Author ORCID Identifier:
Date of Graduation
12-2025
Document Type
Thesis
Degree Name
Master of Science in Chemical Engineering (MSChE)
Degree Level
Graduate
Department
Chemical Engineering
Advisor/Mentor
Monroe, Jacob
Committee Member
Hestekin, Christa
Second Committee Member
Moradi, Mahmou d
Third Committee Member
Wickramasinghe, Rancil
Keywords
Collective Variables; Enhanced Sampling; Heterogeneous Nucleation; Homogeneous Nucleation; Molecular Dynamics; Umbrella Sampling
Abstract
Membrane distillation offers a promising route for desalination but remains limited by salt scaling, where mineral crystals nucleate and grow in concentrated brines. To inform strategies that mitigate scaling, this thesis investigates homogeneous nucleation in supersaturated aqueous NaCl solutions using molecular dynamics and enhanced sampling techniques. Several collective variables (CVs) are tested to capture the early stages of ion aggregation and crystal formation, including ion–ion coordination number, dehydration, and bond orientational order parameters Q₄ and Q₆. Enhanced sampling techniques, such as metadynamics and umbrella sampling, are employed to overcome the high free energy barriers associated with the formation of critical nuclei. All simulations were performed using OpenMM, coupled with PLUMED, and their computational performance was systematically benchmarked. Brute-force MD, without PLUMED, achieved ~8.9 ns/day on a 32-core CPU and 334 ns/day on a GPU. Adding the coordination-number CV reduced speeds to 5.9 ns/day (CPU) and 45.5 ns/day (GPU), while more complex orientational CVs, such as Q₆ and Q₄, further decreased throughput to ~2–3 ns/day. These results reveal a clear trade-off between computational efficiency and the level of fine control over the nucleus structure. The coordination number CV enables efficient sampling but favors amorphous clustering, whereas local orientational order CVs capture crystalline order more accurately at a higher cost. This benchmarking framework provides practical guidance for selecting CVs that strike a balance between fidelity and efficiency, thereby paving the way for future studies of heterogeneous nucleation and scaling mechanisms in membrane distillation systems.
Citation
Abbasi, N. (2025). Unraveling Nucleation Mechanisms in Membrane Distillation: A Molecular Dynamics and Enhanced Sampling Approach. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/6061
