Nanoparticles at liquid interfaces: Rotational dynamics and angular locking

Abstract

Nanoparticles with different surface morphologies that straddle the interface between two immiscible liquids are studied via molecular dynamics simulations. The methodology employed allows us to compute the interfacial free energy at different angular orientations of the nanoparticle. Due to their atomistic nature, the studied nanoparticles present both microscale and macroscale geometrical features and cannot be accurately modeled as a perfectly smooth body (e.g., spheres, cylinders). Under certain physical conditions, microscale features can produce free energy barriers that are much larger than the thermal energy of the surrounding media. The presence of these energy barriers can effectively "lock" the particle at specific angular orientations with respect to the liquid-liquid interface. This work provides new insights on the rotational dynamics of Brownian particles at liquid interfaces and suggests possible strategies to exploit the effects of microscale features with given geometric characteristics.

Curvature capillary migration of microspheres

“…Alternatively, the contact line can be pinned by kinetic trapping at heterogeneities, roughness or other pinning sites on the particle surface [10,[14][15][16][17][18]. Curvature capillary migration depends on the coupling of the particle-sourced distortion with the host interface shape.…”

The equilibrium shape of fluid-fluid interfaces: Derivation and a new numerical method for Young’s and Young-Laplace equations

“…A widely used approach to calculate a minimum energy surface is by means of the Surface Evolver program. 42 But several other approaches, both theoretical and numerical, have been used for studying the fluid-fluid interface shape in different physical problems, e.g., menisci shapes and capillary interactions, [43][44][45][46][47][48][49][50][51][52] droplet shapes, [53][54][55][56][57] diffuse interfaces, [58][59][60] or fluid-fluid interfaces in contact with deformable solids. [61][62][63] In this article, we introduce a new numerical method to obtain the minimum-energy shape of a fluid-fluid interface.…”

Wetting Driven by Thermal Fluctuations on Terraced Nanostructures

“…Hence, periodic energy barriers ∆U − at regular steps s hinder backward random displacements as the liquid-liquid interface undergoes thermal motion along the terraced surface. The time to cross over an energy barrier induced by nanoscale surface features can be predicted via Kramers theory of thermally-activated transitions, as documented in prior work [9,10]. For the present analysis, it suffices to recognize that a mean time T − ∝ exp(∆U − /k B T ) must elapse before observing a backward displacement of the interface over a terrace edge.…”

“…At nanoscales, the interplay between intermolecular forces, Brownian motion, and surface structure can give rise to complex interfacial phenomena that challenge conventional, continuum-based and deterministic, models [4][5][6]. For example, nanoscale surface structures can induce wetting processes governed by thermally-activated transitions between multiple metastable states [7][8][9][10][11][12]. These random transitions lead to directed transport of fluids and solutes when there is directional asymmetry of the energy barriers induced by the physicochemical structure of the confining surfaces [13][14][15].…”

“…The MD techniques employed in this work are extensively described in the literature [34,35] and prior work by the authors [9,10]. The simulated system [see Fig.…”