Nearshore oblique sand bars

The coupling between hydrodynamics and the evolving topography in the surf zone has been theoretically examined for oblique wave incidence. It is shown that positive feedback can lead to the initial growth of several types of rhythmic systems of sand bars. The bars can be down-current oriented or up-current oriented, which means that the offshore end of the bar is shifted down-current or up-current with respect to the shore attachment. In the limit of strong current compared to wave orbital motion, very oblique down-current oriented bars are obtained with a spacing of several times the surf zone width. When wave orbital motions are dominant, systems of up-current oriented bars and crescentic/down-current oriented bars appear with spacings of the order of the surf zone width. The latter feature consists of alternating shoals and troughs at both sides of the break line with the inner shoals being bar-shaped and oblique to the coast. The growth (e-folding) time of the bars ranges from a few hours to a few days and it is favored by constant wave conditions. The range of model parameters leading to growth corresponds to intermediate beach states in between the fully dissipative and the fully reflective situations. Preliminary comparison with field observations shows qualitative agreement.Peer ReviewedPostprint (published version

Deck the Walls with Anisotropic Colloids in Nematic Liquid Crystals.

Nematic liquid crystals (NLCs) offer remarkable opportunities to direct colloids to form complex structures. The elastic energy field that dictates colloid interactions is determined by the NLC director field, which is sensitive to and can be controlled by boundaries including vessel walls and colloid surfaces. By molding the director field via liquid-crystal alignment on these surfaces, elastic energy landscapes can be defined to drive structure formation. We focus on colloids in otherwise defect-free director fields formed near undulating walls. Colloids can be driven along prescribed paths and directed to well-defined docking sites on such wavy boundaries. Colloids that impose strong alignment generate topologically required companion defects. Configurations for homeotropic colloids include a dipolar structure formed by the colloid and its companion hedgehog defect or a quadrupolar structure formed by the colloid and its companion Saturn ring. Adjacent to wavy walls with wavelengths larger than the colloid diameter, spherical particles are attracted to locations along the wall with distortions in the nematic director field that complement those from the colloid. This is the basis of lock-and-key interactions. Here, we study ellipsoidal colloids with homeotropic anchoring near complex undulating walls. The walls impose distortions that decay with distance from the wall to a uniform director in the far field. Ellipsoids form dipolar defect configurations with the colloid's major axis aligned with the far field director. Two distinct quadrupolar defect structures also form, stabilized by confinement; these include the Saturn I configuration with the ellipsoid's major axis aligned with the far field director and the Saturn II configuration with the major axis perpendicular to the far field director. The ellipsoid orientation varies only weakly in bulk and near undulating walls. All configurations are attracted to walls with long, shallow waves. However, for walls with wavelengths that are small compared to the colloid length, Saturn II is repelled, allowing selective docking of aligned objects. Deep, narrow wells prompt the insertion of a vertical ellipsoid. By introducing an opening at the bottom of such a deep well, we study colloids within pores that connect two domains. Ellipsoids with different aspect ratios find different equilibrium positions. An ellipsoid of the right dimension and aspect ratio can plug the pore, creating a class of 2D selective membranes

Modelling long-term morphodynamic evolution of mega-nourishments

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The potential risk induced by climate change in the context of mega-nourishments

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Long-term and large-scale modeling of mega-nourishments

The Sand Engine, ZM (Zandmotor), is a hook-shaped mega-nourishment (21.5 millions m³) located on the Dutch coast with an alongshore length of 2.4 km and an offshore extension of 1 km. The mega-nourishment project was initiated as a coastal protection measure on decadal time scales to maintain the coastline under predicted sea level rise. It follows the philosophy of working in harmony with the forces of nature by taking advantage of the longshore transport as the main distributor of sand along the adjacent coast (Stive et al., 2013). In the present contribution we use the Q2Dmorfo model (van den Berg, et al., 2012) to predict the long-term dynamics of the ZM.Peer ReviewedPostprint (published version

On the physics behind coastal morphodynamic patterns

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Bone-like inducing grafts: in vivo and micro-CT analysis

L'abstract è presente nell'allegato / the abstract is in the attachmen

Nearshore sand bars

This research has been funded by the Spanish government through the research projects CTM2012-35398 (cofunded by FEDER, U.E.) and CTM2015-66225-C2-1-P (MINECO/FEDER).This review summarizes the morphological characteristics and dynamics of nearshore sand bars observed in the surf zone of sandy beaches worldwide, with length scales ranging from tens to hundreds of meters and time scales ranging from hours to weeks. They include shore-parallel bars (straight and crescentic) and transverse bars of different types. Furthermore, the present knowledge on the physical processes behind their formation and development is discussed.Peer ReviewedPostprint (published version

Curvature-driven, One-step Assembly of Reconfigurable Smectic Liquid Crystal "Compound Eye" Lenses

Confined smectic A liquid crystals (SmA LCs) form topological defects called focal conic domains (FCDs) that focus light as gradient-index lenses. Here, we exploit surface curvature to self-assemble FCDs in a single step into a hierarchical structure (coined "flower pattern") molded by the fluid interface that is pinned at the top of a micropillar. The structure resembles the compound eyes of some invertebrates, which consist of hundreds of microlenses on a curved interface, able to focus and construct images in three dimensions. Here we demonstrate that these flowers are indeed "compound eyes" with important features which have not been demonstrated previously in the literature. The eccentric FCDs gradually change in size with radial distance from the edge of the micropillar, resulting in a variable microlens focal length that ranges from a few microns to a few tens of microns within a single "flower". We show that the microlenses can construct a composite 3D image from different depth of field. Moreover, the smectic "compound eye" can be reconfigured by heating and cooling at the LC phase transition temperature; its field of view can be manipulated by tuning the curvature of the LC interface, and the lenses are sensitive to light polarization.Comment: 17 pages, 9 figures (4 supplementary), 3 supplementary movies. This is the pre-peer reviewed version of the article which has been published in its final form in Advanced Optical Materials and is available at http://onlinelibrary.wiley.com/doi/10.1002/adom.201500153/abstrac