Filter by type:

Sort by year:

Frustrated Order on Extrinsic Geometries

Badel L. Mbanga, Gregory M. Grason, Christian D. Santangelo
Journal Paper Phys. Rev. Lett. 108, 017801 (2012)

Abstract

We study, numerically and theoretically, defects in an anisotropic liquid that couple to the extrinsic geometry of a surface. Though the intrinsic geometry tends to confine topological defects to regions of large Gaussian curvature, extrinsic couplings tend to orient the order along the local direction of maximum or minimum bending. This additional frustration is generically unavoidable, and leads to complex ground-state thermodynamics. Using the catenoid as a prototype, we show, in contradistinction to the well-known effects of intrinsic geometry, that extrinsic curvature expels disclinations from the region of maximum curvature above a critical coupling threshold. On catenoids lacking an ‘‘insid-eoutside’’ symmetry, defects are expelled altogether above a critical neck size.

Giant Flexoelectricity of Bent-Core Nematic Liquid Crystals

J. Harden, Badel L. Mbanga, N. Eber, K. Fodor-Csorba, S. Sprunt, J. T. Gleeson, A. Jakli
Journal Paper Phys. Rev. Lett. 97, 157802 (2006)

Abstract

Flexoelectricity is a coupling between orientational deformation and electric polarization. We present a direct method for measuring the flexoelectric coefficients of nematic liquid crystals (NLCs) via the electric current produced by periodic mechanical flexing of the NLC’s bounding surfaces. This method is suitable for measuring the response of bent-core liquid crystals, which are expected to demonstrate a much larger flexoelectric effect than traditional, calamitic liquid crystals. Our results reveal that not only is the bend flexoelectric coefficient of bent-core NLCs gigantic (more than 3 orders of magnitude larger than in calamitics) but also it is much larger than would be expected from microscopic models based on molecular geometry. Thus, bent-core nematic materials can form the basis of a technological breakthrough for conversion between mechanical and electrical energy.

Shape and chirality transitions in off-axis twist nematic elastomer ribbons

Yoshiki Sawa, Kenji Urayama,Toshikazu Takigawa, Vianney Gimenez-Pinto, Badel L. Mbanga, Fangfu Ye, Jonathan V. Selinger, Robin L. B. Selinger
Journal Paper Phys. Rev. E 88, 022502 (2013)

Abstract

Using both experiments and finite element simulations, we explore the shape evolution of off-axis twist nematic elastomer ribbons as a function of temperature. The elastomers are prepared by cross-linking the mesogens with planar anchoring of the director at top and bottom surfaces with a 90o left-handed twist. Shape evolution depends sensitively on the off-axis director orientation at the sample midplane. Both experiments and theoretical studies show that when the director at midplane is parallel to either the ribbon's long or short axes, ribbons form either helicoids or spirals depending on aspect ratio and temperature. Simulation studies show that if the director at midplane is off-axis, ribbons never form helicoids, instead evolving to distorted spiral shapes. Experimental studies for two samples with off-axis geometry show agreement with this prediction. Samples in all these geometries show a remarkable transition from right- to left-handed chiral shapes on change of temperature. Simulations predict off-axis samples also change their macroscopic chirality at fixed temperature, depending on the angular offset. These results provide insight into the mechanisms driving shape evolution and macroscopic chirality, enabling engineering design of these materials for future applications.

The role of curvature anisotropy in the ordering of spheres on an ellipsoid

Christopher J. Burke, Badel L. Mbanga, Zengyi Wei, Patrick T. Spicer and Timothy J. Atherton
Journal Paper Soft Matter, 2015,11, 5872-5882

Abstract

Non-spherical emulsion droplets can be stabilized by densely packed colloidal particles adsorbed at their surface. In order to understand the microstructure of these surface packings, the ordering of hard spheres on ellipsoidal surfaces is determined through large scale computer simulations. Defects in the packing are shown generically to occur most often in regions of strong curvature; however, the relationship between defects and curvature is nontrivial, and the distribution of defects shows secondary maxima for ellipsoids of sufficiently high aspect ratio. As with packings on spherical surfaces, additional defects beyond those required by topology are observed as chains or “scars”. The transition point, however, is found to be softened by the anisotropic curvature which also partially orients the scars. A rich library of symmetric commensurate packings are identified for low particle number. We verify experimentally that ellipsoidal droplets of varying aspect ratio can be arrested by surface-adsorbed colloids.

Modeling elastic instabilities in nematic elastomers

Badel L. Mbanga Fangfu Ye, Jonathan V. Selinger, Robin L. B. Selinger
Journal Paper Phys. Rev. E 82, 051701 (2010)

Abstract

Liquid crystal elastomers are cross-linked polymer networks covalently bonded with liquid crystal mesogens. In the nematic phase, due to strong coupling between mechanical strain and orientational order, these materials display strain-induced instabilities associated with formation and evolution of orientational domains. Using a three-dimensional finite element elastodynamics simulation, we investigate one such instability, the onset of stripe formation in a monodomain film stretched along an axis perpendicular to the nematic director. In our simulation, we observe the formation of striped domains with alternating director rotation. This model allows us to explore the fundamental physics governing dynamic mechanical response of nematic elastomers and also provides a potentially useful computational tool for engineering device applications.

Simulating defect textures on coalescing nematic shells

Badel L. Mbanga, Kate K. Voorhes, Timothy J. Atherton
Journal Paper Phys. Rev. E 89, 052504 (2014)

Abstract

Two nematic shells brought in contact coalesce in order to reduce their interfacial tension. This process proceeds through the creation of a liquid neck-like bridge through which the encapsulated fluid flows. Following this topological transition, We study the defect textures as the combined shell shape evolves. Varying the sizes of the shells, we perform a quasistatic investigation of the director field and the defect valence on the doublet. Regimes are found where positive and negative defects exist due to the large negative Gaussian curvature at the neck. Using large scale computer simulations, we determine how annihilating defect pairs on coalescing shells are selected, and the stage of coalescence at which annihilation occurs.

Modeling liquid crystal elastomers: actuators, pumps, and robots

Robin L. B. Selinger Badel L. Mbanga, Jonathan V. Selinger
Journal Paper Proceedings of the SPIE, Volume 6911, article id. 69110A, 5 pp. (2008)

Abstract

We model the dynamics of shape evolution of liquid crystal elastomers (LCE) in three dimensions using finite element elastodynamics. The model predicts the macroscopic mechanical response induced by changes in nematic order, e.g. by heating or cooling through the isotropic/nematic transition or, in azo-doped materials, by exposure to light. We model the performance of LCE actuator devices including multicomponent actuators, peristaltic pumps and self-propelled robots. The goal of this work is to build a bridge between basic soft matter theory and practical materials engineering/device design. Supported by NSF-DMR-0605889.

Simulations of the polydomain to monodomain transition in nematic elastomers

Badel L. Mbanga, Fangfu Ye, Jonathan V. Selinger, Robin L. B. Selinger
In Preparation To be submitted, 2014

Abstract

Deformation of an initially polydomain nematic elastomer film induces a transition to the monodomain configuration. We model the resulting microstructural evolution and stress-strain response using a recently introduced hybrid particle-finite element elastodynamics simulation approach. We explore how the thermomechanical history of the sample, e.g. its crosslink density and phase at time of network formation, affects the width of the poly-monodomain transition and the associated stress-strain behavior. We find that when the sample is cross-linked in the isotropic phase, the material shows a semi-soft response with a well-defined plateau in the stress-strain curve. By contrast when the sample is cross-linked in the nematic phase, the resulting strong local disorder broadens the transition and the plateau is much less pronounced. This simulation approach allows us to explore the fundamental physics governing dynamic mechanical response of nematic elastomers and also provides a potentially useful computational tool for engineering device applications.

Arrested coalescence of pickering emulsions of bidispersed particles

Badel L. Mbanga, Christopher J. Burke, Patrick T. Spicer, Marco Caggioni, Timothy J. Atherton
In Preparation To be submitted, 2014

Abstract

With applications ranging from food products to cosmetics via targeted drug delivery systems, structured anisotropic colloids provide an efficient way to control the structure, properties and functions of emulsions. When two fluid emulsion droplets are brought in contact, a reduction of the interfacial tension drives their coalescence into a larger droplet of the same total volume and reduced exposed area. This coalescence can be partially or totally hindered by the presence of nano or micron-size particles that coat the interface as in Pickering emulsions. We investigate numerically the dependance of the mechanical stability of these arrested shapes on the particles size, their shape anisotropy, their polydispersity, their interaction with the solvent, and the particle-particle interactions. We discuss structural shape changes that can be induced by tuning the particles interactions after arrest occurs, and provide design parameters for the relevant experiments.

General Purpose Hybrid Particle-Finite Element Elastodynamics Simulation with GPU Implementation

Badel L. Mbanga, Andrew Konya, Robin L. B. Selinger
In Preparation To be submitted, 2014

Abstract

We present a novel fast computational technique for modeling the shape evolution of elastomeric materials at the continuum-level. Our algorithm, a hybrid Particle{Finite Element approach, allows for modeling shape change of samples with complex geometry subject to nonuniform external stimuli. It also captures the dynamics of internal degrees of freedom that can be modulated independently from the polymer matrix such as in liquid crystal elastomers. A dramatic performance improvement is observed when ported onto graphics processing units (GPU).

HYBRID PARTICLE-FINITE ELEMENT ELASTODYNAMICS SIMULATIONS OF NEMATIC LIQUID CRYSTAL ELASTOMERS

Badel L. Mbanga
PhD Dissertation Kent State University | May, 2012 | Document number: kent1334607477
image

Liquid crystal elastomers are cross-linked polymer networks covalently bonded with liquid crystal mesogens. In the nematic phase, due to strong coupling between mechanical strain and orientational order, these materials display strain-induced instabilities associated with formation and evolution of orientational domains. In building a simulation model of these materials, we consider the limit in which the orientational order equilibrates rapidly compared to the strain, so that the local order tensor remains in continuously evolving quasi- static equilibrium as the strain relaxes. Our method allows us to study the onset of stripe formation in a monodomain film stretched along an axis perpendicular to the nematic director, the transition from polydomain to monodomain states, and the interaction of nematic liquid crystal elastomers with external stimuli such as an electric field. We intend through this model to further our understanding of the basic physics governing the dynamic mechanical response of nematic elastomers and also provide a useful computational tool for design and testing of potential engineering device applications.

Committee

  • Prof. Robin Selinger (Adviser)
  • Prof. Eugene Gartland (Committee Member)
  • Prof. Jonathan Selinger (Committee Member)
  • Prof. Xiaoyu Zheng (Committee Member)
  • Prof. Antal Jakli (Committee Member)
  • Size: 138 p.
  • Subject: Physics