Modeling the Assembly of Polymer-Grafted Nanoparticles at Oil–Water Interfaces

Using dissipative particle dynamics (DPD), I model the interfacial adsorption and self-assembly of polymer-grafted nanoparticles at a planar oil–water interface. The amphiphilic core–shell nanoparticles irreversibly adsorb to the interface and create a monolayer covering the interface. The polymer chains of the adsorbed nanoparticles are significantly deformed by surface tension to conform to the interface. I quantitatively characterize the properties of the particle-laden interface and the structure of the monolayer in detail at different surface coverages. I observe that the monolayer of particles grafted with long polymer chains undergoes an intriguing liquid–crystalline–amorphous phase transition in which the relationship between the monolayer structure and the surface tension/pressure of the interface is elucidated. Moreover, my results indicate that the amorphous state at high surface coverage is induced by the anisotropic distribution of the randomly grafted chains on each particle core, which leads to noncircular in-plane morphology formed under excluded volume effects. These studies provide a fundamental understanding of the interfacial behavior of polymer-grafted nanoparticles for achieving complete control of the adsorption and subsequent self-assembly

Modeling Evaporation and Particle Assembly in Colloidal Droplets

Evaporation-induced assembly of nanoparticles in a drying droplet is of great importance in many engineering applications, including printing, coating, and thin film processing. The investigation of particle dynamics in evaporating droplets can provide fundamental hydrodynamic insight for revealing the processing–structure relationship in the particle self-organization induced by solvent evaporation. We develop a free-energy-based multiphase lattice Boltzmann method coupled with Brownian dynamics to simulate evaporating colloidal droplets on solid substrates with specified wetting properties. The influence of interface-bound nanoparticles on the surface tension and evaporation of a flat liquid–vapor interface is first quantified. The results indicate that the particles at the interface reduce surface tension and enhance evaporation flux. For evaporating particle-covered droplets on substrates with different wetting properties, we characterize the increase of evaporate rate via measuring droplet volume. We find that droplet evaporation is determined by the number density and circumferential distribution of interfacial particles. We further correlate particle dynamics and assembly to the evaporation-induced convection in the bulk and on the surface of droplet. Finally, we observe distinct final deposits from evaporating colloidal droplets with bulk-dispersed and interface-bound particles. In addition, the deposit pattern is also influenced by the equilibrium contact angle of droplet

Toward Generating Low-Friction Nanoengineered Surfaces with Liquid–Vapor Interfaces

Using molecular dynamics (MD), we investigate the importance of liquid–vapor interface topography in designing low-friction nanoengineered superhydrophobic surfaces. Shear flow is simulated on patterned surfaces. The relationship between the effective slip length and bubble meniscus curvature is attained by generating entrapped bubbles with large protrusion angles on patterned surfaces with nanoholes. We show that protruding bubbles can induce significant friction, which hinders the slip characteristics produced on liquid–vapor interfaces. By comparing surfaces with nanoholes and nanopillars, we also demonstrate that the continuity of the liquid–vapor interface can greatly influence slip. Our MD results yield an asymptotic behavior of slip length with varying gas fractions, which are found to be consistent with observations from simulations and analytical models produced in continuum studies

Structure and Dynamics of Stimuli-Responsive Nanoparticle Monolayers at Fluid Interfaces

Stimuli-responsive nanoparticles at fluid interfaces offer great potential for realizing on-demand and controllable self-assembly that can benefit various applications. Here, we conducted electrostatic dissipative particle dynamics simulations to provide a fundamental understanding of the microstructure and interfacial dynamics of responsive nanoparticle monolayers at a water–oil interface. The model nanoparticle is functionalized with polyelectrolytes to render the pH sensitivity, which permits further manipulation of the monolayer properties. The monolayer structure was analyzed in great detail through the density and electric field distributions, structure factor, and Voronoi tessellation. Even at a low surface coverage, a continuous disorder-to-order phase transition was observed when the particle’s degree of ionization increases in response to pH changes. The six-neighbor particle fraction and bond orientation order parameter quantitatively characterize the structural transition induced by long-range electrostatic interactions. Adding salt can screen the electrostatic interactions and offer additional control on the monolayer structure. The detailed dynamics of the monolayer in different states was revealed by analyzing mean-squared displacements, in which different diffusion regimes were identified. The self-diffusion of individual particles and the collective dynamics of the whole monolayer were probed and correlated with the structural transition. Our results provide deeper insight into the dynamic behavior of responsive nanoparticle surfactants and lay the groundwork for bottom-up synthesis of novel nanomaterials, responsive emulsions, and microdroplet reactors

Harnessing Interfacially-Active Nanorods to Regenerate Severed Polymer Gels

With newly developed computational approaches, we design a nanocomposite that enables self-regeneration of the gel matrix when a significant portion of the material is severed. The cut instigates the dynamic cascade of cooperative events leading to the regrowth. Specifically, functionalized nanorods localize at the new interface and initiate atom transfer radical polymerization with monomers and cross-linkers in the outer solution. The reaction propagates to form a new cross-linked gel, which can be tuned to resemble the uncut material

Harnessing Interfacially-Active Nanorods to Regenerate Severed Polymer Gels

With newly developed computational approaches, we design a nanocomposite that enables self-regeneration of the gel matrix when a significant portion of the material is severed. The cut instigates the dynamic cascade of cooperative events leading to the regrowth. Specifically, functionalized nanorods localize at the new interface and initiate atom transfer radical polymerization with monomers and cross-linkers in the outer solution. The reaction propagates to form a new cross-linked gel, which can be tuned to resemble the uncut material

Harnessing Interfacially-Active Nanorods to Regenerate Severed Polymer Gels

With newly developed computational approaches, we design a nanocomposite that enables self-regeneration of the gel matrix when a significant portion of the material is severed. The cut instigates the dynamic cascade of cooperative events leading to the regrowth. Specifically, functionalized nanorods localize at the new interface and initiate atom transfer radical polymerization with monomers and cross-linkers in the outer solution. The reaction propagates to form a new cross-linked gel, which can be tuned to resemble the uncut material

Interfacial Targeting of Sessile Droplets Using Electrospray

We report on the use of electrospray atomization to deliver nanoparticles and surfactant directly to the surface of sessile droplets. The particles delivered to the target droplet remained adsorbed at its interface since they arrived solvent-free. Upon complete evaporation, the interface of the target drop was mapped to the underlying substrate, forming a nanoparticle deposit. The use of electrospray permitted the exploration of the interfacial particle transport and the role of surfactants in governing particle motion and deposit structure. When no surfactant was present in the sprayed solution, there was no observable convection of the interfacial particles. When Tween 80, a high-molecular-weight surfactant, was added to the sprayed solution, the surface flow was similarly suppressed. Only when small surfactants (e.g., sodium dodecyl sulfate) were present in the sprayed solution was Marangoni flow, directed toward the droplet apex, induced at the interface. This flow drove the interfacial particles to the apex of the target droplet, creating a particle-dense region at the center of the final deposit. We found that small surfactants were capable of desorbing from the interface at a sufficiently high rate relative to the evaporation time scale of the target droplet. Once inside the drop, the desorbed surfactant was convected to the contact line where it accumulated, inducing a surface tension gradient and a solutal Marangoni flow. Numerical modeling using the lattice Boltzmann–Brownian dynamics method confirmed this mechanism of particle transport and its relationship to deposit structure. The use of sacrificial targets combined with electrospray may provide a unique capability for building colloidal monolayers with organized structure in a scalable way

Self-Healing Vesicles Deposit Lipid-Coated Janus Particles into Nanoscopic Trenches

Using dissipative particle dynamics (DPD) simulations, we model the interaction between nanoscopic lipid vesicles and Janus nanoparticles localized on an adhesive substrate in the presence of an imposed flow. The system is immersed in a hydrophilic solution, and the hydrophilic substrate contains nanoscopic trenches, which are either step- or wedge-shaped. The fluid-driven vesicle successfully picks up Janus particles on the substrate, transports these particles as cargo along the surface, and then drops off the particles into the trenches. For Janus particles with a relatively large hydrophobic region, lipids from the bilayer membrane become detached from the vesicle and bound to the hydrophobic domain of the deposited particle. While the detachment of these lipids rips the vesicle, it provides a coating that effectively shields the hydrophobic portion of the nanoparticle from the outer solution. After the particle has been dropped off, the torn vesicle undergoes structural rearrangement, reforming into a closed structure that resembles its original shape. In effect, the vesicle displays pronounced adaptive behavior, shedding lipids to form a protective coating around the particle and undergoing a self-healing process after the particle has been deposited. This responsive, adaptive behavior is observed in cases involving both the step- and wedge-shaped trenches, but the step trench is more effective at inducing particle drop off. The results reveal that the introduction of grooves or trenches into a hydrophilic surface can facilitate the targeted delivery of amphiphilic particles by self-healing vesicles, which could be used for successive delivery events

Harnessing Fluid-Driven Vesicles To Pick Up and Drop Off Janus Particles

Using dissipative particle dynamics (DPD) simulations, we model the interaction between nanoscopic lipid vesicles and Janus nanoparticles in the presence of an imposed flow. Both the vesicle and Janus nanoparticles are localized on a hydrophilic substrate and immersed in a hydrophilic solution. The fluid-driven vesicle successfully picks up Janus particles on the substrate and transports these particles as cargo along the surface. The vesicle can carry up to four particles as its payload. Hence, the vesicles can act as nanoscopic “vacuum cleaners”, collecting nanoscopic debris localized on the floors of the fluidic devices. Importantly, these studies reveal how an imposed flow can facilitate the incorporation of nanoparticles into nanoscale vesicles. With the introduction of a “sticky” domain on the substrate, the vesicles can also robustly drop off and deposit the particles on the surface. The controlled pickup and delivery of nanoparticles <i>via</i> lipid vesicles can play an important step in the bottom-up assembly of these nanoparticles within small-scale fluidic devices