Engineering
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Preparation of Liquid Crystal Networks for Macroscopic Oscillatory Motion Induced by Light
Chapters
Summary September 20th, 2017
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.
Transcript
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.
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