Biomedical subjects

Gavin A Buxton

Publications and source records attributed to Gavin A Buxton.

8 recordsLinked to original sources

PubMed2006

Multiscale model of miscible polymer blends in porous media: from flow fields to concentration fluctuations.

We have developed a multiscale approach for simulating the concentration fluctuations in a miscible blend subject to complex flow dynamics. We first simulate the hydrodynamics of a fluid as it flows through porous media. In particular, we monitor the velocity gradients as a function of time for a fluid "particle" as it follows a tortuous path through the system. Next, we evolve the structure factor of the spatial concentration fluctuations subject to this flow environment. The velocity gradients experienced by this fluid particle can result in elongation and rotation of the concentration fluctuations. In this manner, we couple the macroscopic flow fields in porous media with the microscopic concentration fluctuations in the polymer blend. We find a close correlation between the tortuous pathways, the velocity gradients in the fluid, and the perturbance of the structure factor from it quiescent state. Furthermore, we find that the concentrations tend to elongate towards the flow direction or at an acute angle with the flow direction.

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PubMed2006

Stress-guided self-assembly in Dutcher films.

Dutcher films consist of a layer of liquid sandwiched between two solid capping layers and can spontaneously self-assemble to form corrugated surfaces. The interplay between the attractive van der Waals forces across the film, and the elastic forces due to the deformation of the capping layers, produces well-defined periodic undulations. We show how computer simulations can capture both the formation of undulations in Dutcher films and the correct periodicity. Furthermore, we simulate Dutcher films which are either compressed or stretched, resulting in the promotion or suppression of undulation growth. In this manner, applied deformations can be shown to guide the self-assembly process in Dutcher films and result in the formation of highly oriented surface corrugations over large distances.

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PubMed2005

Creating structures in polymer blends via a dissolution and phase-separation process.

We show how three-dimensional structures can be formed in polymer blends from pre-existing structures. "Tape" of one polymer is inserted into a matrix of an alternative polymer to form an array of parallelepipeds. We subject this regular structure to partial dissolution in the one-phase region, before quenching the system into the two-phase region. The interplay between dissolution and phase separation can result in complex hierarchic structures. In particular, arrays of microchannels of one polymer species can be formed inside the other polymer.

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PubMed2005

Newtonian fluid meets an elastic solid: coupling lattice Boltzmann and lattice-spring models.

We integrate the lattice Boltzmann model (LBM) and lattice spring model (LSM) to capture the coupling between a compliant bounding surface and the hydrodynamic response of an enclosed fluid. We focus on an elastic, spherical shell filled with a Newtonian fluid where no-slip boundary conditions induce the interaction. We calculate the "breathing mode" oscillations for this system and find good agreement with analytical solutions. Furthermore, we simulate the impact of the fluid-filled, elastic shell on a hard wall and on an adhesive surface. Understanding the dynamics of fluid-filled shells, especially near adhesive surfaces, can be particularly important in the design of microcapsules for pharmaceutical and other technological applications. Our studies reveal that the binding of these capsules to specific surfaces can be sensitive to the physical properties of both the outer shell and the enclosed fluid. The integrated LBM-LSM methodology opens up the possibility of accurately and efficiently capturing the dynamic coupling between fluid flow and a compliant bounding surface in a broad variety of systems.

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PubMed2005

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 Young's 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.

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PubMed2004

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.

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PubMed2004

Using nanoparticles to create self-healing composites.

The need for viable materials for optical communications, display technologies, and biomedical engineering is driving the creation of multilayer composites that combine brittle materials, such as glass, with moldable polymers. However, crack formation is a critical problem in composites where thin brittle films lie in contact with deformable polymer layers. Using computer simulations, we show that adding nanoparticles to the polymers yields materials in which the particles become localized at nanoscale cracks and effectively form "patches" to repair the damaged regions. Through micromechanics simulations, we evaluate the properties of these systems in the undamaged, damaged, and healed states and determine optimal conditions for harnessing nanoparticles to act as responsive, self-assembled "band aids" for composite materials. The results reveal situations where the mechanical properties of the repaired composites can potentially be restored to 75%-100% of the undamaged material.

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PubMed2003

Simulating the morphology and mechanical properties of filled diblock copolymers.

We couple a morphological study of a mixture of diblock copolymers and spherical nanoparticles with a micromechanical simulation to determine how the spatial distribution of the particles affects the mechanical behavior of the composite. The morphological studies are conducted through a hybrid technique, which combines a Cahn-Hilliard (CH) theory for the diblocks and a Brownian dynamics (BD) for the particles. Through these "CH-BD" calculations, we obtain the late-stage morphology of the diblock-particle mixtures. The output of this CH-BD 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 strain fields and overall elastic response of the material. We find that the confinement of nanoparticles within a given domain of a bicontinous diblock mesophase causes the particles to percolate and form essentially a rigid backbone throughout the material. This continuous distribution of fillers significantly increases the reinforcement efficiency of the nanoparticles and dramatically increases the Young's 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.

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