodynamic theory and Stokesian Dynamics simulations with excellent agreement, highlighting our ability to measure accurately many-body HIs. Lastly, we extend our theoretical framework to manipulate colloidal-scale fluid flows. With experimental validation, we compute the required trajectory of a moving particle to induce a desired angular velocity of a probe.

cond-mat.soft↗

arXiv2024

Motility Modulates the Partitioning of Bacteria in Aqueous Two-Phase Systems

We study the partitioning of motile bacteria in an aqueous two-phase mixture of dextran (DEX) and polyethylene glycol (PEG), which can phase separate into DEX-rich and PEG-rich phases. While non-motile bacteria partition exclusively into the DEX-rich phase in all conditions tested, we observed that motile bacteria penetrate the soft DEX/PEG interface and partition variably among the two phases. For our model organism \textit{Bacillus subtilis}, the fraction of motile bacteria in the DEX-rich phase increased from 0.58 to 1 as we increased DEX composition within the two-phase region. We hypothesized that the chemical affinity between DEX and the bacteria cell wall acts to weakly confine the bacteria within the DEX-rich phase; however, motility can generate sufficient mechanical forces to overcome the soft confinement and propel the bacteria into the PEG-rich phase. Using optical tweezers to drag a bacterium across the DEX/PEG interface, we demonstrate that the overall bacteria partitioning is determined by a competition between the interfacial forces and bacterial propulsive forces. Our measurements are supported by a theoretical model of dilute active rods embedded within a periodic soft confinement potential.

cond-mat.soft↗

arXiv2023

Dynamic interfaces for contact-time control of colloidal interactions

Understanding multibody interactions between colloidal particles out of equilibrium has a profound impact on dynamical processes such as colloidal self assembly. However, traditional colloidal interactions are effectively quasi-static on colloidal timescales and cannot be modulated out of equilibrium. A mechanism to dynamically tune the interactions during colloidal contacts can provide new avenues for self assembly and material design. In this work, we develop a framework based on polymer-coated colloids and demonstrate that in-plane surface mobility and mechanical relaxation of polymers at colloidal contact interfaces enable an effective, dynamic interaction. Combining analytical theory, simulations, and optical tweezer experiments, we demonstrate precise control of dynamic pair interactions over a range of pico-Newton forces and seconds timescales. Our model may be used to engineer colloids with exquisite control over the kinetics and thermodynamics of colloidal self-assembly dynamics via interface modulation and nonequilibrium processing.

cond-mat.soft↗