Biomedical subjects

Anna C Balazs

Publications and source records attributed to Anna C Balazs.

At least 19 recordsLinked to original sources

PubMed2006

Designing compliant substrates to regulate the motion of vesicles.

By integrating mesoscale models for hydrodynamics and micromechanics, we examine fluid-driven motion of vesicles on compliant surfaces. The vesicles, modeled as fluid-filled elastic shells, represent biological cells or polymeric microcapsules. Focusing on nonspecific interactions between these vesicles and synthetic substrates, we isolate mechanically and topographically patterned surfaces that transmit stop and go instructions, causing the vesicles to halt at specific locations, and with an increase in the flow velocity, to resume moving. For surfaces containing arrays of compliant posts, the substrates also affect the vesicles' gait, causing them to "crawl," "walk," or "jump." The latter behavior could promote the intermixing of reactants that are encapsulated within the microcapsules. Such control over vesicle dynamics can facilitate various biological assays and fabrication of arrays of mobile microreactors.

Computer Simulation↗

PubMed2006

Exploiting photoinduced reactions in polymer blends to create hierarchically ordered, defect-free materials.

Computer simulations reveal how photoinduced chemical reactions can be exploited to create long-range order in binary and ternary polymeric materials. The process is initiated by shining a spatially uniform light over a photosensitive AB binary blend, which thereby undergoes both a reversible chemical reaction and phase separation. We then introduce a well-collimated, higher intensity light source. Rastering this secondary light over the sample locally increases the reaction rate and causes formation of defect-free, spatially periodic structures. These binary structures resemble either the lamellar or hexagonal phases of microphase-separated diblock copolymers. We measure the regularity of the ordered structures as a function of the relative reaction rates for different values of the rastering speed and determine the optimal conditions for creating defect-free structures in the binary systems. We then add a nonreactive homopolymer C, which is immiscible with both A and B. We show that this component migrates to regions that are illuminated by the secondary, higher intensity light, allowing us to effectively write a pattern of C onto the AB film. Rastering over the ternary blend with this collimated light now leads to hierarchically ordered patterns of A, B, and C. The findings point to a facile, nonintrusive process for manufacturing high quality polymeric devices in a low-cost, efficient manner.

Journal Article↗

PubMed2005

Modeling the flow of fluid/particle mixtures in microchannels: encapsulating nanoparticles within monodisperse droplets.

We develop a hybrid computational approach for simulating mixtures of binary fluids and mobile, submicron particles. The model couples a lattice Boltzmann method for the binary fluid with a Brownian dynamics model for the particles. The particles can exhibit preferential wetting interactions with the different components of the fluid. As an illustration of the method, we carry out simulations in two dimensions to compare the spinodal decomposition of a pure binary fluid with the phase separation of binary blends that contain either fixed or mobile particles. We then isolate conditions where the flow of a binary fluid/particle mixture past surfaces with well-defined asperities leads to the formation of monodisperse droplets, which encapsulate the nanoparticles. The findings provide guidelines for creating multiphase emulsions with well-controlled morphologies.

Journal Article↗

PubMed2005

Local control of periodic pattern formation in binary fluids within microchannels.

Simulations show that a local perturbation (a "hot spot") at the inlet of a microchannel can be a source of traveling concentration waves in a binary fluid and can be exploited to create periodic, oscillatory patterns within the channel. We isolate two distinctly different types of traveling periodic patterns and estimate the wavelength of the structures. Our findings provide the first example of how a local perturbation can be used to create well-defined periodic patterns in two-phase flow within a confined geometry and can lead to new routes for creating gradient materials.

Journal Article↗

PubMed2005

Harnessing light to create defect-free, hierarchically structured polymeric materials.

Computer simulations reveal how photoinduced chemical reactions in polymeric mixtures can be exploited to create long-range order in materials with features that range from the submicron to the nanoscale. The process is initiated by shining a spatially uniform light on a photosensitive AB binary blend, which thereby undergoes both a reversible chemical reaction and a phase separation. When a well-collimated, higher intensity light is rastered over the sample, the system forms defect-free, spatially periodic structures. If a nonreactive homopolymer C is added to the system, this component localizes in regions that are irradiated with a higher intensity light, and one can effectively "write" a pattern of C onto the AB film. Rastering over the ternary blend with the collimated light now leads to hierarchically ordered patterns of A, B, and C. Because our approach involves homopolymers, it significantly expands the range of materials that can be fashioned into a periodic pattern. The findings point to a facile process for manufacturing high-quality polymeric components in an efficient manner.

Letter↗

PubMed2005

Pattern formation arising from condensation of a homogeneous gas into a binary, phase-separating liquid.

We examine the nucleated growth of a binary, immiscible liquid drop within a homogeneous gas. The system couples the growth of the liquid drop with the phase separation of the immiscible components and, thus, can potentially reveal novel pattern formation. To carry out this study, we first characterize the thermodynamic properties of the system in terms of an appropriate Ginzburg-Landau free energy density. By minimizing this free energy, we construct the equilibrium phase diagram for the system. We then use a lattice Boltzmann algorithm to solve the hydrodynamic equations describing the dynamical evolution of the fluid. We observe intriguing tentaclelike structures within the nucleation and growth regime and explore how the formation of these structures depends on the thermodynamic and transport properties of the system. We give scaling laws describing domain growth in both the diffusion- and flow-limited regimes. The results highlight the novel physics that can emerge when there is interplay between the ordering of a density and a concentration field.

Journal Article↗

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.

Journal Article↗

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.

Journal Article↗

PubMed2005

Convection-driven pattern formation in phase-separating binary fluids.

Using a thermal-lattice Boltzmann model, we examine the rich phase behavior that develops when partially miscible fluids evolve in the presence of a vertical temperature gradient, which encompasses the critical temperature T(c) of the mixture. In particular, a binary AB fluid is confined between two plates in a gravitational field. The upper plate is fixed below T(c) and hence, the nearby fluid phase separates into A-rich and B-rich domains. The lower plate is fixed above the temperature T(c), and the surrounding fluid is in the homogeneous phase. A coupling between convection (driven by the temperature gradient) and phase separation gives rise to unique pattern formation. A number of regimes are identified: regularly spaced stripes, convective steady-state columns, the periodic disturbance of these columns, and finally, chaotic dripping from the upper surface. These results highlight dynamical behavior in partially miscible mixtures.

Journal Article↗

PubMed2004

Theoretical model of interfacial polymerization.

We present a theoretical description for the creation of a thin polymeric layer through the interfacial polymerization of two immiscible, low molecular weight liquids. The theory specifically takes into account the effects of polydispersity on the formation of the polymer film at the liquid-liquid interface. Consequently, we can describe the structure of the growing film and the molecular weight distribution of the resultant polymer chains. We focus on a model system where alternating AB copolymers are formed at the interface between phase-separated, low molecular weight species A and B. It is assumed that any A(B) unit can reversibly attach to any available B(A) unit or B(A)-ended chain. The formation of the copolymer layer is described by a system of reaction-diffusion equations, which detail the chemical evolution and diffusive dynamics of the polydisperse mixture of AB copolymers around the interface, and the evolution of the interface itself. Using this model, we determine the effects of the chemical reaction rates and the initial conditions on the kinetics of forming the AB copolymer layer and the structure of this film.

Journal Article↗

PubMed2004

Pattern formation in binary fluids confined between rough, chemically heterogeneous surfaces.

Using a mesoscale model for hydrodynamics, we simulate driven flow of AB binary fluids past surfaces that contain well-defined roughness or asperities. The geometry and wetting properties of the asperities are found to have a dramatic effect on the flow patterns. We isolate conditions where the A fluid forms vertical bands that bridge the asperities and an imposed shear (or pressure gradient) drives the system to form monodisperse droplets of A within the B fluid. The size of the droplets can be tailored by varying the morphology of the asperities. The surfaces needed to create this rich dynamical behavior are used as the stamps in microcontact printing; thus, the parameter space can readily be accessed experimentally, and the predictions suggest an efficient method for forming emulsions with well-controlled morphologies.

Journal Article↗

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.

Journal Article↗

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.

Journal Article↗

PubMed2004

Interdiffusion in a polydisperse polymer blend.

We present a theoretical description of interdiffusion in a binary blend of polymers that exhibit polydispersity in length. The diffusion equations are formulated in terms of the volume fractions and the chain concentrations of the components. This choice of variables is equivalent to the assumption that the local molecular weight distributions of the components are described by the Flory distribution. The Onsager kinetic coefficients are obtained based on the Green-Kubo equation and correspond to the fast-mode interdiffusion theory. As demonstrated by numerical simulations, the resulting equations describe the simultaneous processes of the evolution of blend composition and the relaxation of the local molecular weight distributions of the components. The developed approach can be used to study polymer systems in which the degree of polymerization changes due to interfacial or bulk chemical reactions.

Journal Article↗

PubMed2003

Diffusive intertwining of two fluid phases in chemically patterned microchannels.

Via a coarse-grained model, we simulate the flow of a pressure driven binary AB fluid through a three-dimensional microchannel, which is decorated on both top and bottom with distinct A- and B-like patches. The advection is "frustrated" because A-like patches are placed in the path of the B stream and similarly, B-like patches are placed in the path of the A fluid. A competition between two factors, the advection caused by the imposed flow and the interactions between the confined fluids and the patterned substrates, introduces nonlinearity into the system. This nonlinear behavior gives rise to a temporally periodic state, where the A and B fluids are intertwined. In effect, the simple pattern of chemically distinct patches introduces positive feedback, which is responsible for the instability of the interface separating the injected fluids.

Journal Article↗

PubMed2003

Modeling the self-assembly of copolymer-nanoparticle mixtures confined between solid surfaces.

Using numerical calculations, we undertake the first morphological studies of mixtures of AB diblocks and nanoparticles that are confined between two hard walls. A complex interplay of entropic and enthalpic interactions drives the nonselective particles to localize at the hard walls and A/B interfaces, causing the mixture to spontaneously self-assemble into particle-decorated lamellae that are oriented perpendicular to the surfaces. The film reveals a periodic array of particle "nanowires" that are separated by the nanoscale polymer domains, yielding a vital material for nanodevice fabrication.

Journal Article↗

PubMed2003

Periodic droplet formation in chemically patterned microchannels.

Simulations show that, when a phase-separated binary AB fluid is driven to flow past chemically patterned substrates in a microchannel, the fluid exhibits unique morphological instabilities. For the pattern studied, these instabilities give rise to the simultaneous, periodic formation of monodisperse droplets of A in B and B in A. The system bifurcates between time-independent behavior and different types of regular, nondecaying oscillations in the structural characteristics. The surprisingly complex behavior is observed even in the absence of hydrodynamic interactions and arises from the interplay between the fluid flow and patterned substrate, which introduces nonlinearity into the dynamical system.

Journal Article↗

PubMed2003

Simulating the dynamic behavior of immiscible binary fluids in three-dimensional chemically patterned microchannels.

Using computer simulations, we investigate the behavior of an immiscible binary AB fluid that is driven to flow through a microchannel, which is decorated with a checkerboard pattern of chemically distinct A- and B-like patches on the top and bottom walls. We isolate conditions where the coupling between the imposed flow field and thermodynamic interactions yields a rich interfacial behavior between the A and B fluids and complex velocity patterns over the checkerboard domains. In effect, the A and B fluids undergo extensive mixing in specific regions of the channel, even for low Reynolds number flow. Decreasing the height-to-width ratio of the patterned microchannel enhances the extent of mixing between these two components. On the other hand, the length of the A/B interfaces is optimized in microchannels that have a square cross section. The results provide guidelines for designing microfluidic devices that can be used to effectively intermix multicomponent fluids.

Journal Article↗