Direct ink writing of anisotropic luminescent materials

Luminescent solar concentrators are relatively inexpensive devices proposed to collect, convert, and redirect incident (sun)light for a variety of potential applications. In this work, dichroic dyes are embedded in a liquid crystal elastomer matrix and used as feedstock for direct ink writing. Direct ink writing is a promising and versatile application technique for arbitrarily aligning the dichroic dyes over glass and poly(methyl methacrylate) lightguide surfaces. The resulting prints display anisotropic edge emissions, and suggest usage as striking visual objects, combining localized color and intensity variations when viewed through a polarizer

The interplay between different stimuli in a 4D printed photo-, thermal-, and water-responsive liquid crystal elastomer actuator

Multi-stimuli responsivity in 3D-printed objects is receiving much attention. However, the simultaneous interplay between different environmental stimuli is largely unexplored. In this work, we demonstrate direct ink writing of an oligomeric ink containing an azobenzene photo-switch with an accessible hydrogen bond allowing triple responsivity to light, heat, and water. The resulting printed liquid crystal elastomer performs multiple actuations, the specific response depending on the environmental conditions. Bilayer films formed by printing on a static substrate can rapidly change shape, bending almost 80 degrees if irradiated in air or undergoing a shrinkage of about 50 % of its length when heated. The bilayer film assumes dramatically different shapes in water depending on combined environmental temperature and lighting conditions.</p

Calla Lily flower inspired morphing of flat films to conical tubes

Recently researchers have developed soft actuators capable of morphing into complex shapes taking inspiration from nature. In this paper, we have developed a splay-nematic liquid crystal polymer network tapered actuator that can morph from a flat film to a cone, mimicking the blooming of single petal Calla Lily flower. We have demonstrated the formation of conical tubes through finite element simulations and experiments. The influence of tapering and alignment orientations with respect to the edge of the film on the cones is analyzed through simulations. The design with tapering and splayed alignments oriented at 45° to the edge is found to be the optimal choice for forming conical tubes

Image encoding with unconventional appearance through direct ink writing of a cholesteric liquid crystal oligomer ink

We demonstrate a method combining direct ink writing of a chiral nematic liquid crystal oligomer ink with a photolithography step in a procedure that gives the user great flexibility in the design of striking polymer optics. The printable chiral nematic ink is first made by oligomerizing a reactive nematic liquid crystal monomer with a reactive chiral dopant. This ink can be readily printed using direct ink writing and quickly forms the cholesteric phase after deposition. Increasing the lateral nozzle speed forces the helical chiral nematic alignment into a slanted configuration, which has deviating optical properties compared to conventional planar chiral nematic liquid crystal reflectors—when inspected from the surface normal, these slanted photonics do not display circular polarization dependence, a feature characteristic for chiral nematics. The direction of this slant is determined by the printing direction, and thus highly customizable. For instance, when printing in a single direction, the peak reflected wavelength λmax is only seen at ca. 50° angle of incidence. Writing a back-and-forth pattern turns the object into an “inverse cholesteric” in two dimensions, where the longest reflected wavelength is seen from 50° in either direction. More intricate print path designs lead to more complex appearances. With a two-step photo-crosslinking procedure using masks, two-dimensional designs can be imprinted in direct ink written coatings, resulting in highly complex but programmable reflection patterns

Direct ink writing of 4D structural colors

Additive manufacturing with stimuli-responsive materials—4D printing—is a rapidly growing field, with direct ink writing allowing deposition of a wide variety of materials. The synthesis of a humidity-sensitive cholesteric liquid crystal oligomer ink is reported. With the responsive cholesteric ink, demonstrator devices exhibiting the ink's “four dimensionality” are printed in disparate fashions: as a structural color change or as a preprogrammed deformation mode. After printing, the photonic ink changes color in response to atmospheric humidity, demonstrated as a hydrochromic coating precisely deposited atop a 3D-printed beetle. After activation in aqueous acid, the beetle exhibits vibrant color shifts across the visible spectrum. Alternatively, a scallop-inspired actuator with a 3D-programmed structural color is selectively treated with acid, to allow reversible “opening” and “closing” when exposed to humid and dry air, respectively. The ink enables additive manufacturing of both monolithic and multimaterial stimuli-responsive, shape-changing, structurally colored objects, toward broad application of cholesterics in future “smart,” 4D structurally colored devices

Responsive Photonic Liquid Crystalline Flakes Produced by Ultrasonication

Responsive materials that alter their color in response to environmental changes can be used as optical sensors. Chiral nematic liquid crystals are photonic materials that selectively reflect specific wavelengths of light and have been made environmentally responsive. In this work, we demonstrate the use of ultrasonication of responsive cholesteric liquid crystal network films to form structurally colored flakes that demonstrate color changes when moved from an aqueous to dry environment and back again, suggesting a scalable technique to form quantities of responsive particles that could conceivably be embedded in permeable hosts to allow the optical detection of humidity or certain chemical species

4D Printed Light-Responsive Patterned Liquid Crystal Elastomer Actuators Using a Single Structural Color Ink

Envisioning robotic devices with functionality resulting from processing rather than exclusively material composition, multimodal responsive devices produced from a single azobenzene-functionalized cholesteric liquid crystal elastomer ink are demonstrated. The resulting device displays simultaneous structural color and actuation in response to both ultraviolet and blue light exposures. Through direct ink writing, a microextrusion-based additive manufacturing technique, the liquid crystalline ink is deposited into different effective mesophases—planar and slanted cholesteric, and a uniaxial pseudo-nematic state—by altering only two printing conditions: deposition speed and time before photopolymerization. Comparing the three different printed mesophases, marked differences in optical properties and actuation behaviors are found. Demonstrators based on bilayer architectures present concepts for interactive soft multiactuators deposited in a single printing step from a single ink

4D Printing of Liquid Crystals: What’s Right for Me?

Recent years have seen major advances in the developments of both additive manufacturing concepts and responsive materials. When combined as 4D printing, the process can lead to functional materials and devices for use in health, energy generation, sensing, and soft robots. Among responsive materials, liquid crystals, which can deliver programmed, reversible, rapid responses in both air and underwater, are a prime contender for additive manufacturing, given their ease of use and adaptability to many different applications. In this paper, selected works are compared and analyzed to come to a didactical overview of the liquid crystal-additive manufacturing junction. Reading from front to back gives the reader a comprehensive understanding of the options and challenges in the field, while researchers already experienced in either liquid crystals or additive manufacturing are encouraged to scan through the text to see how they can incorporate additive manufacturing or liquid crystals into their own work. The educational text is closed with proposals for future research in this crossover field

3D anisotropic polyethylene as light-responsive grippers and surfing divers

Untethered soft actuators are usually based on 3D engineered special polymers such as liquid crystal networks or hydrogels that require complex fabrication methods. Here, an easy‐to‐process, anisotropic composite soft actuator based on a simple photothermal dye‐doped polyethylene film is presented. The 3D anisotropic polymer films are prepared by solution‐casting and subsequent thermal solid‐stretching. The resulting soft actuators exhibit large and controllable bending in response to light (UV and/or near‐infrared (NIR)) and are able to act as grippers picking up cargo. Additionally, the films are capable of realizing “diving” and “surfing” locomotion in and over a liquid via the photothermal induced Marangoni effect, yielding a NIR light‐fueled transporter able to pick up cargo. The results open up new possibilities of using commodity polymers in a broad range of applications including untethered soft actuators and robotic devices