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.

The dependence of magnetization on the applied magnetic field and temperature was measured carefully near their Curie temperatures for two perovskite manganite samples: La0.67Ca0.33MnOδ and La0.60Y0.07Ca0.33MnOδ. It is suggested by the results that these materials can be utilized as both the thermal storage (passive regeneration) and as the working material (active regeneration) in an active magnetic regenerative refrigerator with very large temperature span, for their significant entropy change upon the application of a magnetic field and the easily tuned Curie temperatures.

The thermostats in molecular dynamics (MD) simulations of highly confined channel flow may have significant influences on the fidelity of transport phenomena. In this study, we exploit non-equilibrium MD simulations to generate Couette flows with different combinations of thermostat algorithms and strategies. We provide a comprehensive analysis on the effectiveness of three thermostat algorithms Nosé-Hoover chain (NHC), Langevin (LGV) and dissipative particle dynamics (DPD) when applied in three thermostat strategies, thermostating either walls (TW) or fluid (TF), and thermostating both the wall and fluid (TWTF). Our results of thermal and mechanical properties show that the TW strategy more closely resembles experimental conditions. The TF and TWTF systems also produce considerably similar behaviors in weakly sheared systems, but deviate the dynamics in strongly sheared systems due to the isothermal condition. The LGV and DPD thermostats used in the TF and TWTF systems provide vital ways to yield correct dynamics in coarse-grained systems by tuning the fluid transport coefficients. Using conventional NHC thermostat to thermostat fluid only produces correct thermal behaviors in weakly sheared systems, and breaks down due to significant thermal inhomogeneity in strongly sheared systems.

In this paper, nanoscale wetting on groove-patterned surfaces is thoroughly studied using molecular dynamics simulations. The results are compared with Wenzel's and Cassie's predictions to determine whether these continuum theories are still valid at the nanoscale for both hydrophobic and hydrophilic types of surfaces when the droplet size is comparable to the groove size. A system with a liquid mercury droplet and grooved copper substrate is simulated. The wetting properties are determined by measuring contact angles of the liquid droplet at equilibrium states. Correlations are established between the contact angle, roughness factor r, and surface fraction f. The results show that, for hydrophobic surfaces, the contact angle as a function of roughness factor and surface fraction on nanogrooved surfaces obeys the predictions from Wenzel's theory for wetted contacts and Cassie's theory for composite contacts. However, slight deviations occur in composite contacts when a small amount of liquid penetration is observed. The contact angle of this partial wetting cannot be accurately predicted using either Cassie's or Wenzel's theories. For hydrophilic surfaces, only wetted contacts are observed. In most cases, the resulting contact angles are found to be higher than Wenzel's predictions. At the nanoscale, high surface edge density plays an important role, which results in contact line pinning near plateau edges. For both hydrophobic and hydrophilic surfaces, substantial amount of anistropic spreading is found in the direction that is parallel to the grooves, especially at wetted or partially wetted contacts.

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.

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 via lipid vesicles can play an important step in the bottom-up assembly of these nanoparticles within small-scale fluidic devices.

Combining modeling and experiment, we created multilayered gels where each layer was "stacked" on top of the other and covalently interconnected to form mechanically robust materials, which could integrate the properties of the individual layers. In this process, a solution of new initiator, monomer, and cross-linkers was introduced on top of the first gel, and these new components then underwent living (co)polymerization to form the subsequent layer. We simulated this process using dissipative particle dynamics (DPD) to isolate factors that affect the formation and binding of chemically identical gel as well as incompatible layers. Analysis indicates that the covalent bond formation between the different layers is primarily due to reactive chain-ends, rather than residual cross-linkers. In the complementary experiments, we synthesized multilayered gels using either free radical (FRP) or atom transfer radical polymerizations (ATRP) methods. Polymerization results demonstrated that chemically identical materials preserved their structural integrity independent of the polymerization method. For gels encompassing incompatible layers, the contribution of reactive chain-ends plays a particularly important role in the integrity of the material, as indicated by the more mechanically robust systems prepared by ATRP. These studies point to a new approach for combining chemically distinct components into one coherent, multifunctional material as well as an effective method for repairing severed gels. ■ INTRODUCTIONMultifunctional materials address a number of vital technological needs since they allow one material to provide a range of properties and behavior. A challenge in creating these desirable materials is devising an approach for integrating the different components into one cohesive system. Conceptually, one would like to stack the components with different functionalities on top of each other to form the desired product. This approach would have the distinct advantage that it permits new functionalities to be added at will to improve or tailor the utility of the material. To date, however, it remains a considerable challenge to create "stackable materials" that would form a mechanically robust structure. As a step in addressing this challenge, herein we use computational modeling and experimental studies to design multilayered, "stackable" gels, where one layer is effectively "stacked" on top of another. Each gel layer is covalently bound to the neighboring gels, and hence, the system displays considerable mechanical integrity.It is important to recall that researchers have devised means of "gluing" together separated pieces of polymer gels. 1 Recently, Leibler et al. used nanoparticles as a binding agent to successfully attach two severed gels 1b and in this way could heal broken samples. Rather than binding separated sections, our aim is to grow one gel layer on top of another and, thereby, unite chemically distinct gels into a coherent material. Through this approach, we can, for example, stack a hydrophobic ge...

This study investigates shear-induced liquid structure changes in nanoscale Couette flows and their corresponding flow boundary conditions. Molecular dynamics simulations are used to model a liquid argon slab confined between two smooth rigid copper walls with an applied velocity at the upper wall to generate a planar Couette flow. Depending on the applied wall velocity, different liquid structures or the orderings of the liquid at liquid-solid interfaces are identified when reaching steady states. We define three regimes based on the ordering of the liquid structure: Newtonian, layer, and oversheared. Each regime is characterized by the spatial probability distribution and structure factor. These liquid structures are strongly correlated with the liquid velocity and density profiles in the flow. Ultimately, the liquid structures also determine the boundary conditions from pure slip to multilayer locking at liquid-solid interfaces. Our results show that temperature and liquid-solid interaction parameter are also important factors in influencing the liquid structures formed near interfaces.