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  • 1. Fazio, V. S. U.
    et al.
    Lagerwall, S. T.
    Zauls, V.
    Schrader, S.
    Busson, P.
    Hult, Anders
    KTH, Superseded Departments, Polymer Technology.
    Motschmann, H.
    Nonlinear optical properties of a channel waveguide produced with crosslinkable ferroelectric liquid crystals2000In: The European Physical Journal E Soft matter, ISSN 1292-8941, E-ISSN 1292-895X, Vol. 3, no 3, p. 245-251Article in journal (Refereed)
    Abstract [en]

    A binary mixture of ferroelectric liquid crystals (FLCs) was used for the design of a channel waveguide. The FLCs possess two important functionalities: a chromophore with a high hyperpolarizability beta and photoreactive groups. The smectic liquid crystal is aligned in layers parallel to the glass plates in a sandwich geometry. This alignment offers several advantages, such as that moderate electric fields are sufficient to achieve a high degree of polar order. The arrangement was then permanently fixed by photopolymerization which yielded a polar network possessing a high thermal and mechanical stability which did not show any sign of degradation within the monitored period of several months. The linear and nonlinear optical properties have been measured and all four independent components of the nonlinear susceptibility tensor (d) over bar have been determined. The off-resonant d-coefficients are remarkably high and comparable to those of the best known inorganic materials. The alignment led to an inherent channel waveguide for p-polarized light without additional preparation steps. The photopolymerization did not induce scattering sites in the waveguide and the normalized losses were less than 2 dB/cm. The material offers a great potential for the design of nonlinear optical devices such as frequency doublers of low-power laser diodes.

  • 2.
    Khapko, Taras
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Duguet, Yohann
    LIMSI-CNRS, Université Paris-Sud.
    Kreilos, Tobias
    Fachbereich Physik, Philipps-Universität Marburg.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Eckhardt, Bruno
    Fachbereich Physik, Philipps-Universität Marburg.
    Henningson, Dan Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Complexity of localised coherent structures in a boundary-layer flow2014In: The European Physical Journal E Soft matter, ISSN 1292-8941, E-ISSN 1292-895X, Vol. 37, no 32Article in journal (Refereed)
    Abstract [en]

    We study numerically transitional coherent structures in a boundary-layer flow with homogeneous suction at the wall (the so-called asymptotic suction boundary layer ASBL). The dynamics restricted to the laminar-turbulent separatrix is investigated in a spanwise-extended domain that allows for robust localisation of all edge states. We work at fixed Reynolds number and study the edge states as a function of the streamwise period. We demonstrate the complex spatio-temporal dynamics of these localised states, which exhibits multistability and undergoes complex bifurcations leading from periodic to chaotic regimes. It is argued that in all regimes the dynamics restricted to the edge is essentially low-dimensional and non-extensive.

  • 3.
    Manyuhina, Oksana V.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Shaping thin nematic films with competing boundary conditions2014In: The European Physical Journal E Soft matter, ISSN 1292-8941, E-ISSN 1292-895X, Vol. 37, no 6, p. 48-Article in journal (Refereed)
    Abstract [en]

    Free interfaces of liquid crystals tend to minimise both capillarity and anchoring forces. Here we study nematic films in planar and radial geometries with antagonistic anchoring boundary conditions and one deformable interface. Assuming a perturbation ansatz we study possible couplings of the director configuration with the shape of free interfaces. In the long-wavelength limit independent of the surface tension, we find analytically the threshold thickness when the flat film becomes unstable. Next we quantify the bifurcation of a circular ring towards structures with m-fold rotational symmetry, induced by elastic anisotropy of the nematic director in the bulk. We believe that our simplified approach can give additional insight into elastic and capillary phenomena of materials with inherent liquid crystalline order and free interfaces.

  • 4. Nadal, Francois
    et al.
    Pak, On Shun
    Zhu, LaiLai
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Ecole Polytech Fed Lausanne, Switzerland.
    Brandt, Luca
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lauga, Eric
    Rotational propulsion enabled by inertia2014In: The European Physical Journal E Soft matter, ISSN 1292-8941, E-ISSN 1292-895X, Vol. 37, no 7, p. 1-14Article in journal (Refereed)
    Abstract [en]

    The fluid mechanics of small-scale locomotion has recently attracted considerable attention, due to its importance in cell motility and the design of artificial micro-swimmers for biomedical applications. Most studies on the topic consider the ideal limit of zero Reynolds number. In this paper, we investigate a simple propulsion mechanism --an up-down asymmetric dumbbell rotating about its axis of symmetry-- unable to propel in the absence of inertia in a Newtonian fluid. Inertial forces lead to continuous propulsion for all finite values of the Reynolds number. We study computationally its propulsive characteristics as well as analytically in the small-Reynolds-number limit. We also derive the optimal dumbbell geometry. The direction of propulsion enabled by inertia is opposite to that induced by viscoelasticity.

  • 5.
    Rorai, Cecilia
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Touchard, Antoine
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Zhu, Lailai
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Brandt, L.uca
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Motion of an elastic capsule in a constricted microchannel2015In: The European Physical Journal E Soft matter, ISSN 1292-8941, E-ISSN 1292-895X, Vol. 38, no 5Article in journal (Refereed)
    Abstract [en]

    We study the motion of an elastic capsule through a microchannel characterized by a localized constriction. We consider a capsule with a stress-free spherical shape and impose its steady-state configuration in an infinitely long straight channel as the initial condition for our calculations. We report how the capsule deformation, velocity, retention time, and maximum stress of the membrane are affected by the capillary number, Ca , and the constriction shape. We estimate the deformation by measuring the variation of the three-dimensional surface area and a series of alternative quantities easier to extract from experiments. These are the Taylor parameter, the perimeter and the area of the capsule in the spanwise plane. We find that the perimeter is the quantity that best reproduces the behavior of the three-dimensional surface area. This is maximum at the centre of the constriction and shows a second peak after it, whose location depends on the Ca number. We observe that, in general, area-deformation-correlated quantities grow linearly with Ca , while velocity-correlated quantities saturate for large Ca but display a steeper increase for small Ca . The velocity of the capsule divided by the velocity of the flow displays, surprisingly, two different qualitative behaviors for small and large capillary numbers. Finally, we report that longer constrictions and spanwise wall bounded (versus spanwise periodic) domains cause larger deformations and velocities. If the deformation and velocity in the spanwise wall bounded domains are rescaled by the initial equilibrium deformation and velocity, their behavior is undistinguishable from that in a periodic domain. In contrast, a remarkably different behavior is reported in sinusoidally shaped and smoothed rectangular constrictions indicating that the capsule dynamics is particularly sensitive to abrupt changes in the cross section. In a smoothed rectangular constriction larger deformations and velocities occur over a larger distance.

  • 6. Wedin, H.
    et al.
    Bottaro, A.
    Hanifi, Ardeshir
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Zampogna, G.
    Unstable flow structures in the Blasius boundary layer2014In: The European Physical Journal E Soft matter, ISSN 1292-8941, E-ISSN 1292-895X, Vol. 37, no 4, p. 34-Article in journal (Refereed)
    Abstract [en]

    Finite amplitude coherent structures with a reflection symmetry in the spanwise direction of a parallel boundary layer flow are reported together with a preliminary analysis of their stability. The search for the solutions is based on the self-sustaining process originally described by Waleffe (Phys. Fluids 9, 883 (1997)). This requires adding a body force to the Navier-Stokes equations; to locate a relevant nonlinear solution it is necessary to perform a continuation in the nonlinear regime and parameter space in order to render the body force of vanishing amplitude. Some states computed display a spanwise spacing between streaks of the same length scale as turbulence flow structures observed in experiments (S.K. Robinson, Ann. Rev. Fluid Mech. 23, 601 (1991)), and are found to be situated within the buffer layer. The exact coherent structures are unstable to small amplitude perturbations and thus may be part of a set of unstable nonlinear states of possible use to describe the turbulent transition. The nonlinear solutions survive down to a displacement thickness Reynolds number Re-* = 496, displaying a 4-vortex structure and an amplitude of the streamwise root-mean-square velocity of 6% scaled with the free-stream velocity. At this Re-* the exact coherent structure bifurcates supercritically and this is the point where the laminar Blasius flow starts to cohabit the phase space with alternative simple exact solutions of the Navier-Stokes equations.

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