CBE Doctoral Dissertation Defense: “Exploring the Role of Hydrodynamic Interactions in Colloidal Self-Assembly” (Ying-Shuo Peng)

Abstract:

Colloidal self-assembly remains difficult to predict and control, particularly in systems where experimental observations deviate from predictions based on traditional Brownian dynamics models. In these systems, hydrodynamic interactions (HI), which are often neglected or oversimplified, can play a critical role in governing particle dynamics. This dissertation systematically investigates the role of hydrodynamic interactions in controlling self-assembly, nucleation kinetics, and structural transformations in colloidal suspensions. First, the multiparticle collision dynamics coupled with a discrete-particle (MPCD+DP) framework is rigorously validated for resolving short-range hydrodynamic interactions. By examining canonical problems including near-wall diffusion, two-particle dynamics, and diffusion in crowded suspensions, this work demonstrates that accurate resolution of lubrication-like effects requires a sufficiently dense surface discretization. The results establish MPCD+DP as a reliable mesoscale approach for capturing hydrodynamic correlations in complex colloidal environments.

Building on this validated framework, the role of hydrodynamic interactions in crystal nucleation is examined through a detailed analysis of pre-nucleation fluctuations in supersaturated fluid. Contrary to the conventional view that HI primarily rescale particle diffusivity, the results show that HI fundamentally reorganize the dynamics of metastable fluids. Specifically, hydrodynamic coupling enhances long-wavelength collective motion and prolongs the lifetime of density fluctuations, increasing the probability that pre-critical configurations evolve into stable nuclei. A minimal first-passage-time model is developed to quantitatively link fluctuation persistence to nucleation probability, providing a mechanistic explanation for the long-standing discrepancy between experimental and simulation-based nucleation rates.

While hydrodynamic interactions capture the role of collective fluid-mediated motion, they are not sufficient to reproduce the nucleation behavior observed in DNA-grafted colloidal systems. To address this, a compound interaction model is introduced to explicitly incorporate interparticle friction arising from DNA bond rearrangement. The results reveal that friction plays a critical role in pathway selection during nucleation. At weak supersaturation, friction promotes compact cluster formation and accelerates crystallization, while at strong supersaturation, it stabilizes metastable icosahedral and amorphous structures, thereby altering the kinetic landscape of assembly.

The coupled effects of hydrodynamic interactions and friction are investigated in diffusionless transformations of “floppy” colloidal crystals. The results demonstrate a synergistic mechanism in which hydrodynamic interactions promote correlated collective motion, while friction suppresses local defects such as stacking faults. Together, these effects drive the CsCl-BCC crystal transform to defect-free CuAu-FCC rather than RHCP. Moreover, friction is shown to stabilize mechanically unstable parent phases, such as NaCl-type structures, enabling access to transformation pathways that are otherwise kinetically inaccessible.

Overall, this dissertation shows that colloidal self-assembly is governed by an interplay between hydrodynamic interactions and surface friction effects, which together influence nucleation pathways and structural transformations. By explicitly resolving hydrodynamic correlations, this work provides a mechanistic explanation for discrepancies between simulations and experiments and establishes a framework for understanding how dynamic interactions shape self-assembly outcomes in colloidal systems.

Zoom Meeting ID: 541 395 3743