Cees W. M. Bastiaansen

Printed artificial cilia from liquid-crystal network actuators modularly driven by light

Polymeric microactuators are potentially useful in micromechanical systems and lab-on-a-chip systems. However, manufacturing of miniature polymeric actuators has been complicated owing to the necessity of including electrodes for actuation or using lithographic techniques for patterning. Here, we demonstrate that all-polymer microdevices can be fabricated using inkjet printing technology in combination with self-organizing liquid-crystal network actuators. We exploit the self-assembling properties of the liquid crystal to create large strain gradients, and light-driven actuation is chosen to allow simple and remote addressing. By using multiple inks, microactuators with different subunits are created that can be selectively addressed by changing the wavelength of the light. The actuators mimic the motion of natural cilia. These artificial cilia have the potential to create flow and mixing in wet environments such as lab-on-a-chip applications. The process is easily adapted for roll-to-roll fabrication, allowing for large-scale and low-cost production of miniaturized active polymer systems.

Molecular machines: Nanomotor rotates microscale objects

Nanomachines of the future will require molecular-scale motors that can perform work and collectively induce controlled motion of much larger objects. We have designed a synthetic, light-driven molecular motor that is embedded in a liquid-crystal film and can rotate objects placed on the film that exceed the size of the motor molecule by a factor of 10,000. The changes in shape of the motor during the rotary steps cause a remarkable rotational reorganization of the liquid-crystal film and its surface relief, which ultimately causes the rotation of submillimetre-sized particles on the film.

Polarizing energy transfer in photoluminescent materials for display applications

Combinations of sheet polarizers and colour filters form the basis of numerous products — most notably colour liquid-crystal displays, — that require polarized chromatic light. But this combination of elements does not make efficient use of light, as a substantial fraction of the incident light is converted into thermal energy,, limiting the brightness and energy efficiency of the resulting devices. Here we show that these limitations can be overcome by using polymer-based photoluminescent polarizers. Our polarizers operate on a two-step principle: randomly orientated ‘sensitizer’ molecules harvest the incident light by isotropic absorption and then efficiently transfer the energy to a uniaxially orientated photoluminescent polymer, from which coloured light with a high degree of linear polarization is emitted. In principle, isotropic-to-polarized conversion efficiencies approaching unity could be attainable by this approach.

Large amplitude light-induced motion in high elastic modulus polymer actuators

Well-defined gradients in molecular alignment have been used as tools to generate large amplitude, light-induced deformations in stiff polymer networks. These systems are reversible, monolithic and based on a simple one-step self-assembly process. To fabricate the actuators, diacrylate dopants containing azobenzene moieties were blended with liquid crystalline diacrylate hosts and photopolymerized in a twisted configuration. The resulting twisted networks were heavily crosslinked with room temperature elastic moduli on the order of 1 GPa. Regardless of the temperature with respect to the glass transitions, subsequent exposure to UV radiation induced anisotropic expansion/contraction, and simple variations in geometry were used to generate uniaxial bending or helical coiling deformation modes. Because mechanical energy is directly related to elastic modulus, these systems are expected to provide significantly greater work output than contemporary polymer actuator materials.

Anisotropic Drop Morphologies on Corrugated Surfaces

The spreading of liquid drops on surfaces corrugated with micrometer-scale parallel grooves is studied both experimentally and numerically. Because of the surface patterning, the typical final drop shape is no longer spherical. The elongation direction can be either parallel or perpendicular to the direction of the grooves, depending on the initial drop conditions. We interpret this result as a consequence of both the anisotropy of the contact line movement over the surface and the difference in the motion of the advancing and receding contact lines. Parallel to the grooves, we find little hysteresis due to the surface patterning and that the average contact angle approximately conforms to Wenzels law as long as the drop radius is much larger than the typical length scale of the grooves. Perpendicular to the grooves, the contact line can be pinned at the edges of the ridges, leading to large contact angle hysteresis.

Photo-Switchable Surface Topologies in Chiral Nematic Coatings†

An enlightening answer: Dynamic surface photo-responsive topologies of a polymer coating were realized by introducing azobenzene crosslinkers into liquid-crystal polymer networks (see picture). The principle of these coatings is based on breaking the molecular order in the liquid-crystal polymer networks. Under irradiation of UV light the azobenzene compound isomerizes from the trans to the cis conformation.

Printable Optical Sensors Based on H-Bonded Supramolecular Cholesteric Liquid Crystal Networks

A printable H-bonded cholesteric liquid crystal (CLC) polymer film has been fabricated that, after conversion to a hygroscopic polymer salt film, responds to temperature and humidity by changing its reflection color. Fast-responding humidity sensors have been made in which the reflection color changes between green and yellow depending on the relative humidity. The change in reflection band is a result of a change in helix pitch in the film due to absorption and desorption of water, resulting in swelling/deswelling of the film material. When the polymer salt was saturated with water, a red-reflecting film was obtained that can potentially act as a time/temperature integrator. Finally, the films were printed on a foil, showing the potential application of supramolecular CLC materials as low-cost, printable, battery-free optical sensors.

Humidity-responsive liquid crystalline polymer actuators with an asymmetry in the molecular trigger that bend, fold, and curl.

We show a versatile method for the preparation of a variety of humidity-responsive actuators based on a single sheet of a hydrogen-bonded, uniaxially aligned liquid crystal polymer network. In this approach, the asymmetry in the molecular trigger in the anisotropic polymer film plays a dominant role leading to programmed deformation events. The material is locally treated with a potassium hydroxide solution to create the asymmetry in the responsiveness toward humidity, which allows to prepare actuators that bend, fold, or curl.

Room temperature preparation of conductive silver features using spin-coating and inkjet printing

Inkjet printing and spin-coating have been used to prepare patterns using a silver-containing metallo-organic decomposition ink. The patterned ink was reduced to silver by exposure to UV light and subsequent treatment with hydroquinone solution. This process, which took less than a minute, was performed at room temperature, which allowed low glass transition temperature polymeric substrates, such as PET, to be used. The conductivity of the silver patterns was found to be 10% that of bulk silver. The mechanical stability was also measured, with a linear increase in resistance seen for increasing strain, and no significant change in resistance seen after 12 000 cyclic deformations.

Fully Reversible Transition from Wenzel to Cassie—Baxter States on Corrugated Superhydrophobic Surfaces

Liquid drops on textured surfaces show different dynamical behaviors depending on their wetting states. They are extremely mobile when they are supported by composite solid-liquid-air interfaces (Cassie-Baxter state) and immobile when they fully wet the textured surfaces (Wenzel state). By reversibly switching between these two states, it will be possible to achieve control over the fluid dynamics. Unfortunately, these wetting transitions are usually prevented by surface energy barriers. We demonstrate here a new, simple design paradigm consisting of parallel grooves with an appropriate aspect ratio that allows for the controlled, barrierless, reversible switching of the wetting states upon application of electrowetting. We report a direct observation of the barrierless dynamical pathway for the reversible transitions between the Wenzel (collapsed) and Cassie-Baxter (suspended) states and present a theory that accounts for these transitions, including detailed lattice Boltzmann simulations.