Anne Helene Gelebart

Making waves in a photoactive polymer film

Oscillating materials that adapt their shapes in response to external stimuli are of interest for emerging applications in medicine and robotics. For example, liquid-crystal networks can be programmed to undergo stimulus-induced deformations in various geometries, including in response to light. Azobenzene molecules are often incorporated into liquid-crystal polymer films to make them photoresponsive; however, in most cases only the bending responses of these films have been studied, and relaxation after photo-isomerization is rather slow. Modifying the core or adding substituents to the azobenzene moiety can lead to marked changes in photophysical and photochemical properties, providing an opportunity to circumvent the use of a complex set-up that involves multiple light sources, lenses or mirrors. Here, by incorporating azobenzene derivatives with fast cis-to-trans thermal relaxation into liquid-crystal networks, we generate photoactive polymer films that exhibit continuous, directional, macroscopic mechanical waves under constant light illumination, with a feedback loop that is driven by self-shadowing. We explain the mechanism of wave generation using a theoretical model and numerical simulations, which show good qualitative agreement with our experiments. We also demonstrate the potential application of our photoactive films in light-driven locomotion and self-cleaning surfaces, and anticipate further applications in fields such as photomechanical energy harvesting and miniaturized transport.

Mastering the Photothermal Effect in Liquid Crystal Networks: A General Approach for Self-Sustained Mechanical Oscillators

Chemical networks and molecular switches dominate the area of research geared toward macroscopic motion of materials. A counter-intuitive approach to create self-sustained oscillation by light irradiation of ordinary photostabilizers in splay-aligned liquid-crystalline networks made from commercial mesogens is developed. Photostabilizers or any molecules that are able to quickly dissipate the absorbed light through heat, by vibrational and/or rotational modes, can reach self-oscillating macroscopic motion where self-shadowing plays a critical role. The mechanical self-oscillation is linked to temperature oscillations and the asymmetric response over the film thickness. Only a localized responsive zone, acting as hinge, activates the oscillation of a beam-shaped device. The outcome of this research is extended from UV to near-IR actuation, making bulk applications to convert sunlight into mechanical work within reach.

A rewritable, reprogrammable, dual light-responsive polymer actuator

Abstract We report on the fabrication of a rewritable and reprogrammable dual‐photoresponsive liquid crystalline‐based actuator containing an azomerocyanine dye that can be locally converted into the hydroxyazopyridinium form by acid treatment. Each dye absorbs at a different wavelength giving access to programmable actuators, the folding of which can be controlled by using different colors of light. The acidic patterning is reversible and allows the erasing and rewriting of patterns in the polymer film, giving access to reusable, adjustable soft actuators.

Preparation of liquid crystal networks for macroscopic oscillatory motion induced by light

A strategy based on doped liquid crystalline networks is described to create mechanical self-sustained oscillations of plastic films under continuous light irradiation. The photo-excitation of dopants that can quickly dissipate light into heat, coupled with anisotropic thermal expansion and self-shadowing of the film, gives rise to the self-sustained deformation. The oscillations observed are influenced by the dimensions and the modulus of the film, and by the directionality and intensity of the light. The system developed offers applications in energy conversion and harvesting for soft-robotics and automated systems. The general method described here consists of creating free-standing liquid crystalline films and characterizing the mechanical and thermal effects observed. The molecular alignment is achieved using alignment layers (rubbed polyimide), commonly used in the display manufacturing industry. To obtain actuators with large deformation, the mesogens are aligned and polymerized in a splay/bend configuration, i.e., with the director of the liquid crystals (LCs) going gradually from planar to homeotropic through the film thickness. Upon irradiation, the mechanical and thermal oscillations obtained are monitored with a high-speed camera. The results are further quantified by image analysis using an image processing program.

Self-sustained actuation from heat dissipation in liquid crystal polymer networks

ABSTRACT Liquid crystal polymer networks (LCNs) lead the research geared toward macroscopic motion of materials. These actuators are molecularly programed to adapt their shape in response to external stimuli. Non‐photo‐responsive thin films of LCNs covered with heat absorbers (e.g., graphene or ink) are shown to continuously oscillate when exposed to light. The motion is governed by the heat dissipated at the film surface and the anisotropic thermal deformation of the network. The influence of the LC molecular alignment, the film thickness, and the LC matrix on the macroscopic motion is analyzed to probe the limits of the system. The insights gained from these experiments provide not only guidelines to create actuators by photo‐thermal or pure photo‐effects but also a simple method to generate mechanical oscillators for soft robotics and automated systems.

Inter-continental school of geometry and topology in soft matter, optics and biological systems: I-CAMP’14’s review

Stellenbosch, a hometown to wineries in the west of South Africa, hosted the sixth annual Inter-Continental Advanced Materials for Photonics (I-CAMP) in June 2014. This town is located 50 kilometres east of Cape Town and surrounded by Drakenstein and Stellenbosch mountains. Stellenbosch has been named after its founder, Simon van der Stel, the last Commander and first Governor of the Cape Colony, the Dutch settlement at the Cape of Good Hope in South Africa. Stellenbosch University (Universiteit van Stellenbosch and Universiteit Stellenbosch in Afrikaans), a world-class public university located in Stellenbosch, was the venue for the I-CAMP’14, from 15 to 29 June. This university has had numerous notable alumni such as Friedel Sellschop, pioneer in the field of nuclear applied physics, and Uys Krige, famous writer, poet and playwright. The I-CAMP is an annual school that brings together both prominent and junior scientists and allows them to combine advanced education with learning about different cultures worldwide. This summer it mainly welcomed scientists, researchers and professors working in materials science, energy, optics, photonics, biophysics, nanoscience and related fields. Although the I-CAMP’14 took place during the winter in South Africa, the excitement of FIFA World Cup alongwithmoderate temperature allowed attendees to have a great time during this school. The Jonkershoek Nature Reserve and Hottentots-Holland Mountain are incredible places to hike. And Stellenbosch University Botanical Garden is a great example of sightseeing destinations in this town. The social life in Stellenbosch gathers mostly in the city centre, where people enjoy walking, sitting and socialising (Figure 1). The school provided accommodation for attendees at Dagbreek, the largest and second oldest hall of residency of the University of Stellenbosch. The most distinctive element in this hall of residency was its Eiffel Tower (or Die Eiffel), which residents found quite comparable with the French version. The I-CAMP’14, similarly to previous I-CAMPs, was financially sponsored by the International Institute for Complex Adaptive Matter (ICAM-I2CAM), National Science Foundation (NSF) and University of Colorado Boulder (CU-Boulder). Because of scientific contributions from various highly regarded universities and research centres from different countries across the world, it has established itself as one of the worlds best summer schools in terms of technology and scientific techniques. In addition to the aforementioned sponsors, local sponsors contribute support as well. These have in previous years included the National Renewable Energy Laboratory (NREL), the only federal laboratory dedicated to the research, development, commercialisation and deployment of renewable energy and energy efficiency, in 2012, and Isaac Newton Institute of University of Cambridge, a very well-known international research institute for mathematics and theoretical physics, in 2013. This year, the African Institute for Mathematical Sciences (AIMS), a tertiary education and research institute in Muizenberg, South Africa, sponsored and hosted this event for the last 2 days. In 2014, the I-CAMP convened more than 40 undergraduate and graduate students, postdoc fellows and lecturers, from all across the world (Figure 2). The typical educational background of the students was physics; however, chemists, engineers and a few mathematicians attended this event. Although the registration fee was reasonably affordable, thanks to the bountiful support of the sponsors, a considerable number of the participants also received fellowships that included the lodging with complementary breakfast meals as well as conference receptions and school tours for the entire duration of the I-CAMP. Technological advances in the education system are a remarkable characteristic of I-CAMP. The realtime webcast of the lectures gave the opportunity to follow the summer school online for those who could not attend in person. Moreover, the records of the presentations are now archived; thus, they can be watched and downloaded, anytime of the year, from any part of the world. This archive is available online at http://spot.colorado.edu/~smalyukh/icamp2014/ ICAMP2014_Program.html. Liquid Crystals Today, 2015 Vol. 24, No. 1, 15–24, http://dx.doi.org/10.1080/1358314X.2014.973262