Gavin A. Buxton

Computer simulation of polymer solar cells

Computer simulations are shown to be a powerful tool in predicting the response of polymer solar cells. In particular, we show how a drift-diffusion model can capture the transient behaviour of electron, hole and exciton concentrations in heterogeneous devices. Furthermore, computer simulations can reveal interesting new insights into the role of concentration fluxes and internal electric fields. We demonstrate this approach by considering bilayer devices but where the interface is sinusiodal, not planar. To highlight the predictive capabilities of these computer simulations we consider the systematic variation of device morphologies and the effect this has on photovoltaic performance. In this manner, we can correlate the device performance with the devices internal structure and predict how the polymer morphology might be tailored to meet photovoltaic needs.

Lattice spring model of filled polymers and nanocomposites

Mechanical properties of homopolymers containing either solid spheres, rods, or platelets are compared using a three-dimensional lattice spring model, and thus, the effects of filler geometry on the mechanical behavior of the composite are isolated. In addition, the properties of exfoliated and intercalated nanocomposites are examined and the source of the increased reinforcement efficiency in nanocomposites, as a consequence of platelet exfoliation, is elucidated. Viscoelastic deformations are explicitly incorporated in the lattice spring model and, thus, the mechanical response of these filled polymer materials are determined.

Designing oscillating cilia that capture or release microscopic particles.

We use computational modeling to capture the three-dimensional interactions between oscillating, synthetic cilia and a microscopic particle in a fluid-filled microchannel. The synthetic cilia are elastic filaments that are tethered to a substrate and are actuated by a sinusoidal force, which is applied to their free ends. The cilia are arranged in a square pattern, and a neutrally buoyant particle is initially located between these filaments. Our computational studies reveal that, depending on frequency of the beating cilia, the particle can be either driven downward toward the substrate or driven upward and expelled into the fluid above the cilial layer. This behavior mimics the performance of biological cilia used by certain marine animals to extract suspended food particles. The findings uncover a new route for controlling the deposition of microscopic particles in microfluidic devices.

Predicting the self-assembled morphology and mechanical properties of mixtures of diblocks and rod-like nanoparticles

We couple a morphological study of a mixture of diblock copolymers and rod-like, solid nanoparticles with a micromechanical simulation to determine how the spatial distribution and aspect ratio of the particles affects the mechanical behavior of the composite. The morphological studies are conducted through the SCF/DFT technique, which couples the self-consist field theory (SCFT) for the diblocks and a density functional theory (DFT) for parallelepiped particles. Through the SCF/DFT calculations, we obtain the equilibrium morphology of the diblock/particle mixtures. We find that the distribution of particles within the polymers is dependent not only on the relative interaction energies between the particles and the different blocks, but also on the aspect ratio of the rod-like solids. The output of the SCF/DFT model serves as the input to the Lattice Spring Model (LSM), which consists of a three-dimensional network of springs. In particular, the location of the different phases is mapped onto the LSM lattice and the appropriate force constants are assigned to the LSM bonds. A stress is applied to the LSM lattice, and we calculate the local stress and strain fields and overall elastic response of the material. We find that high aspect ratio rods can dramatically increase the Youngs modulus of the material. By integrating the morphological and mechanical models, we can isolate how modifications in physical characteristics of the particles and diblocks affect both the structure of the mixture and the macroscopic behavior of the composite. Thus, we can establish how choices made in the components affect the ultimate performance of the material.

Drug diffusion from polymer core–shell nanoparticles

Polymer core-shell nanoparticles are attractive drug-delivery systems because the drug can be encapsulated inside the core, while the shell properties can be assigned to optimise drug-delivery needs. An elegant approach to such particles is to use polymer gels, which have swelling properties that depend upon conditions such as pH. In this way swelling of the shell, and the drug release, can be targeted to occur at the desired location. We use computer simulations to capture the deformation of polymer core-shell nanoparticles and, subsequently, the drug diffusion from the core of these structures. In particular, we investigate the effects of shell swelling on drug-release rates, where the expanding shell leaves more free space for the drug to diffuse out of the core. Furthermore, we introduce enthalpic interactions and investigate both the physical and chemical barriers to drug release. Therefore, through a combination of structural and fluid simulations we can capture the physics of polymer core-shell nanoparticles and their uses for drug delivery.

Computational Phlebology: The Simulation of a Vein Valve

We present a three-dimensional computer simulation of the dynamics of a vein valve. In particular, we couple the solid mechanics of the vein wall and valve leaflets with the fluid dynamics of the blood flow in the valve. Our model captures the unidirectional nature of blood flow in vein valves; blood is allowed to flow proximally back to the heart, while retrograde blood flow is prohibited through the occlusion of the vein by the valve cusps. Furthermore, we investigate the dynamics of the valve opening area and the blood flow rate through the valve, gaining new insights into the physics of vein valve operation. It is anticipated that through computer simulations we can help raise our understanding of venous hemodynamics and various forms of venous dysfunction.

Synthetic running and tumbling: an autonomous navigation strategy for catalytic nanoswimmers

Equipping miniaturised catalytic swimming devices with the ability to autonomously navigate in response to solution borne stimuli is an attractive route to enabling drug delivery and other transport applications. Here we use simulations of swarms of swimming devices made from pH responsive size changing hydrogels, to investigate their statistical response to pH variations. We find that the velocity modulation associated with size change, a recently discovered fundamental feature for the catalytic swimmer propulsion mechanism, produces a significant rapid statistical accumulation in high pH regions. This is augmented by size change induced modulation of the Brownian rotational rate, which unexpectedly also causes an accumulation in high pH regions, due to a caging effect imposed by the pH gradient. These simulations consequently show the feasibility of using size changing materials to navigate swimming devices autonomously in response to a stimulus within a constant fuel concentration environment.

Modeling the morphology and mechanical properties of sheared ternary mixtures.

Through a combination of simulation techniques, we determine both the structural evolution and mechanical properties of blends formed from immiscible ternary mixtures. In this approach, we first use the lattice Boltzmann method to simulate the phase separation dynamics of A/B/C fluid mixtures for varying compositions within the spinodal region. We also investigate the effect of an imposed shear on the phase ordering of the mixture. We assume that the fluid is quenched sufficiently rapidly that the phase-separated structure is preserved in the resultant solid. Then, the output from our morphological studies serves as the input to the lattice spring model, which is used to simulate the elastic response of solids to an applied deformation. These simulations reveal how the local stress and strain fields and the global Youngs modulus depend on the composition of the blend and the stiffness of the components. By comparing the results for the sheared and unsheared cases, we can isolate optimal processing conditions for enhancing the mechanical performance of the blends. Overall, the findings provide fundamental insight into the relationship between structure, processing, and properties for heterogeneous materials and can yield guidelines for formulating blends with the desired macroscopic mechanical behavior.

Mathematical modeling of microtubule dynamics: Insights into physiology and disease

Computer models of microtubule dynamics have provided the basis for many of the theories on the cellular mechanics of the microtubules, their polymerization kinetics, and the diffusion of tubulin and tau. In the three-dimensional model presented here, we include the effects of tau concentration and the hydrolysis of GTP-tubulin to GDP-tubulin and observe the emergence of microtubule dynamic instability. This integrated approach simulates the essential physics of microtubule dynamics in a cellular environment. The model captures the structure of the microtubules as they undergo steady state dynamic instabilities in this simplified geometry, and also yields the average number, length, and cap size of the microtubules. The model achieves realistic geometries and simulates cellular structures found in degenerating neurons in disease states such as Alzheimer disease. Further, this model can be used to simulate microtubule changes following the addition of antimitotic drugs which have recently attracted attention as chemotherapeutic agents.

Effect of hydrodynamic interactions on the evolution of chemically reactive ternary mixtures

We investigate the structural evolution of an A/B/C ternary mixture in which the A and B components can undergo a reversible chemical reaction to form C. We developed a lattice Boltzmann model for this ternary mixture that allows us to capture both the reaction kinetics and the hydrodynamic interactions within the system. We use this model to study a specific reactive mixture in which C acts as a surfactant, i.e., the formation of C at the A/B interface decreases the interfacial tension between the A and B domains. We found that the dynamics of the system is different for fluids in the diffusive and viscous regimes. In the diffusive regime, the formation of a layer of C at the interface leads to a freezing of the structural evolution in the fluid; the values of the reaction rate constants determine the characteristic domain size in the system. In the viscous regime, where hydrodynamic interactions are important, interfacial reactions cause a slowing down of the domain growth, but do not arrest the evolution of the mixture. The results provide guidelines for controlling the morphology of this complex ternary fluid.