Name: Preparation of Liquid Crystal Networks for Macroscopic Oscillatory Motion Induced by Light (Video) | JoVE Journal
Uploaded: 2017-09-20T15:00:00.000+00:00
Duration: 7 min 56 s
Description: Learn to prepare liquid crystal networks for mechanical oscillation under continuous light irradiation. This method enables photoresponsive films with applications in soft robotics and automated systems.

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5 minutes

This work was financially supported by the Netherlands Organization for Scientific Research (NWO - TOP PUNT Grant: 10018944) and the European Research Council (Vibrate ERC, Grant 669991). A. H. G. acknowledges the funding from the People Program (Marie Curie Actions) of the European Union&#39;s Seventh Framework Program FP7-2013, Grant No. 607602.

NOTE: The overall procedure is detailed in Figure 1.
1. Preparation of Cells
<ol>
	<li>Cleaning of glass plates
	<ol>
		<li>Carefully clean 3 cm x 3 cm glass plates using soap and hot water to remove contaminations (Figure 1A).</li>
		<li>Place the glass plates in a beaker and cover with ethanol 99.5% (Figure 1B).</li>
		<li>Place the beaker in an ultrasonic bath for about 10 min.</li>
		<li>Carefully dry the glass plates with a tissue and blown air.</li>
		<li>Make sure that there is no trace of solvent, dust, or any type of contamination left on the plates. 
		NOTE: The glass plates are now clean and should be manipulated with gloves.</li>
		<li>Place the glass plates in an UV-ozone photoreactor for 20 min in order to remove any organic residues. After the ozone treatment, the plates are ready for the coating step (Figure 1C).</li>
	</ol>
	</li>
	<li>Coating of glass plates 
	NOTE: Two sets of coated plates are prepared: one set with a planar alignment layer and the other with a homeotropic alignment. In a later stage, the cell will be composed of one planar glass plate and one homeotropic glass plate (Figure 1D).
	<ol>
		<li>Air blow a glass plate and place it on the spin coater.</li>
		<li>Deposit the polyimide solution onto the glass plate to cover the entire surface (about 0.5 mL of solution).</li>
		<li>Spin coat the alignment layer according to the following conditions: Program 1: 5 s at 17 x g and acceleration of 11 x g/s; Program 2: 40 s at 420 x g and acceleration of 17 x g/s.</li>
	</ol>
	</li>
	<li>Curing of the alignment layers
	<ol>
		<li>Place the coated glass plates on a hot plate at 110 &#176;C for 10 min in order to remove the majority of the solvent present in the alignment layer mixture (Figure 1E).</li>
		<li>Mark the glass plates (on the non-coated side) with distinctive signs to recognize the homeotropic and the planar alignment layers. A small arrow is usually helpful for the planar glass plate since it also indicates the rubbing direction described in a later stage (Figure 1F).</li>
		<li>Once all the glass plates are coated, and the solvent is removed, place the glass plates in an oven at 180 &#176;C for 1 h to cure the polyimide layer (Figure 1G). 
		CAUTION: This step involves extremely hot temperatures; wear gloves, eyeglasses and appropriate personal protection.</li>
		<li>After baking the glass plates, let them cool down to room temperature. 
		NOTE: To prevent any contamination between each step, it is recommended to place a protective foil over the glass plates.</li>
	</ol>
	</li>
	<li>Rubbing the planar alignment layer
	<ol>
		<li>Rub the glass plates coated with the planar alignment layer in order to create (sub) microchannels in the layer that will guide the LC in one direction. To do so, place the glass plates with the coated side downwards on a velvet cloth. Apply a uniform and soft pressure with two fingers. Carefully drag the glass plate along the surface of the velvet cloth in a straight direction. Lift the glass plate and repeat the same operation three times (Figure 1H). 
		NOTE: It is crucial to rub the plate in a single direction and only while going forward. Going back and forth in a straight direction will result in a poor alignment.</li>
	</ol>
	</li>
	<li>Gluing the cells
	<ol>
		<li>Air blow the glass plates using the air blower.</li>
		<li>Prepare the adhesive by mixing a UV-curing glue with spacers (glass beads) having a well-defined diameter of 20 &#181;m.</li>
		<li>Take a glass plate coated with a planar alignment layer and one glass plate coated with a homeotropic alignment layer. Place two tiny drops of glue at two adjacent corners of the planar glass. Then place two other drops of glue at about 5 mm from the two last corners (Figure 1I).</li>
		<li>Take the homeotropic glass plate and place it on top. Leave a gap of about 4 mm between the edges of the glass plates to provide enough space for the LC mixture. Make sure that the coated sides are facing each other.</li>
		<li>Cure the glue by placing the cell for 2 min under UV light. 
		CAUTION: UV light is hazardous; wear gloves, eyeglasses and appropriate personal protection.</li>
	</ol>
	</li>
</ol>
2. LC Mixture Preparation and Characterization
<ol>
	<li>Components
	<ol>
		<li>Weigh 97.5 mg of the LC diacrylate 1, 2.5 mg of photo-stabilizer and 1 mg of photoinitiator in a brown glass vial (Figure 2). Because of the sensitivity of the initiator, prevent UV light exposure to the mixture as best as possible.</li>
	</ol>
	</li>
	<li>Homogenous mixing of the powders 
	NOTE: This step is performed in a chemical hood.
	<ol>
		<li>Add 3 mL of dichloromethane (DCM) to the above components and shake until the solid is fully dissolved.</li>
		<li>Place the vial on a hot plate at 30 &#176;C for 30 min and add a flow of argon to promote the quick evaporation of the DCM. 
		NOTE: It is recommended to place the vial in vacuum to remove any residual trace of DCM.</li>
	</ol>
	</li>
	<li>Observation under Polarized Optical Microscope (POM) for phase transition determination
	<ol>
		<li>Once the mixture is fully dry, place a small amount (&#177; 10 mg) between two glass plates coated with the planar alignment layer. 
		NOTE: In order to characterize the phases properly, it is recommended to use unrubbed planar glass plates.</li>
		<li>Place the slides in a POM equipped with a hot stage. Heat up the cell until the image turns black (using crossed polarizers) indicating the isotropic phase.</li>
		<li>Slowly cool down the hot plate and note the transition temperatures. For the mixture described above, the isotropic to nematic transition is at 103 &#176;C and the nematic to smectic transition occurs at 86 &#176;C (Figure 3A).</li>
	</ol>
	</li>
</ol>
3. Film Preparation
<ol>
	<li>Filling of the cells (Figure 1J)
	<ol>
		<li>Place the cell on a hot plate with the homeotropic side upwards. Set the temperature to 110 &#176;C (isotropic phase) to facilitate the filling of the cell because the fluid is less viscous than in the nematic phase.</li>
		<li>Place part of the solid mixture on the edge of the cell.</li>
		<li>The solid melts and the liquid mixture flows by capillarity in the cell. Add more mixture at the edge until the cell is filled.</li>
	</ol>
	</li>
	<li>Cooling to the nematic phase and polymerization (Figure 1K)
	<ol>
		<li>Once the cell is filled, slowly cool it down (5 &#176;C/min) to 90 &#176;C to be in the nematic phase.</li>
		<li>As soon as the film is at the right temperature, polymerize the mixture by placing it under UV light at 90 &#176;C for 30 min. 
		CAUTION: UV light is hazardous, it is advised to perform the polymerization in a protected environment.</li>
		<li>A post-baking step is recommended to ensure the full polymerization of the network. Place the cell on a hot plate at 130 &#176;C for about 10 min and let it slowly cool down to room temperature.</li>
	</ol>
	</li>
	<li>Opening of the cell and cutting of the sample
	<ol>
		<li>To open the cell, put a razor blade at one edge and push it in between the two glass plates. The cell opens at once (Figure 1L).</li>
		<li>To peel off the film, lift a small corner with a razor blade. If needed, the glass plate can be placed in hot water to facilitate the removal of the film (Figure 1M).</li>
		<li>Remove the film from the water and gently peel it.</li>
		<li>Cut a stripe along the molecular director (rubbing direction of the planar side) of the film having the following dimensions: 4 mm x 2.5 cm (Figure 1N).</li>
	</ol>
	</li>
</ol>
4. Self-oscillation Observation
<ol>
	<li>Setup in the lab
	<ol>
		<li>Clamp the sample using a self-closing tweezer in such a way that 1.7 cm of the film is free to move.</li>
		<li>Hold the sample vertically and direct the light emitting diode (LED) beam (400 mW/cm2) perpendicular to the sample. Typically, the light is about 20 cm away from the sample. The light should reach the top of the film, below the tweezer (Figure 4). The oscillations obtained are recorded with a high-speed camera (150 frames/s) and analyzed with an image processing program.</li>
	</ol>
	</li>
	<li>Setup with direct sunlight
	<ol>
		<li>Proceed as described above in steps 4.1.1 but instead of using the LED light, focus the sunlight on top of the film with a lens.</li>
	</ol>
	</li>
	<li>Measurement of the thermal effect
	<ol>
		<li>Measure the temperature variation in the oscillating sample using a thermal camera15 (40 frames/s).</li>
	</ol>
	</li>
</ol>

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<li>Hu, Y., Li, Z., Lan, T., Chen, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Photoactuators+for+Direct+Optical-to-Mechanical+Energy+Conversion%3A+From+Nanocomponent+Assembly+to+Macroscopic+Deformation%5BTitle%5D)+AND+%22Adv+Mater%22%5BJournal%5D)">Photoactuators for Direct Optical-to-Mechanical Energy Conversion: From Nanocomponent Assembly to Macroscopic Deformation.</a> Adv Mater. 28 (47), 10548-10556 (2016).</li>
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<li>Ikegami, T., Kageyama, Y., Obara, K., Takeda, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Dissipative+and+autonomous+square-wave+self-oscillation+of+a+macroscopic+hybrid+self-assembly+under+continuous+light+irradiation%5BTitle%5D)+AND+%22Angew+Chem+Int+Ed%22%5BJournal%5D)">Dissipative and autonomous square-wave self-oscillation of a macroscopic hybrid self-assembly under continuous light irradiation.</a> Angew Chem Int Ed. 55 (29), 8239-8243 (2016).</li>
<li>Uchida, E., Azumi, R., Norikane, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Light-induced+crawling+of+crystals+on+a+glass+surface%5BTitle%5D)+AND+%22Nat+Commun%22%5BJournal%5D)">Light-induced crawling of crystals on a glass surface.</a> Nat Commun. 6, 7310(2015).</li>
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<li>Kumar, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((A+chaotic+self-oscillating+sunlight-driven+polymer+actuator%5BTitle%5D)+AND+%22Nat.+Commun%22%5BJournal%5D)">A chaotic self-oscillating sunlight-driven polymer actuator.</a> Nat. Commun. 7, 11975(2016).</li>
<li>Camacho-Lopez, M., Finkelmann, H., Palffy-Muhoray, P., Shelley, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Fast+liquid-crystal+elastomer+swims+into+the+dark%5BTitle%5D)+AND+%22Nat+Mater%22%5BJournal%5D)">Fast liquid-crystal elastomer swims into the dark.</a> Nat Mater. 3 (5), 307-310 (2004).</li>
<li>Kausar, A., Nagano, H., Ogata, T., Nonaka, T., Kurihara, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Photocontrolled+translational+motion+of+a+microscale+solid+object+on+azobenzene-doped+liquid-crystalline+films%5BTitle%5D)+AND+%22Angew+Chem+Int+Ed%22%5BJournal%5D)">Photocontrolled translational motion of a microscale solid object on azobenzene-doped liquid-crystalline films.</a> Angew Chem Int Ed. 48 (12), 2144-2147 (2009).</li>
<li>Yamada, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Photomobile+Polymer+Materials%3A+Towards+Light-Driven+Plastic+Motors%5BTitle%5D)+AND+%22Angew+Chem+Int+Ed%22%5BJournal%5D)">Photomobile Polymer Materials: Towards Light-Driven Plastic Motors.</a> Angew Chem Int Ed. 47 (27), 4986-4988 (2008).</li>
<li>White, T. J., et al. <a target="_blank" href="http://pubs.rsc.org/en/Content/ArticleLanding/2008/SM/b805434g">A high frequency photodriven polymer oscillator.</a> Soft Matter. 4 (9), 1796(2008).</li>
<li>Lee, K. M., et al. <a target="_blank" href="http://onlinelibrary.wiley.com/doi/10.1002/adfm.201100333/abstract">Photodriven, flexural-torsional oscillation of glassy azobenzene liquid crystal polymer networks.</a> Adv Func Mater. 21 (15), 2913-2918 (2011).</li>
<li>Liu, D., Broer, D. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Liquid+crystal+polymer+networks%3A+preparation%2C+properties%2C+and+applications+of+films+with+patterned+molecular+alignment%5BTitle%5D)+AND+%22Langmuir%22%5BJournal%5D)">Liquid crystal polymer networks: preparation, properties, and applications of films with patterned molecular alignment.</a> Langmuir. 30 (45), 13499-13509 (2014).</li>
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<li>Broer, D. J., Mol, G. N. <a target="_blank" href="http://onlinelibrary.wiley.com/doi/10.1002/pen.760310902/full">Anisotropic thermal expansion of densely cross-linked oriented polymer networks.</a> Polym Eng Sci. 31 (9), 625-631 (1991).</li>
<li>Mol, G. N., Harris, K. D., Bastiaansen, C. W. M., Broer, D. J. <a target="_blank" href="http://onlinelibrary.wiley.com/doi/10.1002/adfm.200400503/abstract">Thermo-Mechanical Responses of Liquid-Crystal Networks with a Splayed Molecular Organization.</a> Adv Funct Mater. 15 (7), 1155-1159 (2005).</li>
<li>van Oosten, C. L., Harris, K. D., Bastiaansen, C. W. M., Broer, D. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Glassy+photomechanical+liquid-crystal+network+actuators+for+microscale+devices%5BTitle%5D)+AND+%22Eur+Phys+J+E+Soft+Matter%22%5BJournal%5D)">Glassy photomechanical liquid-crystal network actuators for microscale devices.</a> Eur Phys J E Soft Matter. 23 (3), 329-336 (2007).</li>
<li>Yu, Y., Nakano, M., Ikeda, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Photomechanics%3A+Directed+bending+of+a+polymer+film+by+light%5BTitle%5D)+AND+%22Nature%22%5BJournal%5D)">Photomechanics: Directed bending of a polymer film by light.</a> Nature. 425 (6954), 145(2003).</li>
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The authors have no conflict of interest.

The results described here are comparable with the previous study15 on a LC diacrylate with a spacer of 6 carbons. It shows that the method to obtain oscillation can be applied to films with different mechanical properties16.
The preparation of a photo-thermal responsive LCN is reported. There are a few steps in the described protocol that are critical, such as the rubbing of the planar alignment layers and the preparation of the cell. Indeed, the success of the protocol relies on the high quality of the LC splay alignment, which also limits the application to thin films.
Previously, many examples of photo-actuators based on LCNs that contain a large quantity of photo-switches have been reported11,12,13,19. The main advantages of the method developed here are the limited quantity of dopants needed to observe the actuation (&#60; 5 wt%) and the broad choice of dopants available.15 These results expand the range of potential applications. Moreover, the power of this protocol is the ability to vary the frequency and the amplitude of the oscillation by changing the modulus of the film with a different matrix composition, the dimensions of the stripe and the light intensity.
This methodology can be readily extended to fabricate a broad range of LC materials for automated systems. The protocol described herein paves the way to the development of non-equilibrium systems for soft-robotics and automated materials.

The search towards sustainable energy is an area of growing interest in response to the depletion of fossil energy and climate change. Methods to convert light energy into mechanical work are currently being investigated, such as photovoltaics to generate electricity, biomass to produce fuels, solar water splitting to produce oxygen and hydrogen. However, all these processes fueled by light require multiple steps before their energy production can run machines that perform work. Although these approaches offer a large panel of applications, they require production, storage and transport of intermediates (e.g., electrical potential, chemical fuels). Hence, devices able to convert directly sunlight into macroscopic motion present advantages of simplification.
In the last decades, many examples of photo-actuators have been developed where polymers change shape upon irradiation1,2,3. However, in the large majority of these examples, continuous actuation requires turning the light on/off to switch from one state to the other. So far, only a limited number of photo-responsive materials working out-of-equilibrium has been described4,5,6,7. Systems based on liquid crystal networks (LCNs)8,9,10,11,12,13 are also investigated because of their intrinsic anisotropy that allows for preprogramming the deformation in a controlled fashion14. Recently, it was reported that the photo-thermal effect induced by excitation of photo-stabilizers incorporated in LCN can generate oscillatory motion15.
Here, the method to create LCN films that mechanically oscillate under continuous light irradiation is described. The conception of the films is detailed from the preparation of the cells to the characterization and the polymerization of the LC mixtures. The photo-actuation of the LCN films and the analysis of the motion are also reported. The LCNs are doped with molecules that can quickly dissipate light into heat within the network, which induces anisotropic thermal expansion and subsequent macroscopic deformation of the film. An interplay between self-shadowing, temperature variation and contraction/elongation of the material gives rise to the oscillatory motion15. The precise setup, including the orientation of the light and the sample to obtain this effect is highlighted in the protocol. The oscillation is characterized by its frequency and controlled by the properties of the LCN. To our knowledge, this is the first description of a method for the creation of LCN films that can self-oscillate, by a simple mechanism working with a broad range of dopants.

<table><tbody><tr><td>&lt;strong&gt;Material&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>LC diacrylate (compound &lt;strong&gt;1: Figure 2&lt;/strong&gt;)</td><td>Syncom</td><td>custom synthesis</td><td></td></tr><tr><td>photo-stabilizer</td><td>Ciba</td><td>tinuvin 328</td><td></td></tr><tr><td>photoinitiator</td><td>Ciba</td><td>Irgacure 819</td><td></td></tr><tr><td>Alignment layer planar</td><td>JSR micro</td><td>optimer Al1051</td><td></td></tr><tr><td>Alignment layer homeotropic</td><td>Nissan chemical industry</td><td>Sunever grade 5300</td><td></td></tr><tr><td>&lt;strong&gt;Name&lt;/strong&gt;</td><td>&lt;strong&gt;Company&lt;/strong&gt;</td><td>&lt;strong&gt;Catalog Number&lt;/strong&gt;</td><td>&lt;strong&gt;Comments&lt;/strong&gt;</td></tr><tr><td>&lt;strong&gt;Equipment&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>UV-ozone photoreactor</td><td>Ultra Violet Products, PR-100</td><td>Not available</td><td></td></tr><tr><td>spin coater</td><td>Karl-SUSS</td><td>SUSS RC spin coater CT62 V098</td><td></td></tr><tr><td>UV light</td><td>Gentec</td><td>EXFO-Omnicure S2000</td><td></td></tr><tr><td>micropearl</td><td>Sekisui Chemicals</td><td>SP220-20um</td><td></td></tr><tr><td>Glue</td><td>Gentec</td><td>UVS91</td><td></td></tr><tr><td>LED 365 nm</td><td>Thorlabs</td><td>M365LP1</td><td></td></tr><tr><td>light collimator</td><td>Thorlabs</td><td>SM2F32-A</td><td></td></tr><tr><td>high speed camera</td><td>PCO.</td><td>PCO 5.5 sCMOS camera</td><td></td></tr><tr><td>thermal camera</td><td>Xenics Infrared solution</td><td>Gobi-640-GigE</td><td>used with Xeneth software</td></tr><tr><td>Differential Scanning Calorimeter</td><td>TA instruments</td><td>Q1000</td><td></td></tr><tr><td>Dynamic Mechanical Analyzer</td><td>TA instruments</td><td>Q800</td><td></td></tr><tr><td>Polarized Optical Microscope</td><td>Leica</td><td>DM6000M</td><td></td></tr></tbody></table>

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

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.

The success of the protocol is the observation of the oscillatory motion of the film under light irradiation. Oscillations are large and no misleading result can be seen. Moreover, the oscillations are stable over time (timescale of hours) and little fatigue was observed.
Among others, the quality of the splay alignment is of importance for the accomplishment of the self-sustained actuation (Figure 5A). The gradient in molecular orientation throughout the thickness of the film induces a contraction/expansion at the planar/homeotropic sides of the film upon actuation16,17,18. This asymmetric response enhances the macroscopic motion. The failure of the experiment (absence of bending, small deformation or odd bending) can be explained by a poor LC alignment. First, the film should be transparent. (Figure 5B).  To verify the correct splay alignment in a simple step, the film attached to the glass substrate is observed between crossed polarizers above a diffused white light source(Figure 5C-E). By rotating the film between the cross polarizers from 0° to 45° in the xy plane, the film should sharply change brightness. By pivoting the film out of the plane around the molecular director, the film should change color from black (in plane) to white (out of plane) while viewing from the top. Similar verification steps can be done prior to polymerization by observing the cell through a polarizer on top of the hot plate covered with an aluminum foil. Moreover, when the film is cut in strips, it presents a natural curvature with the center of the curve in the homeotropic side. This is due to the residual stress originating from the polymerization at an elevated temperature, where the expansions of the two sides of the film have opposite signs (Figure 6A). In case the alignment is not successful, the method to prepare the polyimide layers should be reconsidered. The production of those cells is crucial to obtain well-aligned films. The most critical step is the rubbing: a too strong pressure on the plate will remove partially the polyimide layer and result in a very poor command layer for alignment. At room temperature the film is in the glass state (Figure 3B). If the film is soft and/or tacky, it means that the polymerization is not completed, most probably because the irradiation time is too short or the initiator is degraded. The mixture of LC mesogens should be homogeneous and dry before the filling of the cell, because the presence of solvent might influence the phase behavior of the LC mixture. The LC mixture should be aligned prior to polymerization. Precautions should be taken to avoid thermal polymerization during the process of alignment by going through the steps rapidly and not heating the sample above 130 °C for a prolonged time. Filling the cell just above the clearing point is sufficient (110 °C).
The mechanical and thermal oscillations registered by the high-speed camera confirm the success of the presented protocol (Figure 7; Video 1). When the film is clamped at one end, leaving 1.7 cm free to move and irradiated at the planar side with focused light, it unbends towards the flat state in the direction of the light (Figure 6B). The hinge is located at the position of the focus point of the light (Figure 4). The film should move smoothly, perpendicular to the clamp and not on the side. Then the film starts to move continuously with oscillations of frequency 7.6 Hz ± 5% and amplitude 30 ° ± 10% for a film of dimensions 1.7 cm x 0.4 cm x 20 µm. The thermal oscillations measured with the thermal camera present the same frequency (7.4 Hz ± 5%), with a slight phase delay due to the inertia of the film. This frequency f is governed by the dimensions and the modulus of the film15. The amplitude of the oscillations varies with the light intensity and will be influenced by the setup, and in particular, the positioning of the focus point of the light on the sample. The mechanism of oscillation is as follows: 1) the curled film is irradiated with focused light, the dopant absorbs the light and converts it into heat, the film warms up at its hinge and unbends in the direction pre-defined by the LC alignment; 2) the tip shadows the hinge of the film, which induces a decrease of its temperature and its subsequent unbending by relaxation; 3) the hinge is again under irradiation, warms up and the film bends15. The repetition of these successive steps gives rise to the oscillations. The key factors in observing this phenomenon are the photo-thermal effect and the self-shadowing of the film, controlled by the intensity and the position of the focused light (Figure 4). For instance, a slightly tilted lamp will induce a complete curling of the sample. Moreover, too low of a light intensity does not give large bending because the temperature at the hinge is insufficient, while too high of a light intensity on the hinge will induce overshooting (Figure 6C, 180 ° bending of the film). Another requirement for the success of the experiment is to place the setup in an environment protected from wind to avoid perturbation.
<img alt="Thin film deposition process diagram; UV-Ozone treatment; heating stages at 110°C, 180°C; layer stacking." class="xfigimg" src="/files/ftp_upload/56266/56266fig1.jpg" data-altupdatedat="1788250015000" data-alt="Figure 1"> 
Figure 1. The Overall Procedure to Obtain the Splay Aligned LCN in 14 Steps (From A-N). Steps A-C: cleaning of the glass plates; steps D-G: coating of the glass plates to create planar or homeotropic alignment layers; step H: rubbing of the glass plates using a velvet cloth; step I: gluing the plates to form the cell; step J: filling the cell with the LC mixture and alignment in the nematic phase; step K: photo-polymerizing under UV light; steps L-N: opening of the cell and cutting of the film to obtain a strip. <a href="https://cloudflare.jove.com/files/ftp_upload/56266/56266fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Polymer chemical structure with Tinuvin; molecular formula diagram; photostability research." class="xfigimg" src="/files/ftp_upload/56266/56266fig2.jpg" data-altupdatedat="1788250015000" data-alt="Figure 2"> 
Figure 2. Chemical Structures of the Components Used. <a href="https://cloudflare.jove.com/files/ftp_upload/56266/56266fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Differential scanning calorimetry and mechanical analysis graphs of polymers vs. temperature." class="xfigimg" src="/files/ftp_upload/56266/56266fig3.jpg" data-altupdatedat="1788250015000" data-alt="Figure 3"> 
Figure 3. Thermal Characterization of the Monomer Mixture and Polymer Film. A) Differential scanning calorimetry (DSC) of the mixture prior to polymerization to determine the phase transitions. Insets: POM pictures, scale bars: 100 µm. B) Dynamic mechanical thermal analysis (DMTA) measurement of the polymeric film. <a href="https://cloudflare.jove.com/files/ftp_upload/56266/56266fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Light source setup in diagram for optical excitation with responsive film and self-closing tweezers." class="xfigimg" src="/files/ftp_upload/56266/56266fig4.jpg" data-altupdatedat="1788250015000" data-alt="Figure 4"> 
Figure 4. Picture of the Setup Showing the LED on the Left Side, and the Oscillating Film Clamped to the Tweezer in Front of the Light. The inset shows the schematic representation of the bended film and the localized light illumination. The red area corresponds to the warm hinge mentioned in the text. <a href="https://cloudflare.jove.com/files/ftp_upload/56266/56266fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Liquid crystal alignment and optical properties; diagram and experiment showing birefringence effect." class="xfigimg" src="/files/ftp_upload/56266/56266fig5.jpg" data-altupdatedat="1788250015000" data-alt="Figure 5"> 
Figure 5. A) Schematic representation of the splay alignment. B) Picture of the glass cell in front of the TU/e logo showing the transparency and the absence of color of the film. The arrow indicates the rubbing direction of the planar glass plate. C-E) Pictures of the film taken between crossed polarizers showing the characteristics of the splay alignment (Picture D: rotation of 45 ° in the XY plane, Picture E: pivoting out of the xy plane). The molecular director of the alignment is indicated by the red arrow. Scale bar: 1 cm. <a href="https://cloudflare.jove.com/files/ftp_upload/56266/56266fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Microscopic bending process, diagrams A-C; mechanical stress, metal-tweezers, deformation study." class="xfigimg" src="/files/ftp_upload/56266/56266fig6.jpg" data-altupdatedat="1788250015000" data-alt="Figure 6"> 
Figure 6. A) Picture of the film clamped with a tweezer showing a natural curvature with the center of the curve in the homeotropic side. B) Picture of the film going to a flat state upon photo-irradiation (365 nm, 0.52 W/cm2). C) Picture of a film irradiated with too high of a light intensity showing a bending at 180 °. <a href="https://cloudflare.jove.com/files/ftp_upload/56266/56266fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Oscillation graph with amplitude-time data and microscope images, showing vibrational analysis results." class="xfigimg" src="/files/ftp_upload/56266/56266fig7.jpg" data-altupdatedat="1788250015000" data-alt="Figure 7"> 
Figure 7. Mechanical Oscillations of the Tip of the Film Over Time During Irradiation with UV Light (LED 365 nm, 0.52 W/cm2). Insets: Screenshots of the moving film recorded with the high-speed camera. The film geometry is 1.7 cm (length) x 0.4 cm (width) x 20 µm (thick). <a href="https://cloudflare.jove.com/files/ftp_upload/56266/56266fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Temperature oscillation graph; heat distribution analysis; thermographic imaging; scientific chart." class="xfigimg" src="/files/ftp_upload/56266/56266fig8.jpg" data-altupdatedat="1788250015000" data-alt="Figure 8"> 
Figure 8. Thermal Oscillations of the Exposed Area (Hinge) Over Time During Irradiation with UV Light (LED 365 nm, 0.52 W/cm2). Insets: Screenshots of the oscillating film with the temperature profile registered with the thermal camera showing the changes of temperature at the hinge. The film geometry is 1.7 cm (length) x 0.4 cm (width) x 20 µm (thick). <a href="https://cloudflare.jove.com/files/ftp_upload/56266/56266fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

The goal of the protocol is to create liquid crystalline polymer films that can mechanically oscillate under continuous light irradiation. We describe in great detail the conception of free-standing films, from the liquid crystal alignment method to the photo-actuation. The experimental protocol applied to prepare this material is broadly applicable.

A strategy based on doped liquid crystalline networks is described to create mechanical self-sustained oscillations of plastic films under continuous ...

The overall goal of this experimental protocol is to prepare liquid crystal networks that show mechanical oscillation under continuous light irradiation. This method can help answer key questions in material science such as how to get continuous microscopic motion with potential applications in soft robotics and automated systems. The main advantage of this technique is that it's uses and methods are widely used in the splay manufacturing industry with commercially available chemicals.First, carefully clean three by three centimeter glass plates using soap and hot water to remove contaminants. Place the glass plates in a beaker with ethanol. Then place the beaker in an ultrasonic bath for about 10 minutes.Following this, carefully dry the glass plates with a tissue and an air blower. Make sure that there is no trace of solvent, dust, or any type of contamination left on the plates. Place the glass plates in a UV ozone photoreactor for 20 minutes in order to remove organic residues.After ozone treatment, blow air on one of the glass plates and place it on a spin coater. Then deposit approximately 0.5 milliliters of polyamide solution onto the glass plate to cover the entire surface. Spin coat the alignment layer according to the following conditions.After spin coating, mark the glass plates on the non-coated side with distinctive signs to recognize the homeotropic and the planar alignment layers. Place the coated glass plate on a hot plate at 110 degrees Celsius for 10 minutes in order to remove the majority of the solvent present in the alignment layer mixture. Once all the glass plates are coated, place them in an oven at 180 degrees Celsius for one hour to cure the polyamide layer.To create microchannels on the coated glass plates, place each planar plane with the coated side down on a velvet cloth. Apply a uniform and soft pressure with two fingers. Carefully drag the glass plate along the surface of the velvet cloth in a straight direction.Then blow air on the glass plate. Next, prepare an adhesive by mixing a UV curing glue with glass beads having a well-defined diameter of 20 micrometers. Place two tiny drops of glue at two adjacent corners of a glass plate coated with a planar alignment layer.Then place two other drops of glue about five millimeters from the other two corners. Place the glass plate coated with a homeotropic alignment layer on top of the first glass plate leaving a gap of about four millimeters between the edges of the plates to provide enough space for the LC mixture. Then cure the glue by placing the two glass plates or cell under UV light for two minutes.Place the cell on a hot plate with the homeotropic side upwards. Set the temperature to 110 degrees Celsius to facilitate the filling of the cell because the fluid is less viscous than in the nematic phase. Now, place part of the LC mixture on the edge of the cell so that the solid melts and the liquid mixture flows by capillary into the cell.Add more mixture at the edge until the cell is filled. Once the cell is filled, slowly cool it down to 90 degrees Celsius to be in the nematic phase. As soon as the film reaches 90 degrees Celsius, polymerize the mixture by placing the cell under UV light at this temperature for 30 minutes.Following polymerization, place the cell on a hot plate at 130 degrees Celsius for about 10 minutes. After allowing the cell to cool to room temperature, open it by placing a razor blade at one edge and pushing it in between the two glass plates. To peel off the film, lift the corners with the razor blade.Gently peel off the film from the glass plate. Then cut a strip along the molecular director of the film. Next, clamp the film sample using a self-closing tweezer in such a way that 1.7 centimeters of the film is free to move.Hold the sample vertically and direct a light emitting diode beam perpendicular to the sample for self-oscillation observation. The observed oscillatory motion of the film under light irradiation is demonstrated in the presented protocol and the quality of the splay alignment is important for self-sustained actuation. The film should be transparent.To verify the correct splay alignment, the film attached to the glass substrate is observed between crossed polarizers above a diffused white light source. When the film is in strips, it presents a natural curvature with the center of the curve in the homeotropic side which is due to the residual stress originating from the polymerization. The mechanical and thermal oscillations registered by a high speed camera confirm the success of the protocol.When the film is clamped, it unbends toward the flat state in the direction of the light. The film then starts to move continuously with mechanical and thermal oscillations. The key factors in observing this phenomenon are the photothermal effect and the self-shadowing of the film controlled by the intensity and position of the light.Too low of a light intensity does not give large bending because the temperature at the hinge is insufficient while too high of a light intensity on the hinge will induce overshooting. Once mastered, this technique can be done in four hours if it is performed properly. While attempting this procedure, it is important to verify the alignment of the liquid crystalline phase before polymerization to ensure that you obtain stable oscillation.The power of this protocol is the ability to vary the frequency and the amplitude of the oscillation by changing the dimensions of the film and the light intensity. After its development, this method paved the way for researchers in the field of material science to explore other equilibrium motion in soft matter. After watching this video, you should have a good understanding of how to prepare aligned liquid crystalline network to obtain photoresponsive film.Don't forget that working with light can be extremely hazardous and UV protection glasses should always be worn while performing these experiments.

preparation-liquid-crystal-networks-for-macroscopic-oscillatory

Research

JoVE Journal

Engineering

We demonstrate the preparation of siloxane-based and epoxy-based liquid crystal elastomers (LCEs) and LCE nanocomposites. The LCEs are characterized with respect to reversible strain, liquid crystal ordering, and stiffness. As a potential application, we demonstrate their use as shape-responsive substrates in a custom device for active cell culture.

Preparation of Monodomain Liquid Crystal Elastomers and Liquid Crystal Elastomer Nanocomposites

Here, we fabricate 3D polymeric micro/nano structures in which both the shape and the molecular alignment can be engineered with nanometer scale accuracy by the use of direct laser writing. Light induced deformation of several types of liquid crystalline elastomer microstructures can be controlled in the microscopic scale.

Free-form Light Actuators &#8212; Fabrication and Control of Actuation in Microscopic Scale

Here, we present a protocol to trigger an orientational transition of a liquid crystal in response to temperature. Methodologies are described for preparing a sample in order to observe the transition and the detailed transitional evolution.

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

Here, we present the protocols of differential-detection analyses of time-resolved infrared vibrational spectroscopy and electron diffraction which enable observations of the deformations of local structures around photoexcited molecules in a columnar liquid crystal, giving an atomic perspective on the relationship between the structure and the dynamics of this photoactive material.

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

This article describes the microfluidic process and parameters to prepare actuating particles from liquid crystalline elastomers. This process allows the preparation of actuating particles and the variation of their size and shape (from oblate to strongly prolate, core-shell, and Janus morphologies) as well as the magnitude of actuation.

Microfluidic Preparation of Liquid Crystalline Elastomer Actuators

A protocol for the fabrication of a reflective cholesteric liquid crystalline display device containing a redox-responsive chiral dopant allowing quick and low-voltage operation is presented.

An Electrochemical Cholesteric Liquid Crystalline Device for Quick and Low-Voltage Color Modulation

This protocol demonstrates the preparation of a photorheological material that exhibits a solid phase, various liquid crystalline phases, and an isotropic liquid phase by increasing temperature. Presented here are methods for measuring the structure-viscoelasticity relationship of the material.

High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal

Here, we present methods for preparing quasi-two-dimensional (2D) entangled, cross-linked, and liquid crystal actin assemblies from purified proteins.

Tuning the Contractility and Deformation Modes of Active Actin-Based Assemblies In Vitro: From Two-Dimensional Active Networks to Liquid Crystal Drops

Use of photonic crystal slow light waveguides and cavities has been widely adopted by the photonics community in many differing applications. Therefore fabrication and characterization of these devices are of great interest. This paper outlines our fabrication technique and two optical characterization methods, namely: interferometric (waveguides) and resonant scattering (cavities).

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

We report on a process of in situ alteration of HF treated Si (001) surface into a hydrophilic or hydrophobic state by irradiating samples in microfluidic chambers filled with H2O2/H2O solution (0.01%-0.5%) or methanol solutions using pulsed UV laser of a relative low pulse fluence.

Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment

Learn to prepare liquid crystal networks for mechanical oscillation under continuous light irradiation. This method enables photoresponsive films with applications in soft robotics and automated systems.

Preparation of Liquid Crystal Networks for Macroscopic Oscillatory Motion Induced by Light

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