PACS 61.30.Dk -Continuum models and theories of liquid crystal structure PACS 61.30.Gd -Orientational order of liquid crystals; electric and magnetic field effects on order PACS 42.79.Kr -Display devices, liquid-crystal devices Abstract -We study a dilute suspension of magnetic nanoparticles in a nematic-filled micron-sized shallow well with tangent boundary conditions. We use a phenomenological approach to study stable textures dictated by the interplay between confinement, boundary effects and nematic coupling with suspended nanoparticles. We numerically compute the stable textures for both the nematic order parameter and the averaged magnetization, as a function of the coupling strength and a new phenomenological parameter. We observe stable domain walls for the magnetization vector and stable vortices in the nematic host, for appropriate choices of the phenomenological parameters, and these stable patterns may have new technological applications.

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We study a ferromagnetic suspension or a suspension of magnetic nanoparticles in an anisotropic nematic medium, in three different one-dimensional variational settings, ordered in terms of increasing complexity. The three models are featured by a nematic energy, a magnetic energy and a magneto-nematic coupling energy and the experimentally observed patterns are modelled as local or global energy minimizers.We numerically observe polydomains with distinct states of magnetization for weak to moderate magneto-nematic coupling in our models. We demonstrate that these polydomains are stabilised by lowering the temperature (as in Mertelj et al., 2013) and that the polydomain structures lose stability as the magneto-nematic coupling increases. Some exact solutions for prototypical situations are also obtained.

A human pathogen, Neisseria gonorrhoeae (NG), moves on surfaces by attaching and retracting polymeric structures called Type IV pili. The tug-of-war between the pili results in a two-dimensional stochastic motion called twitching motility. In this paper, with the help of real-time NG trajectories, we develop coarse-grained models for their description. The fractal properties of these trajectories are determined and their influence on first passage time and formation of bacterial microcolonies is studied. Our main observations are as follows: (i) NG performs a fast ballistic walk on small time scales and a slow diffusive walk over long time scales with a long crossover region; (ii) there exists a characteristic persistent length l_{p}^{*}, which yields the fastest growth of bacterial aggregates or biofilms. Our simulations reveal that l_{p}^{*}∼L^{0.6}, where L×L is the surface on which the bacteria move; (iii) the morphologies have distinct fractal characteristics as a consequence of the ballistic and diffusive motion of the constituting bacteria.

Bacteria living on surfaces use different types of motility mechanisms to move on the surface in search of food or to form micro-colonies. Twitching is one such form of motility employed by bacteria (e.g. Neisseria gonorrhoeae) in which the polymeric extensions known as type IV pili mediate its movement. Pili extending from cell body adheres to the surface and pulls the bacteria by retraction. The bacterial movement is decided by the two-dimensional "tug-of-war " between the pili attached to the surface. Natural surfaces in which these micro-crawlers dwell are generally spatially inhomogeneous and have varying surface properties. Their motility is known to be affected by the topography of the surfaces. Therefore, it is possible to control bacterial movement by designing structured surfaces which can be potentially utilised for controlling biofilm architecture. In this paper, we numerically investigate the twitching motility in a two-dimensional corrugated channel. We simulate the bacterial movement by two different models: (a) a detailed tug-of-war model which extensively describe the twitching motility of bacteria assisted by pili and (b) a coarse-grained "run-and-tumble" model which depicts the motion of wide-ranging selfpropelled particles. The simulation of bacterial motion through asymmetric corrugated channels using the above models show rectification. The bacterial transport depends on the architecture of the channel. In particular, the variation of the particle current with the geometric parameters of the micro-channels show that we can optimise the particle current for specific values of these parameters. * Konark.Bisht@physics.iitd.ac.in † maratherahul@physics.iitd.ac.in arXiv:1907.05586v2 [physics.bio-ph]