ABSTRACT
We analyze the bending response to light or heat of a solid nematic disk with a director twisted from being radial on the upper surface to be azimuthal on the lower. We find a number of curl lobes determined purely by the geometry of the mechanical frustration that arises during the response.
ABSTRACT
Much recent progress has been made in the study of nematic solids, both glassy and elastomeric, particularly in the realm of stress-free, defect-driven deformation in thin sheets of material. In this paper we consider a subset of texture domains in nematic glasses that are simple to synthesize, and explore the ways that these simple domains may be compatibly combined to yield analogs of the traditional smooth disclination defect textures seen in standard liquid crystals. We calculate the deformation properties of these constructed textures, and show that, subject to the compatibility constraints of the construction, these textures may be further combined to achieve shape blueprinting of three-dimensional structures from flat sheets.
ABSTRACT
A gradient of director through the thickness of a nematic glass cantilever gives a gradient in the large distortions such materials suffer in response to temperature or illumination changes. We first sketch, within isotropic elasticity, how such gradients cause these cantilevers to respond by bending. We then derive the response within the anisotropic elasticity expected for uniaxial solids. Because, in general, spontaneously bending cantilevers have regions of elongation and contraction (with respect to their neutral state), internal stresses are generated, the magnitude of which depends on the anisotropic, fourth rank modulus tensor and in particular on its local alignment arising from the director's spatial distribution. We show that despite elastic complexity, bend is simply linear in the anisotropy of thermo-optical response, with a slope depending on the structure of the modulus tensor, justifying the previous literature on spontaneously bending cantilevers. We also explicitly consider two important director distributions--splay-bend and twist. Splay-bend cantilevers have no anticlastic (double-bend, saddle) response in the isotropic case or for some values of the anisotropic modulus tensor. Twist cantilevers have maximal anticlasticity in the isotropic case which we show to be weakly modified by anisotropy of elastic moduli.
ABSTRACT
Growth control of African trypanosomes in the mammalian host is coupled to differentiation of a non-dividing life cycle stage, the stumpy bloodstream form. We show that a protein kinase with novel domain architecture is important for growth regulation. Zinc finger kinase (ZFK) has a kinase domain related to RAC and S6 kinases flanked by a FYVE-related zinc finger and a phox (PX) homology domain. To investigate the function of the kinase during cyclical development, a stable transformation procedure for bloodstream forms of differentiation-competent (pleomorphic) Trypanosoma brucei strains was established. Deletion of both allelic copies of ZFK by homologous recombination resulted in reduced growth of bloodstream-form parasites in culture, which was correlated with an increased rate of differentiation to the non-dividing stumpy form. Growth and differentiation rates were returned to wild-type level by ectopic ZFK expression. The phenotype is stage-specific, as growth of procyclic (insect form) trypanosomes was unaffected, and Deltazfk/Deltazfk clones were able to undergo full cyclical development in the tsetse fly vector. Deletion of ZFK in a differentiation-defective (monomorphic) strain of T. brucei did not change its growth rate in the bloodstream stage. This suggests a function of ZFK associated with the trypanosomes' decision between either cell cycle progression, as slender bloodstream form, or differentiation to the non-dividing stumpy form.
