Morphogenetic patterns are highly sophisticated dissipative structures. Are they governed by the same general mechanisms as chemical and hydrodynamic patterns? Turings symmetry breaking and Wolperts signalling provide alternative mechanisms. The curr
ent evidence points out that the latter is more relevant but reality is still far more complicated.
We explore a novel strategy of patterning nematic elastomers that does not require inscribing the texture directly. It is based on varying the dopant concentration that, beside shifting the phase transition point, affects the nematic director field v
ia coupling between the gradients of concentration and nematic order parameter. Rotation of the director around a point dopant source causes topological modification manifesting itself in a change of the number of defects. A variety of shapes, dependent on the dopant distribution, are obtained by anisotropic deformation following the nematic--isotropic transition.
We investigate the motion of a flexible Stokesean flagellar swimmer realised as a yarn made of two intertwined elastomer fibres, one active, that can reversibly change its length in response to a local excitation causing transition to the nematic sta
te or swelling, and the other one, a passive isotropic elastomer with identical mechanical properties. A propagating chemical wave may provide an excitation mechanism ensuring a constant length of the excited region. Generally, the swimmer moves along a helical trajectory, and the propagation and rotation velocity are very sensitive to the ratio of the excited region to the pitch of the yarn, as well as to the size of a carried load. External excitation by a moving beam is less effective, unless the direction of the beam is adjusted to rotation of the swimmer.
We consider three-dimensional reshaping of a thin nematoelastic film upon nematic-isotropic transition in the field of a charge one topological defect, leading to either cone or anticone (d-cone) shells. The analysis is based on the relation between
the shell metric and the tensor order parameter under the assumption of no elastic deformation and volume change. The shape of the shell can be modified by doping, creating cones with curved generatrices. Anticones necessarily have an even number of radial creases. The curvature singularity at the apex is resolved due to decay of the nematic order parameter at the defect core.
In this paper we present a comprehensive study of the step-emulsification process for high-throughput production of (sub-)$mu$m-size monodisperse droplets. The microfluidic device combines a Hele-Shaw nanofluidic cell with a step-like outlet to a dee
p and wide reservoir. The proposed theory based on Hele-Shaw hydrodynamics provides the quasi-static shape of the free boundary between the disperse liquid phase engulfed by the co-flowing continuous phase prior to transition to oscillatory step-emulsification at low enough capillary number. At the transition the proposed theory anticipates a simple condition for critical capillary number as a function of the Hele-Shaw cell geometry. The transition threshold is in excellent agreement with experimental data. A simple closed-form expression for the size of the droplets generated in step-emulsification regime derived using simple geometric arguments also shows a very good agreement with the experimental results.
In this communication we address some hydrodynamic aspects of recently revisited drift mechanism of biogenic mixing in the ocean (Katija and Dabiri, Nature vol. 460, pp. 624-626, 2009). The relevance of the locomotion gait at various spatial scales w
ith respect to the drift is discussed. A hydrodynamic scenario of the drift based on unsteady inertial propulsion, typical for most small marine organisms, is proposed. We estimate its effectiveness by taking into account interaction of a swimmer with the turbulent marine environment. Simple scaling arguments are derived to estimate the comparative role of drift-powered mixing with respect to the turbulence. The analysis indicates substantial biomixing effected by relatively small but numerous drifters, such as krill or jellyfish.
