Ultrasound enhanced diffusion in hydrogels: An experimental and non-equilibrium molecular dynamics study

Sebastian E N Price; Caroline Einen; Othonas A Moultos; Thijs J H Vlugt; Catharina de Lange Davies; Erika Eiser; Anders Lervik

doi:10.1063/5.0202182

Ultrasound enhanced diffusion in hydrogels: An experimental and non-equilibrium molecular dynamics study

J Chem Phys. 2024 Apr 21;160(15):154906. doi: 10.1063/5.0202182.

Authors

Sebastian E N Price¹, Caroline Einen², Othonas A Moultos³, Thijs J H Vlugt³, Catharina de Lange Davies⁴, Erika Eiser², Anders Lervik¹

Affiliations

¹ PoreLab and Department of Chemistry, The Norwegian University of Science and Technology, NTNU, N7491 Trondheim, Norway.
² PoreLab and Department of Physics, The Norwegian University of Science and Technology, NTNU, N7491 Trondheim, Norway.
³ Engineering Thermodynamics, Process and Energy Department, Faculty of Mechanical Engineering, Delft University of Technology, Leeghwaterstraat 39, 2628CB Delft, The Netherlands.
⁴ Department of Physics, The Norwegian University of Science and Technology, NTNU, N7491 Trondheim, Norway.

PMID: 38639314
DOI: 10.1063/5.0202182

Abstract

Focused ultrasound has experimentally been found to enhance the diffusion of nanoparticles; our aim with this work is to study this effect closer using both experiments and non-equilibrium molecular dynamics. Measurements from single particle tracking of 40 nm polystyrene nanoparticles in an agarose hydrogel with and without focused ultrasound are presented and compared with a previous experimental study using 100 nm polystyrene nanoparticles. In both cases, we observed an increase in the mean square displacement during focused ultrasound treatment. We developed a coarse-grained non-equilibrium molecular dynamics model with an implicit solvent to investigate the increase in the mean square displacement and its frequency and amplitude dependencies. This model consists of polymer fibers and two sizes of nanoparticles, and the effect of the focused ultrasound was modeled as an external oscillating force field. A comparison between the simulation and experimental results shows similar mean square displacement trends, suggesting that the particle velocity is a significant contributor to the observed ultrasound-enhanced mean square displacement. The resulting diffusion coefficients from the model are compared to the diffusion equation for a two-time continuous time random walk. The model is found to have the same frequency dependency. At lower particle velocity amplitude values, the model has a quadratic relation with the particle velocity amplitude as described by the two-time continuous time random walk derived diffusion equation, but at higher amplitudes, the model deviates, and its diffusion coefficient reaches the non-hindered diffusion coefficient. This observation suggests that at higher ultrasound intensities in hydrogels, the non-hindered diffusion coefficient can be used.