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Liu D.,TU Eindhoven | Broer D.J.,TU Eindhoven | Broer D.J.,Institute for Complex Molecular Systems ICMS
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2015

Based on liquid crystal networks we developed 'smart' coatings with responsive surface topographies. Either by prepatterning or by the formation of self-organized structures they can be switched on and off in a pre-designed manner. Here we provide an overview of our methods to generate coatings that form surface structures upon the actuation by light. The coating oscillates between a flat surface and a surface with pre-designed 3D micro-patterns by modulating a light source. With recent developments in solid state lighting, light is an attractive trigger medium as it can be integrated in a device for local control or can be used remotely for flood or localized exposure. The basic principle of formation of surface topographies is based on the change of molecular organization in ordered liquid crystal polymer networks. The change in order leads to anisotropic dimensional changes with contraction along the director and expansion to the two perpendicular directions and an increase in volume by the formation of free volume. These two effects work in concert to provide local expansion and contraction in the coating steered by the local direction of molecular orientation. The surface deformation, expressed as the height difference between the activated regions and the non-activated regions divided by the initial film thickness, is of the order of 20%. Switching occurs immediately when the light is switched 'on' and 'off' and takes several tens of seconds. © SPIE. Downloading of the abstract is permitted for personal use only 2015.

Liu D.,TU Eindhoven | Broer D.J.,TU Eindhoven | Broer D.J.,Institute for Complex Molecular Systems ICMS
Soft Matter | Year: 2014

We describe a new methodology that enables dynamical control of motion through modulating the friction at coating surfaces by exposing to UV light. The principle is based on reversible switching of the surface topographies of the coating by light. The coating surface transfers from flat in the dark to corrugated in the presence of UV by forming regular ridge-like line gratings. Both the static and the kinetic friction coefficient are investigated in a dynamic manner by switching between the off (flat surface) and the activated (with ridges) state. By dynamically changing the friction, we are able to bring the sample from a static state into motion via UV exposure. When in motion, the friction coefficient can be altered further by modulating the light conditions. For example, a smooth sliding can transfer into an interlocking state, or vice versa. Moreover, we can dynamically reduce the contact area in the interface and thus lowering friction forces. © the Partner Organisations 2014.

Liu D.,TU Eindhoven | Broer D.J.,TU Eindhoven | Broer D.J.,Institute for Complex Molecular Systems ICMS
Angewandte Chemie - International Edition | Year: 2014

Chiral-nematic polymer network coatings form a "fingerprint" texture through self-assembly. For this purpose the molecular helix of the coating is oriented parallel to the substrate. The coating has a flat surface but when actuated by light in the presence of a copolymerized azobenzene compound, 3D fingerprint structures appear in the coating. The helix forms protrusions at the positions where the molecules are aligned parallel to the surface and withdraws at the positions where the orientation is perpendicular. This process proceeds rapidly and is reversible, that is, the fingerprint-shaped protrusions disappear when the light is switched off. The texture in the on-state resembles that of a human fingerprint and is used to manipulate the gripping friction of a robotic finger. The friction coefficient drops by a factor of four to five when the fingerprint switched on because of reduced surface contacts. Science friction: Artificial fingerprints are switched between a flat "off" state and a protruding "on" state. The switching principle is based on a modulated change of liquid-crystal chiral-nematic order, thus creating geometric changes. When illuminated with UV light, the 3D fingerprints are activated by azobenzene and appear in the coating. The surface friction decreases when the fingerprints form and increases when they are switched off. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Evans J.D.,University of Bath | Fernandez A.,University of Bath | Muntean A.,Institute for Complex Molecular Systems ICMS
Multiscale Modeling and Simulation | Year: 2012

We investigate the fast-reaction asymptotics for a one-dimensional reaction-diffusion system describing the penetration of the carbonation reaction in concrete. The technique of matchedasymptotics is used to show that the reaction-diffusion system leads to two distinct classes of sharpinterface models. These correspond to different scalings of a small parameter ε representing the fast-reaction and defined here as the ratio between the characteristic scale of diffusion for the fastest species and the characteristic scale of the carbonation reaction. We explore three conceptually different diffusion regimes in terms of the behavior of the effective diffusivities for the driving chemical species. The limiting models include one-phase and two-phase generalized Stefan moving-boundary problems as well as a nonstandard two-scale (micro-macro) moving-boundary problem - the main result of the paper. Numerical results, supporting the asymptotics, illustrate the behavior of the concentration profiles for relevant parameter regimes. © 2012 Society for Industrial and Applied Mathematics.

Liu D.,TU Eindhoven | Broer D.J.,TU Eindhoven | Broer D.J.,Institute for Complex Molecular Systems ICMS
Langmuir | Year: 2014

Monolithically ordered liquid crystal polymer networks are formed by the photoinitiated polymerization of multifunctional liquid crystal monomers. This paper describes the relevant principles and methods, the basic structure-property relationships in terms of mesogenic properties of the monomers, and the mechanical and optical properties of the polymers. Strategies are discussed to control the molecular orientation by various means and in all three dimensions. The versatility of the process is demonstrated by two examples of films with a patterned molecular order. It is shown that patterned retarders can be made by a two-step polymerization process which is successfully employed in a transflective display principle. A transflective display is a liquid crystal display that operates in both a reflective mode using ambient light and a transmissive mode with light coming from a backlight system. Furthermore, a method is discussed to create a patterned film in a single polymerization process. This film has alternating planar chiral nematic areas next to perpendicularly oriented (so-called homeotropic) areas. When applied as a coating to a substrate, the film changes its surface texture. During exposure to UV light, it switches from a flat to a corrugated state. © 2014 American Chemical Society.

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