Modeling self-assembly and phase behavior in complex mixtures

Nematoelastic crawlers

A propagating "beam" triggering a local phase transition in a nematic elastomer sets it into a crawling motion, which may morph due to buckling. We consider the motion of the various configurations of slender rods and thin stripes with both uniform and splayed nematic order in cross-section and detect the dependence of the gait and speed on flexural rigidity and substrate friction.

Effects of the positive feedback loop in polymerization on the reaction-induced phase separation of polymer mixtures

Phase separation of mixtures composed of a guest polymer dissolved in monomer of a host polymer was induced by photopolymerization. From the polymerization kinetics, it was found that for the host polymer with the glass transition temperature (Tg) higher than the experimental temperature, the Trommsdorff-Norrish effect emerges, providing a method to terminate the phase separation at various intermediate stages of the process and thereby a variety of morphologies can be constructed. For the host polymers with (Tg) lower than the experimental temperature, the freezing process of morphology is not complete, leading to the secondary phase separation inside the domains already generated by the primary process. The kinetics of this secondary phase separation indicates that the peculiar morphology like "sunny-side-up egg"-like structures was generated via spinodal decomposition. The experimental results obtained in this study show that the competition between photopolymerization and phase separation can provide a useful method of materials design in the micro- and sub-micrometer scales.

Self-assembled block copolymer-nanoparticle hybrids: interplay between enthalpy and entropy

The dispersion of nanoparticles in ordered block copolymer nanostructures can provide control over particle location and orientation, and pave the way for engineered nanomaterials that have enhanced mechanical, electrical, or optical properties. Fundamental questions pertaining to the role of enthalpic and entropic particle-polymer interactions remain open and motivate the present work. We consider here a system of 10.6 nm silica nanoparticles (NPs) dispersed in ordered cylinders formed by hydrated poly(ethylene oxide)-poly(propylene oxide) block copolymers (Pluronic P105: EO(37)PO(56)EO(37)). Protonation of silica was used to vary the NP-polymer enthalpic interactions, while polar organic solvents (glycerol, DMSO, ethanol, and DMF) were used to modulate the NP-polymer entropic interactions. The introduction of deprotonated NPs in the place of an equal mass of water did not affect the lattice parameter of the PEO-PPO-PEO block copolymer hexagonal lyotropic liquid crystalline structures. However, the dispersion of protonated NPs led to an increase in the lattice parameter, which was attributed to stronger NP-polymer hydrogen bonding (enthalpic) interactions. Dispersion of protonated NPs into cylindrical structures formed by Pluronic P105 in 80/20 water/organic solvents does not influence the lattice parameter, different from the case of protonated NP in plain water. Organic solvents appear to screen the NP-polymer hydrogen bonding interactions.

Surfactant two-dimensional self-assembly under confinement

Confinement-induced structural rearrangements in supported self-assembled surfactant layers in aqueous salt solutions are investigated using classical density functional theory. The systematic study of the influence of the nature of electrolyte revealed that 2:1 electrolyte stabilizes the hemicylindrical configuration of ionic surfactant layers, while a confinement-induced transition to a tilted monolayer configuration was found in symmetric 1:1 and 2:2 electrolytes. On the basis of this study, we formulate a general model for the energetics of structural rearrangements in supported surfactant layers. This model provides a basis for directed self-assembly of surfactant templates with desired structure and stability for scalable synthesis of nanocomposite functional materials, templated crystal growth, and biomolecule adsorption.

Toxicity testing in the 21st century: a vision and a strategy

With the release of the landmark report Toxicity Testing in the 21st Century: A Vision and a Strategy, the U.S. National Academy of Sciences, in 2007, precipitated a major change in the way toxicity testing is conducted. It envisions increased efficiency in toxicity testing and decreased animal usage by transitioning from current expensive and lengthy in vivo testing with qualitative endpoints to in vitro toxicity pathway assays on human cells or cell lines using robotic high-throughput screening with mechanistic quantitative parameters. Risk assessment in the exposed human population would focus on avoiding significant perturbations in these toxicity pathways. Computational systems biology models would be implemented to determine the dose-response models of perturbations of pathway function. Extrapolation of in vitro results to in vivo human blood and tissue concentrations would be based on pharmacokinetic models for the given exposure condition. This practice would enhance human relevance of test results, and would cover several test agents, compared to traditional toxicological testing strategies. As all the tools that are necessary to implement the vision are currently available or in an advanced stage of development, the key prerequisites to achieving this paradigm shift are a commitment to change in the scientific community, which could be facilitated by a broad discussion of the vision, and obtaining necessary resources to enhance current knowledge of pathway perturbations and pathway assays in humans and to implement computational systems biology models. Implementation of these strategies would result in a new toxicity testing paradigm firmly based on human biology.

Properties and emerging applications of self-assembled structures made from inorganic nanoparticles

Just as nanoparticles display properties that differ from those of bulk samples of the same material, ensembles of nanoparticles can have collective properties that are different to those displayed by individual nanoparticles and bulk samples. Self-assembly has emerged as a powerful technique for controlling the structure and properties of ensembles of inorganic nanoparticles. Here we review different strategies for nanoparticle self-assembly, the properties of self-assembled structures of nanoparticles, and potential applications of such structures. Many of these properties and possible applications rely on our ability to control the interactions between the electronic, magnetic and optical properties of the individual nanoparticles.

Block copolymer nanolithography: translation of molecular level control to nanoscale patterns

The self-asembly of block copolymers is a promising platform for the "bottom-up" fabrication of nanostructured materials and devices. This review covers some of the advances made in this field from the laboratory setting to applications where block copolymers are in use.

Coarse-grained molecular simulation of diffusion and reaction kinetics in a crowded virtual cytoplasm

We present a general-purpose model for biomolecular simulations at the molecular level that incorporates stochasticity, spatial dependence, and volume exclusion, using diffusing and reacting particles with physical dimensions. To validate the model, we first established the formal relationship between the microscopic model parameters (timestep, move length, and reaction probabilities) and the macroscopic coefficients for diffusion and reaction rate. We then compared simulation results with Smoluchowski theory for diffusion-limited irreversible reactions and the best available approximation for diffusion-influenced reversible reactions. To simulate the volumetric effects of a crowded intracellular environment, we created a virtual cytoplasm composed of a heterogeneous population of particles diffusing at rates appropriate to their size. The particle-size distribution was estimated from the relative abundance, mass, and stoichiometries of protein complexes using an experimentally derived proteome catalog from Escherichia coli K12. Simulated diffusion constants exhibited anomalous behavior as a function of time and crowding. Although significant, the volumetric impact of crowding on diffusion cannot fully account for retarded protein mobility in vivo, suggesting that other biophysical factors are at play. The simulated effect of crowding on barnase-barstar dimerization, an experimentally characterized example of a bimolecular association reaction, reveals a biphasic time course, indicating that crowding exerts different effects over different timescales. These observations illustrate that quantitative realism in biosimulation will depend to some extent on mesoscale phenomena that are not currently well understood.